Questions over DSI within DRM.

Questions over DSI within DRM.

[Dave Stevenson]

Hi All

I'm trying to get DSI devices working reliably on the Raspberry Pi,
but I'm hitting a number of places where it isn't clear as to the
expected behaviour within DRM.

Power on state. Many devices want the DSI clock and/or data lanes in
LP-11 state when they are powered up. With the normal calling sequence
of:
- panel/bridge pre_enable calls from connector towards the encoder.
- encoder enable which also enables video.
- panel/bridge enable calls from encoder to connector.
there is no point at which the DSI tx is initialised but not
transmitting video. What DSI states are expected to be adopted at each
point?

On a similar theme, some devices want the clock lane in HS mode early
so they can use it in place of an external oscillator, but the data
lanes still in LP-11. There appears to be no way for the
display/bridge to signal this requirement or it be achieved.

host_transfer calls can supposedly be made at any time, however unless
MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
in high speed mode. If this is before a mode has been set, what
defines the link frequency parameters at this point? Adopting a random
default sounds like a good way to get undefined behaviour.

DSI burst mode needs to set the DSI link frequency independently of
the display mode. How is that meant to be configured? I would have
expected it to come from DT due to link frequency often being chosen
based on EMC restrictions, but I don't see such a thing in any
binding.

As a follow on, bridge devices can support burst mode (eg TI's
SN65DSI83 that's just been merged), so it needs to know the desired
panel timings for the output side of the bridge, but the DSI link
timings to set up the bridge's PLL. What's the correct way for
signalling that? drm_crtc_state->adjusted_mode vs
drm_crtc_state->mode? Except mode is userspace's request, not what has
been validated/updated by the panel/bridge.

vc4 has constraints that the DSI host interface is fed off an integer
divider from a typically 3GHz clock, so the host interface needs to
signal that burst mode is in use even if the panel/bridge doesn't need
to run in burst mode. (This does mean that displays that require a
very precise link frequency can not be supported).
It currently updates the adjusted_mode via drm_encoder_helper_funcs
mode_fixup, but is that the correct thing to do, or is there a better
solution?
I'd have expected the DSI tx to be responsible for configuring burst
mode parameters anyway, so the mechanism required would seem to be
just the normal approach for adopting burst mode if that is defined.

Some DSI host interfaces are implemented as bridges, others are
encoders. Pro's and con's of each? I suspect I'm just missing the
history here.

When it comes to the MIPI_DSI_MODE_* flags, which ones are mutually
exclusive, or are assumed based on others? Does a burst mode DSI sink
set both MIPI_DSI_MODE_VIDEO and MIPI_DSI_MODE_VIDEO_BURST, or just
the latter?
Presumably !MIPI_DSI_MODE_VIDEO signals the of use command mode for
conveying video. So looking at panel-ilitek-ili9881c where it sets
just MIPI_DSI_MODE_VIDEO_SYNC_PULSE means command mode video with sync
pulses? That sounds unlikely.

I have looked for any information that covers this, but failed to find
such, hence calling on all your expertise.

Many thanks for your time,
  Dave

Re: Questions over DSI within DRM.

[Laurent Pinchart]

Hi Dave,

(Expanding the CC list a bit)

On Fri, Jul 02, 2021 at 12:03:31PM +0100, Dave Stevenson wrote:
> Hi All
> 
> I'm trying to get DSI devices working reliably on the Raspberry Pi,
> but I'm hitting a number of places where it isn't clear as to the
> expected behaviour within DRM.

Not a surprise. I dread reading the rest of this e-mail though :-)

> Power on state. Many devices want the DSI clock and/or data lanes in
> LP-11 state when they are powered up.

When they are powered up, or when they are enabled ?

> With the normal calling sequence of:
> - panel/bridge pre_enable calls from connector towards the encoder.
> - encoder enable which also enables video.
> - panel/bridge enable calls from encoder to connector.
> there is no point at which the DSI tx is initialised but not
> transmitting video. What DSI states are expected to be adopted at each
> point?

That's undefined I'm afraid, and it should be documented. The upside is
that you can propose the behaviour that you need :-)

> On a similar theme, some devices want the clock lane in HS mode early
> so they can use it in place of an external oscillator, but the data
> lanes still in LP-11. There appears to be no way for the
> display/bridge to signal this requirement or it be achieved.

You're right. A loooong time ago, the omapdrm driver had an internal
infrastructure that didn't use drm_bridge or drm_panel and instead
required omapdrm-specific drivers for those components. It used to model
the display pipeline in a different way than drm_bridge, with the sync
explicitly setting the source state. A DSI sink could thus control its
enable sequence, interleaving programming of the sink with control of
the source.

Migrating omapdrm to the drm_bridge model took a really large effort,
which makes me believe that transitioning the whole subsystem to
sink-controlled sources would be close to impossible. We could add
DSI-specific operations, or add another enable bridge operation
(post_pre_enable ? :-D). Neither would scale, but it may be enough.

> host_transfer calls can supposedly be made at any time, however unless
> MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
> in high speed mode. If this is before a mode has been set, what
> defines the link frequency parameters at this point? Adopting a random
> default sounds like a good way to get undefined behaviour.
> 
> DSI burst mode needs to set the DSI link frequency independently of
> the display mode. How is that meant to be configured? I would have
> expected it to come from DT due to link frequency often being chosen
> based on EMC restrictions, but I don't see such a thing in any
> binding.

Undefined too. DSI support was added to DRM without any design effort,
it's more a hack than a real solution. The issue with devices that can
be controlled over both DSI and I2C is completely unhandled. So far
nobody has really cared about implementing DSI right as far as I can
tell.

> As a follow on, bridge devices can support burst mode (eg TI's
> SN65DSI83 that's just been merged), so it needs to know the desired
> panel timings for the output side of the bridge, but the DSI link
> timings to set up the bridge's PLL. What's the correct way for
> signalling that? drm_crtc_state->adjusted_mode vs
> drm_crtc_state->mode? Except mode is userspace's request, not what has
> been validated/updated by the panel/bridge.

adjusted_mode is also a bit of a hack, it solves very specific issues,
and its design assumes a single encoder in the chain with no extra
bridges. We should instead add modes to the bridge state, and negotiate
modes along the pipeline the same way we negotiate formats.

> vc4 has constraints that the DSI host interface is fed off an integer
> divider from a typically 3GHz clock, so the host interface needs to
> signal that burst mode is in use even if the panel/bridge doesn't need
> to run in burst mode. (This does mean that displays that require a
> very precise link frequency can not be supported).
> It currently updates the adjusted_mode via drm_encoder_helper_funcs
> mode_fixup, but is that the correct thing to do, or is there a better
> solution?
> I'd have expected the DSI tx to be responsible for configuring burst
> mode parameters anyway, so the mechanism required would seem to be
> just the normal approach for adopting burst mode if that is defined.
> 
> Some DSI host interfaces are implemented as bridges, others are
> encoders. Pro's and con's of each? I suspect I'm just missing the
> history here.

It's indeed history. drm_encoder can't go away as it has been erronously
exposed to userspace, but going forward, everything should be a bridge.
The drm_encoder will still be required, but should just be a dummy,
representing the chain of bridges.

> When it comes to the MIPI_DSI_MODE_* flags, which ones are mutually
> exclusive, or are assumed based on others? Does a burst mode DSI sink
> set both MIPI_DSI_MODE_VIDEO and MIPI_DSI_MODE_VIDEO_BURST, or just
> the latter?
> Presumably !MIPI_DSI_MODE_VIDEO signals the of use command mode for
> conveying video. So looking at panel-ilitek-ili9881c where it sets
> just MIPI_DSI_MODE_VIDEO_SYNC_PULSE means command mode video with sync
> pulses? That sounds unlikely.

I haven't looked at that, I'm afraid I don't know.

> I have looked for any information that covers this, but failed to find
> such, hence calling on all your expertise.

I'm sorry for the lack of solutions to your issues. I can try to help
solving them though.

-- 
Regards,

Laurent Pinchart

Re: Questions over DSI within DRM.

[Jani Nikula]

On Fri, 02 Jul 2021, Laurent Pinchart <laurent.pinchart@ideasonboard.com> wrote:
> On Fri, Jul 02, 2021 at 12:03:31PM +0100, Dave Stevenson wrote:
>> Hi All
>> 
>> I'm trying to get DSI devices working reliably on the Raspberry Pi,
>> but I'm hitting a number of places where it isn't clear as to the
>> expected behaviour within DRM.
>
> Not a surprise. I dread reading the rest of this e-mail though :-)

Indeed.

I'll just note that i915 is *not* a prime example to look at. We
basically just abuse the abstractions as helpers. We have one
parametrized panel driver with both the DSI host and device embedded in
i915, instead of one panel device driver per panel.

>> Power on state. Many devices want the DSI clock and/or data lanes in
>> LP-11 state when they are powered up.
>
> When they are powered up, or when they are enabled ?
>
>> With the normal calling sequence of:
>> - panel/bridge pre_enable calls from connector towards the encoder.
>> - encoder enable which also enables video.
>> - panel/bridge enable calls from encoder to connector.
>> there is no point at which the DSI tx is initialised but not
>> transmitting video. What DSI states are expected to be adopted at each
>> point?
>
> That's undefined I'm afraid, and it should be documented. The upside is
> that you can propose the behaviour that you need :-)
>
>> On a similar theme, some devices want the clock lane in HS mode early
>> so they can use it in place of an external oscillator, but the data
>> lanes still in LP-11. There appears to be no way for the
>> display/bridge to signal this requirement or it be achieved.
>
> You're right. A loooong time ago, the omapdrm driver had an internal
> infrastructure that didn't use drm_bridge or drm_panel and instead
> required omapdrm-specific drivers for those components. It used to model
> the display pipeline in a different way than drm_bridge, with the sync
> explicitly setting the source state. A DSI sink could thus control its
> enable sequence, interleaving programming of the sink with control of
> the source.
>
> Migrating omapdrm to the drm_bridge model took a really large effort,
> which makes me believe that transitioning the whole subsystem to
> sink-controlled sources would be close to impossible. We could add
> DSI-specific operations, or add another enable bridge operation
> (post_pre_enable ? :-D). Neither would scale, but it may be enough.

There's also quite a wide range of DSI abstraction in hardware. IIRC in
omapdrm you need to control the LP and ULPS etc. yourself in the driver,
while in i915 the hardware does quite a bunch of stuff for you. Any drm
level abstraction that does not take this into account will get in the
way.

BR,
Jani.

>> host_transfer calls can supposedly be made at any time, however unless
>> MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
>> in high speed mode. If this is before a mode has been set, what
>> defines the link frequency parameters at this point? Adopting a random
>> default sounds like a good way to get undefined behaviour.
>> 
>> DSI burst mode needs to set the DSI link frequency independently of
>> the display mode. How is that meant to be configured? I would have
>> expected it to come from DT due to link frequency often being chosen
>> based on EMC restrictions, but I don't see such a thing in any
>> binding.
>
> Undefined too. DSI support was added to DRM without any design effort,
> it's more a hack than a real solution. The issue with devices that can
> be controlled over both DSI and I2C is completely unhandled. So far
> nobody has really cared about implementing DSI right as far as I can
> tell.
>
>> As a follow on, bridge devices can support burst mode (eg TI's
>> SN65DSI83 that's just been merged), so it needs to know the desired
>> panel timings for the output side of the bridge, but the DSI link
>> timings to set up the bridge's PLL. What's the correct way for
>> signalling that? drm_crtc_state->adjusted_mode vs
>> drm_crtc_state->mode? Except mode is userspace's request, not what has
>> been validated/updated by the panel/bridge.
>
> adjusted_mode is also a bit of a hack, it solves very specific issues,
> and its design assumes a single encoder in the chain with no extra
> bridges. We should instead add modes to the bridge state, and negotiate
> modes along the pipeline the same way we negotiate formats.
>
>> vc4 has constraints that the DSI host interface is fed off an integer
>> divider from a typically 3GHz clock, so the host interface needs to
>> signal that burst mode is in use even if the panel/bridge doesn't need
>> to run in burst mode. (This does mean that displays that require a
>> very precise link frequency can not be supported).
>> It currently updates the adjusted_mode via drm_encoder_helper_funcs
>> mode_fixup, but is that the correct thing to do, or is there a better
>> solution?
>> I'd have expected the DSI tx to be responsible for configuring burst
>> mode parameters anyway, so the mechanism required would seem to be
>> just the normal approach for adopting burst mode if that is defined.
>> 
>> Some DSI host interfaces are implemented as bridges, others are
>> encoders. Pro's and con's of each? I suspect I'm just missing the
>> history here.
>
> It's indeed history. drm_encoder can't go away as it has been erronously
> exposed to userspace, but going forward, everything should be a bridge.
> The drm_encoder will still be required, but should just be a dummy,
> representing the chain of bridges.
>
>> When it comes to the MIPI_DSI_MODE_* flags, which ones are mutually
>> exclusive, or are assumed based on others? Does a burst mode DSI sink
>> set both MIPI_DSI_MODE_VIDEO and MIPI_DSI_MODE_VIDEO_BURST, or just
>> the latter?
>> Presumably !MIPI_DSI_MODE_VIDEO signals the of use command mode for
>> conveying video. So looking at panel-ilitek-ili9881c where it sets
>> just MIPI_DSI_MODE_VIDEO_SYNC_PULSE means command mode video with sync
>> pulses? That sounds unlikely.
>
> I haven't looked at that, I'm afraid I don't know.
>
>> I have looked for any information that covers this, but failed to find
>> such, hence calling on all your expertise.
>
> I'm sorry for the lack of solutions to your issues. I can try to help
> solving them though.

-- 
Jani Nikula, Intel Open Source Graphics Center

Re: Questions over DSI within DRM.

[Jagan Teki]

On Fri, Jul 2, 2021 at 10:12 PM Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
>
> Hi Dave,
>
> (Expanding the CC list a bit)
>
> On Fri, Jul 02, 2021 at 12:03:31PM +0100, Dave Stevenson wrote:
> > Hi All
> >
> > I'm trying to get DSI devices working reliably on the Raspberry Pi,
> > but I'm hitting a number of places where it isn't clear as to the
> > expected behaviour within DRM.
>
> Not a surprise. I dread reading the rest of this e-mail though :-)
>
> > Power on state. Many devices want the DSI clock and/or data lanes in
> > LP-11 state when they are powered up.
>
> When they are powered up, or when they are enabled ?
>
> > With the normal calling sequence of:
> > - panel/bridge pre_enable calls from connector towards the encoder.
> > - encoder enable which also enables video.
> > - panel/bridge enable calls from encoder to connector.
> > there is no point at which the DSI tx is initialised but not
> > transmitting video. What DSI states are expected to be adopted at each
> > point?
>
> That's undefined I'm afraid, and it should be documented. The upside is
> that you can propose the behaviour that you need :-)
>
> > On a similar theme, some devices want the clock lane in HS mode early
> > so they can use it in place of an external oscillator, but the data
> > lanes still in LP-11. There appears to be no way for the
> > display/bridge to signal this requirement or it be achieved.
>
> You're right. A loooong time ago, the omapdrm driver had an internal
> infrastructure that didn't use drm_bridge or drm_panel and instead
> required omapdrm-specific drivers for those components. It used to model
> the display pipeline in a different way than drm_bridge, with the sync
> explicitly setting the source state. A DSI sink could thus control its
> enable sequence, interleaving programming of the sink with control of
> the source.
>
> Migrating omapdrm to the drm_bridge model took a really large effort,
> which makes me believe that transitioning the whole subsystem to
> sink-controlled sources would be close to impossible. We could add
> DSI-specific operations, or add another enable bridge operation
> (post_pre_enable ? :-D). Neither would scale, but it may be enough.
>
> > host_transfer calls can supposedly be made at any time, however unless
> > MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
> > in high speed mode. If this is before a mode has been set, what
> > defines the link frequency parameters at this point? Adopting a random
> > default sounds like a good way to get undefined behaviour.
> >
> > DSI burst mode needs to set the DSI link frequency independently of
> > the display mode. How is that meant to be configured? I would have
> > expected it to come from DT due to link frequency often being chosen
> > based on EMC restrictions, but I don't see such a thing in any
> > binding.
>
> Undefined too. DSI support was added to DRM without any design effort,
> it's more a hack than a real solution. The issue with devices that can
> be controlled over both DSI and I2C is completely unhandled. So far
> nobody has really cared about implementing DSI right as far as I can
> tell.
>
> > As a follow on, bridge devices can support burst mode (eg TI's
> > SN65DSI83 that's just been merged), so it needs to know the desired
> > panel timings for the output side of the bridge, but the DSI link
> > timings to set up the bridge's PLL. What's the correct way for
> > signalling that? drm_crtc_state->adjusted_mode vs
> > drm_crtc_state->mode? Except mode is userspace's request, not what has
> > been validated/updated by the panel/bridge.
>
> adjusted_mode is also a bit of a hack, it solves very specific issues,
> and its design assumes a single encoder in the chain with no extra
> bridges. We should instead add modes to the bridge state, and negotiate
> modes along the pipeline the same way we negotiate formats.
>
> > vc4 has constraints that the DSI host interface is fed off an integer
> > divider from a typically 3GHz clock, so the host interface needs to
> > signal that burst mode is in use even if the panel/bridge doesn't need
> > to run in burst mode. (This does mean that displays that require a
> > very precise link frequency can not be supported).
> > It currently updates the adjusted_mode via drm_encoder_helper_funcs
> > mode_fixup, but is that the correct thing to do, or is there a better
> > solution?
> > I'd have expected the DSI tx to be responsible for configuring burst
> > mode parameters anyway, so the mechanism required would seem to be
> > just the normal approach for adopting burst mode if that is defined.
> >
> > Some DSI host interfaces are implemented as bridges, others are
> > encoders. Pro's and con's of each? I suspect I'm just missing the
> > history here.
>
> It's indeed history. drm_encoder can't go away as it has been erronously
> exposed to userspace, but going forward, everything should be a bridge.
> The drm_encoder will still be required, but should just be a dummy,
> representing the chain of bridges.

Just to add further on this with some example, thought it might help.

NWL DSI initially started with encoder and eventually become bridge
driven in mainline.
https://lore.kernel.org/lkml/13373313.BzCyiC4ED7@jernej-laptop/t/

Exynos DSI becoming Samsung MIPI DSIM bridge
https://patchwork.kernel.org/project/dri-devel/cover/20210704090230.26489-1-jagan@amarulasolutions.com/

Thanks,
Jagan.

Re: Questions over DSI within DRM.

[Dave Stevenson]

Hi Laurent.

Thanks to you, Jani, and Jagan for your replies.
I'm replying to Laurent's email as it has the greatest number of
discussion points.

Noted that NWL DSI and Exynos DSI have undergone the conversion to
bridges - hopefully I can take those as vague examples.

On Fri, 2 Jul 2021 at 17:42, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
>
> Hi Dave,
>
> (Expanding the CC list a bit)
>
> On Fri, Jul 02, 2021 at 12:03:31PM +0100, Dave Stevenson wrote:
> > Hi All
> >
> > I'm trying to get DSI devices working reliably on the Raspberry Pi,
> > but I'm hitting a number of places where it isn't clear as to the
> > expected behaviour within DRM.
>
> Not a surprise. I dread reading the rest of this e-mail though :-)
>
> > Power on state. Many devices want the DSI clock and/or data lanes in
> > LP-11 state when they are powered up.
>
> When they are powered up, or when they are enabled ?

Generally powered on, so it wants to be in before pre_enable.

Taking SN65DSI83 as an example, the datasheet section 7.4.2
Initialization Sequence says
"After power is applied and stable, the DSI CLK lanes MUST be in HS
state and the DSI data lanes MUST be driven to LP11 state"

It was discussed when Marek's was developing the merged driver that it
seems to be happy to violate this initialisation sequence by starting
video on the data lanes before configuring the DSI83, but is that more
luck than anything else?

Reading the functional spec for Toshiba TC358762 (DSI to DPI bridge
used on the Pi 7" DSI panel), it says:
"1.HW:  Put  D-PHY  data  lane  0 to  ULPS  state
  a)  D-PHY  clock  lane  must  continue  to  supply  clock  for
minimum  20  clocks  before put  the  clock  lane  to  idle
2. HW:  Enable  Clock  Lane  and  Data  Lane  0 D-PHYs
3. HW:  Disable  Data  Lane  1 D-PHY
4. HW:  Transition  to  Operation  mode  when  Data  Lane  0 D-PHY  is
 not  in  ULPS  state.
  a) D-PHY clock lane must supply before Data Lane 0 D-PHY changes to LP state
  b) Host must wait minimum 5200 HSBCLK clock for CORE2 Power to be
stable before transfer any DSI packets."

So there we have a timing constraint from pushing the clock lane into
HS before we can do anything else. Quick calcs do appear to say this
is around 0.5msecs, so possibly it can be ignored.

> > With the normal calling sequence of:
> > - panel/bridge pre_enable calls from connector towards the encoder.
> > - encoder enable which also enables video.
> > - panel/bridge enable calls from encoder to connector.
> > there is no point at which the DSI tx is initialised but not
> > transmitting video. What DSI states are expected to be adopted at each
> > point?
>
> That's undefined I'm afraid, and it should be documented. The upside is
> that you can propose the behaviour that you need :-)

Can we reduce it to one behaviour that is valid for all devices? I
suspect we need at least some set of options :-/

> > On a similar theme, some devices want the clock lane in HS mode early
> > so they can use it in place of an external oscillator, but the data
> > lanes still in LP-11. There appears to be no way for the
> > display/bridge to signal this requirement or it be achieved.
>
> You're right. A loooong time ago, the omapdrm driver had an internal
> infrastructure that didn't use drm_bridge or drm_panel and instead
> required omapdrm-specific drivers for those components. It used to model
> the display pipeline in a different way than drm_bridge, with the sync
> explicitly setting the source state. A DSI sink could thus control its
> enable sequence, interleaving programming of the sink with control of
> the source.
>
> Migrating omapdrm to the drm_bridge model took a really large effort,
> which makes me believe that transitioning the whole subsystem to
> sink-controlled sources would be close to impossible. We could add
> DSI-specific operations, or add another enable bridge operation
> (post_pre_enable ? :-D). Neither would scale, but it may be enough.

I haven't thought it through for all generic cases, but I suspect it's
more a pre_pre_enable that is needed to initialise the PHY etc,
probably from source to sink.
If the panel/bridge can set a flag that can be checked at this point
for whether an early clock is required or not, I think that allows us
to comply with the requirements for a large number of panels/bridges
(LP-11 vs HS config for clock and or data lanes before pre_enable is
called).

pre_enable retains the current behaviour (initialise the chain from
sink to source).
enable then actually starts sending video and enabling outputs.

When I discussed this briefly with Maxime there was a suggestion of
using pm_runtime to be able to power up the pipeline as a whole. If
the bridge driver can use pm_runtime to power up the PHY when
required, then that may solve the issue, however I know too little of
the details to say whether that is actually practical.

> > host_transfer calls can supposedly be made at any time, however unless
> > MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
> > in high speed mode. If this is before a mode has been set, what
> > defines the link frequency parameters at this point? Adopting a random
> > default sounds like a good way to get undefined behaviour.
> >
> > DSI burst mode needs to set the DSI link frequency independently of
> > the display mode. How is that meant to be configured? I would have
> > expected it to come from DT due to link frequency often being chosen
> > based on EMC restrictions, but I don't see such a thing in any
> > binding.
>
> Undefined too. DSI support was added to DRM without any design effort,
> it's more a hack than a real solution. The issue with devices that can
> be controlled over both DSI and I2C is completely unhandled. So far
> nobody has really cared about implementing DSI right as far as I can
> tell.

:-(

Thinking aloud, does having the option to set a burst link frequency
from DT (or ACPI) have any issue for other platforms?
Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
all the defined timings, but drop to LP during blanking" option. The
link frequency has therefore remained a property of the
display/bridge.
dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
through the full detail of the parameters it computes from there.

DSI and I2C controlled devices is yet another issue that I haven't
even looked at.
I think it's more that vc4 wants to ignore DSI should the DSI host
node be enabled in DT, but there's no panel bound to it. One could say
that is a DT error and tough luck, but from a user's perspective that
is a bit harsh.

> > As a follow on, bridge devices can support burst mode (eg TI's
> > SN65DSI83 that's just been merged), so it needs to know the desired
> > panel timings for the output side of the bridge, but the DSI link
> > timings to set up the bridge's PLL. What's the correct way for
> > signalling that? drm_crtc_state->adjusted_mode vs
> > drm_crtc_state->mode? Except mode is userspace's request, not what has
> > been validated/updated by the panel/bridge.
>
> adjusted_mode is also a bit of a hack, it solves very specific issues,
> and its design assumes a single encoder in the chain with no extra
> bridges. We should instead add modes to the bridge state, and negotiate
> modes along the pipeline the same way we negotiate formats.

So as I understand it we already have format negotiation between
bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
so is it possible to extend that to modes?
Are you thinking bridge state that is owned by the framework, or by
the individual bridge drivers?

> > vc4 has constraints that the DSI host interface is fed off an integer
> > divider from a typically 3GHz clock, so the host interface needs to
> > signal that burst mode is in use even if the panel/bridge doesn't need
> > to run in burst mode. (This does mean that displays that require a
> > very precise link frequency can not be supported).
> > It currently updates the adjusted_mode via drm_encoder_helper_funcs
> > mode_fixup, but is that the correct thing to do, or is there a better
> > solution?
> > I'd have expected the DSI tx to be responsible for configuring burst
> > mode parameters anyway, so the mechanism required would seem to be
> > just the normal approach for adopting burst mode if that is defined.
> >
> > Some DSI host interfaces are implemented as bridges, others are
> > encoders. Pro's and con's of each? I suspect I'm just missing the
> > history here.
>
> It's indeed history. drm_encoder can't go away as it has been erronously
> exposed to userspace, but going forward, everything should be a bridge.
> The drm_encoder will still be required, but should just be a dummy,
> representing the chain of bridges.

Thanks to the links from Jagan I can have a look at what is required
to make that change.
I'm not certain it solves any of my issues at present, but at least
brings it into line with the intended implementation.

> > When it comes to the MIPI_DSI_MODE_* flags, which ones are mutually
> > exclusive, or are assumed based on others? Does a burst mode DSI sink
> > set both MIPI_DSI_MODE_VIDEO and MIPI_DSI_MODE_VIDEO_BURST, or just
> > the latter?
> > Presumably !MIPI_DSI_MODE_VIDEO signals the of use command mode for
> > conveying video. So looking at panel-ilitek-ili9881c where it sets
> > just MIPI_DSI_MODE_VIDEO_SYNC_PULSE means command mode video with sync
> > pulses? That sounds unlikely.
>
> I haven't looked at that, I'm afraid I don't know.

I have a suspicion it's been a bit of a scatter-gun in setting flags,
but it'd be nice to have it written down as to the expected behaviour.

> > I have looked for any information that covers this, but failed to find
> > such, hence calling on all your expertise.
>
> I'm sorry for the lack of solutions to your issues. I can try to help
> solving them though.

Any help would be most gratefully received.
I can attack the Pi specific stuff, and delegate framework sections to
Maxime via our contract with him (sorry Maxime!), but having a clear
statement of how this should be achieved is the bit that I'm afraid I
don't have the patience to bat around ad infinitum.

Thanks again.
  Dave

Re: Questions over DSI within DRM.

[Maxime Ripard]

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Hi Dave,

On Mon, Jul 05, 2021 at 04:36:25PM +0100, Dave Stevenson wrote:
> Hi Laurent.
> 
> Thanks to you, Jani, and Jagan for your replies.
> I'm replying to Laurent's email as it has the greatest number of
> discussion points.
> 
> Noted that NWL DSI and Exynos DSI have undergone the conversion to
> bridges - hopefully I can take those as vague examples.
> 
> On Fri, 2 Jul 2021 at 17:42, Laurent Pinchart
> <laurent.pinchart@ideasonboard.com> wrote:
> >
> > Hi Dave,
> >
> > (Expanding the CC list a bit)
> >
> > On Fri, Jul 02, 2021 at 12:03:31PM +0100, Dave Stevenson wrote:
> > > Hi All
> > >
> > > I'm trying to get DSI devices working reliably on the Raspberry Pi,
> > > but I'm hitting a number of places where it isn't clear as to the
> > > expected behaviour within DRM.
> >
> > Not a surprise. I dread reading the rest of this e-mail though :-)
> >
> > > Power on state. Many devices want the DSI clock and/or data lanes in
> > > LP-11 state when they are powered up.
> >
> > When they are powered up, or when they are enabled ?
> 
> Generally powered on, so it wants to be in before pre_enable.
> 
> Taking SN65DSI83 as an example, the datasheet section 7.4.2
> Initialization Sequence says
> "After power is applied and stable, the DSI CLK lanes MUST be in HS
> state and the DSI data lanes MUST be driven to LP11 state"
> 
> It was discussed when Marek's was developing the merged driver that it
> seems to be happy to violate this initialisation sequence by starting
> video on the data lanes before configuring the DSI83, but is that more
> luck than anything else?
> 
> Reading the functional spec for Toshiba TC358762 (DSI to DPI bridge
> used on the Pi 7" DSI panel), it says:
> "1.HW:  Put  D-PHY  data  lane  0 to  ULPS  state
>   a)  D-PHY  clock  lane  must  continue  to  supply  clock  for
> minimum  20  clocks  before put  the  clock  lane  to  idle
> 2. HW:  Enable  Clock  Lane  and  Data  Lane  0 D-PHYs
> 3. HW:  Disable  Data  Lane  1 D-PHY
> 4. HW:  Transition  to  Operation  mode  when  Data  Lane  0 D-PHY  is
>  not  in  ULPS  state.
>   a) D-PHY clock lane must supply before Data Lane 0 D-PHY changes to LP state
>   b) Host must wait minimum 5200 HSBCLK clock for CORE2 Power to be
> stable before transfer any DSI packets."
> 
> So there we have a timing constraint from pushing the clock lane into
> HS before we can do anything else. Quick calcs do appear to say this
> is around 0.5msecs, so possibly it can be ignored.
> 
> > > With the normal calling sequence of:
> > > - panel/bridge pre_enable calls from connector towards the encoder.
> > > - encoder enable which also enables video.
> > > - panel/bridge enable calls from encoder to connector.
> > > there is no point at which the DSI tx is initialised but not
> > > transmitting video. What DSI states are expected to be adopted at each
> > > point?
> >
> > That's undefined I'm afraid, and it should be documented. The upside is
> > that you can propose the behaviour that you need :-)
> 
> Can we reduce it to one behaviour that is valid for all devices? I
> suspect we need at least some set of options :-/

One thing to consider as well is that we might not be able to even
implement the proper behavior due to lack of documentation (or just that
the hardware cannot implement it).

If we multiply the callbacks to accommodate for any panel and downstream
bridges power sequence, in order to properly support any combination
then all drivers will need to implement all the hooks properly (and I'm
guessing we would have some sequence variation too). Chances are we will
end up with a combinatorial explosion of sequences, and this will be a
nightmare to test. And that's again assuming that we can implement them
in the first place.

> > > On a similar theme, some devices want the clock lane in HS mode early
> > > so they can use it in place of an external oscillator, but the data
> > > lanes still in LP-11. There appears to be no way for the
> > > display/bridge to signal this requirement or it be achieved.
> >
> > You're right. A loooong time ago, the omapdrm driver had an internal
> > infrastructure that didn't use drm_bridge or drm_panel and instead
> > required omapdrm-specific drivers for those components. It used to model
> > the display pipeline in a different way than drm_bridge, with the sync
> > explicitly setting the source state. A DSI sink could thus control its
> > enable sequence, interleaving programming of the sink with control of
> > the source.
> >
> > Migrating omapdrm to the drm_bridge model took a really large effort,
> > which makes me believe that transitioning the whole subsystem to
> > sink-controlled sources would be close to impossible. We could add
> > DSI-specific operations, or add another enable bridge operation
> > (post_pre_enable ? :-D). Neither would scale, but it may be enough.
> 
> I haven't thought it through for all generic cases, but I suspect it's
> more a pre_pre_enable that is needed to initialise the PHY etc,
> probably from source to sink.
> If the panel/bridge can set a flag that can be checked at this point
> for whether an early clock is required or not, I think that allows us
> to comply with the requirements for a large number of panels/bridges
> (LP-11 vs HS config for clock and or data lanes before pre_enable is
> called).
> 
> pre_enable retains the current behaviour (initialise the chain from
> sink to source).
> enable then actually starts sending video and enabling outputs.

Flags indeed seem like a more contained option. Another one could be to
have a mipi_dsi_host to (for example) power up the clock lane that would
be called by default before the bridge's enable, or at the downstream
bridge driver discretion before that.

> When I discussed this briefly with Maxime there was a suggestion of
> using pm_runtime to be able to power up the pipeline as a whole. If
> the bridge driver can use pm_runtime to power up the PHY when
> required, then that may solve the issue, however I know too little of
> the details to say whether that is actually practical.

I'm not sure it was about this topic in particular. If I remember well
our discussion, this was about the vc4 driver that tries to circumvent
the framework and call the pre_enable and enable hooks itself because it
wasn't properly powered before and thus any DCS-related call by the
downstream bridge or panel would end up creating a CPU stall.

I suggested to use runtime_pm in the DCS related calls to make sure the
device is powered because there's no relation between the state of the
downstream bridge or panel and whether it can send DCS commands or not.
For all we know, it could do it at probe time.

> > > host_transfer calls can supposedly be made at any time, however unless
> > > MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
> > > in high speed mode. If this is before a mode has been set, what
> > > defines the link frequency parameters at this point? Adopting a random
> > > default sounds like a good way to get undefined behaviour.
> > >
> > > DSI burst mode needs to set the DSI link frequency independently of
> > > the display mode. How is that meant to be configured? I would have
> > > expected it to come from DT due to link frequency often being chosen
> > > based on EMC restrictions, but I don't see such a thing in any
> > > binding.
> >
> > Undefined too. DSI support was added to DRM without any design effort,
> > it's more a hack than a real solution. The issue with devices that can
> > be controlled over both DSI and I2C is completely unhandled. So far
> > nobody has really cared about implementing DSI right as far as I can
> > tell.
> 
> :-(
> 
> Thinking aloud, does having the option to set a burst link frequency
> from DT (or ACPI) have any issue for other platforms?
> Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
> drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
> all the defined timings, but drop to LP during blanking" option. The
> link frequency has therefore remained a property of the
> display/bridge.
> dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
> through the full detail of the parameters it computes from there.

I don't see anything wrong with using link-frequency from the DT to
setup the burst frequency. It's what v4l2 has been using for a while
without any known (to me) drawback, and we're using the same of-graph
bindings, so it shouldn't be too controversial there.

> DSI and I2C controlled devices is yet another issue that I haven't
> even looked at.
> I think it's more that vc4 wants to ignore DSI should the DSI host
> node be enabled in DT, but there's no panel bound to it. One could say
> that is a DT error and tough luck, but from a user's perspective that
> is a bit harsh.

I guess the larger "issue" is that the tree in the DT is done following
the "control" bus, and Linux likes to tie the life cycle of a given
device to its parent bus. Both these decisions make sense, but they
interact in a weird way in some occurrences (like this one, or Allwinner
has an Ethernet PHY controlled through MMIO which end up in the same
case).

I wonder if using device links here could help though.

> > > As a follow on, bridge devices can support burst mode (eg TI's
> > > SN65DSI83 that's just been merged), so it needs to know the desired
> > > panel timings for the output side of the bridge, but the DSI link
> > > timings to set up the bridge's PLL. What's the correct way for
> > > signalling that? drm_crtc_state->adjusted_mode vs
> > > drm_crtc_state->mode? Except mode is userspace's request, not what has
> > > been validated/updated by the panel/bridge.
> >
> > adjusted_mode is also a bit of a hack, it solves very specific issues,
> > and its design assumes a single encoder in the chain with no extra
> > bridges. We should instead add modes to the bridge state, and negotiate
> > modes along the pipeline the same way we negotiate formats.
> 
> So as I understand it we already have format negotiation between
> bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
> so is it possible to extend that to modes?
> Are you thinking bridge state that is owned by the framework, or by
> the individual bridge drivers?

atomic_check is made for that. I guess we could improve its call
sequence to each time a mode is modified along the bridge list we
restart the sequence until all components agree (or reject it entirely
if they can't), but I don't really see why we would need yet another
hook.

Maxime

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Re: Questions over DSI within DRM.

[Dave Stevenson]

Hi Maxime

On Tue, 6 Jul 2021 at 16:13, Maxime Ripard <maxime@cerno.tech> wrote:
>
> Hi Dave,
>
> On Mon, Jul 05, 2021 at 04:36:25PM +0100, Dave Stevenson wrote:
> > Hi Laurent.
> >
> > Thanks to you, Jani, and Jagan for your replies.
> > I'm replying to Laurent's email as it has the greatest number of
> > discussion points.
> >
> > Noted that NWL DSI and Exynos DSI have undergone the conversion to
> > bridges - hopefully I can take those as vague examples.
> >
> > On Fri, 2 Jul 2021 at 17:42, Laurent Pinchart
> > <laurent.pinchart@ideasonboard.com> wrote:
> > >
> > > Hi Dave,
> > >
> > > (Expanding the CC list a bit)
> > >
> > > On Fri, Jul 02, 2021 at 12:03:31PM +0100, Dave Stevenson wrote:
> > > > Hi All
> > > >
> > > > I'm trying to get DSI devices working reliably on the Raspberry Pi,
> > > > but I'm hitting a number of places where it isn't clear as to the
> > > > expected behaviour within DRM.
> > >
> > > Not a surprise. I dread reading the rest of this e-mail though :-)
> > >
> > > > Power on state. Many devices want the DSI clock and/or data lanes in
> > > > LP-11 state when they are powered up.
> > >
> > > When they are powered up, or when they are enabled ?
> >
> > Generally powered on, so it wants to be in before pre_enable.
> >
> > Taking SN65DSI83 as an example, the datasheet section 7.4.2
> > Initialization Sequence says
> > "After power is applied and stable, the DSI CLK lanes MUST be in HS
> > state and the DSI data lanes MUST be driven to LP11 state"
> >
> > It was discussed when Marek's was developing the merged driver that it
> > seems to be happy to violate this initialisation sequence by starting
> > video on the data lanes before configuring the DSI83, but is that more
> > luck than anything else?
> >
> > Reading the functional spec for Toshiba TC358762 (DSI to DPI bridge
> > used on the Pi 7" DSI panel), it says:
> > "1.HW:  Put  D-PHY  data  lane  0 to  ULPS  state
> >   a)  D-PHY  clock  lane  must  continue  to  supply  clock  for
> > minimum  20  clocks  before put  the  clock  lane  to  idle
> > 2. HW:  Enable  Clock  Lane  and  Data  Lane  0 D-PHYs
> > 3. HW:  Disable  Data  Lane  1 D-PHY
> > 4. HW:  Transition  to  Operation  mode  when  Data  Lane  0 D-PHY  is
> >  not  in  ULPS  state.
> >   a) D-PHY clock lane must supply before Data Lane 0 D-PHY changes to LP state
> >   b) Host must wait minimum 5200 HSBCLK clock for CORE2 Power to be
> > stable before transfer any DSI packets."
> >
> > So there we have a timing constraint from pushing the clock lane into
> > HS before we can do anything else. Quick calcs do appear to say this
> > is around 0.5msecs, so possibly it can be ignored.
> >
> > > > With the normal calling sequence of:
> > > > - panel/bridge pre_enable calls from connector towards the encoder.
> > > > - encoder enable which also enables video.
> > > > - panel/bridge enable calls from encoder to connector.
> > > > there is no point at which the DSI tx is initialised but not
> > > > transmitting video. What DSI states are expected to be adopted at each
> > > > point?
> > >
> > > That's undefined I'm afraid, and it should be documented. The upside is
> > > that you can propose the behaviour that you need :-)
> >
> > Can we reduce it to one behaviour that is valid for all devices? I
> > suspect we need at least some set of options :-/
>
> One thing to consider as well is that we might not be able to even
> implement the proper behavior due to lack of documentation (or just that
> the hardware cannot implement it).
>
> If we multiply the callbacks to accommodate for any panel and downstream
> bridges power sequence, in order to properly support any combination
> then all drivers will need to implement all the hooks properly (and I'm
> guessing we would have some sequence variation too). Chances are we will
> end up with a combinatorial explosion of sequences, and this will be a
> nightmare to test. And that's again assuming that we can implement them
> in the first place.

Both possibly true.
At the moment the biggest hole I see is that there is no chance to set
the initial state of the PHY in any shape or form before also starting
video.

Existing drivers that don't implement the new hook to initialise the
PHY state would still work as they currently do (undefined behaviour).

> > > > On a similar theme, some devices want the clock lane in HS mode early
> > > > so they can use it in place of an external oscillator, but the data
> > > > lanes still in LP-11. There appears to be no way for the
> > > > display/bridge to signal this requirement or it be achieved.
> > >
> > > You're right. A loooong time ago, the omapdrm driver had an internal
> > > infrastructure that didn't use drm_bridge or drm_panel and instead
> > > required omapdrm-specific drivers for those components. It used to model
> > > the display pipeline in a different way than drm_bridge, with the sync
> > > explicitly setting the source state. A DSI sink could thus control its
> > > enable sequence, interleaving programming of the sink with control of
> > > the source.
> > >
> > > Migrating omapdrm to the drm_bridge model took a really large effort,
> > > which makes me believe that transitioning the whole subsystem to
> > > sink-controlled sources would be close to impossible. We could add
> > > DSI-specific operations, or add another enable bridge operation
> > > (post_pre_enable ? :-D). Neither would scale, but it may be enough.
> >
> > I haven't thought it through for all generic cases, but I suspect it's
> > more a pre_pre_enable that is needed to initialise the PHY etc,
> > probably from source to sink.
> > If the panel/bridge can set a flag that can be checked at this point
> > for whether an early clock is required or not, I think that allows us
> > to comply with the requirements for a large number of panels/bridges
> > (LP-11 vs HS config for clock and or data lanes before pre_enable is
> > called).
> >
> > pre_enable retains the current behaviour (initialise the chain from
> > sink to source).
> > enable then actually starts sending video and enabling outputs.
>
> Flags indeed seem like a more contained option. Another one could be to
> have a mipi_dsi_host to (for example) power up the clock lane that would
> be called by default before the bridge's enable, or at the downstream
> bridge driver discretion before that.

Which driver will that call?
An extreme example perhaps, but Toshiba make the TC358860 eDP to DSI
bridge chip[1]. So the encoder will be eDP, but the DSI config needs
to go to that bridge. Does that happen automatically within the
framework? I guess so as the bridge will have called
mipi_dsi_host_register for the DSI sink to attach to.

Perhaps a new mipi_dsi_host function to configure the PHY is the
easier solution. If it can allow the sink to request whatever
combination of states from clock and data lanes that it fancies, then
it can be as explicit as required for the initialisation sequence, and
the host driver does its best to comply with the requests.
I'd have a slight query over when and how the host would drop to ULPS
or power off. It probably shouldn't be in post_disable as the sink
hasn't had a chance to finalise everything in its post_disable.
Perhaps pm_runtime with autosuspend is the right call there?

[1] https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html

> > When I discussed this briefly with Maxime there was a suggestion of
> > using pm_runtime to be able to power up the pipeline as a whole. If
> > the bridge driver can use pm_runtime to power up the PHY when
> > required, then that may solve the issue, however I know too little of
> > the details to say whether that is actually practical.
>
> I'm not sure it was about this topic in particular. If I remember well
> our discussion, this was about the vc4 driver that tries to circumvent
> the framework and call the pre_enable and enable hooks itself because it
> wasn't properly powered before and thus any DCS-related call by the
> downstream bridge or panel would end up creating a CPU stall.
>
> I suggested to use runtime_pm in the DCS related calls to make sure the
> device is powered because there's no relation between the state of the
> downstream bridge or panel and whether it can send DCS commands or not.
> For all we know, it could do it at probe time.

pm_runtime is all a bit of a magic black box to me.

We had discussed shifting to using pm_runtime from DCS (and enable)
calls to power up the PHY on demand, and that's what I implemented.
However Laurent flagged up that using
dsi->encoder->crtc->state->adjusted_mode to get the HS clock info
required to send a HS DCS command from that call is deprecated, so how
do we specify the clock rate to use at that point?

> > > > host_transfer calls can supposedly be made at any time, however unless
> > > > MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
> > > > in high speed mode. If this is before a mode has been set, what
> > > > defines the link frequency parameters at this point? Adopting a random
> > > > default sounds like a good way to get undefined behaviour.
> > > >
> > > > DSI burst mode needs to set the DSI link frequency independently of
> > > > the display mode. How is that meant to be configured? I would have
> > > > expected it to come from DT due to link frequency often being chosen
> > > > based on EMC restrictions, but I don't see such a thing in any
> > > > binding.
> > >
> > > Undefined too. DSI support was added to DRM without any design effort,
> > > it's more a hack than a real solution. The issue with devices that can
> > > be controlled over both DSI and I2C is completely unhandled. So far
> > > nobody has really cared about implementing DSI right as far as I can
> > > tell.
> >
> > :-(
> >
> > Thinking aloud, does having the option to set a burst link frequency
> > from DT (or ACPI) have any issue for other platforms?
> > Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
> > drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
> > all the defined timings, but drop to LP during blanking" option. The
> > link frequency has therefore remained a property of the
> > display/bridge.
> > dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
> > through the full detail of the parameters it computes from there.
>
> I don't see anything wrong with using link-frequency from the DT to
> setup the burst frequency. It's what v4l2 has been using for a while
> without any known (to me) drawback, and we're using the same of-graph
> bindings, so it shouldn't be too controversial there.

OK, that sounds like a vague plan.

> > DSI and I2C controlled devices is yet another issue that I haven't
> > even looked at.
> > I think it's more that vc4 wants to ignore DSI should the DSI host
> > node be enabled in DT, but there's no panel bound to it. One could say
> > that is a DT error and tough luck, but from a user's perspective that
> > is a bit harsh.
>
> I guess the larger "issue" is that the tree in the DT is done following
> the "control" bus, and Linux likes to tie the life cycle of a given
> device to its parent bus. Both these decisions make sense, but they
> interact in a weird way in some occurrences (like this one, or Allwinner
> has an Ethernet PHY controlled through MMIO which end up in the same
> case).
>
> I wonder if using device links here could help though.

I really don't know about that one.

> > > > As a follow on, bridge devices can support burst mode (eg TI's
> > > > SN65DSI83 that's just been merged), so it needs to know the desired
> > > > panel timings for the output side of the bridge, but the DSI link
> > > > timings to set up the bridge's PLL. What's the correct way for
> > > > signalling that? drm_crtc_state->adjusted_mode vs
> > > > drm_crtc_state->mode? Except mode is userspace's request, not what has
> > > > been validated/updated by the panel/bridge.
> > >
> > > adjusted_mode is also a bit of a hack, it solves very specific issues,
> > > and its design assumes a single encoder in the chain with no extra
> > > bridges. We should instead add modes to the bridge state, and negotiate
> > > modes along the pipeline the same way we negotiate formats.
> >
> > So as I understand it we already have format negotiation between
> > bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
> > so is it possible to extend that to modes?
> > Are you thinking bridge state that is owned by the framework, or by
> > the individual bridge drivers?
>
> atomic_check is made for that. I guess we could improve its call
> sequence to each time a mode is modified along the bridge list we
> restart the sequence until all components agree (or reject it entirely
> if they can't), but I don't really see why we would need yet another
> hook.

Why do all nodes in the bridge list need to agree? Adjacent nodes need
to agree, but they then also need to retain that agreed timing
somewhere.

Taking SN65DSI8[3|4|5] as an example, it supports burst mode, and the
DSI frequency and timings are permitted to be different from that
which it uses on the LVDS side. The LVDS panel and LVDS side of DSI83
need to agree over the format, and the DSI host and DSI side of DSI83
need to agree, but they may be two different timings.
Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows you to
configure the LVDS rate compared to the DSI rate (the driver currently
goes for 1:1), and registers 0x20 to 0x34 allow you to set the number
of active pixel and blanking on the LVDS side (again currently just
copied across).

The way I'm seeing burst mode as having been interpreted at present is
that it's largely just a flag to say "drop to LP mode between lines".
The timing that needs to be passed to the crtc is therefore going to
be based on the DSI link rate (converted to pixels) with increased
blanking periods.

I guess there are similarities with Media Controller and V4L2 here. A
typical chain there could be:
 sensor -> scaler -> crop -> CSI-2 receiver.
The format on each of those links may be different, but the chain as a
whole needs to be valid. Media Controller largely relies on userspace
to configure all links, but with a DRM chain that isn't really an
option as it's expected that the display chain configures itself.

Thanks
  Dave

Re: Questions over DSI within DRM.

[Maxime Ripard]

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Hi Dave,

On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
> On Tue, 6 Jul 2021 at 16:13, Maxime Ripard <maxime@cerno.tech> wrote:
> > > > > On a similar theme, some devices want the clock lane in HS mode early
> > > > > so they can use it in place of an external oscillator, but the data
> > > > > lanes still in LP-11. There appears to be no way for the
> > > > > display/bridge to signal this requirement or it be achieved.
> > > >
> > > > You're right. A loooong time ago, the omapdrm driver had an internal
> > > > infrastructure that didn't use drm_bridge or drm_panel and instead
> > > > required omapdrm-specific drivers for those components. It used to model
> > > > the display pipeline in a different way than drm_bridge, with the sync
> > > > explicitly setting the source state. A DSI sink could thus control its
> > > > enable sequence, interleaving programming of the sink with control of
> > > > the source.
> > > >
> > > > Migrating omapdrm to the drm_bridge model took a really large effort,
> > > > which makes me believe that transitioning the whole subsystem to
> > > > sink-controlled sources would be close to impossible. We could add
> > > > DSI-specific operations, or add another enable bridge operation
> > > > (post_pre_enable ? :-D). Neither would scale, but it may be enough.
> > >
> > > I haven't thought it through for all generic cases, but I suspect it's
> > > more a pre_pre_enable that is needed to initialise the PHY etc,
> > > probably from source to sink.
> > > If the panel/bridge can set a flag that can be checked at this point
> > > for whether an early clock is required or not, I think that allows us
> > > to comply with the requirements for a large number of panels/bridges
> > > (LP-11 vs HS config for clock and or data lanes before pre_enable is
> > > called).
> > >
> > > pre_enable retains the current behaviour (initialise the chain from
> > > sink to source).
> > > enable then actually starts sending video and enabling outputs.
> >
> > Flags indeed seem like a more contained option. Another one could be to
> > have a mipi_dsi_host to (for example) power up the clock lane that would
> > be called by default before the bridge's enable, or at the downstream
> > bridge driver discretion before that.
> 
> Which driver will that call?

The parent DSI Host

> An extreme example perhaps, but Toshiba make the TC358860 eDP to DSI
> bridge chip[1]. So the encoder will be eDP, but the DSI config needs
> to go to that bridge. Does that happen automatically within the
> framework? I guess so as the bridge will have called
> mipi_dsi_host_register for the DSI sink to attach to.

In that case, whatever sink would be connected to the bridge would call
the bridge clock enable hook if it needs it in its pre_enable, or it
would be called automatically before enable if it doesn't

Would that help?

> Perhaps a new mipi_dsi_host function to configure the PHY is the
> easier solution. If it can allow the sink to request whatever
> combination of states from clock and data lanes that it fancies, then
> it can be as explicit as required for the initialisation sequence, and
> the host driver does its best to comply with the requests.

I don't know, I'm not really fond in general of solutions that try to
cover any single case if we don't need it and / or have today an issue
with this. I'd rather have something that works for the particular
bridge we were discussing, see if it applies to other bridges and modify
it if it doesn't until it works for all our cases. Trying to reason in
all possible cases tend to lead to solutions that are difficult to
maintain and usually over-engineered.

> I'd have a slight query over when and how the host would drop to ULPS
> or power off. It probably shouldn't be in post_disable as the sink
> hasn't had a chance to finalise everything in its post_disable.
>
> Perhaps pm_runtime with autosuspend is the right call there?

pm_runtime semantics mean that once the device is suspended, its power
domains, regulators, clocks, etc. are all shut down, so it doesn't
really fit the low power state expected by DSI

> [1] https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html
> 
> > > When I discussed this briefly with Maxime there was a suggestion of
> > > using pm_runtime to be able to power up the pipeline as a whole. If
> > > the bridge driver can use pm_runtime to power up the PHY when
> > > required, then that may solve the issue, however I know too little of
> > > the details to say whether that is actually practical.
> >
> > I'm not sure it was about this topic in particular. If I remember well
> > our discussion, this was about the vc4 driver that tries to circumvent
> > the framework and call the pre_enable and enable hooks itself because it
> > wasn't properly powered before and thus any DCS-related call by the
> > downstream bridge or panel would end up creating a CPU stall.
> >
> > I suggested to use runtime_pm in the DCS related calls to make sure the
> > device is powered because there's no relation between the state of the
> > downstream bridge or panel and whether it can send DCS commands or not.
> > For all we know, it could do it at probe time.
> 
> pm_runtime is all a bit of a magic black box to me.
> 
> We had discussed shifting to using pm_runtime from DCS (and enable)
> calls to power up the PHY on demand, and that's what I implemented.
> However Laurent flagged up that using
> dsi->encoder->crtc->state->adjusted_mode to get the HS clock info
> required to send a HS DCS command from that call is deprecated, so how
> do we specify the clock rate to use at that point?

I guess the most sensible would be to have a custom bridge state, and
add a pointer to the current bridge state in struct drm_bridge. Then, as
long as you have a bridge pointer you have a way to get the current
state associated to it, and since we already have atomic_duplicate_state
/ atomic_destroy_state we can create our own structure around it storing
whatever we want.

> > > > > host_transfer calls can supposedly be made at any time, however unless
> > > > > MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
> > > > > in high speed mode. If this is before a mode has been set, what
> > > > > defines the link frequency parameters at this point? Adopting a random
> > > > > default sounds like a good way to get undefined behaviour.
> > > > >
> > > > > DSI burst mode needs to set the DSI link frequency independently of
> > > > > the display mode. How is that meant to be configured? I would have
> > > > > expected it to come from DT due to link frequency often being chosen
> > > > > based on EMC restrictions, but I don't see such a thing in any
> > > > > binding.
> > > >
> > > > Undefined too. DSI support was added to DRM without any design effort,
> > > > it's more a hack than a real solution. The issue with devices that can
> > > > be controlled over both DSI and I2C is completely unhandled. So far
> > > > nobody has really cared about implementing DSI right as far as I can
> > > > tell.
> > >
> > > :-(
> > >
> > > Thinking aloud, does having the option to set a burst link frequency
> > > from DT (or ACPI) have any issue for other platforms?
> > > Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
> > > drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
> > > all the defined timings, but drop to LP during blanking" option. The
> > > link frequency has therefore remained a property of the
> > > display/bridge.
> > > dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
> > > through the full detail of the parameters it computes from there.
> >
> > I don't see anything wrong with using link-frequency from the DT to
> > setup the burst frequency. It's what v4l2 has been using for a while
> > without any known (to me) drawback, and we're using the same of-graph
> > bindings, so it shouldn't be too controversial there.
> 
> OK, that sounds like a vague plan.
> 
> > > DSI and I2C controlled devices is yet another issue that I haven't
> > > even looked at.
> > > I think it's more that vc4 wants to ignore DSI should the DSI host
> > > node be enabled in DT, but there's no panel bound to it. One could say
> > > that is a DT error and tough luck, but from a user's perspective that
> > > is a bit harsh.
> >
> > I guess the larger "issue" is that the tree in the DT is done following
> > the "control" bus, and Linux likes to tie the life cycle of a given
> > device to its parent bus. Both these decisions make sense, but they
> > interact in a weird way in some occurrences (like this one, or Allwinner
> > has an Ethernet PHY controlled through MMIO which end up in the same
> > case).
> >
> > I wonder if using device links here could help though.
> 
> I really don't know about that one.

It's a piece of infrastructure that was created at first (I think?) to
model the power dependency between devices that don't have a parent /
child relationship. For example, if you use DMA, you probably want to
keep the IOMMU powered as long as you are, but it is in a completely
separate branch of the "device tree" (not one from the DTB, the one that
linux DM creates).

It was later expanded to also cover probe order and make sure a supplier
would probe before its consumer, effectively making EPROBE_DEFER
obsolete.

The second part is still fairly new, but I think we can solve this by
adding a device link between the DSI host and whatever is at the end of
the OF-Graph endpoint.

> > > > > As a follow on, bridge devices can support burst mode (eg TI's
> > > > > SN65DSI83 that's just been merged), so it needs to know the desired
> > > > > panel timings for the output side of the bridge, but the DSI link
> > > > > timings to set up the bridge's PLL. What's the correct way for
> > > > > signalling that? drm_crtc_state->adjusted_mode vs
> > > > > drm_crtc_state->mode? Except mode is userspace's request, not what has
> > > > > been validated/updated by the panel/bridge.
> > > >
> > > > adjusted_mode is also a bit of a hack, it solves very specific issues,
> > > > and its design assumes a single encoder in the chain with no extra
> > > > bridges. We should instead add modes to the bridge state, and negotiate
> > > > modes along the pipeline the same way we negotiate formats.
> > >
> > > So as I understand it we already have format negotiation between
> > > bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
> > > so is it possible to extend that to modes?
> > > Are you thinking bridge state that is owned by the framework, or by
> > > the individual bridge drivers?
> >
> > atomic_check is made for that. I guess we could improve its call
> > sequence to each time a mode is modified along the bridge list we
> > restart the sequence until all components agree (or reject it entirely
> > if they can't), but I don't really see why we would need yet another
> > hook.
> 
> Why do all nodes in the bridge list need to agree? Adjacent nodes need
> to agree, but they then also need to retain that agreed timing
> somewhere.

We might have mutually exclusive requirements though? Let's use the
example of the VC4 HDMI driver that can't have odd horizontal timings,
and assume it's a constraint of our DSI driver instead.

Then, we have a DSI->LVDS bridge, a LVDS->RGB bridge and a panel (which
is a bit ridiculous, but whatever). If the LVDS->RGB bridge can't have
even horizontal timings, then you just can't display it, even though
they are not adjacent (unless the bridge in the middle can modify the
timings between the input and output, but that's not always possible).

Similarly, if for the RGB panel we need to increase a bit some timings
to accommodate for a larger pixel clock and end up above what the DSI
host can provide, we're also done.

> Taking SN65DSI8[3|4|5] as an example, it supports burst mode, and the
> DSI frequency and timings are permitted to be different from that
> which it uses on the LVDS side. The LVDS panel and LVDS side of DSI83
> need to agree over the format, and the DSI host and DSI side of DSI83
> need to agree, but they may be two different timings.
> Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows you to
> configure the LVDS rate compared to the DSI rate (the driver currently
> goes for 1:1), and registers 0x20 to 0x34 allow you to set the number
> of active pixel and blanking on the LVDS side (again currently just
> copied across).
>
> The way I'm seeing burst mode as having been interpreted at present is
> that it's largely just a flag to say "drop to LP mode between lines".
> The timing that needs to be passed to the crtc is therefore going to
> be based on the DSI link rate (converted to pixels) with increased
> blanking periods.
> 
> I guess there are similarities with Media Controller and V4L2 here. A
> typical chain there could be:
>  sensor -> scaler -> crop -> CSI-2 receiver.
> The format on each of those links may be different, but the chain as a
> whole needs to be valid. Media Controller largely relies on userspace
> to configure all links, but with a DRM chain that isn't really an
> option as it's expected that the display chain configures itself.

Also, the userspace has no concept of media sub-devices in DRM, so it
just sets the mode on the whole DRM/KMS device, unlike what v4l2 does.
In v4l2, afaik, if you ended up with the above scenarios it would just
be rejected when you set the format on the link, letting the userspace
figure it out. We can't really do that here

Maxime

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Re: Questions over DSI within DRM.

[Laurent Pinchart]

Hello,

Reviving a bit of an old thread.

On Thu, Jul 15, 2021 at 11:50:22AM +0200, Maxime Ripard wrote:
> On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
> > On Tue, 6 Jul 2021 at 16:13, Maxime Ripard wrote:
> > > > > > On a similar theme, some devices want the clock lane in HS mode early
> > > > > > so they can use it in place of an external oscillator, but the data
> > > > > > lanes still in LP-11. There appears to be no way for the
> > > > > > display/bridge to signal this requirement or it be achieved.
> > > > >
> > > > > You're right. A loooong time ago, the omapdrm driver had an internal
> > > > > infrastructure that didn't use drm_bridge or drm_panel and instead
> > > > > required omapdrm-specific drivers for those components. It used to model
> > > > > the display pipeline in a different way than drm_bridge, with the sync
> > > > > explicitly setting the source state. A DSI sink could thus control its
> > > > > enable sequence, interleaving programming of the sink with control of
> > > > > the source.
> > > > >
> > > > > Migrating omapdrm to the drm_bridge model took a really large effort,
> > > > > which makes me believe that transitioning the whole subsystem to
> > > > > sink-controlled sources would be close to impossible. We could add
> > > > > DSI-specific operations, or add another enable bridge operation
> > > > > (post_pre_enable ? :-D). Neither would scale, but it may be enough.
> > > >
> > > > I haven't thought it through for all generic cases, but I suspect it's
> > > > more a pre_pre_enable that is needed to initialise the PHY etc,
> > > > probably from source to sink.

I believe it could be implemented as a pre-pre-enable indeed. It feels
like a bit of a hack, as the next time we need more fine-grained control
over the startup sequence, we'll have to add a pre-pre-pre-enable. Given
that the startup sequence requirements come from the sink device, it
would make sense to let it explicitly control the initialization,
instead of driving it from the source. I don't think we'll be able to
rework the bridge API in that direction though, so I'm fine with a hack.

> > > > If the panel/bridge can set a flag that can be checked at this point
> > > > for whether an early clock is required or not, I think that allows us
> > > > to comply with the requirements for a large number of panels/bridges
> > > > (LP-11 vs HS config for clock and or data lanes before pre_enable is
> > > > called).
> > > >
> > > > pre_enable retains the current behaviour (initialise the chain from
> > > > sink to source).
> > > > enable then actually starts sending video and enabling outputs.
> > >
> > > Flags indeed seem like a more contained option. Another one could be to
> > > have a mipi_dsi_host to (for example) power up the clock lane that would
> > > be called by default before the bridge's enable, or at the downstream
> > > bridge driver discretion before that.
> > 
> > Which driver will that call?
> 
> The parent DSI Host
> 
> > An extreme example perhaps, but Toshiba make the TC358860 eDP to DSI
> > bridge chip[1]. So the encoder will be eDP, but the DSI config needs
> > to go to that bridge. Does that happen automatically within the
> > framework? I guess so as the bridge will have called
> > mipi_dsi_host_register for the DSI sink to attach to.
> 
> In that case, whatever sink would be connected to the bridge would call
> the bridge clock enable hook if it needs it in its pre_enable, or it
> would be called automatically before enable if it doesn't
> 
> Would that help?

Sounds good to me, in theory at least (let's see what issues we'll run
into in practice :-)).

Has anyone given it a try, or is planning to ?

> > Perhaps a new mipi_dsi_host function to configure the PHY is the
> > easier solution. If it can allow the sink to request whatever
> > combination of states from clock and data lanes that it fancies, then
> > it can be as explicit as required for the initialisation sequence, and
> > the host driver does its best to comply with the requests.
> 
> I don't know, I'm not really fond in general of solutions that try to
> cover any single case if we don't need it and / or have today an issue
> with this. I'd rather have something that works for the particular
> bridge we were discussing, see if it applies to other bridges and modify
> it if it doesn't until it works for all our cases. Trying to reason in
> all possible cases tend to lead to solutions that are difficult to
> maintain and usually over-engineered.

A DSI host clock enable operation or a DSI host PHY configuration
operation both fit in the same place in the grand scheme of things, so I
don't mind either. We should be able to easily move from a more specific
operation to a more generic one if the need arises.

> > I'd have a slight query over when and how the host would drop to ULPS
> > or power off. It probably shouldn't be in post_disable as the sink
> > hasn't had a chance to finalise everything in its post_disable.
> >
> > Perhaps pm_runtime with autosuspend is the right call there?
> 
> pm_runtime semantics mean that once the device is suspended, its power
> domains, regulators, clocks, etc. are all shut down, so it doesn't
> really fit the low power state expected by DSI
> 
> > [1] https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html
> > 
> > > > When I discussed this briefly with Maxime there was a suggestion of
> > > > using pm_runtime to be able to power up the pipeline as a whole. If
> > > > the bridge driver can use pm_runtime to power up the PHY when
> > > > required, then that may solve the issue, however I know too little of
> > > > the details to say whether that is actually practical.
> > >
> > > I'm not sure it was about this topic in particular. If I remember well
> > > our discussion, this was about the vc4 driver that tries to circumvent
> > > the framework and call the pre_enable and enable hooks itself because it
> > > wasn't properly powered before and thus any DCS-related call by the
> > > downstream bridge or panel would end up creating a CPU stall.
> > >
> > > I suggested to use runtime_pm in the DCS related calls to make sure the
> > > device is powered because there's no relation between the state of the
> > > downstream bridge or panel and whether it can send DCS commands or not.
> > > For all we know, it could do it at probe time.
> > 
> > pm_runtime is all a bit of a magic black box to me.
> > 
> > We had discussed shifting to using pm_runtime from DCS (and enable)
> > calls to power up the PHY on demand, and that's what I implemented.
> > However Laurent flagged up that using
> > dsi->encoder->crtc->state->adjusted_mode to get the HS clock info
> > required to send a HS DCS command from that call is deprecated, so how
> > do we specify the clock rate to use at that point?
> 
> I guess the most sensible would be to have a custom bridge state, and
> add a pointer to the current bridge state in struct drm_bridge. Then, as
> long as you have a bridge pointer you have a way to get the current
> state associated to it, and since we already have atomic_duplicate_state
> / atomic_destroy_state we can create our own structure around it storing
> whatever we want.

That's a good point. It would only be needed if we use runtime PM to
work around the initialization sequence issue, not if we implement a DSI
host clock enable/disable operation, right ?

> > > > > > host_transfer calls can supposedly be made at any time, however unless
> > > > > > MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
> > > > > > in high speed mode. If this is before a mode has been set, what
> > > > > > defines the link frequency parameters at this point? Adopting a random
> > > > > > default sounds like a good way to get undefined behaviour.
> > > > > >
> > > > > > DSI burst mode needs to set the DSI link frequency independently of
> > > > > > the display mode. How is that meant to be configured? I would have
> > > > > > expected it to come from DT due to link frequency often being chosen
> > > > > > based on EMC restrictions, but I don't see such a thing in any
> > > > > > binding.
> > > > >
> > > > > Undefined too. DSI support was added to DRM without any design effort,
> > > > > it's more a hack than a real solution. The issue with devices that can
> > > > > be controlled over both DSI and I2C is completely unhandled. So far
> > > > > nobody has really cared about implementing DSI right as far as I can
> > > > > tell.
> > > >
> > > > :-(
> > > >
> > > > Thinking aloud, does having the option to set a burst link frequency
> > > > from DT (or ACPI) have any issue for other platforms?
> > > > Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
> > > > drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
> > > > all the defined timings, but drop to LP during blanking" option. The
> > > > link frequency has therefore remained a property of the
> > > > display/bridge.
> > > > dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
> > > > through the full detail of the parameters it computes from there.
> > >
> > > I don't see anything wrong with using link-frequency from the DT to
> > > setup the burst frequency. It's what v4l2 has been using for a while
> > > without any known (to me) drawback, and we're using the same of-graph
> > > bindings, so it shouldn't be too controversial there.

How would that frequency we picked in practice ? Do panels typically
support a range of HS frequencies for DCS HS transfers ?

> > OK, that sounds like a vague plan.
> > 
> > > > DSI and I2C controlled devices is yet another issue that I haven't
> > > > even looked at.
> > > > I think it's more that vc4 wants to ignore DSI should the DSI host
> > > > node be enabled in DT, but there's no panel bound to it. One could say
> > > > that is a DT error and tough luck, but from a user's perspective that
> > > > is a bit harsh.
> > >
> > > I guess the larger "issue" is that the tree in the DT is done following
> > > the "control" bus, and Linux likes to tie the life cycle of a given
> > > device to its parent bus. Both these decisions make sense, but they
> > > interact in a weird way in some occurrences (like this one, or Allwinner
> > > has an Ethernet PHY controlled through MMIO which end up in the same
> > > case).
> > >
> > > I wonder if using device links here could help though.
> > 
> > I really don't know about that one.
> 
> It's a piece of infrastructure that was created at first (I think?) to
> model the power dependency between devices that don't have a parent /
> child relationship. For example, if you use DMA, you probably want to
> keep the IOMMU powered as long as you are, but it is in a completely
> separate branch of the "device tree" (not one from the DTB, the one that
> linux DM creates).
> 
> It was later expanded to also cover probe order and make sure a supplier
> would probe before its consumer, effectively making EPROBE_DEFER
> obsolete.
> 
> The second part is still fairly new, but I think we can solve this by
> adding a device link between the DSI host and whatever is at the end of
> the OF-Graph endpoint.
> 
> > > > > > As a follow on, bridge devices can support burst mode (eg TI's
> > > > > > SN65DSI83 that's just been merged), so it needs to know the desired
> > > > > > panel timings for the output side of the bridge, but the DSI link
> > > > > > timings to set up the bridge's PLL. What's the correct way for
> > > > > > signalling that? drm_crtc_state->adjusted_mode vs
> > > > > > drm_crtc_state->mode? Except mode is userspace's request, not what has
> > > > > > been validated/updated by the panel/bridge.
> > > > >
> > > > > adjusted_mode is also a bit of a hack, it solves very specific issues,
> > > > > and its design assumes a single encoder in the chain with no extra
> > > > > bridges. We should instead add modes to the bridge state, and negotiate
> > > > > modes along the pipeline the same way we negotiate formats.
> > > >
> > > > So as I understand it we already have format negotiation between
> > > > bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
> > > > so is it possible to extend that to modes?
> > > > Are you thinking bridge state that is owned by the framework, or by
> > > > the individual bridge drivers?
> > >
> > > atomic_check is made for that. I guess we could improve its call
> > > sequence to each time a mode is modified along the bridge list we
> > > restart the sequence until all components agree (or reject it entirely
> > > if they can't), but I don't really see why we would need yet another
> > > hook.

Isn't this what atomic_get_output_bus_fmts() and
atomic_get_input_bus_fmts() implement ?

> > Why do all nodes in the bridge list need to agree? Adjacent nodes need
> > to agree, but they then also need to retain that agreed timing
> > somewhere.
> 
> We might have mutually exclusive requirements though? Let's use the
> example of the VC4 HDMI driver that can't have odd horizontal timings,
> and assume it's a constraint of our DSI driver instead.
> 
> Then, we have a DSI->LVDS bridge, a LVDS->RGB bridge and a panel (which
> is a bit ridiculous, but whatever). If the LVDS->RGB bridge can't have
> even horizontal timings, then you just can't display it, even though
> they are not adjacent (unless the bridge in the middle can modify the
> timings between the input and output, but that's not always possible).
> 
> Similarly, if for the RGB panel we need to increase a bit some timings
> to accommodate for a larger pixel clock and end up above what the DSI
> host can provide, we're also done.

The hard part will be to figure out a good heuristics to perform the
negotiation without going back and forth (at least not in a way that
would require too many iterations, and certainly avoiding infinite
loops). That will be an interesting problem to solve, but maybe we'll be
lucky and a simple approach will work for the use cases we have to
support today.

> > Taking SN65DSI8[3|4|5] as an example, it supports burst mode, and the
> > DSI frequency and timings are permitted to be different from that
> > which it uses on the LVDS side. The LVDS panel and LVDS side of DSI83
> > need to agree over the format, and the DSI host and DSI side of DSI83
> > need to agree, but they may be two different timings.
> > Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows you to
> > configure the LVDS rate compared to the DSI rate (the driver currently
> > goes for 1:1), and registers 0x20 to 0x34 allow you to set the number
> > of active pixel and blanking on the LVDS side (again currently just
> > copied across).
> >
> > The way I'm seeing burst mode as having been interpreted at present is
> > that it's largely just a flag to say "drop to LP mode between lines".
> > The timing that needs to be passed to the crtc is therefore going to
> > be based on the DSI link rate (converted to pixels) with increased
> > blanking periods.
> > 
> > I guess there are similarities with Media Controller and V4L2 here. A
> > typical chain there could be:
> >  sensor -> scaler -> crop -> CSI-2 receiver.
> > The format on each of those links may be different, but the chain as a
> > whole needs to be valid. Media Controller largely relies on userspace
> > to configure all links, but with a DRM chain that isn't really an
> > option as it's expected that the display chain configures itself.
> 
> Also, the userspace has no concept of media sub-devices in DRM, so it
> just sets the mode on the whole DRM/KMS device, unlike what v4l2 does.
> In v4l2, afaik, if you ended up with the above scenarios it would just
> be rejected when you set the format on the link, letting the userspace
> figure it out. We can't really do that here

I wonder how long we'll be able to keep userspace out of the picture to
configure the internals of the pipeline. I don't want to be the first
person who will have a use case that requires this.

-- 
Regards,

Laurent Pinchart

Re: Questions over DSI within DRM.

[Andrzej Hajda]

Hi,

Thanks Laurent for reviving the thread, I have missed it entirely.


On 03.10.2021 16:16, Laurent Pinchart wrote:
> Hello,
> 
> Reviving a bit of an old thread.
> 
> On Thu, Jul 15, 2021 at 11:50:22AM +0200, Maxime Ripard wrote:
>> On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
>>> On Tue, 6 Jul 2021 at 16:13, Maxime Ripard wrote:
>>>>>>> On a similar theme, some devices want the clock lane in HS mode early
>>>>>>> so they can use it in place of an external oscillator, but the data
>>>>>>> lanes still in LP-11. There appears to be no way for the
>>>>>>> display/bridge to signal this requirement or it be achieved.
>>>>>>
>>>>>> You're right. A loooong time ago, the omapdrm driver had an internal
>>>>>> infrastructure that didn't use drm_bridge or drm_panel and instead
>>>>>> required omapdrm-specific drivers for those components. It used to model
>>>>>> the display pipeline in a different way than drm_bridge, with the sync
>>>>>> explicitly setting the source state. A DSI sink could thus control its
>>>>>> enable sequence, interleaving programming of the sink with control of
>>>>>> the source.
>>>>>>
>>>>>> Migrating omapdrm to the drm_bridge model took a really large effort,
>>>>>> which makes me believe that transitioning the whole subsystem to
>>>>>> sink-controlled sources would be close to impossible. We could add
>>>>>> DSI-specific operations, or add another enable bridge operation
>>>>>> (post_pre_enable ? :-D). Neither would scale, but it may be enough.
>>>>>
>>>>> I haven't thought it through for all generic cases, but I suspect it's
>>>>> more a pre_pre_enable that is needed to initialise the PHY etc,
>>>>> probably from source to sink.
> 
> I believe it could be implemented as a pre-pre-enable indeed. It feels
> like a bit of a hack, as the next time we need more fine-grained control
> over the startup sequence, we'll have to add a pre-pre-pre-enable. Given
> that the startup sequence requirements come from the sink device, it
> would make sense to let it explicitly control the initialization,
> instead of driving it from the source. I don't think we'll be able to
> rework the bridge API in that direction though, so I'm fine with a hack.

As I remember I have suggested in similar discussion [1] adding to 
mipi_dsi_host_ops requested operations:
power_on - power on DSI bus (do we really need it?)
init - enter LP11 (or HS-stop state if I remember correctly)
and then call them from the right place in DSI device, probably 
pre_enable callback.

This way we could avoid polluting drm_bridge_ops, with DSI specific stuff.

[1]: 
https://lore.kernel.org/dri-devel/6700c90f-d0e0-5cbf-1616-0c1d158441b1@samsung.com/#t


Sorry for addressing only this issue, but I need to read whole thread, 
to re-read whole thread, and today it is too late for me :)

Regards
Andrzej

Re: Questions over DSI within DRM.

[Dave Stevenson]

Hi Laurent

On Sun, 3 Oct 2021 at 15:16, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
>
> Hello,
>
> Reviving a bit of an old thread.

I'd been looking at reviving this conversation too as I've moved
further on with DSI on the Pi, and converting from an encoder to a
bridge

> On Thu, Jul 15, 2021 at 11:50:22AM +0200, Maxime Ripard wrote:
> > On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
> > > On Tue, 6 Jul 2021 at 16:13, Maxime Ripard wrote:
> > > > > > > On a similar theme, some devices want the clock lane in HS mode early
> > > > > > > so they can use it in place of an external oscillator, but the data
> > > > > > > lanes still in LP-11. There appears to be no way for the
> > > > > > > display/bridge to signal this requirement or it be achieved.
> > > > > >
> > > > > > You're right. A loooong time ago, the omapdrm driver had an internal
> > > > > > infrastructure that didn't use drm_bridge or drm_panel and instead
> > > > > > required omapdrm-specific drivers for those components. It used to model
> > > > > > the display pipeline in a different way than drm_bridge, with the sync
> > > > > > explicitly setting the source state. A DSI sink could thus control its
> > > > > > enable sequence, interleaving programming of the sink with control of
> > > > > > the source.
> > > > > >
> > > > > > Migrating omapdrm to the drm_bridge model took a really large effort,
> > > > > > which makes me believe that transitioning the whole subsystem to
> > > > > > sink-controlled sources would be close to impossible. We could add
> > > > > > DSI-specific operations, or add another enable bridge operation
> > > > > > (post_pre_enable ? :-D). Neither would scale, but it may be enough.
> > > > >
> > > > > I haven't thought it through for all generic cases, but I suspect it's
> > > > > more a pre_pre_enable that is needed to initialise the PHY etc,
> > > > > probably from source to sink.
>
> I believe it could be implemented as a pre-pre-enable indeed. It feels
> like a bit of a hack, as the next time we need more fine-grained control
> over the startup sequence, we'll have to add a pre-pre-pre-enable. Given
> that the startup sequence requirements come from the sink device, it
> would make sense to let it explicitly control the initialization,
> instead of driving it from the source. I don't think we'll be able to
> rework the bridge API in that direction though, so I'm fine with a hack.

There are pros and cons to both approaches.
You're in a much better place to make that sort of call than I am, so
I'll take your advice.

Implementing a DSI host op function may mean an update to a number of
existing DSI host drivers, but it would be cleaner. It's also what
Andrzej has suggested.

Thinking it through, a function that requests clock lane frequency and
state (ULPS or LP-11 predominantly), and data lane state (again ULPS
or LP-11) should allow the required behaviour for most of the bridges
I'm aware of. Most want either LP-11, or HS clock at a known
frequency. Giving the option for setting back into ULPS also allows
for power saving/standby mechanisms should the need arise.
Does it need a way to pass back the actual DSI frequency being used,
in a similar vein to mode_fixup? That allows for the bridge to request
the display clock, but the burst mode link frequency to be returned
(I'm assuming that's a property of the DSI host only, and not the
bridge).

I'm having a discussion with someone who wants to run SN65DSI85 in the
two independent LVDS display mode. That requires a DSI HS clock on
DSI-A even if only panel B is active, so with this extra function that
would be achievable as well.

Thoughts?

> > > > > If the panel/bridge can set a flag that can be checked at this point
> > > > > for whether an early clock is required or not, I think that allows us
> > > > > to comply with the requirements for a large number of panels/bridges
> > > > > (LP-11 vs HS config for clock and or data lanes before pre_enable is
> > > > > called).
> > > > >
> > > > > pre_enable retains the current behaviour (initialise the chain from
> > > > > sink to source).
> > > > > enable then actually starts sending video and enabling outputs.
> > > >
> > > > Flags indeed seem like a more contained option. Another one could be to
> > > > have a mipi_dsi_host to (for example) power up the clock lane that would
> > > > be called by default before the bridge's enable, or at the downstream
> > > > bridge driver discretion before that.
> > >
> > > Which driver will that call?
> >
> > The parent DSI Host
> >
> > > An extreme example perhaps, but Toshiba make the TC358860 eDP to DSI
> > > bridge chip[1]. So the encoder will be eDP, but the DSI config needs
> > > to go to that bridge. Does that happen automatically within the
> > > framework? I guess so as the bridge will have called
> > > mipi_dsi_host_register for the DSI sink to attach to.
> >
> > In that case, whatever sink would be connected to the bridge would call
> > the bridge clock enable hook if it needs it in its pre_enable, or it
> > would be called automatically before enable if it doesn't
> >
> > Would that help?
>
> Sounds good to me, in theory at least (let's see what issues we'll run
> into in practice :-)).
>
> Has anyone given it a try, or is planning to ?
>
> > > Perhaps a new mipi_dsi_host function to configure the PHY is the
> > > easier solution. If it can allow the sink to request whatever
> > > combination of states from clock and data lanes that it fancies, then
> > > it can be as explicit as required for the initialisation sequence, and
> > > the host driver does its best to comply with the requests.
> >
> > I don't know, I'm not really fond in general of solutions that try to
> > cover any single case if we don't need it and / or have today an issue
> > with this. I'd rather have something that works for the particular
> > bridge we were discussing, see if it applies to other bridges and modify
> > it if it doesn't until it works for all our cases. Trying to reason in
> > all possible cases tend to lead to solutions that are difficult to
> > maintain and usually over-engineered.
>
> A DSI host clock enable operation or a DSI host PHY configuration
> operation both fit in the same place in the grand scheme of things, so I
> don't mind either. We should be able to easily move from a more specific
> operation to a more generic one if the need arises.
>
> > > I'd have a slight query over when and how the host would drop to ULPS
> > > or power off. It probably shouldn't be in post_disable as the sink
> > > hasn't had a chance to finalise everything in its post_disable.
> > >
> > > Perhaps pm_runtime with autosuspend is the right call there?
> >
> > pm_runtime semantics mean that once the device is suspended, its power
> > domains, regulators, clocks, etc. are all shut down, so it doesn't
> > really fit the low power state expected by DSI
> >
> > > [1] https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html
> > >
> > > > > When I discussed this briefly with Maxime there was a suggestion of
> > > > > using pm_runtime to be able to power up the pipeline as a whole. If
> > > > > the bridge driver can use pm_runtime to power up the PHY when
> > > > > required, then that may solve the issue, however I know too little of
> > > > > the details to say whether that is actually practical.
> > > >
> > > > I'm not sure it was about this topic in particular. If I remember well
> > > > our discussion, this was about the vc4 driver that tries to circumvent
> > > > the framework and call the pre_enable and enable hooks itself because it
> > > > wasn't properly powered before and thus any DCS-related call by the
> > > > downstream bridge or panel would end up creating a CPU stall.
> > > >
> > > > I suggested to use runtime_pm in the DCS related calls to make sure the
> > > > device is powered because there's no relation between the state of the
> > > > downstream bridge or panel and whether it can send DCS commands or not.
> > > > For all we know, it could do it at probe time.
> > >
> > > pm_runtime is all a bit of a magic black box to me.
> > >
> > > We had discussed shifting to using pm_runtime from DCS (and enable)
> > > calls to power up the PHY on demand, and that's what I implemented.
> > > However Laurent flagged up that using
> > > dsi->encoder->crtc->state->adjusted_mode to get the HS clock info
> > > required to send a HS DCS command from that call is deprecated, so how
> > > do we specify the clock rate to use at that point?
> >
> > I guess the most sensible would be to have a custom bridge state, and
> > add a pointer to the current bridge state in struct drm_bridge. Then, as
> > long as you have a bridge pointer you have a way to get the current
> > state associated to it, and since we already have atomic_duplicate_state
> > / atomic_destroy_state we can create our own structure around it storing
> > whatever we want.
>
> That's a good point. It would only be needed if we use runtime PM to
> work around the initialization sequence issue, not if we implement a DSI
> host clock enable/disable operation, right ?

Obviously this only works with atomic bridges otherwise you have no
state, but I assume all bridges should be heading that route now.

> > > > > > > host_transfer calls can supposedly be made at any time, however unless
> > > > > > > MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
> > > > > > > in high speed mode. If this is before a mode has been set, what
> > > > > > > defines the link frequency parameters at this point? Adopting a random
> > > > > > > default sounds like a good way to get undefined behaviour.
> > > > > > >
> > > > > > > DSI burst mode needs to set the DSI link frequency independently of
> > > > > > > the display mode. How is that meant to be configured? I would have
> > > > > > > expected it to come from DT due to link frequency often being chosen
> > > > > > > based on EMC restrictions, but I don't see such a thing in any
> > > > > > > binding.
> > > > > >
> > > > > > Undefined too. DSI support was added to DRM without any design effort,
> > > > > > it's more a hack than a real solution. The issue with devices that can
> > > > > > be controlled over both DSI and I2C is completely unhandled. So far
> > > > > > nobody has really cared about implementing DSI right as far as I can
> > > > > > tell.
> > > > >
> > > > > :-(
> > > > >
> > > > > Thinking aloud, does having the option to set a burst link frequency
> > > > > from DT (or ACPI) have any issue for other platforms?
> > > > > Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
> > > > > drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
> > > > > all the defined timings, but drop to LP during blanking" option. The
> > > > > link frequency has therefore remained a property of the
> > > > > display/bridge.
> > > > > dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
> > > > > through the full detail of the parameters it computes from there.
> > > >
> > > > I don't see anything wrong with using link-frequency from the DT to
> > > > setup the burst frequency. It's what v4l2 has been using for a while
> > > > without any known (to me) drawback, and we're using the same of-graph
> > > > bindings, so it shouldn't be too controversial there.
>
> How would that frequency we picked in practice ? Do panels typically
> support a range of HS frequencies for DCS HS transfers ?

I was thinking more of bridges where they often run a PLL off the
incoming DSI clock, and then have a FIFO with potentially different
clock rates and timings on input and output. SN65DSI83 supports that,
as do the Toshiba bridge chips I'm currently looking at.
You need to know the DSI link frequency to configure the PLL
correctly, but then the bridge output timing is a different matter.

> > > OK, that sounds like a vague plan.
> > >
> > > > > DSI and I2C controlled devices is yet another issue that I haven't
> > > > > even looked at.
> > > > > I think it's more that vc4 wants to ignore DSI should the DSI host
> > > > > node be enabled in DT, but there's no panel bound to it. One could say
> > > > > that is a DT error and tough luck, but from a user's perspective that
> > > > > is a bit harsh.
> > > >
> > > > I guess the larger "issue" is that the tree in the DT is done following
> > > > the "control" bus, and Linux likes to tie the life cycle of a given
> > > > device to its parent bus. Both these decisions make sense, but they
> > > > interact in a weird way in some occurrences (like this one, or Allwinner
> > > > has an Ethernet PHY controlled through MMIO which end up in the same
> > > > case).
> > > >
> > > > I wonder if using device links here could help though.
> > >
> > > I really don't know about that one.
> >
> > It's a piece of infrastructure that was created at first (I think?) to
> > model the power dependency between devices that don't have a parent /
> > child relationship. For example, if you use DMA, you probably want to
> > keep the IOMMU powered as long as you are, but it is in a completely
> > separate branch of the "device tree" (not one from the DTB, the one that
> > linux DM creates).
> >
> > It was later expanded to also cover probe order and make sure a supplier
> > would probe before its consumer, effectively making EPROBE_DEFER
> > obsolete.
> >
> > The second part is still fairly new, but I think we can solve this by
> > adding a device link between the DSI host and whatever is at the end of
> > the OF-Graph endpoint.
> >
> > > > > > > As a follow on, bridge devices can support burst mode (eg TI's
> > > > > > > SN65DSI83 that's just been merged), so it needs to know the desired
> > > > > > > panel timings for the output side of the bridge, but the DSI link
> > > > > > > timings to set up the bridge's PLL. What's the correct way for
> > > > > > > signalling that? drm_crtc_state->adjusted_mode vs
> > > > > > > drm_crtc_state->mode? Except mode is userspace's request, not what has
> > > > > > > been validated/updated by the panel/bridge.
> > > > > >
> > > > > > adjusted_mode is also a bit of a hack, it solves very specific issues,
> > > > > > and its design assumes a single encoder in the chain with no extra
> > > > > > bridges. We should instead add modes to the bridge state, and negotiate
> > > > > > modes along the pipeline the same way we negotiate formats.
> > > > >
> > > > > So as I understand it we already have format negotiation between
> > > > > bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
> > > > > so is it possible to extend that to modes?
> > > > > Are you thinking bridge state that is owned by the framework, or by
> > > > > the individual bridge drivers?
> > > >
> > > > atomic_check is made for that. I guess we could improve its call
> > > > sequence to each time a mode is modified along the bridge list we
> > > > restart the sequence until all components agree (or reject it entirely
> > > > if they can't), but I don't really see why we would need yet another
> > > > hook.
>
> Isn't this what atomic_get_output_bus_fmts() and
> atomic_get_input_bus_fmts() implement ?

Those negotiate a single u32 bus format between nodes, not a complete timing.

> > > Why do all nodes in the bridge list need to agree? Adjacent nodes need
> > > to agree, but they then also need to retain that agreed timing
> > > somewhere.
> >
> > We might have mutually exclusive requirements though? Let's use the
> > example of the VC4 HDMI driver that can't have odd horizontal timings,
> > and assume it's a constraint of our DSI driver instead.
> >
> > Then, we have a DSI->LVDS bridge, a LVDS->RGB bridge and a panel (which
> > is a bit ridiculous, but whatever). If the LVDS->RGB bridge can't have
> > even horizontal timings, then you just can't display it, even though
> > they are not adjacent (unless the bridge in the middle can modify the
> > timings between the input and output, but that's not always possible).
> >
> > Similarly, if for the RGB panel we need to increase a bit some timings
> > to accommodate for a larger pixel clock and end up above what the DSI
> > host can provide, we're also done.
>
> The hard part will be to figure out a good heuristics to perform the
> negotiation without going back and forth (at least not in a way that
> would require too many iterations, and certainly avoiding infinite
> loops). That will be an interesting problem to solve, but maybe we'll be
> lucky and a simple approach will work for the use cases we have to
> support today.

One to kick into the long grass possibly then.

For burst mode and bridges retiming things, generally I think it works
if you consider a single pass through a mode_fixup equivalent starting
at the panel.
The panel advertises what it wants.
The closest bridge (eg DSI83) fixup amends panel's requested mode
based on any constraints that it has, stores a copy, and passes that
down the chain.
The next bridge (eg vc4_dsi) amends the mode based on restrictions it
has (eg burst mode link frequency). That's the end of the chain in
this case, so that's the mode that the crtc needs to be programmed
with.

The modes then need to be passed back up the chain so that eg DSI83
knows the link frequency in use, and can therefore configure the PLL
appropriately.

The bit it would be nice to fix is that burst mode has effectively
increased the horizontal front porch, but that could be fixed within
the bridge so that the panel gets the timing it requested.

> > > Taking SN65DSI8[3|4|5] as an example, it supports burst mode, and the
> > > DSI frequency and timings are permitted to be different from that
> > > which it uses on the LVDS side. The LVDS panel and LVDS side of DSI83
> > > need to agree over the format, and the DSI host and DSI side of DSI83
> > > need to agree, but they may be two different timings.
> > > Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows you to
> > > configure the LVDS rate compared to the DSI rate (the driver currently
> > > goes for 1:1), and registers 0x20 to 0x34 allow you to set the number
> > > of active pixel and blanking on the LVDS side (again currently just
> > > copied across).
> > >
> > > The way I'm seeing burst mode as having been interpreted at present is
> > > that it's largely just a flag to say "drop to LP mode between lines".
> > > The timing that needs to be passed to the crtc is therefore going to
> > > be based on the DSI link rate (converted to pixels) with increased
> > > blanking periods.
> > >
> > > I guess there are similarities with Media Controller and V4L2 here. A
> > > typical chain there could be:
> > >  sensor -> scaler -> crop -> CSI-2 receiver.
> > > The format on each of those links may be different, but the chain as a
> > > whole needs to be valid. Media Controller largely relies on userspace
> > > to configure all links, but with a DRM chain that isn't really an
> > > option as it's expected that the display chain configures itself.
> >
> > Also, the userspace has no concept of media sub-devices in DRM, so it
> > just sets the mode on the whole DRM/KMS device, unlike what v4l2 does.
> > In v4l2, afaik, if you ended up with the above scenarios it would just
> > be rejected when you set the format on the link, letting the userspace
> > figure it out. We can't really do that here
>
> I wonder how long we'll be able to keep userspace out of the picture to
> configure the internals of the pipeline. I don't want to be the first
> person who will have a use case that requires this.

I suspect none of us want to be the first one to hit this scenario!

As I've just posted on the other thread about SN65DSI83, I've hit a
major stumbling block with the current design where vc4_dsi (and
Exynos) breaks the bridge_chain so that it gets called first and can
do initialisation, and then it calls down the chain.
Having now converted to a DSI bridge driver, it works fine with
connected non-atomic bridges, but fails with atomic bridges.
drm_atomic_add_encoder_bridges adds the state for all the bridges the
framework is aware of. With the split chain it misses adding the state
of our "hidden" bridges, and we can't add the extra state from our
atomic_duplicate_state call as we don't have the state to add to (we
just return our state to be added).
This bumps up the priority for us of finding a suitable solution for
this initialisation issue, so I'll start looking at how feasible a new
DSI host function is.

Thanks
  Dave

Re: Questions over DSI within DRM.

[Andrzej Hajda]

On 05.10.2021 13:23, Dave Stevenson wrote:
> Hi Laurent
> 
> On Sun, 3 Oct 2021 at 15:16, Laurent Pinchart
> <laurent.pinchart@ideasonboard.com> wrote:
>>
>> Hello,
>>
>> Reviving a bit of an old thread.
> 
> I'd been looking at reviving this conversation too as I've moved
> further on with DSI on the Pi, and converting from an encoder to a
> bridge
> 
>> On Thu, Jul 15, 2021 at 11:50:22AM +0200, Maxime Ripard wrote:
>>> On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
>>>> On Tue, 6 Jul 2021 at 16:13, Maxime Ripard wrote:
>>>>>>>> On a similar theme, some devices want the clock lane in HS mode early
>>>>>>>> so they can use it in place of an external oscillator, but the data
>>>>>>>> lanes still in LP-11. There appears to be no way for the
>>>>>>>> display/bridge to signal this requirement or it be achieved.
>>>>>>>
>>>>>>> You're right. A loooong time ago, the omapdrm driver had an internal
>>>>>>> infrastructure that didn't use drm_bridge or drm_panel and instead
>>>>>>> required omapdrm-specific drivers for those components. It used to model
>>>>>>> the display pipeline in a different way than drm_bridge, with the sync
>>>>>>> explicitly setting the source state. A DSI sink could thus control its
>>>>>>> enable sequence, interleaving programming of the sink with control of
>>>>>>> the source.
>>>>>>>
>>>>>>> Migrating omapdrm to the drm_bridge model took a really large effort,
>>>>>>> which makes me believe that transitioning the whole subsystem to
>>>>>>> sink-controlled sources would be close to impossible. We could add
>>>>>>> DSI-specific operations, or add another enable bridge operation
>>>>>>> (post_pre_enable ? :-D). Neither would scale, but it may be enough.
>>>>>>
>>>>>> I haven't thought it through for all generic cases, but I suspect it's
>>>>>> more a pre_pre_enable that is needed to initialise the PHY etc,
>>>>>> probably from source to sink.
>>
>> I believe it could be implemented as a pre-pre-enable indeed. It feels
>> like a bit of a hack, as the next time we need more fine-grained control
>> over the startup sequence, we'll have to add a pre-pre-pre-enable. Given
>> that the startup sequence requirements come from the sink device, it
>> would make sense to let it explicitly control the initialization,
>> instead of driving it from the source. I don't think we'll be able to
>> rework the bridge API in that direction though, so I'm fine with a hack.
> 
> There are pros and cons to both approaches.
> You're in a much better place to make that sort of call than I am, so
> I'll take your advice.
> 
> Implementing a DSI host op function may mean an update to a number of
> existing DSI host drivers, 

Why? You just add new op to mipi_dsi_host_ops and create appropriate 
helper, which in case of NULL will perform default action, either:
- return -ENOSYS,
- try to emulate by calling mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET)

The latter is just wild guess, but I suspect it could work (even now) as 
an alternate way of entering into stop state.


> but it would be cleaner. It's also what
> Andrzej has suggested.
> 
> Thinking it through, a function that requests clock lane frequency and
> state (ULPS or LP-11 predominantly), and data lane state (again ULPS
> or LP-11) should allow the required behaviour for most of the bridges
> I'm aware of. Most want either LP-11, or HS clock at a known
> frequency.

LP-11 is quite reasonable, but regarding frequency I am not sure who 
should manage it? DSI device datasheets known to me usually mentioned 
max frequency, and it is covered by mipi_dsi_device.hs_rate. Min 
frequency is usually determined by amount of data we need to transfer 
per second.
Do you need to set frequency from device, or just to know actual frequency?
In former case (if it is really necessary !!!) you should request 
frequency range anyway, as host usually is able to set only discrete 
number of frequencies. But it need to be clearly specified how and when 
this op can be used - more ops, more questions about interaction between 
them.
In latter case another mipi host op should be quite easy to implement.


> Giving the option for setting back into ULPS also allows
> for power saving/standby mechanisms should the need arise.

Another op :)

Regards
Andrzej


> Does it need a way to pass back the actual DSI frequency being used,
> in a similar vein to mode_fixup? That allows for the bridge to request
> the display clock, but the burst mode link frequency to be returned
> (I'm assuming that's a property of the DSI host only, and not the
> bridge).
> 
> I'm having a discussion with someone who wants to run SN65DSI85 in the
> two independent LVDS display mode. That requires a DSI HS clock on
> DSI-A even if only panel B is active, so with this extra function that
> would be achievable as well.
> 
> Thoughts?
> 
>>>>>> If the panel/bridge can set a flag that can be checked at this point
>>>>>> for whether an early clock is required or not, I think that allows us
>>>>>> to comply with the requirements for a large number of panels/bridges
>>>>>> (LP-11 vs HS config for clock and or data lanes before pre_enable is
>>>>>> called).
>>>>>>
>>>>>> pre_enable retains the current behaviour (initialise the chain from
>>>>>> sink to source).
>>>>>> enable then actually starts sending video and enabling outputs.
>>>>>
>>>>> Flags indeed seem like a more contained option. Another one could be to
>>>>> have a mipi_dsi_host to (for example) power up the clock lane that would
>>>>> be called by default before the bridge's enable, or at the downstream
>>>>> bridge driver discretion before that.
>>>>
>>>> Which driver will that call?
>>>
>>> The parent DSI Host
>>>
>>>> An extreme example perhaps, but Toshiba make the TC358860 eDP to DSI
>>>> bridge chip[1]. So the encoder will be eDP, but the DSI config needs
>>>> to go to that bridge. Does that happen automatically within the
>>>> framework? I guess so as the bridge will have called
>>>> mipi_dsi_host_register for the DSI sink to attach to.
>>>
>>> In that case, whatever sink would be connected to the bridge would call
>>> the bridge clock enable hook if it needs it in its pre_enable, or it
>>> would be called automatically before enable if it doesn't
>>>
>>> Would that help?
>>
>> Sounds good to me, in theory at least (let's see what issues we'll run
>> into in practice :-)).
>>
>> Has anyone given it a try, or is planning to ?
>>
>>>> Perhaps a new mipi_dsi_host function to configure the PHY is the
>>>> easier solution. If it can allow the sink to request whatever
>>>> combination of states from clock and data lanes that it fancies, then
>>>> it can be as explicit as required for the initialisation sequence, and
>>>> the host driver does its best to comply with the requests.
>>>
>>> I don't know, I'm not really fond in general of solutions that try to
>>> cover any single case if we don't need it and / or have today an issue
>>> with this. I'd rather have something that works for the particular
>>> bridge we were discussing, see if it applies to other bridges and modify
>>> it if it doesn't until it works for all our cases. Trying to reason in
>>> all possible cases tend to lead to solutions that are difficult to
>>> maintain and usually over-engineered.
>>
>> A DSI host clock enable operation or a DSI host PHY configuration
>> operation both fit in the same place in the grand scheme of things, so I
>> don't mind either. We should be able to easily move from a more specific
>> operation to a more generic one if the need arises.
>>
>>>> I'd have a slight query over when and how the host would drop to ULPS
>>>> or power off. It probably shouldn't be in post_disable as the sink
>>>> hasn't had a chance to finalise everything in its post_disable.
>>>>
>>>> Perhaps pm_runtime with autosuspend is the right call there?
>>>
>>> pm_runtime semantics mean that once the device is suspended, its power
>>> domains, regulators, clocks, etc. are all shut down, so it doesn't
>>> really fit the low power state expected by DSI
>>>
>>>> [1] https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html
>>>>
>>>>>> When I discussed this briefly with Maxime there was a suggestion of
>>>>>> using pm_runtime to be able to power up the pipeline as a whole. If
>>>>>> the bridge driver can use pm_runtime to power up the PHY when
>>>>>> required, then that may solve the issue, however I know too little of
>>>>>> the details to say whether that is actually practical.
>>>>>
>>>>> I'm not sure it was about this topic in particular. If I remember well
>>>>> our discussion, this was about the vc4 driver that tries to circumvent
>>>>> the framework and call the pre_enable and enable hooks itself because it
>>>>> wasn't properly powered before and thus any DCS-related call by the
>>>>> downstream bridge or panel would end up creating a CPU stall.
>>>>>
>>>>> I suggested to use runtime_pm in the DCS related calls to make sure the
>>>>> device is powered because there's no relation between the state of the
>>>>> downstream bridge or panel and whether it can send DCS commands or not.
>>>>> For all we know, it could do it at probe time.
>>>>
>>>> pm_runtime is all a bit of a magic black box to me.
>>>>
>>>> We had discussed shifting to using pm_runtime from DCS (and enable)
>>>> calls to power up the PHY on demand, and that's what I implemented.
>>>> However Laurent flagged up that using
>>>> dsi->encoder->crtc->state->adjusted_mode to get the HS clock info
>>>> required to send a HS DCS command from that call is deprecated, so how
>>>> do we specify the clock rate to use at that point?
>>>
>>> I guess the most sensible would be to have a custom bridge state, and
>>> add a pointer to the current bridge state in struct drm_bridge. Then, as
>>> long as you have a bridge pointer you have a way to get the current
>>> state associated to it, and since we already have atomic_duplicate_state
>>> / atomic_destroy_state we can create our own structure around it storing
>>> whatever we want.
>>
>> That's a good point. It would only be needed if we use runtime PM to
>> work around the initialization sequence issue, not if we implement a DSI
>> host clock enable/disable operation, right ?
> 
> Obviously this only works with atomic bridges otherwise you have no
> state, but I assume all bridges should be heading that route now.
> 
>>>>>>>> host_transfer calls can supposedly be made at any time, however unless
>>>>>>>> MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
>>>>>>>> in high speed mode. If this is before a mode has been set, what
>>>>>>>> defines the link frequency parameters at this point? Adopting a random
>>>>>>>> default sounds like a good way to get undefined behaviour.
>>>>>>>>
>>>>>>>> DSI burst mode needs to set the DSI link frequency independently of
>>>>>>>> the display mode. How is that meant to be configured? I would have
>>>>>>>> expected it to come from DT due to link frequency often being chosen
>>>>>>>> based on EMC restrictions, but I don't see such a thing in any
>>>>>>>> binding.
>>>>>>>
>>>>>>> Undefined too. DSI support was added to DRM without any design effort,
>>>>>>> it's more a hack than a real solution. The issue with devices that can
>>>>>>> be controlled over both DSI and I2C is completely unhandled. So far
>>>>>>> nobody has really cared about implementing DSI right as far as I can
>>>>>>> tell.
>>>>>>
>>>>>> :-(
>>>>>>
>>>>>> Thinking aloud, does having the option to set a burst link frequency
>>>>>> from DT (or ACPI) have any issue for other platforms?
>>>>>> Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
>>>>>> drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
>>>>>> all the defined timings, but drop to LP during blanking" option. The
>>>>>> link frequency has therefore remained a property of the
>>>>>> display/bridge.
>>>>>> dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
>>>>>> through the full detail of the parameters it computes from there.
>>>>>
>>>>> I don't see anything wrong with using link-frequency from the DT to
>>>>> setup the burst frequency. It's what v4l2 has been using for a while
>>>>> without any known (to me) drawback, and we're using the same of-graph
>>>>> bindings, so it shouldn't be too controversial there.
>>
>> How would that frequency we picked in practice ? Do panels typically
>> support a range of HS frequencies for DCS HS transfers ?
> 
> I was thinking more of bridges where they often run a PLL off the
> incoming DSI clock, and then have a FIFO with potentially different
> clock rates and timings on input and output. SN65DSI83 supports that,
> as do the Toshiba bridge chips I'm currently looking at.
> You need to know the DSI link frequency to configure the PLL
> correctly, but then the bridge output timing is a different matter.
> 
>>>> OK, that sounds like a vague plan.
>>>>
>>>>>> DSI and I2C controlled devices is yet another issue that I haven't
>>>>>> even looked at.
>>>>>> I think it's more that vc4 wants to ignore DSI should the DSI host
>>>>>> node be enabled in DT, but there's no panel bound to it. One could say
>>>>>> that is a DT error and tough luck, but from a user's perspective that
>>>>>> is a bit harsh.
>>>>>
>>>>> I guess the larger "issue" is that the tree in the DT is done following
>>>>> the "control" bus, and Linux likes to tie the life cycle of a given
>>>>> device to its parent bus. Both these decisions make sense, but they
>>>>> interact in a weird way in some occurrences (like this one, or Allwinner
>>>>> has an Ethernet PHY controlled through MMIO which end up in the same
>>>>> case).
>>>>>
>>>>> I wonder if using device links here could help though.
>>>>
>>>> I really don't know about that one.
>>>
>>> It's a piece of infrastructure that was created at first (I think?) to
>>> model the power dependency between devices that don't have a parent /
>>> child relationship. For example, if you use DMA, you probably want to
>>> keep the IOMMU powered as long as you are, but it is in a completely
>>> separate branch of the "device tree" (not one from the DTB, the one that
>>> linux DM creates).
>>>
>>> It was later expanded to also cover probe order and make sure a supplier
>>> would probe before its consumer, effectively making EPROBE_DEFER
>>> obsolete.
>>>
>>> The second part is still fairly new, but I think we can solve this by
>>> adding a device link between the DSI host and whatever is at the end of
>>> the OF-Graph endpoint.
>>>
>>>>>>>> As a follow on, bridge devices can support burst mode (eg TI's
>>>>>>>> SN65DSI83 that's just been merged), so it needs to know the desired
>>>>>>>> panel timings for the output side of the bridge, but the DSI link
>>>>>>>> timings to set up the bridge's PLL. What's the correct way for
>>>>>>>> signalling that? drm_crtc_state->adjusted_mode vs
>>>>>>>> drm_crtc_state->mode? Except mode is userspace's request, not what has
>>>>>>>> been validated/updated by the panel/bridge.
>>>>>>>
>>>>>>> adjusted_mode is also a bit of a hack, it solves very specific issues,
>>>>>>> and its design assumes a single encoder in the chain with no extra
>>>>>>> bridges. We should instead add modes to the bridge state, and negotiate
>>>>>>> modes along the pipeline the same way we negotiate formats.
>>>>>>
>>>>>> So as I understand it we already have format negotiation between
>>>>>> bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
>>>>>> so is it possible to extend that to modes?
>>>>>> Are you thinking bridge state that is owned by the framework, or by
>>>>>> the individual bridge drivers?
>>>>>
>>>>> atomic_check is made for that. I guess we could improve its call
>>>>> sequence to each time a mode is modified along the bridge list we
>>>>> restart the sequence until all components agree (or reject it entirely
>>>>> if they can't), but I don't really see why we would need yet another
>>>>> hook.
>>
>> Isn't this what atomic_get_output_bus_fmts() and
>> atomic_get_input_bus_fmts() implement ?
> 
> Those negotiate a single u32 bus format between nodes, not a complete timing.
> 
>>>> Why do all nodes in the bridge list need to agree? Adjacent nodes need
>>>> to agree, but they then also need to retain that agreed timing
>>>> somewhere.
>>>
>>> We might have mutually exclusive requirements though? Let's use the
>>> example of the VC4 HDMI driver that can't have odd horizontal timings,
>>> and assume it's a constraint of our DSI driver instead.
>>>
>>> Then, we have a DSI->LVDS bridge, a LVDS->RGB bridge and a panel (which
>>> is a bit ridiculous, but whatever). If the LVDS->RGB bridge can't have
>>> even horizontal timings, then you just can't display it, even though
>>> they are not adjacent (unless the bridge in the middle can modify the
>>> timings between the input and output, but that's not always possible).
>>>
>>> Similarly, if for the RGB panel we need to increase a bit some timings
>>> to accommodate for a larger pixel clock and end up above what the DSI
>>> host can provide, we're also done.
>>
>> The hard part will be to figure out a good heuristics to perform the
>> negotiation without going back and forth (at least not in a way that
>> would require too many iterations, and certainly avoiding infinite
>> loops). That will be an interesting problem to solve, but maybe we'll be
>> lucky and a simple approach will work for the use cases we have to
>> support today.
> 
> One to kick into the long grass possibly then.
> 
> For burst mode and bridges retiming things, generally I think it works
> if you consider a single pass through a mode_fixup equivalent starting
> at the panel.
> The panel advertises what it wants.
> The closest bridge (eg DSI83) fixup amends panel's requested mode
> based on any constraints that it has, stores a copy, and passes that
> down the chain.
> The next bridge (eg vc4_dsi) amends the mode based on restrictions it
> has (eg burst mode link frequency). That's the end of the chain in
> this case, so that's the mode that the crtc needs to be programmed
> with.
> 
> The modes then need to be passed back up the chain so that eg DSI83
> knows the link frequency in use, and can therefore configure the PLL
> appropriately.
> 
> The bit it would be nice to fix is that burst mode has effectively
> increased the horizontal front porch, but that could be fixed within
> the bridge so that the panel gets the timing it requested.
> 
>>>> Taking SN65DSI8[3|4|5] as an example, it supports burst mode, and the
>>>> DSI frequency and timings are permitted to be different from that
>>>> which it uses on the LVDS side. The LVDS panel and LVDS side of DSI83
>>>> need to agree over the format, and the DSI host and DSI side of DSI83
>>>> need to agree, but they may be two different timings.
>>>> Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows you to
>>>> configure the LVDS rate compared to the DSI rate (the driver currently
>>>> goes for 1:1), and registers 0x20 to 0x34 allow you to set the number
>>>> of active pixel and blanking on the LVDS side (again currently just
>>>> copied across).
>>>>
>>>> The way I'm seeing burst mode as having been interpreted at present is
>>>> that it's largely just a flag to say "drop to LP mode between lines".
>>>> The timing that needs to be passed to the crtc is therefore going to
>>>> be based on the DSI link rate (converted to pixels) with increased
>>>> blanking periods.
>>>>
>>>> I guess there are similarities with Media Controller and V4L2 here. A
>>>> typical chain there could be:
>>>>   sensor -> scaler -> crop -> CSI-2 receiver.
>>>> The format on each of those links may be different, but the chain as a
>>>> whole needs to be valid. Media Controller largely relies on userspace
>>>> to configure all links, but with a DRM chain that isn't really an
>>>> option as it's expected that the display chain configures itself.
>>>
>>> Also, the userspace has no concept of media sub-devices in DRM, so it
>>> just sets the mode on the whole DRM/KMS device, unlike what v4l2 does.
>>> In v4l2, afaik, if you ended up with the above scenarios it would just
>>> be rejected when you set the format on the link, letting the userspace
>>> figure it out. We can't really do that here
>>
>> I wonder how long we'll be able to keep userspace out of the picture to
>> configure the internals of the pipeline. I don't want to be the first
>> person who will have a use case that requires this.
> 
> I suspect none of us want to be the first one to hit this scenario!
> 
> As I've just posted on the other thread about SN65DSI83, I've hit a
> major stumbling block with the current design where vc4_dsi (and
> Exynos) breaks the bridge_chain so that it gets called first and can
> do initialisation, and then it calls down the chain.
> Having now converted to a DSI bridge driver, it works fine with
> connected non-atomic bridges, but fails with atomic bridges.
> drm_atomic_add_encoder_bridges adds the state for all the bridges the
> framework is aware of. With the split chain it misses adding the state
> of our "hidden" bridges, and we can't add the extra state from our
> atomic_duplicate_state call as we don't have the state to add to (we
> just return our state to be added).
> This bumps up the priority for us of finding a suitable solution for
> this initialisation issue, so I'll start looking at how feasible a new
> DSI host function is.
> 
> Thanks
>    Dave
> 

Re: Questions over DSI within DRM.

[Dave Stevenson]

Hi Andrzej

Thanks for joining in the discussion.

On Tue, 5 Oct 2021 at 16:08, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
>
> On 05.10.2021 13:23, Dave Stevenson wrote:
> > Hi Laurent
> >
> > On Sun, 3 Oct 2021 at 15:16, Laurent Pinchart
> > <laurent.pinchart@ideasonboard.com> wrote:
> >>
> >> Hello,
> >>
> >> Reviving a bit of an old thread.
> >
> > I'd been looking at reviving this conversation too as I've moved
> > further on with DSI on the Pi, and converting from an encoder to a
> > bridge
> >
> >> On Thu, Jul 15, 2021 at 11:50:22AM +0200, Maxime Ripard wrote:
> >>> On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
> >>>> On Tue, 6 Jul 2021 at 16:13, Maxime Ripard wrote:
> >>>>>>>> On a similar theme, some devices want the clock lane in HS mode early
> >>>>>>>> so they can use it in place of an external oscillator, but the data
> >>>>>>>> lanes still in LP-11. There appears to be no way for the
> >>>>>>>> display/bridge to signal this requirement or it be achieved.
> >>>>>>>
> >>>>>>> You're right. A loooong time ago, the omapdrm driver had an internal
> >>>>>>> infrastructure that didn't use drm_bridge or drm_panel and instead
> >>>>>>> required omapdrm-specific drivers for those components. It used to model
> >>>>>>> the display pipeline in a different way than drm_bridge, with the sync
> >>>>>>> explicitly setting the source state. A DSI sink could thus control its
> >>>>>>> enable sequence, interleaving programming of the sink with control of
> >>>>>>> the source.
> >>>>>>>
> >>>>>>> Migrating omapdrm to the drm_bridge model took a really large effort,
> >>>>>>> which makes me believe that transitioning the whole subsystem to
> >>>>>>> sink-controlled sources would be close to impossible. We could add
> >>>>>>> DSI-specific operations, or add another enable bridge operation
> >>>>>>> (post_pre_enable ? :-D). Neither would scale, but it may be enough.
> >>>>>>
> >>>>>> I haven't thought it through for all generic cases, but I suspect it's
> >>>>>> more a pre_pre_enable that is needed to initialise the PHY etc,
> >>>>>> probably from source to sink.
> >>
> >> I believe it could be implemented as a pre-pre-enable indeed. It feels
> >> like a bit of a hack, as the next time we need more fine-grained control
> >> over the startup sequence, we'll have to add a pre-pre-pre-enable. Given
> >> that the startup sequence requirements come from the sink device, it
> >> would make sense to let it explicitly control the initialization,
> >> instead of driving it from the source. I don't think we'll be able to
> >> rework the bridge API in that direction though, so I'm fine with a hack.
> >
> > There are pros and cons to both approaches.
> > You're in a much better place to make that sort of call than I am, so
> > I'll take your advice.
> >
> > Implementing a DSI host op function may mean an update to a number of
> > existing DSI host drivers,
>
> Why? You just add new op to mipi_dsi_host_ops and create appropriate
> helper, which in case of NULL will perform default action, either:
> - return -ENOSYS,
> - try to emulate by calling mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET)
>
> The latter is just wild guess, but I suspect it could work (even now) as
> an alternate way of entering into stop state.

If we're looking at fixing bridges to use the new DSI state call and
then initialise in "pre_enable" (ie before video is started), then
making that change to the bridge will require support for the DSI
state call or risk regressions.
Some may have been getting away with it by making the initialisation
call in "enable" whilst video is active - that sounds like the issue
Marek was trying to log in his patch to SN65DSI83.

> > but it would be cleaner. It's also what
> > Andrzej has suggested.
> >
> > Thinking it through, a function that requests clock lane frequency and
> > state (ULPS or LP-11 predominantly), and data lane state (again ULPS
> > or LP-11) should allow the required behaviour for most of the bridges
> > I'm aware of. Most want either LP-11, or HS clock at a known
> > frequency.
>
> LP-11 is quite reasonable, but regarding frequency I am not sure who
> should manage it? DSI device datasheets known to me usually mentioned
> max frequency, and it is covered by mipi_dsi_device.hs_rate. Min
> frequency is usually determined by amount of data we need to transfer
> per second.
> Do you need to set frequency from device, or just to know actual frequency?
> In former case (if it is really necessary !!!) you should request
> frequency range anyway, as host usually is able to set only discrete
> number of frequencies. But it need to be clearly specified how and when
> this op can be used - more ops, more questions about interaction between
> them.
> In latter case another mipi host op should be quite easy to implement.

If a burst mode frequency is specified in struct mipi_dsi_device
hs_rate then there is no issue.
If not, then you'll generally want to use the pixel clock, and the DSI
host hasn't necessarily been told the video mode at the point the
bridge/panel requests a new DSI state.

> > Giving the option for setting back into ULPS also allows
> > for power saving/standby mechanisms should the need arise.
>
> Another op :)

Does it need to be, or can it be one op as
enum mipi_dsi_lane_state{
 DSI_ULPS,
 DSI_LP11,
 DSI_HS,
};

int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
       enum mipi_dsi_lane_state clock_lane_state,
       u64 clock_lane_freq,
       enum mipi_dsi_lane_state data_lane_state)

  Dave

> Regards
> Andrzej
>
>
> > Does it need a way to pass back the actual DSI frequency being used,
> > in a similar vein to mode_fixup? That allows for the bridge to request
> > the display clock, but the burst mode link frequency to be returned
> > (I'm assuming that's a property of the DSI host only, and not the
> > bridge).
> >
> > I'm having a discussion with someone who wants to run SN65DSI85 in the
> > two independent LVDS display mode. That requires a DSI HS clock on
> > DSI-A even if only panel B is active, so with this extra function that
> > would be achievable as well.
> >
> > Thoughts?
> >
> >>>>>> If the panel/bridge can set a flag that can be checked at this point
> >>>>>> for whether an early clock is required or not, I think that allows us
> >>>>>> to comply with the requirements for a large number of panels/bridges
> >>>>>> (LP-11 vs HS config for clock and or data lanes before pre_enable is
> >>>>>> called).
> >>>>>>
> >>>>>> pre_enable retains the current behaviour (initialise the chain from
> >>>>>> sink to source).
> >>>>>> enable then actually starts sending video and enabling outputs.
> >>>>>
> >>>>> Flags indeed seem like a more contained option. Another one could be to
> >>>>> have a mipi_dsi_host to (for example) power up the clock lane that would
> >>>>> be called by default before the bridge's enable, or at the downstream
> >>>>> bridge driver discretion before that.
> >>>>
> >>>> Which driver will that call?
> >>>
> >>> The parent DSI Host
> >>>
> >>>> An extreme example perhaps, but Toshiba make the TC358860 eDP to DSI
> >>>> bridge chip[1]. So the encoder will be eDP, but the DSI config needs
> >>>> to go to that bridge. Does that happen automatically within the
> >>>> framework? I guess so as the bridge will have called
> >>>> mipi_dsi_host_register for the DSI sink to attach to.
> >>>
> >>> In that case, whatever sink would be connected to the bridge would call
> >>> the bridge clock enable hook if it needs it in its pre_enable, or it
> >>> would be called automatically before enable if it doesn't
> >>>
> >>> Would that help?
> >>
> >> Sounds good to me, in theory at least (let's see what issues we'll run
> >> into in practice :-)).
> >>
> >> Has anyone given it a try, or is planning to ?
> >>
> >>>> Perhaps a new mipi_dsi_host function to configure the PHY is the
> >>>> easier solution. If it can allow the sink to request whatever
> >>>> combination of states from clock and data lanes that it fancies, then
> >>>> it can be as explicit as required for the initialisation sequence, and
> >>>> the host driver does its best to comply with the requests.
> >>>
> >>> I don't know, I'm not really fond in general of solutions that try to
> >>> cover any single case if we don't need it and / or have today an issue
> >>> with this. I'd rather have something that works for the particular
> >>> bridge we were discussing, see if it applies to other bridges and modify
> >>> it if it doesn't until it works for all our cases. Trying to reason in
> >>> all possible cases tend to lead to solutions that are difficult to
> >>> maintain and usually over-engineered.
> >>
> >> A DSI host clock enable operation or a DSI host PHY configuration
> >> operation both fit in the same place in the grand scheme of things, so I
> >> don't mind either. We should be able to easily move from a more specific
> >> operation to a more generic one if the need arises.
> >>
> >>>> I'd have a slight query over when and how the host would drop to ULPS
> >>>> or power off. It probably shouldn't be in post_disable as the sink
> >>>> hasn't had a chance to finalise everything in its post_disable.
> >>>>
> >>>> Perhaps pm_runtime with autosuspend is the right call there?
> >>>
> >>> pm_runtime semantics mean that once the device is suspended, its power
> >>> domains, regulators, clocks, etc. are all shut down, so it doesn't
> >>> really fit the low power state expected by DSI
> >>>
> >>>> [1] https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html
> >>>>
> >>>>>> When I discussed this briefly with Maxime there was a suggestion of
> >>>>>> using pm_runtime to be able to power up the pipeline as a whole. If
> >>>>>> the bridge driver can use pm_runtime to power up the PHY when
> >>>>>> required, then that may solve the issue, however I know too little of
> >>>>>> the details to say whether that is actually practical.
> >>>>>
> >>>>> I'm not sure it was about this topic in particular. If I remember well
> >>>>> our discussion, this was about the vc4 driver that tries to circumvent
> >>>>> the framework and call the pre_enable and enable hooks itself because it
> >>>>> wasn't properly powered before and thus any DCS-related call by the
> >>>>> downstream bridge or panel would end up creating a CPU stall.
> >>>>>
> >>>>> I suggested to use runtime_pm in the DCS related calls to make sure the
> >>>>> device is powered because there's no relation between the state of the
> >>>>> downstream bridge or panel and whether it can send DCS commands or not.
> >>>>> For all we know, it could do it at probe time.
> >>>>
> >>>> pm_runtime is all a bit of a magic black box to me.
> >>>>
> >>>> We had discussed shifting to using pm_runtime from DCS (and enable)
> >>>> calls to power up the PHY on demand, and that's what I implemented.
> >>>> However Laurent flagged up that using
> >>>> dsi->encoder->crtc->state->adjusted_mode to get the HS clock info
> >>>> required to send a HS DCS command from that call is deprecated, so how
> >>>> do we specify the clock rate to use at that point?
> >>>
> >>> I guess the most sensible would be to have a custom bridge state, and
> >>> add a pointer to the current bridge state in struct drm_bridge. Then, as
> >>> long as you have a bridge pointer you have a way to get the current
> >>> state associated to it, and since we already have atomic_duplicate_state
> >>> / atomic_destroy_state we can create our own structure around it storing
> >>> whatever we want.
> >>
> >> That's a good point. It would only be needed if we use runtime PM to
> >> work around the initialization sequence issue, not if we implement a DSI
> >> host clock enable/disable operation, right ?
> >
> > Obviously this only works with atomic bridges otherwise you have no
> > state, but I assume all bridges should be heading that route now.
> >
> >>>>>>>> host_transfer calls can supposedly be made at any time, however unless
> >>>>>>>> MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
> >>>>>>>> in high speed mode. If this is before a mode has been set, what
> >>>>>>>> defines the link frequency parameters at this point? Adopting a random
> >>>>>>>> default sounds like a good way to get undefined behaviour.
> >>>>>>>>
> >>>>>>>> DSI burst mode needs to set the DSI link frequency independently of
> >>>>>>>> the display mode. How is that meant to be configured? I would have
> >>>>>>>> expected it to come from DT due to link frequency often being chosen
> >>>>>>>> based on EMC restrictions, but I don't see such a thing in any
> >>>>>>>> binding.
> >>>>>>>
> >>>>>>> Undefined too. DSI support was added to DRM without any design effort,
> >>>>>>> it's more a hack than a real solution. The issue with devices that can
> >>>>>>> be controlled over both DSI and I2C is completely unhandled. So far
> >>>>>>> nobody has really cared about implementing DSI right as far as I can
> >>>>>>> tell.
> >>>>>>
> >>>>>> :-(
> >>>>>>
> >>>>>> Thinking aloud, does having the option to set a burst link frequency
> >>>>>> from DT (or ACPI) have any issue for other platforms?
> >>>>>> Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
> >>>>>> drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
> >>>>>> all the defined timings, but drop to LP during blanking" option. The
> >>>>>> link frequency has therefore remained a property of the
> >>>>>> display/bridge.
> >>>>>> dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
> >>>>>> through the full detail of the parameters it computes from there.
> >>>>>
> >>>>> I don't see anything wrong with using link-frequency from the DT to
> >>>>> setup the burst frequency. It's what v4l2 has been using for a while
> >>>>> without any known (to me) drawback, and we're using the same of-graph
> >>>>> bindings, so it shouldn't be too controversial there.
> >>
> >> How would that frequency we picked in practice ? Do panels typically
> >> support a range of HS frequencies for DCS HS transfers ?
> >
> > I was thinking more of bridges where they often run a PLL off the
> > incoming DSI clock, and then have a FIFO with potentially different
> > clock rates and timings on input and output. SN65DSI83 supports that,
> > as do the Toshiba bridge chips I'm currently looking at.
> > You need to know the DSI link frequency to configure the PLL
> > correctly, but then the bridge output timing is a different matter.
> >
> >>>> OK, that sounds like a vague plan.
> >>>>
> >>>>>> DSI and I2C controlled devices is yet another issue that I haven't
> >>>>>> even looked at.
> >>>>>> I think it's more that vc4 wants to ignore DSI should the DSI host
> >>>>>> node be enabled in DT, but there's no panel bound to it. One could say
> >>>>>> that is a DT error and tough luck, but from a user's perspective that
> >>>>>> is a bit harsh.
> >>>>>
> >>>>> I guess the larger "issue" is that the tree in the DT is done following
> >>>>> the "control" bus, and Linux likes to tie the life cycle of a given
> >>>>> device to its parent bus. Both these decisions make sense, but they
> >>>>> interact in a weird way in some occurrences (like this one, or Allwinner
> >>>>> has an Ethernet PHY controlled through MMIO which end up in the same
> >>>>> case).
> >>>>>
> >>>>> I wonder if using device links here could help though.
> >>>>
> >>>> I really don't know about that one.
> >>>
> >>> It's a piece of infrastructure that was created at first (I think?) to
> >>> model the power dependency between devices that don't have a parent /
> >>> child relationship. For example, if you use DMA, you probably want to
> >>> keep the IOMMU powered as long as you are, but it is in a completely
> >>> separate branch of the "device tree" (not one from the DTB, the one that
> >>> linux DM creates).
> >>>
> >>> It was later expanded to also cover probe order and make sure a supplier
> >>> would probe before its consumer, effectively making EPROBE_DEFER
> >>> obsolete.
> >>>
> >>> The second part is still fairly new, but I think we can solve this by
> >>> adding a device link between the DSI host and whatever is at the end of
> >>> the OF-Graph endpoint.
> >>>
> >>>>>>>> As a follow on, bridge devices can support burst mode (eg TI's
> >>>>>>>> SN65DSI83 that's just been merged), so it needs to know the desired
> >>>>>>>> panel timings for the output side of the bridge, but the DSI link
> >>>>>>>> timings to set up the bridge's PLL. What's the correct way for
> >>>>>>>> signalling that? drm_crtc_state->adjusted_mode vs
> >>>>>>>> drm_crtc_state->mode? Except mode is userspace's request, not what has
> >>>>>>>> been validated/updated by the panel/bridge.
> >>>>>>>
> >>>>>>> adjusted_mode is also a bit of a hack, it solves very specific issues,
> >>>>>>> and its design assumes a single encoder in the chain with no extra
> >>>>>>> bridges. We should instead add modes to the bridge state, and negotiate
> >>>>>>> modes along the pipeline the same way we negotiate formats.
> >>>>>>
> >>>>>> So as I understand it we already have format negotiation between
> >>>>>> bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
> >>>>>> so is it possible to extend that to modes?
> >>>>>> Are you thinking bridge state that is owned by the framework, or by
> >>>>>> the individual bridge drivers?
> >>>>>
> >>>>> atomic_check is made for that. I guess we could improve its call
> >>>>> sequence to each time a mode is modified along the bridge list we
> >>>>> restart the sequence until all components agree (or reject it entirely
> >>>>> if they can't), but I don't really see why we would need yet another
> >>>>> hook.
> >>
> >> Isn't this what atomic_get_output_bus_fmts() and
> >> atomic_get_input_bus_fmts() implement ?
> >
> > Those negotiate a single u32 bus format between nodes, not a complete timing.
> >
> >>>> Why do all nodes in the bridge list need to agree? Adjacent nodes need
> >>>> to agree, but they then also need to retain that agreed timing
> >>>> somewhere.
> >>>
> >>> We might have mutually exclusive requirements though? Let's use the
> >>> example of the VC4 HDMI driver that can't have odd horizontal timings,
> >>> and assume it's a constraint of our DSI driver instead.
> >>>
> >>> Then, we have a DSI->LVDS bridge, a LVDS->RGB bridge and a panel (which
> >>> is a bit ridiculous, but whatever). If the LVDS->RGB bridge can't have
> >>> even horizontal timings, then you just can't display it, even though
> >>> they are not adjacent (unless the bridge in the middle can modify the
> >>> timings between the input and output, but that's not always possible).
> >>>
> >>> Similarly, if for the RGB panel we need to increase a bit some timings
> >>> to accommodate for a larger pixel clock and end up above what the DSI
> >>> host can provide, we're also done.
> >>
> >> The hard part will be to figure out a good heuristics to perform the
> >> negotiation without going back and forth (at least not in a way that
> >> would require too many iterations, and certainly avoiding infinite
> >> loops). That will be an interesting problem to solve, but maybe we'll be
> >> lucky and a simple approach will work for the use cases we have to
> >> support today.
> >
> > One to kick into the long grass possibly then.
> >
> > For burst mode and bridges retiming things, generally I think it works
> > if you consider a single pass through a mode_fixup equivalent starting
> > at the panel.
> > The panel advertises what it wants.
> > The closest bridge (eg DSI83) fixup amends panel's requested mode
> > based on any constraints that it has, stores a copy, and passes that
> > down the chain.
> > The next bridge (eg vc4_dsi) amends the mode based on restrictions it
> > has (eg burst mode link frequency). That's the end of the chain in
> > this case, so that's the mode that the crtc needs to be programmed
> > with.
> >
> > The modes then need to be passed back up the chain so that eg DSI83
> > knows the link frequency in use, and can therefore configure the PLL
> > appropriately.
> >
> > The bit it would be nice to fix is that burst mode has effectively
> > increased the horizontal front porch, but that could be fixed within
> > the bridge so that the panel gets the timing it requested.
> >
> >>>> Taking SN65DSI8[3|4|5] as an example, it supports burst mode, and the
> >>>> DSI frequency and timings are permitted to be different from that
> >>>> which it uses on the LVDS side. The LVDS panel and LVDS side of DSI83
> >>>> need to agree over the format, and the DSI host and DSI side of DSI83
> >>>> need to agree, but they may be two different timings.
> >>>> Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows you to
> >>>> configure the LVDS rate compared to the DSI rate (the driver currently
> >>>> goes for 1:1), and registers 0x20 to 0x34 allow you to set the number
> >>>> of active pixel and blanking on the LVDS side (again currently just
> >>>> copied across).
> >>>>
> >>>> The way I'm seeing burst mode as having been interpreted at present is
> >>>> that it's largely just a flag to say "drop to LP mode between lines".
> >>>> The timing that needs to be passed to the crtc is therefore going to
> >>>> be based on the DSI link rate (converted to pixels) with increased
> >>>> blanking periods.
> >>>>
> >>>> I guess there are similarities with Media Controller and V4L2 here. A
> >>>> typical chain there could be:
> >>>>   sensor -> scaler -> crop -> CSI-2 receiver.
> >>>> The format on each of those links may be different, but the chain as a
> >>>> whole needs to be valid. Media Controller largely relies on userspace
> >>>> to configure all links, but with a DRM chain that isn't really an
> >>>> option as it's expected that the display chain configures itself.
> >>>
> >>> Also, the userspace has no concept of media sub-devices in DRM, so it
> >>> just sets the mode on the whole DRM/KMS device, unlike what v4l2 does.
> >>> In v4l2, afaik, if you ended up with the above scenarios it would just
> >>> be rejected when you set the format on the link, letting the userspace
> >>> figure it out. We can't really do that here
> >>
> >> I wonder how long we'll be able to keep userspace out of the picture to
> >> configure the internals of the pipeline. I don't want to be the first
> >> person who will have a use case that requires this.
> >
> > I suspect none of us want to be the first one to hit this scenario!
> >
> > As I've just posted on the other thread about SN65DSI83, I've hit a
> > major stumbling block with the current design where vc4_dsi (and
> > Exynos) breaks the bridge_chain so that it gets called first and can
> > do initialisation, and then it calls down the chain.
> > Having now converted to a DSI bridge driver, it works fine with
> > connected non-atomic bridges, but fails with atomic bridges.
> > drm_atomic_add_encoder_bridges adds the state for all the bridges the
> > framework is aware of. With the split chain it misses adding the state
> > of our "hidden" bridges, and we can't add the extra state from our
> > atomic_duplicate_state call as we don't have the state to add to (we
> > just return our state to be added).
> > This bumps up the priority for us of finding a suitable solution for
> > this initialisation issue, so I'll start looking at how feasible a new
> > DSI host function is.
> >
> > Thanks
> >    Dave
> >
>

Re: Questions over DSI within DRM.

[Andrzej Hajda]

On 05.10.2021 17:32, Dave Stevenson wrote:
> Hi Andrzej
> 
> Thanks for joining in the discussion.
> 
> On Tue, 5 Oct 2021 at 16:08, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
>>
>> On 05.10.2021 13:23, Dave Stevenson wrote:
>>> Hi Laurent
>>>
>>> On Sun, 3 Oct 2021 at 15:16, Laurent Pinchart
>>> <laurent.pinchart@ideasonboard.com> wrote:
>>>>
>>>> Hello,
>>>>
>>>> Reviving a bit of an old thread.
>>>
>>> I'd been looking at reviving this conversation too as I've moved
>>> further on with DSI on the Pi, and converting from an encoder to a
>>> bridge
>>>
>>>> On Thu, Jul 15, 2021 at 11:50:22AM +0200, Maxime Ripard wrote:
>>>>> On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
>>>>>> On Tue, 6 Jul 2021 at 16:13, Maxime Ripard wrote:
>>>>>>>>>> On a similar theme, some devices want the clock lane in HS mode early
>>>>>>>>>> so they can use it in place of an external oscillator, but the data
>>>>>>>>>> lanes still in LP-11. There appears to be no way for the
>>>>>>>>>> display/bridge to signal this requirement or it be achieved.
>>>>>>>>>
>>>>>>>>> You're right. A loooong time ago, the omapdrm driver had an internal
>>>>>>>>> infrastructure that didn't use drm_bridge or drm_panel and instead
>>>>>>>>> required omapdrm-specific drivers for those components. It used to model
>>>>>>>>> the display pipeline in a different way than drm_bridge, with the sync
>>>>>>>>> explicitly setting the source state. A DSI sink could thus control its
>>>>>>>>> enable sequence, interleaving programming of the sink with control of
>>>>>>>>> the source.
>>>>>>>>>
>>>>>>>>> Migrating omapdrm to the drm_bridge model took a really large effort,
>>>>>>>>> which makes me believe that transitioning the whole subsystem to
>>>>>>>>> sink-controlled sources would be close to impossible. We could add
>>>>>>>>> DSI-specific operations, or add another enable bridge operation
>>>>>>>>> (post_pre_enable ? :-D). Neither would scale, but it may be enough.
>>>>>>>>
>>>>>>>> I haven't thought it through for all generic cases, but I suspect it's
>>>>>>>> more a pre_pre_enable that is needed to initialise the PHY etc,
>>>>>>>> probably from source to sink.
>>>>
>>>> I believe it could be implemented as a pre-pre-enable indeed. It feels
>>>> like a bit of a hack, as the next time we need more fine-grained control
>>>> over the startup sequence, we'll have to add a pre-pre-pre-enable. Given
>>>> that the startup sequence requirements come from the sink device, it
>>>> would make sense to let it explicitly control the initialization,
>>>> instead of driving it from the source. I don't think we'll be able to
>>>> rework the bridge API in that direction though, so I'm fine with a hack.
>>>
>>> There are pros and cons to both approaches.
>>> You're in a much better place to make that sort of call than I am, so
>>> I'll take your advice.
>>>
>>> Implementing a DSI host op function may mean an update to a number of
>>> existing DSI host drivers,
>>
>> Why? You just add new op to mipi_dsi_host_ops and create appropriate
>> helper, which in case of NULL will perform default action, either:
>> - return -ENOSYS,
>> - try to emulate by calling mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET)
>>
>> The latter is just wild guess, but I suspect it could work (even now) as
>> an alternate way of entering into stop state.
> 
> If we're looking at fixing bridges to use the new DSI state call and
> then initialise in "pre_enable" (ie before video is started), then
> making that change to the bridge will require support for the DSI
> state call or risk regressions.

I think there is misunderstanding somewhere, I do not know where, yet :)
My idea is as follow:
1. Add dsi_host.set_state() op to dsi host accompanied with helper, lets 
call it mipi_dsi_host_set_state for now.
2. Implement dsi_host.set_state in DSI host you are working with.
3. In body of your bridge pre_enable you call mipi_dsi_host_set_state in 
proper place.

1 and 2 do not introduce any regression.
3 can introduce regression only if your bridge is used with another dsi 
host but only on such platforms.

Why do you think we need to 'fix' it everywhere?

> Some may have been getting away with it by making the initialisation
> call in "enable" whilst video is active - that sounds like the issue
> Marek was trying to log in his patch to SN65DSI83.
> 
>>> but it would be cleaner. It's also what
>>> Andrzej has suggested.
>>>
>>> Thinking it through, a function that requests clock lane frequency and
>>> state (ULPS or LP-11 predominantly), and data lane state (again ULPS
>>> or LP-11) should allow the required behaviour for most of the bridges
>>> I'm aware of. Most want either LP-11, or HS clock at a known
>>> frequency.
>>
>> LP-11 is quite reasonable, but regarding frequency I am not sure who
>> should manage it? DSI device datasheets known to me usually mentioned
>> max frequency, and it is covered by mipi_dsi_device.hs_rate. Min
>> frequency is usually determined by amount of data we need to transfer
>> per second.
>> Do you need to set frequency from device, or just to know actual frequency?
>> In former case (if it is really necessary !!!) you should request
>> frequency range anyway, as host usually is able to set only discrete
>> number of frequencies. But it need to be clearly specified how and when
>> this op can be used - more ops, more questions about interaction between
>> them.
>> In latter case another mipi host op should be quite easy to implement.
> 
> If a burst mode frequency is specified in struct mipi_dsi_device
> hs_rate then there is no issue.

hs_rate should be called rather max_hs_rate as it describes only top 
limit of HS clock, decision which clock to choose is on DSI host side.

> If not, then you'll generally want to use the pixel clock,

Probably not, often we want to use rate higher than pixel clock 
(time-compression in DSI spec), to leave time gaps for LP mode.

> and the DSI
> host hasn't necessarily been told the video mode at the point the
> bridge/panel requests a new DSI state.

pre_enable and enable ops are called after modeset, so video mode is known.

> 
>>> Giving the option for setting back into ULPS also allows
>>> for power saving/standby mechanisms should the need arise.
>>
>> Another op :)
> 
> Does it need to be, or can it be one op as
> enum mipi_dsi_lane_state{
>   DSI_ULPS,
>   DSI_LP11,
>   DSI_HS,
> };
> 
> int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
>         enum mipi_dsi_lane_state clock_lane_state,
>         u64 clock_lane_freq,
>         enum mipi_dsi_lane_state data_lane_state)

I still do not know why do you want to set freq explicitly, ie why DSI 
host could not choose it, based on bridge constraints, DSI host 
constraints and requested video mode.

Regarding different modes for clock and data lanes there exists DSI mode 
flag MIPI_DSI_CLOCK_NON_CONTINUOUS which determines if host in LP-11 
data lanes state can also put clock lane into LP-11. Is there need for 
sth more?
I'd like to avoid possibility of nonsense(?) configurations like clock 
in LP-11 and data lanes in HS.

Regards
Andrzej



> 
>    Dave
> 
>> Regards
>> Andrzej
>>
>>
>>> Does it need a way to pass back the actual DSI frequency being used,
>>> in a similar vein to mode_fixup? That allows for the bridge to request
>>> the display clock, but the burst mode link frequency to be returned
>>> (I'm assuming that's a property of the DSI host only, and not the
>>> bridge).
>>>
>>> I'm having a discussion with someone who wants to run SN65DSI85 in the
>>> two independent LVDS display mode. That requires a DSI HS clock on
>>> DSI-A even if only panel B is active, so with this extra function that
>>> would be achievable as well.
>>>
>>> Thoughts?
>>>
>>>>>>>> If the panel/bridge can set a flag that can be checked at this point
>>>>>>>> for whether an early clock is required or not, I think that allows us
>>>>>>>> to comply with the requirements for a large number of panels/bridges
>>>>>>>> (LP-11 vs HS config for clock and or data lanes before pre_enable is
>>>>>>>> called).
>>>>>>>>
>>>>>>>> pre_enable retains the current behaviour (initialise the chain from
>>>>>>>> sink to source).
>>>>>>>> enable then actually starts sending video and enabling outputs.
>>>>>>>
>>>>>>> Flags indeed seem like a more contained option. Another one could be to
>>>>>>> have a mipi_dsi_host to (for example) power up the clock lane that would
>>>>>>> be called by default before the bridge's enable, or at the downstream
>>>>>>> bridge driver discretion before that.
>>>>>>
>>>>>> Which driver will that call?
>>>>>
>>>>> The parent DSI Host
>>>>>
>>>>>> An extreme example perhaps, but Toshiba make the TC358860 eDP to DSI
>>>>>> bridge chip[1]. So the encoder will be eDP, but the DSI config needs
>>>>>> to go to that bridge. Does that happen automatically within the
>>>>>> framework? I guess so as the bridge will have called
>>>>>> mipi_dsi_host_register for the DSI sink to attach to.
>>>>>
>>>>> In that case, whatever sink would be connected to the bridge would call
>>>>> the bridge clock enable hook if it needs it in its pre_enable, or it
>>>>> would be called automatically before enable if it doesn't
>>>>>
>>>>> Would that help?
>>>>
>>>> Sounds good to me, in theory at least (let's see what issues we'll run
>>>> into in practice :-)).
>>>>
>>>> Has anyone given it a try, or is planning to ?
>>>>
>>>>>> Perhaps a new mipi_dsi_host function to configure the PHY is the
>>>>>> easier solution. If it can allow the sink to request whatever
>>>>>> combination of states from clock and data lanes that it fancies, then
>>>>>> it can be as explicit as required for the initialisation sequence, and
>>>>>> the host driver does its best to comply with the requests.
>>>>>
>>>>> I don't know, I'm not really fond in general of solutions that try to
>>>>> cover any single case if we don't need it and / or have today an issue
>>>>> with this. I'd rather have something that works for the particular
>>>>> bridge we were discussing, see if it applies to other bridges and modify
>>>>> it if it doesn't until it works for all our cases. Trying to reason in
>>>>> all possible cases tend to lead to solutions that are difficult to
>>>>> maintain and usually over-engineered.
>>>>
>>>> A DSI host clock enable operation or a DSI host PHY configuration
>>>> operation both fit in the same place in the grand scheme of things, so I
>>>> don't mind either. We should be able to easily move from a more specific
>>>> operation to a more generic one if the need arises.
>>>>
>>>>>> I'd have a slight query over when and how the host would drop to ULPS
>>>>>> or power off. It probably shouldn't be in post_disable as the sink
>>>>>> hasn't had a chance to finalise everything in its post_disable.
>>>>>>
>>>>>> Perhaps pm_runtime with autosuspend is the right call there?
>>>>>
>>>>> pm_runtime semantics mean that once the device is suspended, its power
>>>>> domains, regulators, clocks, etc. are all shut down, so it doesn't
>>>>> really fit the low power state expected by DSI
>>>>>
>>>>>> [1] https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html
>>>>>>
>>>>>>>> When I discussed this briefly with Maxime there was a suggestion of
>>>>>>>> using pm_runtime to be able to power up the pipeline as a whole. If
>>>>>>>> the bridge driver can use pm_runtime to power up the PHY when
>>>>>>>> required, then that may solve the issue, however I know too little of
>>>>>>>> the details to say whether that is actually practical.
>>>>>>>
>>>>>>> I'm not sure it was about this topic in particular. If I remember well
>>>>>>> our discussion, this was about the vc4 driver that tries to circumvent
>>>>>>> the framework and call the pre_enable and enable hooks itself because it
>>>>>>> wasn't properly powered before and thus any DCS-related call by the
>>>>>>> downstream bridge or panel would end up creating a CPU stall.
>>>>>>>
>>>>>>> I suggested to use runtime_pm in the DCS related calls to make sure the
>>>>>>> device is powered because there's no relation between the state of the
>>>>>>> downstream bridge or panel and whether it can send DCS commands or not.
>>>>>>> For all we know, it could do it at probe time.
>>>>>>
>>>>>> pm_runtime is all a bit of a magic black box to me.
>>>>>>
>>>>>> We had discussed shifting to using pm_runtime from DCS (and enable)
>>>>>> calls to power up the PHY on demand, and that's what I implemented.
>>>>>> However Laurent flagged up that using
>>>>>> dsi->encoder->crtc->state->adjusted_mode to get the HS clock info
>>>>>> required to send a HS DCS command from that call is deprecated, so how
>>>>>> do we specify the clock rate to use at that point?
>>>>>
>>>>> I guess the most sensible would be to have a custom bridge state, and
>>>>> add a pointer to the current bridge state in struct drm_bridge. Then, as
>>>>> long as you have a bridge pointer you have a way to get the current
>>>>> state associated to it, and since we already have atomic_duplicate_state
>>>>> / atomic_destroy_state we can create our own structure around it storing
>>>>> whatever we want.
>>>>
>>>> That's a good point. It would only be needed if we use runtime PM to
>>>> work around the initialization sequence issue, not if we implement a DSI
>>>> host clock enable/disable operation, right ?
>>>
>>> Obviously this only works with atomic bridges otherwise you have no
>>> state, but I assume all bridges should be heading that route now.
>>>
>>>>>>>>>> host_transfer calls can supposedly be made at any time, however unless
>>>>>>>>>> MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
>>>>>>>>>> in high speed mode. If this is before a mode has been set, what
>>>>>>>>>> defines the link frequency parameters at this point? Adopting a random
>>>>>>>>>> default sounds like a good way to get undefined behaviour.
>>>>>>>>>>
>>>>>>>>>> DSI burst mode needs to set the DSI link frequency independently of
>>>>>>>>>> the display mode. How is that meant to be configured? I would have
>>>>>>>>>> expected it to come from DT due to link frequency often being chosen
>>>>>>>>>> based on EMC restrictions, but I don't see such a thing in any
>>>>>>>>>> binding.
>>>>>>>>>
>>>>>>>>> Undefined too. DSI support was added to DRM without any design effort,
>>>>>>>>> it's more a hack than a real solution. The issue with devices that can
>>>>>>>>> be controlled over both DSI and I2C is completely unhandled. So far
>>>>>>>>> nobody has really cared about implementing DSI right as far as I can
>>>>>>>>> tell.
>>>>>>>>
>>>>>>>> :-(
>>>>>>>>
>>>>>>>> Thinking aloud, does having the option to set a burst link frequency
>>>>>>>> from DT (or ACPI) have any issue for other platforms?
>>>>>>>> Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
>>>>>>>> drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
>>>>>>>> all the defined timings, but drop to LP during blanking" option. The
>>>>>>>> link frequency has therefore remained a property of the
>>>>>>>> display/bridge.
>>>>>>>> dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
>>>>>>>> through the full detail of the parameters it computes from there.
>>>>>>>
>>>>>>> I don't see anything wrong with using link-frequency from the DT to
>>>>>>> setup the burst frequency. It's what v4l2 has been using for a while
>>>>>>> without any known (to me) drawback, and we're using the same of-graph
>>>>>>> bindings, so it shouldn't be too controversial there.
>>>>
>>>> How would that frequency we picked in practice ? Do panels typically
>>>> support a range of HS frequencies for DCS HS transfers ?
>>>
>>> I was thinking more of bridges where they often run a PLL off the
>>> incoming DSI clock, and then have a FIFO with potentially different
>>> clock rates and timings on input and output. SN65DSI83 supports that,
>>> as do the Toshiba bridge chips I'm currently looking at.
>>> You need to know the DSI link frequency to configure the PLL
>>> correctly, but then the bridge output timing is a different matter.
>>>
>>>>>> OK, that sounds like a vague plan.
>>>>>>
>>>>>>>> DSI and I2C controlled devices is yet another issue that I haven't
>>>>>>>> even looked at.
>>>>>>>> I think it's more that vc4 wants to ignore DSI should the DSI host
>>>>>>>> node be enabled in DT, but there's no panel bound to it. One could say
>>>>>>>> that is a DT error and tough luck, but from a user's perspective that
>>>>>>>> is a bit harsh.
>>>>>>>
>>>>>>> I guess the larger "issue" is that the tree in the DT is done following
>>>>>>> the "control" bus, and Linux likes to tie the life cycle of a given
>>>>>>> device to its parent bus. Both these decisions make sense, but they
>>>>>>> interact in a weird way in some occurrences (like this one, or Allwinner
>>>>>>> has an Ethernet PHY controlled through MMIO which end up in the same
>>>>>>> case).
>>>>>>>
>>>>>>> I wonder if using device links here could help though.
>>>>>>
>>>>>> I really don't know about that one.
>>>>>
>>>>> It's a piece of infrastructure that was created at first (I think?) to
>>>>> model the power dependency between devices that don't have a parent /
>>>>> child relationship. For example, if you use DMA, you probably want to
>>>>> keep the IOMMU powered as long as you are, but it is in a completely
>>>>> separate branch of the "device tree" (not one from the DTB, the one that
>>>>> linux DM creates).
>>>>>
>>>>> It was later expanded to also cover probe order and make sure a supplier
>>>>> would probe before its consumer, effectively making EPROBE_DEFER
>>>>> obsolete.
>>>>>
>>>>> The second part is still fairly new, but I think we can solve this by
>>>>> adding a device link between the DSI host and whatever is at the end of
>>>>> the OF-Graph endpoint.
>>>>>
>>>>>>>>>> As a follow on, bridge devices can support burst mode (eg TI's
>>>>>>>>>> SN65DSI83 that's just been merged), so it needs to know the desired
>>>>>>>>>> panel timings for the output side of the bridge, but the DSI link
>>>>>>>>>> timings to set up the bridge's PLL. What's the correct way for
>>>>>>>>>> signalling that? drm_crtc_state->adjusted_mode vs
>>>>>>>>>> drm_crtc_state->mode? Except mode is userspace's request, not what has
>>>>>>>>>> been validated/updated by the panel/bridge.
>>>>>>>>>
>>>>>>>>> adjusted_mode is also a bit of a hack, it solves very specific issues,
>>>>>>>>> and its design assumes a single encoder in the chain with no extra
>>>>>>>>> bridges. We should instead add modes to the bridge state, and negotiate
>>>>>>>>> modes along the pipeline the same way we negotiate formats.
>>>>>>>>
>>>>>>>> So as I understand it we already have format negotiation between
>>>>>>>> bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
>>>>>>>> so is it possible to extend that to modes?
>>>>>>>> Are you thinking bridge state that is owned by the framework, or by
>>>>>>>> the individual bridge drivers?
>>>>>>>
>>>>>>> atomic_check is made for that. I guess we could improve its call
>>>>>>> sequence to each time a mode is modified along the bridge list we
>>>>>>> restart the sequence until all components agree (or reject it entirely
>>>>>>> if they can't), but I don't really see why we would need yet another
>>>>>>> hook.
>>>>
>>>> Isn't this what atomic_get_output_bus_fmts() and
>>>> atomic_get_input_bus_fmts() implement ?
>>>
>>> Those negotiate a single u32 bus format between nodes, not a complete timing.
>>>
>>>>>> Why do all nodes in the bridge list need to agree? Adjacent nodes need
>>>>>> to agree, but they then also need to retain that agreed timing
>>>>>> somewhere.
>>>>>
>>>>> We might have mutually exclusive requirements though? Let's use the
>>>>> example of the VC4 HDMI driver that can't have odd horizontal timings,
>>>>> and assume it's a constraint of our DSI driver instead.
>>>>>
>>>>> Then, we have a DSI->LVDS bridge, a LVDS->RGB bridge and a panel (which
>>>>> is a bit ridiculous, but whatever). If the LVDS->RGB bridge can't have
>>>>> even horizontal timings, then you just can't display it, even though
>>>>> they are not adjacent (unless the bridge in the middle can modify the
>>>>> timings between the input and output, but that's not always possible).
>>>>>
>>>>> Similarly, if for the RGB panel we need to increase a bit some timings
>>>>> to accommodate for a larger pixel clock and end up above what the DSI
>>>>> host can provide, we're also done.
>>>>
>>>> The hard part will be to figure out a good heuristics to perform the
>>>> negotiation without going back and forth (at least not in a way that
>>>> would require too many iterations, and certainly avoiding infinite
>>>> loops). That will be an interesting problem to solve, but maybe we'll be
>>>> lucky and a simple approach will work for the use cases we have to
>>>> support today.
>>>
>>> One to kick into the long grass possibly then.
>>>
>>> For burst mode and bridges retiming things, generally I think it works
>>> if you consider a single pass through a mode_fixup equivalent starting
>>> at the panel.
>>> The panel advertises what it wants.
>>> The closest bridge (eg DSI83) fixup amends panel's requested mode
>>> based on any constraints that it has, stores a copy, and passes that
>>> down the chain.
>>> The next bridge (eg vc4_dsi) amends the mode based on restrictions it
>>> has (eg burst mode link frequency). That's the end of the chain in
>>> this case, so that's the mode that the crtc needs to be programmed
>>> with.
>>>
>>> The modes then need to be passed back up the chain so that eg DSI83
>>> knows the link frequency in use, and can therefore configure the PLL
>>> appropriately.
>>>
>>> The bit it would be nice to fix is that burst mode has effectively
>>> increased the horizontal front porch, but that could be fixed within
>>> the bridge so that the panel gets the timing it requested.
>>>
>>>>>> Taking SN65DSI8[3|4|5] as an example, it supports burst mode, and the
>>>>>> DSI frequency and timings are permitted to be different from that
>>>>>> which it uses on the LVDS side. The LVDS panel and LVDS side of DSI83
>>>>>> need to agree over the format, and the DSI host and DSI side of DSI83
>>>>>> need to agree, but they may be two different timings.
>>>>>> Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows you to
>>>>>> configure the LVDS rate compared to the DSI rate (the driver currently
>>>>>> goes for 1:1), and registers 0x20 to 0x34 allow you to set the number
>>>>>> of active pixel and blanking on the LVDS side (again currently just
>>>>>> copied across).
>>>>>>
>>>>>> The way I'm seeing burst mode as having been interpreted at present is
>>>>>> that it's largely just a flag to say "drop to LP mode between lines".
>>>>>> The timing that needs to be passed to the crtc is therefore going to
>>>>>> be based on the DSI link rate (converted to pixels) with increased
>>>>>> blanking periods.
>>>>>>
>>>>>> I guess there are similarities with Media Controller and V4L2 here. A
>>>>>> typical chain there could be:
>>>>>>    sensor -> scaler -> crop -> CSI-2 receiver.
>>>>>> The format on each of those links may be different, but the chain as a
>>>>>> whole needs to be valid. Media Controller largely relies on userspace
>>>>>> to configure all links, but with a DRM chain that isn't really an
>>>>>> option as it's expected that the display chain configures itself.
>>>>>
>>>>> Also, the userspace has no concept of media sub-devices in DRM, so it
>>>>> just sets the mode on the whole DRM/KMS device, unlike what v4l2 does.
>>>>> In v4l2, afaik, if you ended up with the above scenarios it would just
>>>>> be rejected when you set the format on the link, letting the userspace
>>>>> figure it out. We can't really do that here
>>>>
>>>> I wonder how long we'll be able to keep userspace out of the picture to
>>>> configure the internals of the pipeline. I don't want to be the first
>>>> person who will have a use case that requires this.
>>>
>>> I suspect none of us want to be the first one to hit this scenario!
>>>
>>> As I've just posted on the other thread about SN65DSI83, I've hit a
>>> major stumbling block with the current design where vc4_dsi (and
>>> Exynos) breaks the bridge_chain so that it gets called first and can
>>> do initialisation, and then it calls down the chain.
>>> Having now converted to a DSI bridge driver, it works fine with
>>> connected non-atomic bridges, but fails with atomic bridges.
>>> drm_atomic_add_encoder_bridges adds the state for all the bridges the
>>> framework is aware of. With the split chain it misses adding the state
>>> of our "hidden" bridges, and we can't add the extra state from our
>>> atomic_duplicate_state call as we don't have the state to add to (we
>>> just return our state to be added).
>>> This bumps up the priority for us of finding a suitable solution for
>>> this initialisation issue, so I'll start looking at how feasible a new
>>> DSI host function is.
>>>
>>> Thanks
>>>     Dave
>>>
>>

Re: Questions over DSI within DRM.

[Dave Stevenson]

On Tue, 5 Oct 2021 at 22:03, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
>
> On 05.10.2021 17:32, Dave Stevenson wrote:
> > Hi Andrzej
> >
> > Thanks for joining in the discussion.
> >
> > On Tue, 5 Oct 2021 at 16:08, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
> >>
> >> On 05.10.2021 13:23, Dave Stevenson wrote:
> >>> Hi Laurent
> >>>
> >>> On Sun, 3 Oct 2021 at 15:16, Laurent Pinchart
> >>> <laurent.pinchart@ideasonboard.com> wrote:
> >>>>
> >>>> Hello,
> >>>>
> >>>> Reviving a bit of an old thread.
> >>>
> >>> I'd been looking at reviving this conversation too as I've moved
> >>> further on with DSI on the Pi, and converting from an encoder to a
> >>> bridge
> >>>
> >>>> On Thu, Jul 15, 2021 at 11:50:22AM +0200, Maxime Ripard wrote:
> >>>>> On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
> >>>>>> On Tue, 6 Jul 2021 at 16:13, Maxime Ripard wrote:
> >>>>>>>>>> On a similar theme, some devices want the clock lane in HS mode early
> >>>>>>>>>> so they can use it in place of an external oscillator, but the data
> >>>>>>>>>> lanes still in LP-11. There appears to be no way for the
> >>>>>>>>>> display/bridge to signal this requirement or it be achieved.
> >>>>>>>>>
> >>>>>>>>> You're right. A loooong time ago, the omapdrm driver had an internal
> >>>>>>>>> infrastructure that didn't use drm_bridge or drm_panel and instead
> >>>>>>>>> required omapdrm-specific drivers for those components. It used to model
> >>>>>>>>> the display pipeline in a different way than drm_bridge, with the sync
> >>>>>>>>> explicitly setting the source state. A DSI sink could thus control its
> >>>>>>>>> enable sequence, interleaving programming of the sink with control of
> >>>>>>>>> the source.
> >>>>>>>>>
> >>>>>>>>> Migrating omapdrm to the drm_bridge model took a really large effort,
> >>>>>>>>> which makes me believe that transitioning the whole subsystem to
> >>>>>>>>> sink-controlled sources would be close to impossible. We could add
> >>>>>>>>> DSI-specific operations, or add another enable bridge operation
> >>>>>>>>> (post_pre_enable ? :-D). Neither would scale, but it may be enough.
> >>>>>>>>
> >>>>>>>> I haven't thought it through for all generic cases, but I suspect it's
> >>>>>>>> more a pre_pre_enable that is needed to initialise the PHY etc,
> >>>>>>>> probably from source to sink.
> >>>>
> >>>> I believe it could be implemented as a pre-pre-enable indeed. It feels
> >>>> like a bit of a hack, as the next time we need more fine-grained control
> >>>> over the startup sequence, we'll have to add a pre-pre-pre-enable. Given
> >>>> that the startup sequence requirements come from the sink device, it
> >>>> would make sense to let it explicitly control the initialization,
> >>>> instead of driving it from the source. I don't think we'll be able to
> >>>> rework the bridge API in that direction though, so I'm fine with a hack.
> >>>
> >>> There are pros and cons to both approaches.
> >>> You're in a much better place to make that sort of call than I am, so
> >>> I'll take your advice.
> >>>
> >>> Implementing a DSI host op function may mean an update to a number of
> >>> existing DSI host drivers,
> >>
> >> Why? You just add new op to mipi_dsi_host_ops and create appropriate
> >> helper, which in case of NULL will perform default action, either:
> >> - return -ENOSYS,
> >> - try to emulate by calling mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET)
> >>
> >> The latter is just wild guess, but I suspect it could work (even now) as
> >> an alternate way of entering into stop state.
> >
> > If we're looking at fixing bridges to use the new DSI state call and
> > then initialise in "pre_enable" (ie before video is started), then
> > making that change to the bridge will require support for the DSI
> > state call or risk regressions.
>
> I think there is misunderstanding somewhere, I do not know where, yet :)
> My idea is as follow:
> 1. Add dsi_host.set_state() op to dsi host accompanied with helper, lets
> call it mipi_dsi_host_set_state for now.
> 2. Implement dsi_host.set_state in DSI host you are working with.
> 3. In body of your bridge pre_enable you call mipi_dsi_host_set_state in
> proper place.
>
> 1 and 2 do not introduce any regression.
> 3 can introduce regression only if your bridge is used with another dsi
> host but only on such platforms.
>
> Why do you think we need to 'fix' it everywhere?

I'm looking at sn65dsi83 which is already working against other
platforms (I don't know which). If it is fixed to request the DSI
state and initialise in pre_enable, then those platforms need updating
too.
If mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET) can be used to wake
the host up then that would be great, but I don't have access to those
platforms to test.

> > Some may have been getting away with it by making the initialisation
> > call in "enable" whilst video is active - that sounds like the issue
> > Marek was trying to log in his patch to SN65DSI83.
> >
> >>> but it would be cleaner. It's also what
> >>> Andrzej has suggested.
> >>>
> >>> Thinking it through, a function that requests clock lane frequency and
> >>> state (ULPS or LP-11 predominantly), and data lane state (again ULPS
> >>> or LP-11) should allow the required behaviour for most of the bridges
> >>> I'm aware of. Most want either LP-11, or HS clock at a known
> >>> frequency.
> >>
> >> LP-11 is quite reasonable, but regarding frequency I am not sure who
> >> should manage it? DSI device datasheets known to me usually mentioned
> >> max frequency, and it is covered by mipi_dsi_device.hs_rate. Min
> >> frequency is usually determined by amount of data we need to transfer
> >> per second.
> >> Do you need to set frequency from device, or just to know actual frequency?
> >> In former case (if it is really necessary !!!) you should request
> >> frequency range anyway, as host usually is able to set only discrete
> >> number of frequencies. But it need to be clearly specified how and when
> >> this op can be used - more ops, more questions about interaction between
> >> them.
> >> In latter case another mipi host op should be quite easy to implement.
> >
> > If a burst mode frequency is specified in struct mipi_dsi_device
> > hs_rate then there is no issue.
>
> hs_rate should be called rather max_hs_rate as it describes only top
> limit of HS clock, decision which clock to choose is on DSI host side.

Indeed, I've just realised that one.

> > If not, then you'll generally want to use the pixel clock,
>
> Probably not, often we want to use rate higher than pixel clock
> (time-compression in DSI spec), to leave time gaps for LP mode.

Only if MIPI_DSI_MODE_VIDEO_BURST is set in the mode_flags, otherwise
you should be running at the pixel clock. (Slight assumption as there
is no definition of what MIPI_DSI_MODE_VIDEO_BURST really means in the
documentation).

> > and the DSI
> > host hasn't necessarily been told the video mode at the point the
> > bridge/panel requests a new DSI state.
>
> pre_enable and enable ops are called after modeset, so video mode is known.

Thanks, I wasn't aware of that. It didn't help that our DSI driver
didn't implement a mode_set function as it did all the work in
pre_enable having looked back at the crtc for the mode.

It looks like there is a slight quirk that there is an encoder
atomic_mode_set, but not a bridge one. If we're implementing the
atomic API is it correct to update our bridge's internal state (ie the
hs_rate) from a (atomic_)mode_set? That's a different discussion
though.

> >
> >>> Giving the option for setting back into ULPS also allows
> >>> for power saving/standby mechanisms should the need arise.
> >>
> >> Another op :)
> >
> > Does it need to be, or can it be one op as
> > enum mipi_dsi_lane_state{
> >   DSI_ULPS,
> >   DSI_LP11,
> >   DSI_HS,
> > };
> >
> > int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
> >         enum mipi_dsi_lane_state clock_lane_state,
> >         u64 clock_lane_freq,
> >         enum mipi_dsi_lane_state data_lane_state)
>
> I still do not know why do you want to set freq explicitly, ie why DSI
> host could not choose it, based on bridge constraints, DSI host
> constraints and requested video mode.

I'm partly trying to piece together what information is already around
in order to be able to make a good decision on the frequency.
If we have (max_)hs_rate defined and a mode, then we can make a
decision within the DSI host driver.

Adding a link-frequencies DT property would allow the system
configuration to be defined rather than some random heuristics.

But otherwise I do now agree with you that we probably don't need an
explicit frequency from the bridge/panel driver.

There is still the situation documented in [1] that the transfer
function "can be called no matter the state the host is in", so that
would include as part of attach when we don't have a video mode. Any
transfer without the MIPI_DSI_MSG_USE_LPM flag is then at an undefined
HS speed. There are likely very few cases where that matters, so a
WARN from the DSI host driver is probably sufficient for now.

[1] https://www.kernel.org/doc/html/latest/gpu/drm-kms-helpers.html?highlight=dsi#c.mipi_dsi_host_ops

> Regarding different modes for clock and data lanes there exists DSI mode
> flag MIPI_DSI_CLOCK_NON_CONTINUOUS which determines if host in LP-11
> data lanes state can also put clock lane into LP-11. Is there need for
> sth more?
> I'd like to avoid possibility of nonsense(?) configurations like clock
> in LP-11 and data lanes in HS.

Having had a first pass at implementing this, then I certainly agree
with altering the potential states I'd previously suggested.
    DSI_STANDBY,
    DSI_ULPS,
    DSI_ACTIVE,
Seems more sensible now.
For the clock lane, as you say, MIPI_DSI_CLOCK_NON_CONTINUOUS
determines whether DSI_ACTIVE means LP-11 or HS.
The data lanes follow the clock lane in state, so DSI_ACTIVE means
LP-11, sending LP commands, or HS if video is actively being
transmitted.

Having just DSI_ACTIVE also avoids the nonsense you rightly raise of
LP clock but HS data.

So it can be condensed down to a single state.
int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
        enum mipi_dsi_lane_state state);
Does that seem reasonable?

I guess the other thing to do is document what is meant by each flag.
Currently the documentation is rather lacking in the intended
behaviour, which is where I started with this thread.

Thanks.
  Dave

> Regards
> Andrzej
>
>
>
> >
> >    Dave
> >
> >> Regards
> >> Andrzej
> >>
> >>
> >>> Does it need a way to pass back the actual DSI frequency being used,
> >>> in a similar vein to mode_fixup? That allows for the bridge to request
> >>> the display clock, but the burst mode link frequency to be returned
> >>> (I'm assuming that's a property of the DSI host only, and not the
> >>> bridge).
> >>>
> >>> I'm having a discussion with someone who wants to run SN65DSI85 in the
> >>> two independent LVDS display mode. That requires a DSI HS clock on
> >>> DSI-A even if only panel B is active, so with this extra function that
> >>> would be achievable as well.
> >>>
> >>> Thoughts?
> >>>
> >>>>>>>> If the panel/bridge can set a flag that can be checked at this point
> >>>>>>>> for whether an early clock is required or not, I think that allows us
> >>>>>>>> to comply with the requirements for a large number of panels/bridges
> >>>>>>>> (LP-11 vs HS config for clock and or data lanes before pre_enable is
> >>>>>>>> called).
> >>>>>>>>
> >>>>>>>> pre_enable retains the current behaviour (initialise the chain from
> >>>>>>>> sink to source).
> >>>>>>>> enable then actually starts sending video and enabling outputs.
> >>>>>>>
> >>>>>>> Flags indeed seem like a more contained option. Another one could be to
> >>>>>>> have a mipi_dsi_host to (for example) power up the clock lane that would
> >>>>>>> be called by default before the bridge's enable, or at the downstream
> >>>>>>> bridge driver discretion before that.
> >>>>>>
> >>>>>> Which driver will that call?
> >>>>>
> >>>>> The parent DSI Host
> >>>>>
> >>>>>> An extreme example perhaps, but Toshiba make the TC358860 eDP to DSI
> >>>>>> bridge chip[1]. So the encoder will be eDP, but the DSI config needs
> >>>>>> to go to that bridge. Does that happen automatically within the
> >>>>>> framework? I guess so as the bridge will have called
> >>>>>> mipi_dsi_host_register for the DSI sink to attach to.
> >>>>>
> >>>>> In that case, whatever sink would be connected to the bridge would call
> >>>>> the bridge clock enable hook if it needs it in its pre_enable, or it
> >>>>> would be called automatically before enable if it doesn't
> >>>>>
> >>>>> Would that help?
> >>>>
> >>>> Sounds good to me, in theory at least (let's see what issues we'll run
> >>>> into in practice :-)).
> >>>>
> >>>> Has anyone given it a try, or is planning to ?
> >>>>
> >>>>>> Perhaps a new mipi_dsi_host function to configure the PHY is the
> >>>>>> easier solution. If it can allow the sink to request whatever
> >>>>>> combination of states from clock and data lanes that it fancies, then
> >>>>>> it can be as explicit as required for the initialisation sequence, and
> >>>>>> the host driver does its best to comply with the requests.
> >>>>>
> >>>>> I don't know, I'm not really fond in general of solutions that try to
> >>>>> cover any single case if we don't need it and / or have today an issue
> >>>>> with this. I'd rather have something that works for the particular
> >>>>> bridge we were discussing, see if it applies to other bridges and modify
> >>>>> it if it doesn't until it works for all our cases. Trying to reason in
> >>>>> all possible cases tend to lead to solutions that are difficult to
> >>>>> maintain and usually over-engineered.
> >>>>
> >>>> A DSI host clock enable operation or a DSI host PHY configuration
> >>>> operation both fit in the same place in the grand scheme of things, so I
> >>>> don't mind either. We should be able to easily move from a more specific
> >>>> operation to a more generic one if the need arises.
> >>>>
> >>>>>> I'd have a slight query over when and how the host would drop to ULPS
> >>>>>> or power off. It probably shouldn't be in post_disable as the sink
> >>>>>> hasn't had a chance to finalise everything in its post_disable.
> >>>>>>
> >>>>>> Perhaps pm_runtime with autosuspend is the right call there?
> >>>>>
> >>>>> pm_runtime semantics mean that once the device is suspended, its power
> >>>>> domains, regulators, clocks, etc. are all shut down, so it doesn't
> >>>>> really fit the low power state expected by DSI
> >>>>>
> >>>>>> [1] https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html
> >>>>>>
> >>>>>>>> When I discussed this briefly with Maxime there was a suggestion of
> >>>>>>>> using pm_runtime to be able to power up the pipeline as a whole. If
> >>>>>>>> the bridge driver can use pm_runtime to power up the PHY when
> >>>>>>>> required, then that may solve the issue, however I know too little of
> >>>>>>>> the details to say whether that is actually practical.
> >>>>>>>
> >>>>>>> I'm not sure it was about this topic in particular. If I remember well
> >>>>>>> our discussion, this was about the vc4 driver that tries to circumvent
> >>>>>>> the framework and call the pre_enable and enable hooks itself because it
> >>>>>>> wasn't properly powered before and thus any DCS-related call by the
> >>>>>>> downstream bridge or panel would end up creating a CPU stall.
> >>>>>>>
> >>>>>>> I suggested to use runtime_pm in the DCS related calls to make sure the
> >>>>>>> device is powered because there's no relation between the state of the
> >>>>>>> downstream bridge or panel and whether it can send DCS commands or not.
> >>>>>>> For all we know, it could do it at probe time.
> >>>>>>
> >>>>>> pm_runtime is all a bit of a magic black box to me.
> >>>>>>
> >>>>>> We had discussed shifting to using pm_runtime from DCS (and enable)
> >>>>>> calls to power up the PHY on demand, and that's what I implemented.
> >>>>>> However Laurent flagged up that using
> >>>>>> dsi->encoder->crtc->state->adjusted_mode to get the HS clock info
> >>>>>> required to send a HS DCS command from that call is deprecated, so how
> >>>>>> do we specify the clock rate to use at that point?
> >>>>>
> >>>>> I guess the most sensible would be to have a custom bridge state, and
> >>>>> add a pointer to the current bridge state in struct drm_bridge. Then, as
> >>>>> long as you have a bridge pointer you have a way to get the current
> >>>>> state associated to it, and since we already have atomic_duplicate_state
> >>>>> / atomic_destroy_state we can create our own structure around it storing
> >>>>> whatever we want.
> >>>>
> >>>> That's a good point. It would only be needed if we use runtime PM to
> >>>> work around the initialization sequence issue, not if we implement a DSI
> >>>> host clock enable/disable operation, right ?
> >>>
> >>> Obviously this only works with atomic bridges otherwise you have no
> >>> state, but I assume all bridges should be heading that route now.
> >>>
> >>>>>>>>>> host_transfer calls can supposedly be made at any time, however unless
> >>>>>>>>>> MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
> >>>>>>>>>> in high speed mode. If this is before a mode has been set, what
> >>>>>>>>>> defines the link frequency parameters at this point? Adopting a random
> >>>>>>>>>> default sounds like a good way to get undefined behaviour.
> >>>>>>>>>>
> >>>>>>>>>> DSI burst mode needs to set the DSI link frequency independently of
> >>>>>>>>>> the display mode. How is that meant to be configured? I would have
> >>>>>>>>>> expected it to come from DT due to link frequency often being chosen
> >>>>>>>>>> based on EMC restrictions, but I don't see such a thing in any
> >>>>>>>>>> binding.
> >>>>>>>>>
> >>>>>>>>> Undefined too. DSI support was added to DRM without any design effort,
> >>>>>>>>> it's more a hack than a real solution. The issue with devices that can
> >>>>>>>>> be controlled over both DSI and I2C is completely unhandled. So far
> >>>>>>>>> nobody has really cared about implementing DSI right as far as I can
> >>>>>>>>> tell.
> >>>>>>>>
> >>>>>>>> :-(
> >>>>>>>>
> >>>>>>>> Thinking aloud, does having the option to set a burst link frequency
> >>>>>>>> from DT (or ACPI) have any issue for other platforms?
> >>>>>>>> Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
> >>>>>>>> drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
> >>>>>>>> all the defined timings, but drop to LP during blanking" option. The
> >>>>>>>> link frequency has therefore remained a property of the
> >>>>>>>> display/bridge.
> >>>>>>>> dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
> >>>>>>>> through the full detail of the parameters it computes from there.
> >>>>>>>
> >>>>>>> I don't see anything wrong with using link-frequency from the DT to
> >>>>>>> setup the burst frequency. It's what v4l2 has been using for a while
> >>>>>>> without any known (to me) drawback, and we're using the same of-graph
> >>>>>>> bindings, so it shouldn't be too controversial there.
> >>>>
> >>>> How would that frequency we picked in practice ? Do panels typically
> >>>> support a range of HS frequencies for DCS HS transfers ?
> >>>
> >>> I was thinking more of bridges where they often run a PLL off the
> >>> incoming DSI clock, and then have a FIFO with potentially different
> >>> clock rates and timings on input and output. SN65DSI83 supports that,
> >>> as do the Toshiba bridge chips I'm currently looking at.
> >>> You need to know the DSI link frequency to configure the PLL
> >>> correctly, but then the bridge output timing is a different matter.
> >>>
> >>>>>> OK, that sounds like a vague plan.
> >>>>>>
> >>>>>>>> DSI and I2C controlled devices is yet another issue that I haven't
> >>>>>>>> even looked at.
> >>>>>>>> I think it's more that vc4 wants to ignore DSI should the DSI host
> >>>>>>>> node be enabled in DT, but there's no panel bound to it. One could say
> >>>>>>>> that is a DT error and tough luck, but from a user's perspective that
> >>>>>>>> is a bit harsh.
> >>>>>>>
> >>>>>>> I guess the larger "issue" is that the tree in the DT is done following
> >>>>>>> the "control" bus, and Linux likes to tie the life cycle of a given
> >>>>>>> device to its parent bus. Both these decisions make sense, but they
> >>>>>>> interact in a weird way in some occurrences (like this one, or Allwinner
> >>>>>>> has an Ethernet PHY controlled through MMIO which end up in the same
> >>>>>>> case).
> >>>>>>>
> >>>>>>> I wonder if using device links here could help though.
> >>>>>>
> >>>>>> I really don't know about that one.
> >>>>>
> >>>>> It's a piece of infrastructure that was created at first (I think?) to
> >>>>> model the power dependency between devices that don't have a parent /
> >>>>> child relationship. For example, if you use DMA, you probably want to
> >>>>> keep the IOMMU powered as long as you are, but it is in a completely
> >>>>> separate branch of the "device tree" (not one from the DTB, the one that
> >>>>> linux DM creates).
> >>>>>
> >>>>> It was later expanded to also cover probe order and make sure a supplier
> >>>>> would probe before its consumer, effectively making EPROBE_DEFER
> >>>>> obsolete.
> >>>>>
> >>>>> The second part is still fairly new, but I think we can solve this by
> >>>>> adding a device link between the DSI host and whatever is at the end of
> >>>>> the OF-Graph endpoint.
> >>>>>
> >>>>>>>>>> As a follow on, bridge devices can support burst mode (eg TI's
> >>>>>>>>>> SN65DSI83 that's just been merged), so it needs to know the desired
> >>>>>>>>>> panel timings for the output side of the bridge, but the DSI link
> >>>>>>>>>> timings to set up the bridge's PLL. What's the correct way for
> >>>>>>>>>> signalling that? drm_crtc_state->adjusted_mode vs
> >>>>>>>>>> drm_crtc_state->mode? Except mode is userspace's request, not what has
> >>>>>>>>>> been validated/updated by the panel/bridge.
> >>>>>>>>>
> >>>>>>>>> adjusted_mode is also a bit of a hack, it solves very specific issues,
> >>>>>>>>> and its design assumes a single encoder in the chain with no extra
> >>>>>>>>> bridges. We should instead add modes to the bridge state, and negotiate
> >>>>>>>>> modes along the pipeline the same way we negotiate formats.
> >>>>>>>>
> >>>>>>>> So as I understand it we already have format negotiation between
> >>>>>>>> bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
> >>>>>>>> so is it possible to extend that to modes?
> >>>>>>>> Are you thinking bridge state that is owned by the framework, or by
> >>>>>>>> the individual bridge drivers?
> >>>>>>>
> >>>>>>> atomic_check is made for that. I guess we could improve its call
> >>>>>>> sequence to each time a mode is modified along the bridge list we
> >>>>>>> restart the sequence until all components agree (or reject it entirely
> >>>>>>> if they can't), but I don't really see why we would need yet another
> >>>>>>> hook.
> >>>>
> >>>> Isn't this what atomic_get_output_bus_fmts() and
> >>>> atomic_get_input_bus_fmts() implement ?
> >>>
> >>> Those negotiate a single u32 bus format between nodes, not a complete timing.
> >>>
> >>>>>> Why do all nodes in the bridge list need to agree? Adjacent nodes need
> >>>>>> to agree, but they then also need to retain that agreed timing
> >>>>>> somewhere.
> >>>>>
> >>>>> We might have mutually exclusive requirements though? Let's use the
> >>>>> example of the VC4 HDMI driver that can't have odd horizontal timings,
> >>>>> and assume it's a constraint of our DSI driver instead.
> >>>>>
> >>>>> Then, we have a DSI->LVDS bridge, a LVDS->RGB bridge and a panel (which
> >>>>> is a bit ridiculous, but whatever). If the LVDS->RGB bridge can't have
> >>>>> even horizontal timings, then you just can't display it, even though
> >>>>> they are not adjacent (unless the bridge in the middle can modify the
> >>>>> timings between the input and output, but that's not always possible).
> >>>>>
> >>>>> Similarly, if for the RGB panel we need to increase a bit some timings
> >>>>> to accommodate for a larger pixel clock and end up above what the DSI
> >>>>> host can provide, we're also done.
> >>>>
> >>>> The hard part will be to figure out a good heuristics to perform the
> >>>> negotiation without going back and forth (at least not in a way that
> >>>> would require too many iterations, and certainly avoiding infinite
> >>>> loops). That will be an interesting problem to solve, but maybe we'll be
> >>>> lucky and a simple approach will work for the use cases we have to
> >>>> support today.
> >>>
> >>> One to kick into the long grass possibly then.
> >>>
> >>> For burst mode and bridges retiming things, generally I think it works
> >>> if you consider a single pass through a mode_fixup equivalent starting
> >>> at the panel.
> >>> The panel advertises what it wants.
> >>> The closest bridge (eg DSI83) fixup amends panel's requested mode
> >>> based on any constraints that it has, stores a copy, and passes that
> >>> down the chain.
> >>> The next bridge (eg vc4_dsi) amends the mode based on restrictions it
> >>> has (eg burst mode link frequency). That's the end of the chain in
> >>> this case, so that's the mode that the crtc needs to be programmed
> >>> with.
> >>>
> >>> The modes then need to be passed back up the chain so that eg DSI83
> >>> knows the link frequency in use, and can therefore configure the PLL
> >>> appropriately.
> >>>
> >>> The bit it would be nice to fix is that burst mode has effectively
> >>> increased the horizontal front porch, but that could be fixed within
> >>> the bridge so that the panel gets the timing it requested.
> >>>
> >>>>>> Taking SN65DSI8[3|4|5] as an example, it supports burst mode, and the
> >>>>>> DSI frequency and timings are permitted to be different from that
> >>>>>> which it uses on the LVDS side. The LVDS panel and LVDS side of DSI83
> >>>>>> need to agree over the format, and the DSI host and DSI side of DSI83
> >>>>>> need to agree, but they may be two different timings.
> >>>>>> Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows you to
> >>>>>> configure the LVDS rate compared to the DSI rate (the driver currently
> >>>>>> goes for 1:1), and registers 0x20 to 0x34 allow you to set the number
> >>>>>> of active pixel and blanking on the LVDS side (again currently just
> >>>>>> copied across).
> >>>>>>
> >>>>>> The way I'm seeing burst mode as having been interpreted at present is
> >>>>>> that it's largely just a flag to say "drop to LP mode between lines".
> >>>>>> The timing that needs to be passed to the crtc is therefore going to
> >>>>>> be based on the DSI link rate (converted to pixels) with increased
> >>>>>> blanking periods.
> >>>>>>
> >>>>>> I guess there are similarities with Media Controller and V4L2 here. A
> >>>>>> typical chain there could be:
> >>>>>>    sensor -> scaler -> crop -> CSI-2 receiver.
> >>>>>> The format on each of those links may be different, but the chain as a
> >>>>>> whole needs to be valid. Media Controller largely relies on userspace
> >>>>>> to configure all links, but with a DRM chain that isn't really an
> >>>>>> option as it's expected that the display chain configures itself.
> >>>>>
> >>>>> Also, the userspace has no concept of media sub-devices in DRM, so it
> >>>>> just sets the mode on the whole DRM/KMS device, unlike what v4l2 does.
> >>>>> In v4l2, afaik, if you ended up with the above scenarios it would just
> >>>>> be rejected when you set the format on the link, letting the userspace
> >>>>> figure it out. We can't really do that here
> >>>>
> >>>> I wonder how long we'll be able to keep userspace out of the picture to
> >>>> configure the internals of the pipeline. I don't want to be the first
> >>>> person who will have a use case that requires this.
> >>>
> >>> I suspect none of us want to be the first one to hit this scenario!
> >>>
> >>> As I've just posted on the other thread about SN65DSI83, I've hit a
> >>> major stumbling block with the current design where vc4_dsi (and
> >>> Exynos) breaks the bridge_chain so that it gets called first and can
> >>> do initialisation, and then it calls down the chain.
> >>> Having now converted to a DSI bridge driver, it works fine with
> >>> connected non-atomic bridges, but fails with atomic bridges.
> >>> drm_atomic_add_encoder_bridges adds the state for all the bridges the
> >>> framework is aware of. With the split chain it misses adding the state
> >>> of our "hidden" bridges, and we can't add the extra state from our
> >>> atomic_duplicate_state call as we don't have the state to add to (we
> >>> just return our state to be added).
> >>> This bumps up the priority for us of finding a suitable solution for
> >>> this initialisation issue, so I'll start looking at how feasible a new
> >>> DSI host function is.
> >>>
> >>> Thanks
> >>>     Dave
> >>>
> >>
>

Re: Questions over DSI within DRM.

[Andrzej Hajda]

On 07.10.2021 13:07, Dave Stevenson wrote:
> On Tue, 5 Oct 2021 at 22:03, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
>>
>> On 05.10.2021 17:32, Dave Stevenson wrote:
>>> Hi Andrzej
>>>
>>> Thanks for joining in the discussion.
>>>
>>> On Tue, 5 Oct 2021 at 16:08, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
>>>>
>>>> On 05.10.2021 13:23, Dave Stevenson wrote:
>>>>> Hi Laurent
>>>>>
>>>>> On Sun, 3 Oct 2021 at 15:16, Laurent Pinchart
>>>>> <laurent.pinchart@ideasonboard.com> wrote:
>>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> Reviving a bit of an old thread.
>>>>>
>>>>> I'd been looking at reviving this conversation too as I've moved
>>>>> further on with DSI on the Pi, and converting from an encoder to a
>>>>> bridge
>>>>>
>>>>>> On Thu, Jul 15, 2021 at 11:50:22AM +0200, Maxime Ripard wrote:
>>>>>>> On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
>>>>>>>> On Tue, 6 Jul 2021 at 16:13, Maxime Ripard wrote:
>>>>>>>>>>>> On a similar theme, some devices want the clock lane in HS mode early
>>>>>>>>>>>> so they can use it in place of an external oscillator, but the data
>>>>>>>>>>>> lanes still in LP-11. There appears to be no way for the
>>>>>>>>>>>> display/bridge to signal this requirement or it be achieved.
>>>>>>>>>>>
>>>>>>>>>>> You're right. A loooong time ago, the omapdrm driver had an internal
>>>>>>>>>>> infrastructure that didn't use drm_bridge or drm_panel and instead
>>>>>>>>>>> required omapdrm-specific drivers for those components. It used to model
>>>>>>>>>>> the display pipeline in a different way than drm_bridge, with the sync
>>>>>>>>>>> explicitly setting the source state. A DSI sink could thus control its
>>>>>>>>>>> enable sequence, interleaving programming of the sink with control of
>>>>>>>>>>> the source.
>>>>>>>>>>>
>>>>>>>>>>> Migrating omapdrm to the drm_bridge model took a really large effort,
>>>>>>>>>>> which makes me believe that transitioning the whole subsystem to
>>>>>>>>>>> sink-controlled sources would be close to impossible. We could add
>>>>>>>>>>> DSI-specific operations, or add another enable bridge operation
>>>>>>>>>>> (post_pre_enable ? :-D). Neither would scale, but it may be enough.
>>>>>>>>>>
>>>>>>>>>> I haven't thought it through for all generic cases, but I suspect it's
>>>>>>>>>> more a pre_pre_enable that is needed to initialise the PHY etc,
>>>>>>>>>> probably from source to sink.
>>>>>>
>>>>>> I believe it could be implemented as a pre-pre-enable indeed. It feels
>>>>>> like a bit of a hack, as the next time we need more fine-grained control
>>>>>> over the startup sequence, we'll have to add a pre-pre-pre-enable. Given
>>>>>> that the startup sequence requirements come from the sink device, it
>>>>>> would make sense to let it explicitly control the initialization,
>>>>>> instead of driving it from the source. I don't think we'll be able to
>>>>>> rework the bridge API in that direction though, so I'm fine with a hack.
>>>>>
>>>>> There are pros and cons to both approaches.
>>>>> You're in a much better place to make that sort of call than I am, so
>>>>> I'll take your advice.
>>>>>
>>>>> Implementing a DSI host op function may mean an update to a number of
>>>>> existing DSI host drivers,
>>>>
>>>> Why? You just add new op to mipi_dsi_host_ops and create appropriate
>>>> helper, which in case of NULL will perform default action, either:
>>>> - return -ENOSYS,
>>>> - try to emulate by calling mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET)
>>>>
>>>> The latter is just wild guess, but I suspect it could work (even now) as
>>>> an alternate way of entering into stop state.
>>>
>>> If we're looking at fixing bridges to use the new DSI state call and
>>> then initialise in "pre_enable" (ie before video is started), then
>>> making that change to the bridge will require support for the DSI
>>> state call or risk regressions.
>>
>> I think there is misunderstanding somewhere, I do not know where, yet :)
>> My idea is as follow:
>> 1. Add dsi_host.set_state() op to dsi host accompanied with helper, lets
>> call it mipi_dsi_host_set_state for now.
>> 2. Implement dsi_host.set_state in DSI host you are working with.
>> 3. In body of your bridge pre_enable you call mipi_dsi_host_set_state in
>> proper place.
>>
>> 1 and 2 do not introduce any regression.
>> 3 can introduce regression only if your bridge is used with another dsi
>> host but only on such platforms.
>>
>> Why do you think we need to 'fix' it everywhere?
> 
> I'm looking at sn65dsi83 which is already working against other
> platforms (I don't know which). If it is fixed to request the DSI
> state and initialise in pre_enable, then those platforms need updating
> too.
> If mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET) can be used to wake
> the host up then that would be great, but I don't have access to those
> platforms to test.
> 
>>> Some may have been getting away with it by making the initialisation
>>> call in "enable" whilst video is active - that sounds like the issue
>>> Marek was trying to log in his patch to SN65DSI83.
>>>
>>>>> but it would be cleaner. It's also what
>>>>> Andrzej has suggested.
>>>>>
>>>>> Thinking it through, a function that requests clock lane frequency and
>>>>> state (ULPS or LP-11 predominantly), and data lane state (again ULPS
>>>>> or LP-11) should allow the required behaviour for most of the bridges
>>>>> I'm aware of. Most want either LP-11, or HS clock at a known
>>>>> frequency.
>>>>
>>>> LP-11 is quite reasonable, but regarding frequency I am not sure who
>>>> should manage it? DSI device datasheets known to me usually mentioned
>>>> max frequency, and it is covered by mipi_dsi_device.hs_rate. Min
>>>> frequency is usually determined by amount of data we need to transfer
>>>> per second.
>>>> Do you need to set frequency from device, or just to know actual frequency?
>>>> In former case (if it is really necessary !!!) you should request
>>>> frequency range anyway, as host usually is able to set only discrete
>>>> number of frequencies. But it need to be clearly specified how and when
>>>> this op can be used - more ops, more questions about interaction between
>>>> them.
>>>> In latter case another mipi host op should be quite easy to implement.
>>>
>>> If a burst mode frequency is specified in struct mipi_dsi_device
>>> hs_rate then there is no issue.
>>
>> hs_rate should be called rather max_hs_rate as it describes only top
>> limit of HS clock, decision which clock to choose is on DSI host side.
> 
> Indeed, I've just realised that one.
> 
>>> If not, then you'll generally want to use the pixel clock,
>>
>> Probably not, often we want to use rate higher than pixel clock
>> (time-compression in DSI spec), to leave time gaps for LP mode.
> 
> Only if MIPI_DSI_MODE_VIDEO_BURST is set in the mode_flags, otherwise
> you should be running at the pixel clock. (Slight assumption as there
> is no definition of what MIPI_DSI_MODE_VIDEO_BURST really means in the
> documentation).

Specification describes it clearly [1], grep for 'burst'.

[1]: https://hifpga.com/upfiles/15552304546301142.pdf

> 
>>> and the DSI
>>> host hasn't necessarily been told the video mode at the point the
>>> bridge/panel requests a new DSI state.
>>
>> pre_enable and enable ops are called after modeset, so video mode is known.
> 
> Thanks, I wasn't aware of that. It didn't help that our DSI driver
> didn't implement a mode_set function as it did all the work in
> pre_enable having looked back at the crtc for the mode.
> 
> It looks like there is a slight quirk that there is an encoder
> atomic_mode_set, but not a bridge one. If we're implementing the
> atomic API is it correct to update our bridge's internal state (ie the
> hs_rate) from a (atomic_)mode_set? That's a different discussion
> though.

It shouldn't matter, the mode should be stored in atomic state. In 
pre-enable you should have all required info: videomode and constraints.

> 
>>>
>>>>> Giving the option for setting back into ULPS also allows
>>>>> for power saving/standby mechanisms should the need arise.
>>>>
>>>> Another op :)
>>>
>>> Does it need to be, or can it be one op as
>>> enum mipi_dsi_lane_state{
>>>    DSI_ULPS,
>>>    DSI_LP11,
>>>    DSI_HS,
>>> };
>>>
>>> int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
>>>          enum mipi_dsi_lane_state clock_lane_state,
>>>          u64 clock_lane_freq,
>>>          enum mipi_dsi_lane_state data_lane_state)
>>
>> I still do not know why do you want to set freq explicitly, ie why DSI
>> host could not choose it, based on bridge constraints, DSI host
>> constraints and requested video mode.
> 
> I'm partly trying to piece together what information is already around
> in order to be able to make a good decision on the frequency.
> If we have (max_)hs_rate defined and a mode, then we can make a
> decision within the DSI host driver.
> 
> Adding a link-frequencies DT property would allow the system
> configuration to be defined rather than some random heuristics.

Exynos DSI used samsung,burst-clock-frequency property, I am not sure of 
others, but it was rather copy/paste solution from vendor code. Maybe it 
is OK this way.

> 
> But otherwise I do now agree with you that we probably don't need an
> explicit frequency from the bridge/panel driver.
> 
> There is still the situation documented in [1] that the transfer
> function "can be called no matter the state the host is in", so that
> would include as part of attach when we don't have a video mode. Any
> transfer without the MIPI_DSI_MSG_USE_LPM flag is then at an undefined
> HS speed. There are likely very few cases where that matters, so a
> WARN from the DSI host driver is probably sufficient for now.

But we have already constraints from both players - host and device, so 
it should not be an issue to use max allowed speed.

> 
> [1] https://www.kernel.org/doc/html/latest/gpu/drm-kms-helpers.html?highlight=dsi#c.mipi_dsi_host_ops
> 
>> Regarding different modes for clock and data lanes there exists DSI mode
>> flag MIPI_DSI_CLOCK_NON_CONTINUOUS which determines if host in LP-11
>> data lanes state can also put clock lane into LP-11. Is there need for
>> sth more?
>> I'd like to avoid possibility of nonsense(?) configurations like clock
>> in LP-11 and data lanes in HS.
> 
> Having had a first pass at implementing this, then I certainly agree
> with altering the potential states I'd previously suggested.
>      DSI_STANDBY,

Specification uses LP11 or STOP state to describe LP-11, so I would 
prefer to stick to one of it.

>      DSI_ULPS,

I am not sure but I hope this is LP-00.

>      DSI_ACTIVE,

I am not sure about this one, this would mean start video transmission 
in HS, as there is no HS stop state. It does not look like desirable in 
pre-enable state. 'HS idle' state is just LP-11.

> Seems more sensible now
> For the clock lane, as you say, MIPI_DSI_CLOCK_NON_CONTINUOUS
> determines whether DSI_ACTIVE means LP-11 or HS.
> The data lanes follow the clock lane in state, so DSI_ACTIVE means
> LP-11, sending LP commands, or HS if video is actively being
> transmitted.

I think you are messing up in above sentence :)
For definition look for 'continuous clock' phrase in spec above [1].

> 
> Having just DSI_ACTIVE also avoids the nonsense you rightly raise of
> LP clock but HS data.
> 
> So it can be condensed down to a single state.
> int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
>          enum mipi_dsi_lane_state state);
> Does that seem reasonable?

Yes, but if we stay with two states I wonder if it wouldn't be better to 
use two separate ops, just matter of taste.

> 
> I guess the other thing to do is document what is meant by each flag.
> Currently the documentation is rather lacking in the intended
> behaviour, which is where I started with this thread.

Yes, at least in case spec is not clear about it.

Regards
Andrzej


> 
> Thanks.
>    Dave
> 
>> Regards
>> Andrzej
>>
>>
>>
>>>
>>>     Dave
>>>
>>>> Regards
>>>> Andrzej
>>>>
>>>>
>>>>> Does it need a way to pass back the actual DSI frequency being used,
>>>>> in a similar vein to mode_fixup? That allows for the bridge to request
>>>>> the display clock, but the burst mode link frequency to be returned
>>>>> (I'm assuming that's a property of the DSI host only, and not the
>>>>> bridge).
>>>>>
>>>>> I'm having a discussion with someone who wants to run SN65DSI85 in the
>>>>> two independent LVDS display mode. That requires a DSI HS clock on
>>>>> DSI-A even if only panel B is active, so with this extra function that
>>>>> would be achievable as well.
>>>>>
>>>>> Thoughts?
>>>>>
>>>>>>>>>> If the panel/bridge can set a flag that can be checked at this point
>>>>>>>>>> for whether an early clock is required or not, I think that allows us
>>>>>>>>>> to comply with the requirements for a large number of panels/bridges
>>>>>>>>>> (LP-11 vs HS config for clock and or data lanes before pre_enable is
>>>>>>>>>> called).
>>>>>>>>>>
>>>>>>>>>> pre_enable retains the current behaviour (initialise the chain from
>>>>>>>>>> sink to source).
>>>>>>>>>> enable then actually starts sending video and enabling outputs.
>>>>>>>>>
>>>>>>>>> Flags indeed seem like a more contained option. Another one could be to
>>>>>>>>> have a mipi_dsi_host to (for example) power up the clock lane that would
>>>>>>>>> be called by default before the bridge's enable, or at the downstream
>>>>>>>>> bridge driver discretion before that.
>>>>>>>>
>>>>>>>> Which driver will that call?
>>>>>>>
>>>>>>> The parent DSI Host
>>>>>>>
>>>>>>>> An extreme example perhaps, but Toshiba make the TC358860 eDP to DSI
>>>>>>>> bridge chip[1]. So the encoder will be eDP, but the DSI config needs
>>>>>>>> to go to that bridge. Does that happen automatically within the
>>>>>>>> framework? I guess so as the bridge will have called
>>>>>>>> mipi_dsi_host_register for the DSI sink to attach to.
>>>>>>>
>>>>>>> In that case, whatever sink would be connected to the bridge would call
>>>>>>> the bridge clock enable hook if it needs it in its pre_enable, or it
>>>>>>> would be called automatically before enable if it doesn't
>>>>>>>
>>>>>>> Would that help?
>>>>>>
>>>>>> Sounds good to me, in theory at least (let's see what issues we'll run
>>>>>> into in practice :-)).
>>>>>>
>>>>>> Has anyone given it a try, or is planning to ?
>>>>>>
>>>>>>>> Perhaps a new mipi_dsi_host function to configure the PHY is the
>>>>>>>> easier solution. If it can allow the sink to request whatever
>>>>>>>> combination of states from clock and data lanes that it fancies, then
>>>>>>>> it can be as explicit as required for the initialisation sequence, and
>>>>>>>> the host driver does its best to comply with the requests.
>>>>>>>
>>>>>>> I don't know, I'm not really fond in general of solutions that try to
>>>>>>> cover any single case if we don't need it and / or have today an issue
>>>>>>> with this. I'd rather have something that works for the particular
>>>>>>> bridge we were discussing, see if it applies to other bridges and modify
>>>>>>> it if it doesn't until it works for all our cases. Trying to reason in
>>>>>>> all possible cases tend to lead to solutions that are difficult to
>>>>>>> maintain and usually over-engineered.
>>>>>>
>>>>>> A DSI host clock enable operation or a DSI host PHY configuration
>>>>>> operation both fit in the same place in the grand scheme of things, so I
>>>>>> don't mind either. We should be able to easily move from a more specific
>>>>>> operation to a more generic one if the need arises.
>>>>>>
>>>>>>>> I'd have a slight query over when and how the host would drop to ULPS
>>>>>>>> or power off. It probably shouldn't be in post_disable as the sink
>>>>>>>> hasn't had a chance to finalise everything in its post_disable.
>>>>>>>>
>>>>>>>> Perhaps pm_runtime with autosuspend is the right call there?
>>>>>>>
>>>>>>> pm_runtime semantics mean that once the device is suspended, its power
>>>>>>> domains, regulators, clocks, etc. are all shut down, so it doesn't
>>>>>>> really fit the low power state expected by DSI
>>>>>>>
>>>>>>>> [1] https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html
>>>>>>>>
>>>>>>>>>> When I discussed this briefly with Maxime there was a suggestion of
>>>>>>>>>> using pm_runtime to be able to power up the pipeline as a whole. If
>>>>>>>>>> the bridge driver can use pm_runtime to power up the PHY when
>>>>>>>>>> required, then that may solve the issue, however I know too little of
>>>>>>>>>> the details to say whether that is actually practical.
>>>>>>>>>
>>>>>>>>> I'm not sure it was about this topic in particular. If I remember well
>>>>>>>>> our discussion, this was about the vc4 driver that tries to circumvent
>>>>>>>>> the framework and call the pre_enable and enable hooks itself because it
>>>>>>>>> wasn't properly powered before and thus any DCS-related call by the
>>>>>>>>> downstream bridge or panel would end up creating a CPU stall.
>>>>>>>>>
>>>>>>>>> I suggested to use runtime_pm in the DCS related calls to make sure the
>>>>>>>>> device is powered because there's no relation between the state of the
>>>>>>>>> downstream bridge or panel and whether it can send DCS commands or not.
>>>>>>>>> For all we know, it could do it at probe time.
>>>>>>>>
>>>>>>>> pm_runtime is all a bit of a magic black box to me.
>>>>>>>>
>>>>>>>> We had discussed shifting to using pm_runtime from DCS (and enable)
>>>>>>>> calls to power up the PHY on demand, and that's what I implemented.
>>>>>>>> However Laurent flagged up that using
>>>>>>>> dsi->encoder->crtc->state->adjusted_mode to get the HS clock info
>>>>>>>> required to send a HS DCS command from that call is deprecated, so how
>>>>>>>> do we specify the clock rate to use at that point?
>>>>>>>
>>>>>>> I guess the most sensible would be to have a custom bridge state, and
>>>>>>> add a pointer to the current bridge state in struct drm_bridge. Then, as
>>>>>>> long as you have a bridge pointer you have a way to get the current
>>>>>>> state associated to it, and since we already have atomic_duplicate_state
>>>>>>> / atomic_destroy_state we can create our own structure around it storing
>>>>>>> whatever we want.
>>>>>>
>>>>>> That's a good point. It would only be needed if we use runtime PM to
>>>>>> work around the initialization sequence issue, not if we implement a DSI
>>>>>> host clock enable/disable operation, right ?
>>>>>
>>>>> Obviously this only works with atomic bridges otherwise you have no
>>>>> state, but I assume all bridges should be heading that route now.
>>>>>
>>>>>>>>>>>> host_transfer calls can supposedly be made at any time, however unless
>>>>>>>>>>>> MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
>>>>>>>>>>>> in high speed mode. If this is before a mode has been set, what
>>>>>>>>>>>> defines the link frequency parameters at this point? Adopting a random
>>>>>>>>>>>> default sounds like a good way to get undefined behaviour.
>>>>>>>>>>>>
>>>>>>>>>>>> DSI burst mode needs to set the DSI link frequency independently of
>>>>>>>>>>>> the display mode. How is that meant to be configured? I would have
>>>>>>>>>>>> expected it to come from DT due to link frequency often being chosen
>>>>>>>>>>>> based on EMC restrictions, but I don't see such a thing in any
>>>>>>>>>>>> binding.
>>>>>>>>>>>
>>>>>>>>>>> Undefined too. DSI support was added to DRM without any design effort,
>>>>>>>>>>> it's more a hack than a real solution. The issue with devices that can
>>>>>>>>>>> be controlled over both DSI and I2C is completely unhandled. So far
>>>>>>>>>>> nobody has really cared about implementing DSI right as far as I can
>>>>>>>>>>> tell.
>>>>>>>>>>
>>>>>>>>>> :-(
>>>>>>>>>>
>>>>>>>>>> Thinking aloud, does having the option to set a burst link frequency
>>>>>>>>>> from DT (or ACPI) have any issue for other platforms?
>>>>>>>>>> Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
>>>>>>>>>> drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
>>>>>>>>>> all the defined timings, but drop to LP during blanking" option. The
>>>>>>>>>> link frequency has therefore remained a property of the
>>>>>>>>>> display/bridge.
>>>>>>>>>> dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
>>>>>>>>>> through the full detail of the parameters it computes from there.
>>>>>>>>>
>>>>>>>>> I don't see anything wrong with using link-frequency from the DT to
>>>>>>>>> setup the burst frequency. It's what v4l2 has been using for a while
>>>>>>>>> without any known (to me) drawback, and we're using the same of-graph
>>>>>>>>> bindings, so it shouldn't be too controversial there.
>>>>>>
>>>>>> How would that frequency we picked in practice ? Do panels typically
>>>>>> support a range of HS frequencies for DCS HS transfers ?
>>>>>
>>>>> I was thinking more of bridges where they often run a PLL off the
>>>>> incoming DSI clock, and then have a FIFO with potentially different
>>>>> clock rates and timings on input and output. SN65DSI83 supports that,
>>>>> as do the Toshiba bridge chips I'm currently looking at.
>>>>> You need to know the DSI link frequency to configure the PLL
>>>>> correctly, but then the bridge output timing is a different matter.
>>>>>
>>>>>>>> OK, that sounds like a vague plan.
>>>>>>>>
>>>>>>>>>> DSI and I2C controlled devices is yet another issue that I haven't
>>>>>>>>>> even looked at.
>>>>>>>>>> I think it's more that vc4 wants to ignore DSI should the DSI host
>>>>>>>>>> node be enabled in DT, but there's no panel bound to it. One could say
>>>>>>>>>> that is a DT error and tough luck, but from a user's perspective that
>>>>>>>>>> is a bit harsh.
>>>>>>>>>
>>>>>>>>> I guess the larger "issue" is that the tree in the DT is done following
>>>>>>>>> the "control" bus, and Linux likes to tie the life cycle of a given
>>>>>>>>> device to its parent bus. Both these decisions make sense, but they
>>>>>>>>> interact in a weird way in some occurrences (like this one, or Allwinner
>>>>>>>>> has an Ethernet PHY controlled through MMIO which end up in the same
>>>>>>>>> case).
>>>>>>>>>
>>>>>>>>> I wonder if using device links here could help though.
>>>>>>>>
>>>>>>>> I really don't know about that one.
>>>>>>>
>>>>>>> It's a piece of infrastructure that was created at first (I think?) to
>>>>>>> model the power dependency between devices that don't have a parent /
>>>>>>> child relationship. For example, if you use DMA, you probably want to
>>>>>>> keep the IOMMU powered as long as you are, but it is in a completely
>>>>>>> separate branch of the "device tree" (not one from the DTB, the one that
>>>>>>> linux DM creates).
>>>>>>>
>>>>>>> It was later expanded to also cover probe order and make sure a supplier
>>>>>>> would probe before its consumer, effectively making EPROBE_DEFER
>>>>>>> obsolete.
>>>>>>>
>>>>>>> The second part is still fairly new, but I think we can solve this by
>>>>>>> adding a device link between the DSI host and whatever is at the end of
>>>>>>> the OF-Graph endpoint.
>>>>>>>
>>>>>>>>>>>> As a follow on, bridge devices can support burst mode (eg TI's
>>>>>>>>>>>> SN65DSI83 that's just been merged), so it needs to know the desired
>>>>>>>>>>>> panel timings for the output side of the bridge, but the DSI link
>>>>>>>>>>>> timings to set up the bridge's PLL. What's the correct way for
>>>>>>>>>>>> signalling that? drm_crtc_state->adjusted_mode vs
>>>>>>>>>>>> drm_crtc_state->mode? Except mode is userspace's request, not what has
>>>>>>>>>>>> been validated/updated by the panel/bridge.
>>>>>>>>>>>
>>>>>>>>>>> adjusted_mode is also a bit of a hack, it solves very specific issues,
>>>>>>>>>>> and its design assumes a single encoder in the chain with no extra
>>>>>>>>>>> bridges. We should instead add modes to the bridge state, and negotiate
>>>>>>>>>>> modes along the pipeline the same way we negotiate formats.
>>>>>>>>>>
>>>>>>>>>> So as I understand it we already have format negotiation between
>>>>>>>>>> bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
>>>>>>>>>> so is it possible to extend that to modes?
>>>>>>>>>> Are you thinking bridge state that is owned by the framework, or by
>>>>>>>>>> the individual bridge drivers?
>>>>>>>>>
>>>>>>>>> atomic_check is made for that. I guess we could improve its call
>>>>>>>>> sequence to each time a mode is modified along the bridge list we
>>>>>>>>> restart the sequence until all components agree (or reject it entirely
>>>>>>>>> if they can't), but I don't really see why we would need yet another
>>>>>>>>> hook.
>>>>>>
>>>>>> Isn't this what atomic_get_output_bus_fmts() and
>>>>>> atomic_get_input_bus_fmts() implement ?
>>>>>
>>>>> Those negotiate a single u32 bus format between nodes, not a complete timing.
>>>>>
>>>>>>>> Why do all nodes in the bridge list need to agree? Adjacent nodes need
>>>>>>>> to agree, but they then also need to retain that agreed timing
>>>>>>>> somewhere.
>>>>>>>
>>>>>>> We might have mutually exclusive requirements though? Let's use the
>>>>>>> example of the VC4 HDMI driver that can't have odd horizontal timings,
>>>>>>> and assume it's a constraint of our DSI driver instead.
>>>>>>>
>>>>>>> Then, we have a DSI->LVDS bridge, a LVDS->RGB bridge and a panel (which
>>>>>>> is a bit ridiculous, but whatever). If the LVDS->RGB bridge can't have
>>>>>>> even horizontal timings, then you just can't display it, even though
>>>>>>> they are not adjacent (unless the bridge in the middle can modify the
>>>>>>> timings between the input and output, but that's not always possible).
>>>>>>>
>>>>>>> Similarly, if for the RGB panel we need to increase a bit some timings
>>>>>>> to accommodate for a larger pixel clock and end up above what the DSI
>>>>>>> host can provide, we're also done.
>>>>>>
>>>>>> The hard part will be to figure out a good heuristics to perform the
>>>>>> negotiation without going back and forth (at least not in a way that
>>>>>> would require too many iterations, and certainly avoiding infinite
>>>>>> loops). That will be an interesting problem to solve, but maybe we'll be
>>>>>> lucky and a simple approach will work for the use cases we have to
>>>>>> support today.
>>>>>
>>>>> One to kick into the long grass possibly then.
>>>>>
>>>>> For burst mode and bridges retiming things, generally I think it works
>>>>> if you consider a single pass through a mode_fixup equivalent starting
>>>>> at the panel.
>>>>> The panel advertises what it wants.
>>>>> The closest bridge (eg DSI83) fixup amends panel's requested mode
>>>>> based on any constraints that it has, stores a copy, and passes that
>>>>> down the chain.
>>>>> The next bridge (eg vc4_dsi) amends the mode based on restrictions it
>>>>> has (eg burst mode link frequency). That's the end of the chain in
>>>>> this case, so that's the mode that the crtc needs to be programmed
>>>>> with.
>>>>>
>>>>> The modes then need to be passed back up the chain so that eg DSI83
>>>>> knows the link frequency in use, and can therefore configure the PLL
>>>>> appropriately.
>>>>>
>>>>> The bit it would be nice to fix is that burst mode has effectively
>>>>> increased the horizontal front porch, but that could be fixed within
>>>>> the bridge so that the panel gets the timing it requested.
>>>>>
>>>>>>>> Taking SN65DSI8[3|4|5] as an example, it supports burst mode, and the
>>>>>>>> DSI frequency and timings are permitted to be different from that
>>>>>>>> which it uses on the LVDS side. The LVDS panel and LVDS side of DSI83
>>>>>>>> need to agree over the format, and the DSI host and DSI side of DSI83
>>>>>>>> need to agree, but they may be two different timings.
>>>>>>>> Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows you to
>>>>>>>> configure the LVDS rate compared to the DSI rate (the driver currently
>>>>>>>> goes for 1:1), and registers 0x20 to 0x34 allow you to set the number
>>>>>>>> of active pixel and blanking on the LVDS side (again currently just
>>>>>>>> copied across).
>>>>>>>>
>>>>>>>> The way I'm seeing burst mode as having been interpreted at present is
>>>>>>>> that it's largely just a flag to say "drop to LP mode between lines".
>>>>>>>> The timing that needs to be passed to the crtc is therefore going to
>>>>>>>> be based on the DSI link rate (converted to pixels) with increased
>>>>>>>> blanking periods.
>>>>>>>>
>>>>>>>> I guess there are similarities with Media Controller and V4L2 here. A
>>>>>>>> typical chain there could be:
>>>>>>>>     sensor -> scaler -> crop -> CSI-2 receiver.
>>>>>>>> The format on each of those links may be different, but the chain as a
>>>>>>>> whole needs to be valid. Media Controller largely relies on userspace
>>>>>>>> to configure all links, but with a DRM chain that isn't really an
>>>>>>>> option as it's expected that the display chain configures itself.
>>>>>>>
>>>>>>> Also, the userspace has no concept of media sub-devices in DRM, so it
>>>>>>> just sets the mode on the whole DRM/KMS device, unlike what v4l2 does.
>>>>>>> In v4l2, afaik, if you ended up with the above scenarios it would just
>>>>>>> be rejected when you set the format on the link, letting the userspace
>>>>>>> figure it out. We can't really do that here
>>>>>>
>>>>>> I wonder how long we'll be able to keep userspace out of the picture to
>>>>>> configure the internals of the pipeline. I don't want to be the first
>>>>>> person who will have a use case that requires this.
>>>>>
>>>>> I suspect none of us want to be the first one to hit this scenario!
>>>>>
>>>>> As I've just posted on the other thread about SN65DSI83, I've hit a
>>>>> major stumbling block with the current design where vc4_dsi (and
>>>>> Exynos) breaks the bridge_chain so that it gets called first and can
>>>>> do initialisation, and then it calls down the chain.
>>>>> Having now converted to a DSI bridge driver, it works fine with
>>>>> connected non-atomic bridges, but fails with atomic bridges.
>>>>> drm_atomic_add_encoder_bridges adds the state for all the bridges the
>>>>> framework is aware of. With the split chain it misses adding the state
>>>>> of our "hidden" bridges, and we can't add the extra state from our
>>>>> atomic_duplicate_state call as we don't have the state to add to (we
>>>>> just return our state to be added).
>>>>> This bumps up the priority for us of finding a suitable solution for
>>>>> this initialisation issue, so I'll start looking at how feasible a new
>>>>> DSI host function is.
>>>>>
>>>>> Thanks
>>>>>      Dave
>>>>>
>>>>
>>

Re: Questions over DSI within DRM.

[Dave Stevenson]

On Thu, 7 Oct 2021 at 21:19, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
>
> On 07.10.2021 13:07, Dave Stevenson wrote:
> > On Tue, 5 Oct 2021 at 22:03, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
> >>
> >> On 05.10.2021 17:32, Dave Stevenson wrote:
> >>> Hi Andrzej
> >>>
> >>> Thanks for joining in the discussion.
> >>>
> >>> On Tue, 5 Oct 2021 at 16:08, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
> >>>>
> >>>> On 05.10.2021 13:23, Dave Stevenson wrote:
> >>>>> Hi Laurent
> >>>>>
> >>>>> On Sun, 3 Oct 2021 at 15:16, Laurent Pinchart
> >>>>> <laurent.pinchart@ideasonboard.com> wrote:
> >>>>>>
> >>>>>> Hello,
> >>>>>>
> >>>>>> Reviving a bit of an old thread.
> >>>>>
> >>>>> I'd been looking at reviving this conversation too as I've moved
> >>>>> further on with DSI on the Pi, and converting from an encoder to a
> >>>>> bridge
> >>>>>
> >>>>>> On Thu, Jul 15, 2021 at 11:50:22AM +0200, Maxime Ripard wrote:
> >>>>>>> On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
> >>>>>>>> On Tue, 6 Jul 2021 at 16:13, Maxime Ripard wrote:
> >>>>>>>>>>>> On a similar theme, some devices want the clock lane in HS mode early
> >>>>>>>>>>>> so they can use it in place of an external oscillator, but the data
> >>>>>>>>>>>> lanes still in LP-11. There appears to be no way for the
> >>>>>>>>>>>> display/bridge to signal this requirement or it be achieved.
> >>>>>>>>>>>
> >>>>>>>>>>> You're right. A loooong time ago, the omapdrm driver had an internal
> >>>>>>>>>>> infrastructure that didn't use drm_bridge or drm_panel and instead
> >>>>>>>>>>> required omapdrm-specific drivers for those components. It used to model
> >>>>>>>>>>> the display pipeline in a different way than drm_bridge, with the sync
> >>>>>>>>>>> explicitly setting the source state. A DSI sink could thus control its
> >>>>>>>>>>> enable sequence, interleaving programming of the sink with control of
> >>>>>>>>>>> the source.
> >>>>>>>>>>>
> >>>>>>>>>>> Migrating omapdrm to the drm_bridge model took a really large effort,
> >>>>>>>>>>> which makes me believe that transitioning the whole subsystem to
> >>>>>>>>>>> sink-controlled sources would be close to impossible. We could add
> >>>>>>>>>>> DSI-specific operations, or add another enable bridge operation
> >>>>>>>>>>> (post_pre_enable ? :-D). Neither would scale, but it may be enough.
> >>>>>>>>>>
> >>>>>>>>>> I haven't thought it through for all generic cases, but I suspect it's
> >>>>>>>>>> more a pre_pre_enable that is needed to initialise the PHY etc,
> >>>>>>>>>> probably from source to sink.
> >>>>>>
> >>>>>> I believe it could be implemented as a pre-pre-enable indeed. It feels
> >>>>>> like a bit of a hack, as the next time we need more fine-grained control
> >>>>>> over the startup sequence, we'll have to add a pre-pre-pre-enable. Given
> >>>>>> that the startup sequence requirements come from the sink device, it
> >>>>>> would make sense to let it explicitly control the initialization,
> >>>>>> instead of driving it from the source. I don't think we'll be able to
> >>>>>> rework the bridge API in that direction though, so I'm fine with a hack.
> >>>>>
> >>>>> There are pros and cons to both approaches.
> >>>>> You're in a much better place to make that sort of call than I am, so
> >>>>> I'll take your advice.
> >>>>>
> >>>>> Implementing a DSI host op function may mean an update to a number of
> >>>>> existing DSI host drivers,
> >>>>
> >>>> Why? You just add new op to mipi_dsi_host_ops and create appropriate
> >>>> helper, which in case of NULL will perform default action, either:
> >>>> - return -ENOSYS,
> >>>> - try to emulate by calling mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET)
> >>>>
> >>>> The latter is just wild guess, but I suspect it could work (even now) as
> >>>> an alternate way of entering into stop state.
> >>>
> >>> If we're looking at fixing bridges to use the new DSI state call and
> >>> then initialise in "pre_enable" (ie before video is started), then
> >>> making that change to the bridge will require support for the DSI
> >>> state call or risk regressions.
> >>
> >> I think there is misunderstanding somewhere, I do not know where, yet :)
> >> My idea is as follow:
> >> 1. Add dsi_host.set_state() op to dsi host accompanied with helper, lets
> >> call it mipi_dsi_host_set_state for now.
> >> 2. Implement dsi_host.set_state in DSI host you are working with.
> >> 3. In body of your bridge pre_enable you call mipi_dsi_host_set_state in
> >> proper place.
> >>
> >> 1 and 2 do not introduce any regression.
> >> 3 can introduce regression only if your bridge is used with another dsi
> >> host but only on such platforms.
> >>
> >> Why do you think we need to 'fix' it everywhere?
> >
> > I'm looking at sn65dsi83 which is already working against other
> > platforms (I don't know which). If it is fixed to request the DSI
> > state and initialise in pre_enable, then those platforms need updating
> > too.
> > If mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET) can be used to wake
> > the host up then that would be great, but I don't have access to those
> > platforms to test.
> >
> >>> Some may have been getting away with it by making the initialisation
> >>> call in "enable" whilst video is active - that sounds like the issue
> >>> Marek was trying to log in his patch to SN65DSI83.
> >>>
> >>>>> but it would be cleaner. It's also what
> >>>>> Andrzej has suggested.
> >>>>>
> >>>>> Thinking it through, a function that requests clock lane frequency and
> >>>>> state (ULPS or LP-11 predominantly), and data lane state (again ULPS
> >>>>> or LP-11) should allow the required behaviour for most of the bridges
> >>>>> I'm aware of. Most want either LP-11, or HS clock at a known
> >>>>> frequency.
> >>>>
> >>>> LP-11 is quite reasonable, but regarding frequency I am not sure who
> >>>> should manage it? DSI device datasheets known to me usually mentioned
> >>>> max frequency, and it is covered by mipi_dsi_device.hs_rate. Min
> >>>> frequency is usually determined by amount of data we need to transfer
> >>>> per second.
> >>>> Do you need to set frequency from device, or just to know actual frequency?
> >>>> In former case (if it is really necessary !!!) you should request
> >>>> frequency range anyway, as host usually is able to set only discrete
> >>>> number of frequencies. But it need to be clearly specified how and when
> >>>> this op can be used - more ops, more questions about interaction between
> >>>> them.
> >>>> In latter case another mipi host op should be quite easy to implement.
> >>>
> >>> If a burst mode frequency is specified in struct mipi_dsi_device
> >>> hs_rate then there is no issue.
> >>
> >> hs_rate should be called rather max_hs_rate as it describes only top
> >> limit of HS clock, decision which clock to choose is on DSI host side.
> >
> > Indeed, I've just realised that one.
> >
> >>> If not, then you'll generally want to use the pixel clock,
> >>
> >> Probably not, often we want to use rate higher than pixel clock
> >> (time-compression in DSI spec), to leave time gaps for LP mode.
> >
> > Only if MIPI_DSI_MODE_VIDEO_BURST is set in the mode_flags, otherwise
> > you should be running at the pixel clock. (Slight assumption as there
> > is no definition of what MIPI_DSI_MODE_VIDEO_BURST really means in the
> > documentation).
>
> Specification describes it clearly [1], grep for 'burst'.
>
> [1]: https://hifpga.com/upfiles/15552304546301142.pdf

The MIPI DSI spec defines the concept of burst, it doesn't define the
Linux kernel implementation of it.

In a Linux DSI host driver:
- How should a DSI host driver choose the link frequency to enable
"blocks of pixel data can be transferred in a shorter time"? hs_rate
defines the max rate, and the pixel clock the minimum. Pick any number
in between?
- How does a DSI device driver get told the link frequency? Many
bridge chips run their PLLs off the DSI HS clock, so need to know how
to convert that into a desired pixel clock.
- 8.11.4 "Following HS pixel data transmission, the bus may stay in HS
Mode for sending blanking packets or go to Low Power Mode, during
which it may remain idle, i.e. the host processor remains in LP-11
state, or LP transmission may take place in either direction. ".
Option in the DSI spec. How is that mode chosen by the DSI device
driver for the host driver to implement?
- 8.11.1 "To enable PHY synchronization the host processor should
periodically end HS transmission and drive the Data Lanes to the LP
state. This transition should take place at least once per frame;
shown as LPM in the figures in this section." Another option within
the DSI spec as to whether it is once per frame or more frequently.
How does a DSI device driver signal which mode is required to the DSI
host driver? (And I've just had a device that is fussy about this
one).

I know we all hate writing documentation, but saying the parent
specification defines how it should be implemented on a system isn't
true, and developing/updating a DSI driver results in these sorts of
questions.

> >>> and the DSI
> >>> host hasn't necessarily been told the video mode at the point the
> >>> bridge/panel requests a new DSI state.
> >>
> >> pre_enable and enable ops are called after modeset, so video mode is known.
> >
> > Thanks, I wasn't aware of that. It didn't help that our DSI driver
> > didn't implement a mode_set function as it did all the work in
> > pre_enable having looked back at the crtc for the mode.
> >
> > It looks like there is a slight quirk that there is an encoder
> > atomic_mode_set, but not a bridge one. If we're implementing the
> > atomic API is it correct to update our bridge's internal state (ie the
> > hs_rate) from a (atomic_)mode_set? That's a different discussion
> > though.
>
> It shouldn't matter, the mode should be stored in atomic state. In
> pre-enable you should have all required info: videomode and constraints.

Sorry, perhaps I didn't describe my scenario well enough.
Atomic DSI host bridge driver. Atomic DSI bridge/panel driver.

mode_set gets called on all nodes.
pre_enables are called starting at the furthest end from the encoder,
so the bridge/panel driver pre_enable gets called before the DSI host
pre_enable. The bridge/panel wants to power up the DSI host using the
new API. The DSI host bridge hasn't had the pre_enable called yet, and
currently we're not proposing an atomic_set_state. At that point, what
parameters should the DSI host driver be using to determine the link
frequency as it doesn't have a state?

Perhaps we should be proposing an atomic_set_state which takes a
struct drm_atomic_state *. How would that work if called from a
non-atomic bridge driver though?

> >
> >>>
> >>>>> Giving the option for setting back into ULPS also allows
> >>>>> for power saving/standby mechanisms should the need arise.
> >>>>
> >>>> Another op :)
> >>>
> >>> Does it need to be, or can it be one op as
> >>> enum mipi_dsi_lane_state{
> >>>    DSI_ULPS,
> >>>    DSI_LP11,
> >>>    DSI_HS,
> >>> };
> >>>
> >>> int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
> >>>          enum mipi_dsi_lane_state clock_lane_state,
> >>>          u64 clock_lane_freq,
> >>>          enum mipi_dsi_lane_state data_lane_state)
> >>
> >> I still do not know why do you want to set freq explicitly, ie why DSI
> >> host could not choose it, based on bridge constraints, DSI host
> >> constraints and requested video mode.
> >
> > I'm partly trying to piece together what information is already around
> > in order to be able to make a good decision on the frequency.
> > If we have (max_)hs_rate defined and a mode, then we can make a
> > decision within the DSI host driver.
> >
> > Adding a link-frequencies DT property would allow the system
> > configuration to be defined rather than some random heuristics.
>
> Exynos DSI used samsung,burst-clock-frequency property, I am not sure of
> others, but it was rather copy/paste solution from vendor code. Maybe it
> is OK this way.

Just for interest's sake I had a quick look for how other drivers configure it.
hs_rate is only used by ST-Ericsson MCDE and OMAP.
iMX8 nwl-dsi, Synopsys dw-mipi-dsi, hisilicon dw_drm_dsi, mtk_dsi, and
msm/dsi_host all set magic flags in the hardware based on
MIPI_DSI_MODE_VIDEO_BURST, and appear to have no reporting of the
frequency used.
Exynos appears to require vendor-specific DT property
"samsung,burst-clock-frequency".

It would be nice to have a standardised and documented approach.

It's obviously not valid for me to reuse a Samsung specific property
on a non-Samsung platform, and duplicating it as a different
vendor-specific property would imply that it shouldn't be
vendor-specific.
From V4L2 there is already the defined link-frequencies DT property to
set CSI2 HS rates, so it would seem reasonable for that to be used for
DSI as well.

> >
> > But otherwise I do now agree with you that we probably don't need an
> > explicit frequency from the bridge/panel driver.
> >
> > There is still the situation documented in [1] that the transfer
> > function "can be called no matter the state the host is in", so that
> > would include as part of attach when we don't have a video mode. Any
> > transfer without the MIPI_DSI_MSG_USE_LPM flag is then at an undefined
> > HS speed. There are likely very few cases where that matters, so a
> > WARN from the DSI host driver is probably sufficient for now.
>
> But we have already constraints from both players - host and device, so
> it should not be an issue to use max allowed speed.

For devices that set MIPI_DSI_MODE_VIDEO_BURST we can if hs_rate is
set or we otherwise assume 500MHz.
This is an issue for devices without MIPI_DSI_MODE_VIDEO_BURST set
where they are expecting their pixel rate. Under these conditions we
don't know what that pixel rate is.

> >
> > [1] https://www.kernel.org/doc/html/latest/gpu/drm-kms-helpers.html?highlight=dsi#c.mipi_dsi_host_ops
> >
> >> Regarding different modes for clock and data lanes there exists DSI mode
> >> flag MIPI_DSI_CLOCK_NON_CONTINUOUS which determines if host in LP-11
> >> data lanes state can also put clock lane into LP-11. Is there need for
> >> sth more?
> >> I'd like to avoid possibility of nonsense(?) configurations like clock
> >> in LP-11 and data lanes in HS.
> >
> > Having had a first pass at implementing this, then I certainly agree
> > with altering the potential states I'd previously suggested.
> >      DSI_STANDBY,
>
> Specification uses LP11 or STOP state to describe LP-11, so I would
> prefer to stick to one of it.

Sorry, I was meaning standby as in power off the PHY. DSI_OFF is
probably more accurate.
An LP-11 / STOP state would be DSI_ACTIVE.

> >      DSI_ULPS,
>
> I am not sure but I hope this is LP-00.

It is, but also it needs the relevant escape sequence to enter/exit
it. You can't simply just switch to LP-00.
I'm looking more at link states than physical states.

> >      DSI_ACTIVE,
>
> I am not sure about this one, this would mean start video transmission
> in HS, as there is no HS stop state. It does not look like desirable in
> pre-enable state. 'HS idle' state is just LP-11.
>
> > Seems more sensible now
> > For the clock lane, as you say, MIPI_DSI_CLOCK_NON_CONTINUOUS
> > determines whether DSI_ACTIVE means LP-11 or HS.
> > The data lanes follow the clock lane in state, so DSI_ACTIVE means
> > LP-11, sending LP commands, or HS if video is actively being
> > transmitted.
>
> I think you are messing up in above sentence :)
> For definition look for 'continuous clock' phrase in spec above [1].

Not sure I agree with you based on my clarification of what I meant by
DSI_STANDBY.

> > Having just DSI_ACTIVE also avoids the nonsense you rightly raise of
> > LP clock but HS data.
> >
> > So it can be condensed down to a single state.
> > int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
> >          enum mipi_dsi_lane_state state);
> > Does that seem reasonable?
>
> Yes, but if we stay with two states I wonder if it wouldn't be better to
> use two separate ops, just matter of taste.

Well there is the question of whether ULPS is actually useful or not
over powering down the PHY, but by making it a single op taking an
enum means that even if we ignore it now then it could be added.
I am aware of DSI devices that will switch to standby when they see a
ULPS entry sequence, so being able to request that from disable would
be reasonable.

If I step back and asked you for a proposal of an API that allows a
DSI device to ask the bridge to enable itself based on the existing
flags and parameters, what would you have it look like?

> >
> > I guess the other thing to do is document what is meant by each flag.
> > Currently the documentation is rather lacking in the intended
> > behaviour, which is where I started with this thread.
>
> Yes, at least in case spec is not clear about it.

Again, the spec does not determine the implementation details under a
specific operating system.

I suspect I'm in the minority where we have a relatively open platform
and more and more random non-mainline kernel developers are coming
along and wanting to add this or that DSI device. If it isn't
documented somewhere as to how to configure this stuff then they go
down the totally wrong path, and both the platform and the DRM
subsystem look bad.
When you have drivers merged into the mainline kernel such as [1]
which set mode_flags to just MIPI_DSI_MODE_VIDEO_BURST (presumably
command mode but higher link frequency?), or [2] which sets just
MIPI_DSI_MODE_VIDEO_SYNC_PULSE (command mode with sync pulses?), it
would appear that it isn't clear to experienced developers who are
submitting DSI device drivers as to what these flags mean, nor which
are valid in various combinations. If they get it wrong, nor is it
picked up during review, then relative newbies have no hope.

Laurent acknowledged at the start of this thread that a lot of stuff
isn't defined. I (or others working on our behalf) can put the work
into document it, but that documentation needs to be correct.

Thanks
  Dave

[1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/panel/panel-feiyang-fy07024di26a30d.c#L226
I can't find a datasheet for this one, but it seems unlikely.
[2] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/panel/panel-ilitek-ili9881c.c#L673
I'm fairly certain that ILI9881 doesn't support command mode.

> Regards
> Andrzej
>
>
> >
> > Thanks.
> >    Dave
> >
> >> Regards
> >> Andrzej
> >>
> >>
> >>
> >>>
> >>>     Dave
> >>>
> >>>> Regards
> >>>> Andrzej
> >>>>
> >>>>
> >>>>> Does it need a way to pass back the actual DSI frequency being used,
> >>>>> in a similar vein to mode_fixup? That allows for the bridge to request
> >>>>> the display clock, but the burst mode link frequency to be returned
> >>>>> (I'm assuming that's a property of the DSI host only, and not the
> >>>>> bridge).
> >>>>>
> >>>>> I'm having a discussion with someone who wants to run SN65DSI85 in the
> >>>>> two independent LVDS display mode. That requires a DSI HS clock on
> >>>>> DSI-A even if only panel B is active, so with this extra function that
> >>>>> would be achievable as well.
> >>>>>
> >>>>> Thoughts?
> >>>>>
> >>>>>>>>>> If the panel/bridge can set a flag that can be checked at this point
> >>>>>>>>>> for whether an early clock is required or not, I think that allows us
> >>>>>>>>>> to comply with the requirements for a large number of panels/bridges
> >>>>>>>>>> (LP-11 vs HS config for clock and or data lanes before pre_enable is
> >>>>>>>>>> called).
> >>>>>>>>>>
> >>>>>>>>>> pre_enable retains the current behaviour (initialise the chain from
> >>>>>>>>>> sink to source).
> >>>>>>>>>> enable then actually starts sending video and enabling outputs.
> >>>>>>>>>
> >>>>>>>>> Flags indeed seem like a more contained option. Another one could be to
> >>>>>>>>> have a mipi_dsi_host to (for example) power up the clock lane that would
> >>>>>>>>> be called by default before the bridge's enable, or at the downstream
> >>>>>>>>> bridge driver discretion before that.
> >>>>>>>>
> >>>>>>>> Which driver will that call?
> >>>>>>>
> >>>>>>> The parent DSI Host
> >>>>>>>
> >>>>>>>> An extreme example perhaps, but Toshiba make the TC358860 eDP to DSI
> >>>>>>>> bridge chip[1]. So the encoder will be eDP, but the DSI config needs
> >>>>>>>> to go to that bridge. Does that happen automatically within the
> >>>>>>>> framework? I guess so as the bridge will have called
> >>>>>>>> mipi_dsi_host_register for the DSI sink to attach to.
> >>>>>>>
> >>>>>>> In that case, whatever sink would be connected to the bridge would call
> >>>>>>> the bridge clock enable hook if it needs it in its pre_enable, or it
> >>>>>>> would be called automatically before enable if it doesn't
> >>>>>>>
> >>>>>>> Would that help?
> >>>>>>
> >>>>>> Sounds good to me, in theory at least (let's see what issues we'll run
> >>>>>> into in practice :-)).
> >>>>>>
> >>>>>> Has anyone given it a try, or is planning to ?
> >>>>>>
> >>>>>>>> Perhaps a new mipi_dsi_host function to configure the PHY is the
> >>>>>>>> easier solution. If it can allow the sink to request whatever
> >>>>>>>> combination of states from clock and data lanes that it fancies, then
> >>>>>>>> it can be as explicit as required for the initialisation sequence, and
> >>>>>>>> the host driver does its best to comply with the requests.
> >>>>>>>
> >>>>>>> I don't know, I'm not really fond in general of solutions that try to
> >>>>>>> cover any single case if we don't need it and / or have today an issue
> >>>>>>> with this. I'd rather have something that works for the particular
> >>>>>>> bridge we were discussing, see if it applies to other bridges and modify
> >>>>>>> it if it doesn't until it works for all our cases. Trying to reason in
> >>>>>>> all possible cases tend to lead to solutions that are difficult to
> >>>>>>> maintain and usually over-engineered.
> >>>>>>
> >>>>>> A DSI host clock enable operation or a DSI host PHY configuration
> >>>>>> operation both fit in the same place in the grand scheme of things, so I
> >>>>>> don't mind either. We should be able to easily move from a more specific
> >>>>>> operation to a more generic one if the need arises.
> >>>>>>
> >>>>>>>> I'd have a slight query over when and how the host would drop to ULPS
> >>>>>>>> or power off. It probably shouldn't be in post_disable as the sink
> >>>>>>>> hasn't had a chance to finalise everything in its post_disable.
> >>>>>>>>
> >>>>>>>> Perhaps pm_runtime with autosuspend is the right call there?
> >>>>>>>
> >>>>>>> pm_runtime semantics mean that once the device is suspended, its power
> >>>>>>> domains, regulators, clocks, etc. are all shut down, so it doesn't
> >>>>>>> really fit the low power state expected by DSI
> >>>>>>>
> >>>>>>>> [1] https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html
> >>>>>>>>
> >>>>>>>>>> When I discussed this briefly with Maxime there was a suggestion of
> >>>>>>>>>> using pm_runtime to be able to power up the pipeline as a whole. If
> >>>>>>>>>> the bridge driver can use pm_runtime to power up the PHY when
> >>>>>>>>>> required, then that may solve the issue, however I know too little of
> >>>>>>>>>> the details to say whether that is actually practical.
> >>>>>>>>>
> >>>>>>>>> I'm not sure it was about this topic in particular. If I remember well
> >>>>>>>>> our discussion, this was about the vc4 driver that tries to circumvent
> >>>>>>>>> the framework and call the pre_enable and enable hooks itself because it
> >>>>>>>>> wasn't properly powered before and thus any DCS-related call by the
> >>>>>>>>> downstream bridge or panel would end up creating a CPU stall.
> >>>>>>>>>
> >>>>>>>>> I suggested to use runtime_pm in the DCS related calls to make sure the
> >>>>>>>>> device is powered because there's no relation between the state of the
> >>>>>>>>> downstream bridge or panel and whether it can send DCS commands or not.
> >>>>>>>>> For all we know, it could do it at probe time.
> >>>>>>>>
> >>>>>>>> pm_runtime is all a bit of a magic black box to me.
> >>>>>>>>
> >>>>>>>> We had discussed shifting to using pm_runtime from DCS (and enable)
> >>>>>>>> calls to power up the PHY on demand, and that's what I implemented.
> >>>>>>>> However Laurent flagged up that using
> >>>>>>>> dsi->encoder->crtc->state->adjusted_mode to get the HS clock info
> >>>>>>>> required to send a HS DCS command from that call is deprecated, so how
> >>>>>>>> do we specify the clock rate to use at that point?
> >>>>>>>
> >>>>>>> I guess the most sensible would be to have a custom bridge state, and
> >>>>>>> add a pointer to the current bridge state in struct drm_bridge. Then, as
> >>>>>>> long as you have a bridge pointer you have a way to get the current
> >>>>>>> state associated to it, and since we already have atomic_duplicate_state
> >>>>>>> / atomic_destroy_state we can create our own structure around it storing
> >>>>>>> whatever we want.
> >>>>>>
> >>>>>> That's a good point. It would only be needed if we use runtime PM to
> >>>>>> work around the initialization sequence issue, not if we implement a DSI
> >>>>>> host clock enable/disable operation, right ?
> >>>>>
> >>>>> Obviously this only works with atomic bridges otherwise you have no
> >>>>> state, but I assume all bridges should be heading that route now.
> >>>>>
> >>>>>>>>>>>> host_transfer calls can supposedly be made at any time, however unless
> >>>>>>>>>>>> MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
> >>>>>>>>>>>> in high speed mode. If this is before a mode has been set, what
> >>>>>>>>>>>> defines the link frequency parameters at this point? Adopting a random
> >>>>>>>>>>>> default sounds like a good way to get undefined behaviour.
> >>>>>>>>>>>>
> >>>>>>>>>>>> DSI burst mode needs to set the DSI link frequency independently of
> >>>>>>>>>>>> the display mode. How is that meant to be configured? I would have
> >>>>>>>>>>>> expected it to come from DT due to link frequency often being chosen
> >>>>>>>>>>>> based on EMC restrictions, but I don't see such a thing in any
> >>>>>>>>>>>> binding.
> >>>>>>>>>>>
> >>>>>>>>>>> Undefined too. DSI support was added to DRM without any design effort,
> >>>>>>>>>>> it's more a hack than a real solution. The issue with devices that can
> >>>>>>>>>>> be controlled over both DSI and I2C is completely unhandled. So far
> >>>>>>>>>>> nobody has really cared about implementing DSI right as far as I can
> >>>>>>>>>>> tell.
> >>>>>>>>>>
> >>>>>>>>>> :-(
> >>>>>>>>>>
> >>>>>>>>>> Thinking aloud, does having the option to set a burst link frequency
> >>>>>>>>>> from DT (or ACPI) have any issue for other platforms?
> >>>>>>>>>> Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
> >>>>>>>>>> drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
> >>>>>>>>>> all the defined timings, but drop to LP during blanking" option. The
> >>>>>>>>>> link frequency has therefore remained a property of the
> >>>>>>>>>> display/bridge.
> >>>>>>>>>> dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
> >>>>>>>>>> through the full detail of the parameters it computes from there.
> >>>>>>>>>
> >>>>>>>>> I don't see anything wrong with using link-frequency from the DT to
> >>>>>>>>> setup the burst frequency. It's what v4l2 has been using for a while
> >>>>>>>>> without any known (to me) drawback, and we're using the same of-graph
> >>>>>>>>> bindings, so it shouldn't be too controversial there.
> >>>>>>
> >>>>>> How would that frequency we picked in practice ? Do panels typically
> >>>>>> support a range of HS frequencies for DCS HS transfers ?
> >>>>>
> >>>>> I was thinking more of bridges where they often run a PLL off the
> >>>>> incoming DSI clock, and then have a FIFO with potentially different
> >>>>> clock rates and timings on input and output. SN65DSI83 supports that,
> >>>>> as do the Toshiba bridge chips I'm currently looking at.
> >>>>> You need to know the DSI link frequency to configure the PLL
> >>>>> correctly, but then the bridge output timing is a different matter.
> >>>>>
> >>>>>>>> OK, that sounds like a vague plan.
> >>>>>>>>
> >>>>>>>>>> DSI and I2C controlled devices is yet another issue that I haven't
> >>>>>>>>>> even looked at.
> >>>>>>>>>> I think it's more that vc4 wants to ignore DSI should the DSI host
> >>>>>>>>>> node be enabled in DT, but there's no panel bound to it. One could say
> >>>>>>>>>> that is a DT error and tough luck, but from a user's perspective that
> >>>>>>>>>> is a bit harsh.
> >>>>>>>>>
> >>>>>>>>> I guess the larger "issue" is that the tree in the DT is done following
> >>>>>>>>> the "control" bus, and Linux likes to tie the life cycle of a given
> >>>>>>>>> device to its parent bus. Both these decisions make sense, but they
> >>>>>>>>> interact in a weird way in some occurrences (like this one, or Allwinner
> >>>>>>>>> has an Ethernet PHY controlled through MMIO which end up in the same
> >>>>>>>>> case).
> >>>>>>>>>
> >>>>>>>>> I wonder if using device links here could help though.
> >>>>>>>>
> >>>>>>>> I really don't know about that one.
> >>>>>>>
> >>>>>>> It's a piece of infrastructure that was created at first (I think?) to
> >>>>>>> model the power dependency between devices that don't have a parent /
> >>>>>>> child relationship. For example, if you use DMA, you probably want to
> >>>>>>> keep the IOMMU powered as long as you are, but it is in a completely
> >>>>>>> separate branch of the "device tree" (not one from the DTB, the one that
> >>>>>>> linux DM creates).
> >>>>>>>
> >>>>>>> It was later expanded to also cover probe order and make sure a supplier
> >>>>>>> would probe before its consumer, effectively making EPROBE_DEFER
> >>>>>>> obsolete.
> >>>>>>>
> >>>>>>> The second part is still fairly new, but I think we can solve this by
> >>>>>>> adding a device link between the DSI host and whatever is at the end of
> >>>>>>> the OF-Graph endpoint.
> >>>>>>>
> >>>>>>>>>>>> As a follow on, bridge devices can support burst mode (eg TI's
> >>>>>>>>>>>> SN65DSI83 that's just been merged), so it needs to know the desired
> >>>>>>>>>>>> panel timings for the output side of the bridge, but the DSI link
> >>>>>>>>>>>> timings to set up the bridge's PLL. What's the correct way for
> >>>>>>>>>>>> signalling that? drm_crtc_state->adjusted_mode vs
> >>>>>>>>>>>> drm_crtc_state->mode? Except mode is userspace's request, not what has
> >>>>>>>>>>>> been validated/updated by the panel/bridge.
> >>>>>>>>>>>
> >>>>>>>>>>> adjusted_mode is also a bit of a hack, it solves very specific issues,
> >>>>>>>>>>> and its design assumes a single encoder in the chain with no extra
> >>>>>>>>>>> bridges. We should instead add modes to the bridge state, and negotiate
> >>>>>>>>>>> modes along the pipeline the same way we negotiate formats.
> >>>>>>>>>>
> >>>>>>>>>> So as I understand it we already have format negotiation between
> >>>>>>>>>> bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
> >>>>>>>>>> so is it possible to extend that to modes?
> >>>>>>>>>> Are you thinking bridge state that is owned by the framework, or by
> >>>>>>>>>> the individual bridge drivers?
> >>>>>>>>>
> >>>>>>>>> atomic_check is made for that. I guess we could improve its call
> >>>>>>>>> sequence to each time a mode is modified along the bridge list we
> >>>>>>>>> restart the sequence until all components agree (or reject it entirely
> >>>>>>>>> if they can't), but I don't really see why we would need yet another
> >>>>>>>>> hook.
> >>>>>>
> >>>>>> Isn't this what atomic_get_output_bus_fmts() and
> >>>>>> atomic_get_input_bus_fmts() implement ?
> >>>>>
> >>>>> Those negotiate a single u32 bus format between nodes, not a complete timing.
> >>>>>
> >>>>>>>> Why do all nodes in the bridge list need to agree? Adjacent nodes need
> >>>>>>>> to agree, but they then also need to retain that agreed timing
> >>>>>>>> somewhere.
> >>>>>>>
> >>>>>>> We might have mutually exclusive requirements though? Let's use the
> >>>>>>> example of the VC4 HDMI driver that can't have odd horizontal timings,
> >>>>>>> and assume it's a constraint of our DSI driver instead.
> >>>>>>>
> >>>>>>> Then, we have a DSI->LVDS bridge, a LVDS->RGB bridge and a panel (which
> >>>>>>> is a bit ridiculous, but whatever). If the LVDS->RGB bridge can't have
> >>>>>>> even horizontal timings, then you just can't display it, even though
> >>>>>>> they are not adjacent (unless the bridge in the middle can modify the
> >>>>>>> timings between the input and output, but that's not always possible).
> >>>>>>>
> >>>>>>> Similarly, if for the RGB panel we need to increase a bit some timings
> >>>>>>> to accommodate for a larger pixel clock and end up above what the DSI
> >>>>>>> host can provide, we're also done.
> >>>>>>
> >>>>>> The hard part will be to figure out a good heuristics to perform the
> >>>>>> negotiation without going back and forth (at least not in a way that
> >>>>>> would require too many iterations, and certainly avoiding infinite
> >>>>>> loops). That will be an interesting problem to solve, but maybe we'll be
> >>>>>> lucky and a simple approach will work for the use cases we have to
> >>>>>> support today.
> >>>>>
> >>>>> One to kick into the long grass possibly then.
> >>>>>
> >>>>> For burst mode and bridges retiming things, generally I think it works
> >>>>> if you consider a single pass through a mode_fixup equivalent starting
> >>>>> at the panel.
> >>>>> The panel advertises what it wants.
> >>>>> The closest bridge (eg DSI83) fixup amends panel's requested mode
> >>>>> based on any constraints that it has, stores a copy, and passes that
> >>>>> down the chain.
> >>>>> The next bridge (eg vc4_dsi) amends the mode based on restrictions it
> >>>>> has (eg burst mode link frequency). That's the end of the chain in
> >>>>> this case, so that's the mode that the crtc needs to be programmed
> >>>>> with.
> >>>>>
> >>>>> The modes then need to be passed back up the chain so that eg DSI83
> >>>>> knows the link frequency in use, and can therefore configure the PLL
> >>>>> appropriately.
> >>>>>
> >>>>> The bit it would be nice to fix is that burst mode has effectively
> >>>>> increased the horizontal front porch, but that could be fixed within
> >>>>> the bridge so that the panel gets the timing it requested.
> >>>>>
> >>>>>>>> Taking SN65DSI8[3|4|5] as an example, it supports burst mode, and the
> >>>>>>>> DSI frequency and timings are permitted to be different from that
> >>>>>>>> which it uses on the LVDS side. The LVDS panel and LVDS side of DSI83
> >>>>>>>> need to agree over the format, and the DSI host and DSI side of DSI83
> >>>>>>>> need to agree, but they may be two different timings.
> >>>>>>>> Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows you to
> >>>>>>>> configure the LVDS rate compared to the DSI rate (the driver currently
> >>>>>>>> goes for 1:1), and registers 0x20 to 0x34 allow you to set the number
> >>>>>>>> of active pixel and blanking on the LVDS side (again currently just
> >>>>>>>> copied across).
> >>>>>>>>
> >>>>>>>> The way I'm seeing burst mode as having been interpreted at present is
> >>>>>>>> that it's largely just a flag to say "drop to LP mode between lines".
> >>>>>>>> The timing that needs to be passed to the crtc is therefore going to
> >>>>>>>> be based on the DSI link rate (converted to pixels) with increased
> >>>>>>>> blanking periods.
> >>>>>>>>
> >>>>>>>> I guess there are similarities with Media Controller and V4L2 here. A
> >>>>>>>> typical chain there could be:
> >>>>>>>>     sensor -> scaler -> crop -> CSI-2 receiver.
> >>>>>>>> The format on each of those links may be different, but the chain as a
> >>>>>>>> whole needs to be valid. Media Controller largely relies on userspace
> >>>>>>>> to configure all links, but with a DRM chain that isn't really an
> >>>>>>>> option as it's expected that the display chain configures itself.
> >>>>>>>
> >>>>>>> Also, the userspace has no concept of media sub-devices in DRM, so it
> >>>>>>> just sets the mode on the whole DRM/KMS device, unlike what v4l2 does.
> >>>>>>> In v4l2, afaik, if you ended up with the above scenarios it would just
> >>>>>>> be rejected when you set the format on the link, letting the userspace
> >>>>>>> figure it out. We can't really do that here
> >>>>>>
> >>>>>> I wonder how long we'll be able to keep userspace out of the picture to
> >>>>>> configure the internals of the pipeline. I don't want to be the first
> >>>>>> person who will have a use case that requires this.
> >>>>>
> >>>>> I suspect none of us want to be the first one to hit this scenario!
> >>>>>
> >>>>> As I've just posted on the other thread about SN65DSI83, I've hit a
> >>>>> major stumbling block with the current design where vc4_dsi (and
> >>>>> Exynos) breaks the bridge_chain so that it gets called first and can
> >>>>> do initialisation, and then it calls down the chain.
> >>>>> Having now converted to a DSI bridge driver, it works fine with
> >>>>> connected non-atomic bridges, but fails with atomic bridges.
> >>>>> drm_atomic_add_encoder_bridges adds the state for all the bridges the
> >>>>> framework is aware of. With the split chain it misses adding the state
> >>>>> of our "hidden" bridges, and we can't add the extra state from our
> >>>>> atomic_duplicate_state call as we don't have the state to add to (we
> >>>>> just return our state to be added).
> >>>>> This bumps up the priority for us of finding a suitable solution for
> >>>>> this initialisation issue, so I'll start looking at how feasible a new
> >>>>> DSI host function is.
> >>>>>
> >>>>> Thanks
> >>>>>      Dave
> >>>>>
> >>>>
> >>
>

Re: Questions over DSI within DRM.

[Andrzej Hajda]

On 08.10.2021 19:33, Dave Stevenson wrote:
> On Thu, 7 Oct 2021 at 21:19, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
>>
>> On 07.10.2021 13:07, Dave Stevenson wrote:
>>> On Tue, 5 Oct 2021 at 22:03, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
>>>>
>>>> On 05.10.2021 17:32, Dave Stevenson wrote:
>>>>> Hi Andrzej
>>>>>
>>>>> Thanks for joining in the discussion.
>>>>>
>>>>> On Tue, 5 Oct 2021 at 16:08, Andrzej Hajda <andrzej.hajda@gmail.com> wrote:
>>>>>>
>>>>>> On 05.10.2021 13:23, Dave Stevenson wrote:
>>>>>>> Hi Laurent
>>>>>>>
>>>>>>> On Sun, 3 Oct 2021 at 15:16, Laurent Pinchart
>>>>>>> <laurent.pinchart@ideasonboard.com> wrote:
>>>>>>>>
>>>>>>>> Hello,
>>>>>>>>
>>>>>>>> Reviving a bit of an old thread.
>>>>>>>
>>>>>>> I'd been looking at reviving this conversation too as I've moved
>>>>>>> further on with DSI on the Pi, and converting from an encoder to a
>>>>>>> bridge
>>>>>>>
>>>>>>>> On Thu, Jul 15, 2021 at 11:50:22AM +0200, Maxime Ripard wrote:
>>>>>>>>> On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
>>>>>>>>>> On Tue, 6 Jul 2021 at 16:13, Maxime Ripard wrote:
>>>>>>>>>>>>>> On a similar theme, some devices want the clock lane in HS mode early
>>>>>>>>>>>>>> so they can use it in place of an external oscillator, but the data
>>>>>>>>>>>>>> lanes still in LP-11. There appears to be no way for the
>>>>>>>>>>>>>> display/bridge to signal this requirement or it be achieved.
>>>>>>>>>>>>>
>>>>>>>>>>>>> You're right. A loooong time ago, the omapdrm driver had an internal
>>>>>>>>>>>>> infrastructure that didn't use drm_bridge or drm_panel and instead
>>>>>>>>>>>>> required omapdrm-specific drivers for those components. It used to model
>>>>>>>>>>>>> the display pipeline in a different way than drm_bridge, with the sync
>>>>>>>>>>>>> explicitly setting the source state. A DSI sink could thus control its
>>>>>>>>>>>>> enable sequence, interleaving programming of the sink with control of
>>>>>>>>>>>>> the source.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Migrating omapdrm to the drm_bridge model took a really large effort,
>>>>>>>>>>>>> which makes me believe that transitioning the whole subsystem to
>>>>>>>>>>>>> sink-controlled sources would be close to impossible. We could add
>>>>>>>>>>>>> DSI-specific operations, or add another enable bridge operation
>>>>>>>>>>>>> (post_pre_enable ? :-D). Neither would scale, but it may be enough.
>>>>>>>>>>>>
>>>>>>>>>>>> I haven't thought it through for all generic cases, but I suspect it's
>>>>>>>>>>>> more a pre_pre_enable that is needed to initialise the PHY etc,
>>>>>>>>>>>> probably from source to sink.
>>>>>>>>
>>>>>>>> I believe it could be implemented as a pre-pre-enable indeed. It feels
>>>>>>>> like a bit of a hack, as the next time we need more fine-grained control
>>>>>>>> over the startup sequence, we'll have to add a pre-pre-pre-enable. Given
>>>>>>>> that the startup sequence requirements come from the sink device, it
>>>>>>>> would make sense to let it explicitly control the initialization,
>>>>>>>> instead of driving it from the source. I don't think we'll be able to
>>>>>>>> rework the bridge API in that direction though, so I'm fine with a hack.
>>>>>>>
>>>>>>> There are pros and cons to both approaches.
>>>>>>> You're in a much better place to make that sort of call than I am, so
>>>>>>> I'll take your advice.
>>>>>>>
>>>>>>> Implementing a DSI host op function may mean an update to a number of
>>>>>>> existing DSI host drivers,
>>>>>>
>>>>>> Why? You just add new op to mipi_dsi_host_ops and create appropriate
>>>>>> helper, which in case of NULL will perform default action, either:
>>>>>> - return -ENOSYS,
>>>>>> - try to emulate by calling mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET)
>>>>>>
>>>>>> The latter is just wild guess, but I suspect it could work (even now) as
>>>>>> an alternate way of entering into stop state.
>>>>>
>>>>> If we're looking at fixing bridges to use the new DSI state call and
>>>>> then initialise in "pre_enable" (ie before video is started), then
>>>>> making that change to the bridge will require support for the DSI
>>>>> state call or risk regressions.
>>>>
>>>> I think there is misunderstanding somewhere, I do not know where, yet :)
>>>> My idea is as follow:
>>>> 1. Add dsi_host.set_state() op to dsi host accompanied with helper, lets
>>>> call it mipi_dsi_host_set_state for now.
>>>> 2. Implement dsi_host.set_state in DSI host you are working with.
>>>> 3. In body of your bridge pre_enable you call mipi_dsi_host_set_state in
>>>> proper place.
>>>>
>>>> 1 and 2 do not introduce any regression.
>>>> 3 can introduce regression only if your bridge is used with another dsi
>>>> host but only on such platforms.
>>>>
>>>> Why do you think we need to 'fix' it everywhere?
>>>
>>> I'm looking at sn65dsi83 which is already working against other
>>> platforms (I don't know which). If it is fixed to request the DSI
>>> state and initialise in pre_enable, then those platforms need updating
>>> too.
>>> If mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET) can be used to wake
>>> the host up then that would be great, but I don't have access to those
>>> platforms to test.
>>>
>>>>> Some may have been getting away with it by making the initialisation
>>>>> call in "enable" whilst video is active - that sounds like the issue
>>>>> Marek was trying to log in his patch to SN65DSI83.
>>>>>
>>>>>>> but it would be cleaner. It's also what
>>>>>>> Andrzej has suggested.
>>>>>>>
>>>>>>> Thinking it through, a function that requests clock lane frequency and
>>>>>>> state (ULPS or LP-11 predominantly), and data lane state (again ULPS
>>>>>>> or LP-11) should allow the required behaviour for most of the bridges
>>>>>>> I'm aware of. Most want either LP-11, or HS clock at a known
>>>>>>> frequency.
>>>>>>
>>>>>> LP-11 is quite reasonable, but regarding frequency I am not sure who
>>>>>> should manage it? DSI device datasheets known to me usually mentioned
>>>>>> max frequency, and it is covered by mipi_dsi_device.hs_rate. Min
>>>>>> frequency is usually determined by amount of data we need to transfer
>>>>>> per second.
>>>>>> Do you need to set frequency from device, or just to know actual frequency?
>>>>>> In former case (if it is really necessary !!!) you should request
>>>>>> frequency range anyway, as host usually is able to set only discrete
>>>>>> number of frequencies. But it need to be clearly specified how and when
>>>>>> this op can be used - more ops, more questions about interaction between
>>>>>> them.
>>>>>> In latter case another mipi host op should be quite easy to implement.
>>>>>
>>>>> If a burst mode frequency is specified in struct mipi_dsi_device
>>>>> hs_rate then there is no issue.
>>>>
>>>> hs_rate should be called rather max_hs_rate as it describes only top
>>>> limit of HS clock, decision which clock to choose is on DSI host side.
>>>
>>> Indeed, I've just realised that one.
>>>
>>>>> If not, then you'll generally want to use the pixel clock,
>>>>
>>>> Probably not, often we want to use rate higher than pixel clock
>>>> (time-compression in DSI spec), to leave time gaps for LP mode.
>>>
>>> Only if MIPI_DSI_MODE_VIDEO_BURST is set in the mode_flags, otherwise
>>> you should be running at the pixel clock. (Slight assumption as there
>>> is no definition of what MIPI_DSI_MODE_VIDEO_BURST really means in the
>>> documentation).
>>
>> Specification describes it clearly [1], grep for 'burst'.
>>
>> [1]: https://hifpga.com/upfiles/15552304546301142.pdf
> 
> The MIPI DSI spec defines the concept of burst, it doesn't define the
> Linux kernel implementation of it.
> 
> In a Linux DSI host driver:
> - How should a DSI host driver choose the link frequency to enable
> "blocks of pixel data can be transferred in a shorter time"? hs_rate
> defines the max rate, and the pixel clock the minimum. Pick any number
> in between?
> - How does a DSI device driver get told the link frequency? Many
> bridge chips run their PLLs off the DSI HS clock, so need to know how
> to convert that into a desired pixel clock.
> - 8.11.4 "Following HS pixel data transmission, the bus may stay in HS
> Mode for sending blanking packets or go to Low Power Mode, during
> which it may remain idle, i.e. the host processor remains in LP-11
> state, or LP transmission may take place in either direction. ".
> Option in the DSI spec. How is that mode chosen by the DSI device
> driver for the host driver to implement?
> - 8.11.1 "To enable PHY synchronization the host processor should
> periodically end HS transmission and drive the Data Lanes to the LP
> state. This transition should take place at least once per frame;
> shown as LPM in the figures in this section." Another option within
> the DSI spec as to whether it is once per frame or more frequently.
> How does a DSI device driver signal which mode is required to the DSI
> host driver? (And I've just had a device that is fussy about this
> one).

I think all these questions are rather to hw platform team, if they 
confirm that hw works well with specific clock and other hw settings 
developer should obey it. This is probably why almost nobody is using 
for example hs_rate constraint - developer receives straightforward 
instruction with fixed set of parameters (or just set of magic numbers 
without explanation) to program on both DSI host and device - no place 
for hesitation :)
As I understand your bridge is re-used in multiple configurations thus 
you need more intelligence in the driver. Am I right? I think the best 
would be to just start implementing missing bits.

> 
> I know we all hate writing documentation, but saying the parent
> specification defines how it should be implemented on a system isn't
> true, and developing/updating a DSI driver results in these sorts of
> questions.
> 
>>>>> and the DSI
>>>>> host hasn't necessarily been told the video mode at the point the
>>>>> bridge/panel requests a new DSI state.
>>>>
>>>> pre_enable and enable ops are called after modeset, so video mode is known.
>>>
>>> Thanks, I wasn't aware of that. It didn't help that our DSI driver
>>> didn't implement a mode_set function as it did all the work in
>>> pre_enable having looked back at the crtc for the mode.
>>>
>>> It looks like there is a slight quirk that there is an encoder
>>> atomic_mode_set, but not a bridge one. If we're implementing the
>>> atomic API is it correct to update our bridge's internal state (ie the
>>> hs_rate) from a (atomic_)mode_set? That's a different discussion
>>> though.
>>
>> It shouldn't matter, the mode should be stored in atomic state. In
>> pre-enable you should have all required info: videomode and constraints.
> 
> Sorry, perhaps I didn't describe my scenario well enough.
> Atomic DSI host bridge driver. Atomic DSI bridge/panel driver.
> 
> mode_set gets called on all nodes.
> pre_enables are called starting at the furthest end from the encoder,
> so the bridge/panel driver pre_enable gets called before the DSI host
> pre_enable. The bridge/panel wants to power up the DSI host using the
> new API. The DSI host bridge hasn't had the pre_enable called yet, and
> currently we're not proposing an atomic_set_state. At that point, what
> parameters should the DSI host driver be using to determine the link
> frequency as it doesn't have a state?

First of all it has access to device's hs_rate, so it can use it.
But it has also access to crtc state at bridge.encoder->crtc->state. 
Which should be already set by mode_set - any pre_enable is called after 
mode_set.

> 
> Perhaps we should be proposing an atomic_set_state which takes a
> struct drm_atomic_state *. How would that work if called from a
> non-atomic bridge driver though?
> 
>>>
>>>>>
>>>>>>> Giving the option for setting back into ULPS also allows
>>>>>>> for power saving/standby mechanisms should the need arise.
>>>>>>
>>>>>> Another op :)
>>>>>
>>>>> Does it need to be, or can it be one op as
>>>>> enum mipi_dsi_lane_state{
>>>>>     DSI_ULPS,
>>>>>     DSI_LP11,
>>>>>     DSI_HS,
>>>>> };
>>>>>
>>>>> int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
>>>>>           enum mipi_dsi_lane_state clock_lane_state,
>>>>>           u64 clock_lane_freq,
>>>>>           enum mipi_dsi_lane_state data_lane_state)
>>>>
>>>> I still do not know why do you want to set freq explicitly, ie why DSI
>>>> host could not choose it, based on bridge constraints, DSI host
>>>> constraints and requested video mode.
>>>
>>> I'm partly trying to piece together what information is already around
>>> in order to be able to make a good decision on the frequency.
>>> If we have (max_)hs_rate defined and a mode, then we can make a
>>> decision within the DSI host driver.
>>>
>>> Adding a link-frequencies DT property would allow the system
>>> configuration to be defined rather than some random heuristics.
>>
>> Exynos DSI used samsung,burst-clock-frequency property, I am not sure of
>> others, but it was rather copy/paste solution from vendor code. Maybe it
>> is OK this way.
> 
> Just for interest's sake I had a quick look for how other drivers configure it.
> hs_rate is only used by ST-Ericsson MCDE and OMAP.
> iMX8 nwl-dsi, Synopsys dw-mipi-dsi, hisilicon dw_drm_dsi, mtk_dsi, and
> msm/dsi_host all set magic flags in the hardware based on
> MIPI_DSI_MODE_VIDEO_BURST, and appear to have no reporting of the
> frequency used.
> Exynos appears to require vendor-specific DT property
> "samsung,burst-clock-frequency".
> 
> It would be nice to have a standardised and documented approach.
> 
> It's obviously not valid for me to reuse a Samsung specific property
> on a non-Samsung platform, and duplicating it as a different
> vendor-specific property would imply that it shouldn't be
> vendor-specific.
>  From V4L2 there is already the defined link-frequencies DT property to
> set CSI2 HS rates, so it would seem reasonable for that to be used for
> DSI as well.

Sounds OK.

> 
>>>
>>> But otherwise I do now agree with you that we probably don't need an
>>> explicit frequency from the bridge/panel driver.
>>>
>>> There is still the situation documented in [1] that the transfer
>>> function "can be called no matter the state the host is in", so that
>>> would include as part of attach when we don't have a video mode. Any
>>> transfer without the MIPI_DSI_MSG_USE_LPM flag is then at an undefined
>>> HS speed. There are likely very few cases where that matters, so a
>>> WARN from the DSI host driver is probably sufficient for now.
>>
>> But we have already constraints from both players - host and device, so
>> it should not be an issue to use max allowed speed.
> 
> For devices that set MIPI_DSI_MODE_VIDEO_BURST we can if hs_rate is
> set or we otherwise assume 500MHz.

Maybe better would be fix drivers to provide real limit.


> This is an issue for devices without MIPI_DSI_MODE_VIDEO_BURST set
> where they are expecting their pixel rate. Under these conditions we
> don't know what that pixel rate 
> 
>>>
>>> [1] https://www.kernel.org/doc/html/latest/gpu/drm-kms-helpers.html?highlight=dsi#c.mipi_dsi_host_ops
>>>
>>>> Regarding different modes for clock and data lanes there exists DSI mode
>>>> flag MIPI_DSI_CLOCK_NON_CONTINUOUS which determines if host in LP-11
>>>> data lanes state can also put clock lane into LP-11. Is there need for
>>>> sth more?
>>>> I'd like to avoid possibility of nonsense(?) configurations like clock
>>>> in LP-11 and data lanes in HS.
>>>
>>> Having had a first pass at implementing this, then I certainly agree
>>> with altering the potential states I'd previously suggested.
>>>       DSI_STANDBY,
>>
>> Specification uses LP11 or STOP state to describe LP-11, so I would
>> prefer to stick to one of it.
> 
> Sorry, I was meaning standby as in power off the PHY. DSI_OFF is
> probably more accurate.
> An LP-11 / STOP state would be DSI_ACTIVE.
> 
>>>       DSI_ULPS,
>>
>> I am not sure but I hope this is LP-00.
> 
> It is, but also it needs the relevant escape sequence to enter/exit
> it. You can't simply just switch to LP-00.
> I'm looking more at link states than physical states.
> 
>>>       DSI_ACTIVE,
>>
>> I am not sure about this one, this would mean start video transmission
>> in HS, as there is no HS stop state. It does not look like desirable in
>> pre-enable state. 'HS idle' state is just LP-11.
>>
>>> Seems more sensible now
>>> For the clock lane, as you say, MIPI_DSI_CLOCK_NON_CONTINUOUS
>>> determines whether DSI_ACTIVE means LP-11 or HS.
>>> The data lanes follow the clock lane in state, so DSI_ACTIVE means
>>> LP-11, sending LP commands, or HS if video is actively being
>>> transmitted.
>>
>> I think you are messing up in above sentence :)
>> For definition look for 'continuous clock' phrase in spec above [1].
> 
> Not sure I agree with you based on my clarification of what I meant by
> DSI_STANDBY.
> 
>>> Having just DSI_ACTIVE also avoids the nonsense you rightly raise of
>>> LP clock but HS data.
>>>
>>> So it can be condensed down to a single state.
>>> int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
>>>           enum mipi_dsi_lane_state state);
>>> Does that seem reasonable?
>>
>> Yes, but if we stay with two states I wonder if it wouldn't be better to
>> use two separate ops, just matter of taste.
> 
> Well there is the question of whether ULPS is actually useful or not
> over powering down the PHY, but by making it a single op taking an
> enum means that even if we ignore it now then it could be added.
> I am aware of DSI devices that will switch to standby when they see a
> ULPS entry sequence, so being able to request that from disable would
> be reasonable.
> 
> If I step back and asked you for a proposal of an API that allows a
> DSI device to ask the bridge to enable itself based on the existing
> flags and parameters, what would you have it look like?

> 
>>>
>>> I guess the other thing to do is document what is meant by each flag.
>>> Currently the documentation is rather lacking in the intended
>>> behaviour, which is where I started with this thread.
>>
>> Yes, at least in case spec is not clear about it.
> 
> Again, the spec does not determine the implementation details under a
> specific operating system.
> 
> I suspect I'm in the minority where we have a relatively open platform
> and more and more random non-mainline kernel developers are coming
> along and wanting to add this or that DSI device. If it isn't
> documented somewhere as to how to configure this stuff then they go
> down the totally wrong path, and both the platform and the DRM
> subsystem look bad.

This is classic issue - it is hard to write framework/docs for things 
you cannot test/implement. It is left for devs who need it :)


> When you have drivers merged into the mainline kernel such as [1]
> which set mode_flags to just MIPI_DSI_MODE_VIDEO_BURST (presumably
> command mode but higher link frequency?), or [2] which sets just
> MIPI_DSI_MODE_VIDEO_SYNC_PULSE (command mode with sync pulses?), it
> would appear that it isn't clear to experienced developers who are
> submitting DSI device drivers as to what these flags mean, nor which
> are valid in various combinations. If they get it wrong, nor is it
> picked up during review, then relative newbies have no hope.
> 
> Laurent acknowledged at the start of this thread that a lot of stuff
> isn't defined. I (or others working on our behalf) can put the work
> into document it, but that documentation needs to be correct.
> 
> Thanks
>    Dave
> 
> [1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/panel/panel-feiyang-fy07024di26a30d.c#L226
> I can't find a datasheet for this one, but it seems unlikely.

http://files.pine64.org/doc/datasheet/pine64/FY07024DI26A30-D_feiyang_LCD_panel.pdf


Regards
Andrzej


> [2] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/panel/panel-ilitek-ili9881c.c#L673
> I'm fairly certain that ILI9881 doesn't support command mode.
> 
>> Regards
>> Andrzej
>>
>>
>>>
>>> Thanks.
>>>     Dave
>>>
>>>> Regards
>>>> Andrzej
>>>>
>>>>
>>>>
>>>>>
>>>>>      Dave
>>>>>
>>>>>> Regards
>>>>>> Andrzej
>>>>>>
>>>>>>
>>>>>>> Does it need a way to pass back the actual DSI frequency being used,
>>>>>>> in a similar vein to mode_fixup? That allows for the bridge to request
>>>>>>> the display clock, but the burst mode link frequency to be returned
>>>>>>> (I'm assuming that's a property of the DSI host only, and not the
>>>>>>> bridge).
>>>>>>>
>>>>>>> I'm having a discussion with someone who wants to run SN65DSI85 in the
>>>>>>> two independent LVDS display mode. That requires a DSI HS clock on
>>>>>>> DSI-A even if only panel B is active, so with this extra function that
>>>>>>> would be achievable as well.
>>>>>>>
>>>>>>> Thoughts?
>>>>>>>
>>>>>>>>>>>> If the panel/bridge can set a flag that can be checked at this point
>>>>>>>>>>>> for whether an early clock is required or not, I think that allows us
>>>>>>>>>>>> to comply with the requirements for a large number of panels/bridges
>>>>>>>>>>>> (LP-11 vs HS config for clock and or data lanes before pre_enable is
>>>>>>>>>>>> called).
>>>>>>>>>>>>
>>>>>>>>>>>> pre_enable retains the current behaviour (initialise the chain from
>>>>>>>>>>>> sink to source).
>>>>>>>>>>>> enable then actually starts sending video and enabling outputs.
>>>>>>>>>>>
>>>>>>>>>>> Flags indeed seem like a more contained option. Another one could be to
>>>>>>>>>>> have a mipi_dsi_host to (for example) power up the clock lane that would
>>>>>>>>>>> be called by default before the bridge's enable, or at the downstream
>>>>>>>>>>> bridge driver discretion before that.
>>>>>>>>>>
>>>>>>>>>> Which driver will that call?
>>>>>>>>>
>>>>>>>>> The parent DSI Host
>>>>>>>>>
>>>>>>>>>> An extreme example perhaps, but Toshiba make the TC358860 eDP to DSI
>>>>>>>>>> bridge chip[1]. So the encoder will be eDP, but the DSI config needs
>>>>>>>>>> to go to that bridge. Does that happen automatically within the
>>>>>>>>>> framework? I guess so as the bridge will have called
>>>>>>>>>> mipi_dsi_host_register for the DSI sink to attach to.
>>>>>>>>>
>>>>>>>>> In that case, whatever sink would be connected to the bridge would call
>>>>>>>>> the bridge clock enable hook if it needs it in its pre_enable, or it
>>>>>>>>> would be called automatically before enable if it doesn't
>>>>>>>>>
>>>>>>>>> Would that help?
>>>>>>>>
>>>>>>>> Sounds good to me, in theory at least (let's see what issues we'll run
>>>>>>>> into in practice :-)).
>>>>>>>>
>>>>>>>> Has anyone given it a try, or is planning to ?
>>>>>>>>
>>>>>>>>>> Perhaps a new mipi_dsi_host function to configure the PHY is the
>>>>>>>>>> easier solution. If it can allow the sink to request whatever
>>>>>>>>>> combination of states from clock and data lanes that it fancies, then
>>>>>>>>>> it can be as explicit as required for the initialisation sequence, and
>>>>>>>>>> the host driver does its best to comply with the requests.
>>>>>>>>>
>>>>>>>>> I don't know, I'm not really fond in general of solutions that try to
>>>>>>>>> cover any single case if we don't need it and / or have today an issue
>>>>>>>>> with this. I'd rather have something that works for the particular
>>>>>>>>> bridge we were discussing, see if it applies to other bridges and modify
>>>>>>>>> it if it doesn't until it works for all our cases. Trying to reason in
>>>>>>>>> all possible cases tend to lead to solutions that are difficult to
>>>>>>>>> maintain and usually over-engineered.
>>>>>>>>
>>>>>>>> A DSI host clock enable operation or a DSI host PHY configuration
>>>>>>>> operation both fit in the same place in the grand scheme of things, so I
>>>>>>>> don't mind either. We should be able to easily move from a more specific
>>>>>>>> operation to a more generic one if the need arises.
>>>>>>>>
>>>>>>>>>> I'd have a slight query over when and how the host would drop to ULPS
>>>>>>>>>> or power off. It probably shouldn't be in post_disable as the sink
>>>>>>>>>> hasn't had a chance to finalise everything in its post_disable.
>>>>>>>>>>
>>>>>>>>>> Perhaps pm_runtime with autosuspend is the right call there?
>>>>>>>>>
>>>>>>>>> pm_runtime semantics mean that once the device is suspended, its power
>>>>>>>>> domains, regulators, clocks, etc. are all shut down, so it doesn't
>>>>>>>>> really fit the low power state expected by DSI
>>>>>>>>>
>>>>>>>>>> [1] https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html
>>>>>>>>>>
>>>>>>>>>>>> When I discussed this briefly with Maxime there was a suggestion of
>>>>>>>>>>>> using pm_runtime to be able to power up the pipeline as a whole. If
>>>>>>>>>>>> the bridge driver can use pm_runtime to power up the PHY when
>>>>>>>>>>>> required, then that may solve the issue, however I know too little of
>>>>>>>>>>>> the details to say whether that is actually practical.
>>>>>>>>>>>
>>>>>>>>>>> I'm not sure it was about this topic in particular. If I remember well
>>>>>>>>>>> our discussion, this was about the vc4 driver that tries to circumvent
>>>>>>>>>>> the framework and call the pre_enable and enable hooks itself because it
>>>>>>>>>>> wasn't properly powered before and thus any DCS-related call by the
>>>>>>>>>>> downstream bridge or panel would end up creating a CPU stall.
>>>>>>>>>>>
>>>>>>>>>>> I suggested to use runtime_pm in the DCS related calls to make sure the
>>>>>>>>>>> device is powered because there's no relation between the state of the
>>>>>>>>>>> downstream bridge or panel and whether it can send DCS commands or not.
>>>>>>>>>>> For all we know, it could do it at probe time.
>>>>>>>>>>
>>>>>>>>>> pm_runtime is all a bit of a magic black box to me.
>>>>>>>>>>
>>>>>>>>>> We had discussed shifting to using pm_runtime from DCS (and enable)
>>>>>>>>>> calls to power up the PHY on demand, and that's what I implemented.
>>>>>>>>>> However Laurent flagged up that using
>>>>>>>>>> dsi->encoder->crtc->state->adjusted_mode to get the HS clock info
>>>>>>>>>> required to send a HS DCS command from that call is deprecated, so how
>>>>>>>>>> do we specify the clock rate to use at that point?
>>>>>>>>>
>>>>>>>>> I guess the most sensible would be to have a custom bridge state, and
>>>>>>>>> add a pointer to the current bridge state in struct drm_bridge. Then, as
>>>>>>>>> long as you have a bridge pointer you have a way to get the current
>>>>>>>>> state associated to it, and since we already have atomic_duplicate_state
>>>>>>>>> / atomic_destroy_state we can create our own structure around it storing
>>>>>>>>> whatever we want.
>>>>>>>>
>>>>>>>> That's a good point. It would only be needed if we use runtime PM to
>>>>>>>> work around the initialization sequence issue, not if we implement a DSI
>>>>>>>> host clock enable/disable operation, right ?
>>>>>>>
>>>>>>> Obviously this only works with atomic bridges otherwise you have no
>>>>>>> state, but I assume all bridges should be heading that route now.
>>>>>>>
>>>>>>>>>>>>>> host_transfer calls can supposedly be made at any time, however unless
>>>>>>>>>>>>>> MIPI_DSI_MSG_USE_LPM is set in the message then we're meant to send it
>>>>>>>>>>>>>> in high speed mode. If this is before a mode has been set, what
>>>>>>>>>>>>>> defines the link frequency parameters at this point? Adopting a random
>>>>>>>>>>>>>> default sounds like a good way to get undefined behaviour.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> DSI burst mode needs to set the DSI link frequency independently of
>>>>>>>>>>>>>> the display mode. How is that meant to be configured? I would have
>>>>>>>>>>>>>> expected it to come from DT due to link frequency often being chosen
>>>>>>>>>>>>>> based on EMC restrictions, but I don't see such a thing in any
>>>>>>>>>>>>>> binding.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Undefined too. DSI support was added to DRM without any design effort,
>>>>>>>>>>>>> it's more a hack than a real solution. The issue with devices that can
>>>>>>>>>>>>> be controlled over both DSI and I2C is completely unhandled. So far
>>>>>>>>>>>>> nobody has really cared about implementing DSI right as far as I can
>>>>>>>>>>>>> tell.
>>>>>>>>>>>>
>>>>>>>>>>>> :-(
>>>>>>>>>>>>
>>>>>>>>>>>> Thinking aloud, does having the option to set a burst link frequency
>>>>>>>>>>>> from DT (or ACPI) have any issue for other platforms?
>>>>>>>>>>>> Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the various
>>>>>>>>>>>> drivers, all except stm/dw_mipi_dsi-stm.c appear to take it as a "use
>>>>>>>>>>>> all the defined timings, but drop to LP during blanking" option. The
>>>>>>>>>>>> link frequency has therefore remained a property of the
>>>>>>>>>>>> display/bridge.
>>>>>>>>>>>> dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't followed
>>>>>>>>>>>> through the full detail of the parameters it computes from there.
>>>>>>>>>>>
>>>>>>>>>>> I don't see anything wrong with using link-frequency from the DT to
>>>>>>>>>>> setup the burst frequency. It's what v4l2 has been using for a while
>>>>>>>>>>> without any known (to me) drawback, and we're using the same of-graph
>>>>>>>>>>> bindings, so it shouldn't be too controversial there.
>>>>>>>>
>>>>>>>> How would that frequency we picked in practice ? Do panels typically
>>>>>>>> support a range of HS frequencies for DCS HS transfers ?
>>>>>>>
>>>>>>> I was thinking more of bridges where they often run a PLL off the
>>>>>>> incoming DSI clock, and then have a FIFO with potentially different
>>>>>>> clock rates and timings on input and output. SN65DSI83 supports that,
>>>>>>> as do the Toshiba bridge chips I'm currently looking at.
>>>>>>> You need to know the DSI link frequency to configure the PLL
>>>>>>> correctly, but then the bridge output timing is a different matter.
>>>>>>>
>>>>>>>>>> OK, that sounds like a vague plan.
>>>>>>>>>>
>>>>>>>>>>>> DSI and I2C controlled devices is yet another issue that I haven't
>>>>>>>>>>>> even looked at.
>>>>>>>>>>>> I think it's more that vc4 wants to ignore DSI should the DSI host
>>>>>>>>>>>> node be enabled in DT, but there's no panel bound to it. One could say
>>>>>>>>>>>> that is a DT error and tough luck, but from a user's perspective that
>>>>>>>>>>>> is a bit harsh.
>>>>>>>>>>>
>>>>>>>>>>> I guess the larger "issue" is that the tree in the DT is done following
>>>>>>>>>>> the "control" bus, and Linux likes to tie the life cycle of a given
>>>>>>>>>>> device to its parent bus. Both these decisions make sense, but they
>>>>>>>>>>> interact in a weird way in some occurrences (like this one, or Allwinner
>>>>>>>>>>> has an Ethernet PHY controlled through MMIO which end up in the same
>>>>>>>>>>> case).
>>>>>>>>>>>
>>>>>>>>>>> I wonder if using device links here could help though.
>>>>>>>>>>
>>>>>>>>>> I really don't know about that one.
>>>>>>>>>
>>>>>>>>> It's a piece of infrastructure that was created at first (I think?) to
>>>>>>>>> model the power dependency between devices that don't have a parent /
>>>>>>>>> child relationship. For example, if you use DMA, you probably want to
>>>>>>>>> keep the IOMMU powered as long as you are, but it is in a completely
>>>>>>>>> separate branch of the "device tree" (not one from the DTB, the one that
>>>>>>>>> linux DM creates).
>>>>>>>>>
>>>>>>>>> It was later expanded to also cover probe order and make sure a supplier
>>>>>>>>> would probe before its consumer, effectively making EPROBE_DEFER
>>>>>>>>> obsolete.
>>>>>>>>>
>>>>>>>>> The second part is still fairly new, but I think we can solve this by
>>>>>>>>> adding a device link between the DSI host and whatever is at the end of
>>>>>>>>> the OF-Graph endpoint.
>>>>>>>>>
>>>>>>>>>>>>>> As a follow on, bridge devices can support burst mode (eg TI's
>>>>>>>>>>>>>> SN65DSI83 that's just been merged), so it needs to know the desired
>>>>>>>>>>>>>> panel timings for the output side of the bridge, but the DSI link
>>>>>>>>>>>>>> timings to set up the bridge's PLL. What's the correct way for
>>>>>>>>>>>>>> signalling that? drm_crtc_state->adjusted_mode vs
>>>>>>>>>>>>>> drm_crtc_state->mode? Except mode is userspace's request, not what has
>>>>>>>>>>>>>> been validated/updated by the panel/bridge.
>>>>>>>>>>>>>
>>>>>>>>>>>>> adjusted_mode is also a bit of a hack, it solves very specific issues,
>>>>>>>>>>>>> and its design assumes a single encoder in the chain with no extra
>>>>>>>>>>>>> bridges. We should instead add modes to the bridge state, and negotiate
>>>>>>>>>>>>> modes along the pipeline the same way we negotiate formats.
>>>>>>>>>>>>
>>>>>>>>>>>> So as I understand it we already have format negotiation between
>>>>>>>>>>>> bridges via atomic_get_output_bus_fmts and atomic_get_input_bus_fmts,
>>>>>>>>>>>> so is it possible to extend that to modes?
>>>>>>>>>>>> Are you thinking bridge state that is owned by the framework, or by
>>>>>>>>>>>> the individual bridge drivers?
>>>>>>>>>>>
>>>>>>>>>>> atomic_check is made for that. I guess we could improve its call
>>>>>>>>>>> sequence to each time a mode is modified along the bridge list we
>>>>>>>>>>> restart the sequence until all components agree (or reject it entirely
>>>>>>>>>>> if they can't), but I don't really see why we would need yet another
>>>>>>>>>>> hook.
>>>>>>>>
>>>>>>>> Isn't this what atomic_get_output_bus_fmts() and
>>>>>>>> atomic_get_input_bus_fmts() implement ?
>>>>>>>
>>>>>>> Those negotiate a single u32 bus format between nodes, not a complete timing.
>>>>>>>
>>>>>>>>>> Why do all nodes in the bridge list need to agree? Adjacent nodes need
>>>>>>>>>> to agree, but they then also need to retain that agreed timing
>>>>>>>>>> somewhere.
>>>>>>>>>
>>>>>>>>> We might have mutually exclusive requirements though? Let's use the
>>>>>>>>> example of the VC4 HDMI driver that can't have odd horizontal timings,
>>>>>>>>> and assume it's a constraint of our DSI driver instead.
>>>>>>>>>
>>>>>>>>> Then, we have a DSI->LVDS bridge, a LVDS->RGB bridge and a panel (which
>>>>>>>>> is a bit ridiculous, but whatever). If the LVDS->RGB bridge can't have
>>>>>>>>> even horizontal timings, then you just can't display it, even though
>>>>>>>>> they are not adjacent (unless the bridge in the middle can modify the
>>>>>>>>> timings between the input and output, but that's not always possible).
>>>>>>>>>
>>>>>>>>> Similarly, if for the RGB panel we need to increase a bit some timings
>>>>>>>>> to accommodate for a larger pixel clock and end up above what the DSI
>>>>>>>>> host can provide, we're also done.
>>>>>>>>
>>>>>>>> The hard part will be to figure out a good heuristics to perform the
>>>>>>>> negotiation without going back and forth (at least not in a way that
>>>>>>>> would require too many iterations, and certainly avoiding infinite
>>>>>>>> loops). That will be an interesting problem to solve, but maybe we'll be
>>>>>>>> lucky and a simple approach will work for the use cases we have to
>>>>>>>> support today.
>>>>>>>
>>>>>>> One to kick into the long grass possibly then.
>>>>>>>
>>>>>>> For burst mode and bridges retiming things, generally I think it works
>>>>>>> if you consider a single pass through a mode_fixup equivalent starting
>>>>>>> at the panel.
>>>>>>> The panel advertises what it wants.
>>>>>>> The closest bridge (eg DSI83) fixup amends panel's requested mode
>>>>>>> based on any constraints that it has, stores a copy, and passes that
>>>>>>> down the chain.
>>>>>>> The next bridge (eg vc4_dsi) amends the mode based on restrictions it
>>>>>>> has (eg burst mode link frequency). That's the end of the chain in
>>>>>>> this case, so that's the mode that the crtc needs to be programmed
>>>>>>> with.
>>>>>>>
>>>>>>> The modes then need to be passed back up the chain so that eg DSI83
>>>>>>> knows the link frequency in use, and can therefore configure the PLL
>>>>>>> appropriately.
>>>>>>>
>>>>>>> The bit it would be nice to fix is that burst mode has effectively
>>>>>>> increased the horizontal front porch, but that could be fixed within
>>>>>>> the bridge so that the panel gets the timing it requested.
>>>>>>>
>>>>>>>>>> Taking SN65DSI8[3|4|5] as an example, it supports burst mode, and the
>>>>>>>>>> DSI frequency and timings are permitted to be different from that
>>>>>>>>>> which it uses on the LVDS side. The LVDS panel and LVDS side of DSI83
>>>>>>>>>> need to agree over the format, and the DSI host and DSI side of DSI83
>>>>>>>>>> need to agree, but they may be two different timings.
>>>>>>>>>> Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows you to
>>>>>>>>>> configure the LVDS rate compared to the DSI rate (the driver currently
>>>>>>>>>> goes for 1:1), and registers 0x20 to 0x34 allow you to set the number
>>>>>>>>>> of active pixel and blanking on the LVDS side (again currently just
>>>>>>>>>> copied across).
>>>>>>>>>>
>>>>>>>>>> The way I'm seeing burst mode as having been interpreted at present is
>>>>>>>>>> that it's largely just a flag to say "drop to LP mode between lines".
>>>>>>>>>> The timing that needs to be passed to the crtc is therefore going to
>>>>>>>>>> be based on the DSI link rate (converted to pixels) with increased
>>>>>>>>>> blanking periods.
>>>>>>>>>>
>>>>>>>>>> I guess there are similarities with Media Controller and V4L2 here. A
>>>>>>>>>> typical chain there could be:
>>>>>>>>>>      sensor -> scaler -> crop -> CSI-2 receiver.
>>>>>>>>>> The format on each of those links may be different, but the chain as a
>>>>>>>>>> whole needs to be valid. Media Controller largely relies on userspace
>>>>>>>>>> to configure all links, but with a DRM chain that isn't really an
>>>>>>>>>> option as it's expected that the display chain configures itself.
>>>>>>>>>
>>>>>>>>> Also, the userspace has no concept of media sub-devices in DRM, so it
>>>>>>>>> just sets the mode on the whole DRM/KMS device, unlike what v4l2 does.
>>>>>>>>> In v4l2, afaik, if you ended up with the above scenarios it would just
>>>>>>>>> be rejected when you set the format on the link, letting the userspace
>>>>>>>>> figure it out. We can't really do that here
>>>>>>>>
>>>>>>>> I wonder how long we'll be able to keep userspace out of the picture to
>>>>>>>> configure the internals of the pipeline. I don't want to be the first
>>>>>>>> person who will have a use case that requires this.
>>>>>>>
>>>>>>> I suspect none of us want to be the first one to hit this scenario!
>>>>>>>
>>>>>>> As I've just posted on the other thread about SN65DSI83, I've hit a
>>>>>>> major stumbling block with the current design where vc4_dsi (and
>>>>>>> Exynos) breaks the bridge_chain so that it gets called first and can
>>>>>>> do initialisation, and then it calls down the chain.
>>>>>>> Having now converted to a DSI bridge driver, it works fine with
>>>>>>> connected non-atomic bridges, but fails with atomic bridges.
>>>>>>> drm_atomic_add_encoder_bridges adds the state for all the bridges the
>>>>>>> framework is aware of. With the split chain it misses adding the state
>>>>>>> of our "hidden" bridges, and we can't add the extra state from our
>>>>>>> atomic_duplicate_state call as we don't have the state to add to (we
>>>>>>> just return our state to be added).
>>>>>>> This bumps up the priority for us of finding a suitable solution for
>>>>>>> this initialisation issue, so I'll start looking at how feasible a new
>>>>>>> DSI host function is.
>>>>>>>
>>>>>>> Thanks
>>>>>>>       Dave
>>>>>>>
>>>>>>
>>>>
>>

Re: Questions over DSI within DRM.

[Andrzej Hajda]

On 12.10.2021 00:13, Andrzej Hajda wrote:
> On 08.10.2021 19:33, Dave Stevenson wrote:
>> On Thu, 7 Oct 2021 at 21:19, Andrzej Hajda <andrzej.hajda@gmail.com> 
>> wrote:
>>>
>>> On 07.10.2021 13:07, Dave Stevenson wrote:
>>>> On Tue, 5 Oct 2021 at 22:03, Andrzej Hajda <andrzej.hajda@gmail.com> 
>>>> wrote:
>>>>>
>>>>> On 05.10.2021 17:32, Dave Stevenson wrote:
>>>>>> Hi Andrzej
>>>>>>
>>>>>> Thanks for joining in the discussion.
>>>>>>
>>>>>> On Tue, 5 Oct 2021 at 16:08, Andrzej Hajda 
>>>>>> <andrzej.hajda@gmail.com> wrote:
>>>>>>>
>>>>>>> On 05.10.2021 13:23, Dave Stevenson wrote:
>>>>>>>> Hi Laurent
>>>>>>>>
>>>>>>>> On Sun, 3 Oct 2021 at 15:16, Laurent Pinchart
>>>>>>>> <laurent.pinchart@ideasonboard.com> wrote:
>>>>>>>>>
>>>>>>>>> Hello,
>>>>>>>>>
>>>>>>>>> Reviving a bit of an old thread.
>>>>>>>>
>>>>>>>> I'd been looking at reviving this conversation too as I've moved
>>>>>>>> further on with DSI on the Pi, and converting from an encoder to a
>>>>>>>> bridge
>>>>>>>>
>>>>>>>>> On Thu, Jul 15, 2021 at 11:50:22AM +0200, Maxime Ripard wrote:
>>>>>>>>>> On Tue, Jul 06, 2021 at 05:44:58PM +0100, Dave Stevenson wrote:
>>>>>>>>>>> On Tue, 6 Jul 2021 at 16:13, Maxime Ripard wrote:
>>>>>>>>>>>>>>> On a similar theme, some devices want the clock lane in 
>>>>>>>>>>>>>>> HS mode early
>>>>>>>>>>>>>>> so they can use it in place of an external oscillator, 
>>>>>>>>>>>>>>> but the data
>>>>>>>>>>>>>>> lanes still in LP-11. There appears to be no way for the
>>>>>>>>>>>>>>> display/bridge to signal this requirement or it be achieved.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> You're right. A loooong time ago, the omapdrm driver had 
>>>>>>>>>>>>>> an internal
>>>>>>>>>>>>>> infrastructure that didn't use drm_bridge or drm_panel and 
>>>>>>>>>>>>>> instead
>>>>>>>>>>>>>> required omapdrm-specific drivers for those components. It 
>>>>>>>>>>>>>> used to model
>>>>>>>>>>>>>> the display pipeline in a different way than drm_bridge, 
>>>>>>>>>>>>>> with the sync
>>>>>>>>>>>>>> explicitly setting the source state. A DSI sink could thus 
>>>>>>>>>>>>>> control its
>>>>>>>>>>>>>> enable sequence, interleaving programming of the sink with 
>>>>>>>>>>>>>> control of
>>>>>>>>>>>>>> the source.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Migrating omapdrm to the drm_bridge model took a really 
>>>>>>>>>>>>>> large effort,
>>>>>>>>>>>>>> which makes me believe that transitioning the whole 
>>>>>>>>>>>>>> subsystem to
>>>>>>>>>>>>>> sink-controlled sources would be close to impossible. We 
>>>>>>>>>>>>>> could add
>>>>>>>>>>>>>> DSI-specific operations, or add another enable bridge 
>>>>>>>>>>>>>> operation
>>>>>>>>>>>>>> (post_pre_enable ? :-D). Neither would scale, but it may 
>>>>>>>>>>>>>> be enough.
>>>>>>>>>>>>>
>>>>>>>>>>>>> I haven't thought it through for all generic cases, but I 
>>>>>>>>>>>>> suspect it's
>>>>>>>>>>>>> more a pre_pre_enable that is needed to initialise the PHY 
>>>>>>>>>>>>> etc,
>>>>>>>>>>>>> probably from source to sink.
>>>>>>>>>
>>>>>>>>> I believe it could be implemented as a pre-pre-enable indeed. 
>>>>>>>>> It feels
>>>>>>>>> like a bit of a hack, as the next time we need more 
>>>>>>>>> fine-grained control
>>>>>>>>> over the startup sequence, we'll have to add a 
>>>>>>>>> pre-pre-pre-enable. Given
>>>>>>>>> that the startup sequence requirements come from the sink 
>>>>>>>>> device, it
>>>>>>>>> would make sense to let it explicitly control the initialization,
>>>>>>>>> instead of driving it from the source. I don't think we'll be 
>>>>>>>>> able to
>>>>>>>>> rework the bridge API in that direction though, so I'm fine 
>>>>>>>>> with a hack.
>>>>>>>>
>>>>>>>> There are pros and cons to both approaches.
>>>>>>>> You're in a much better place to make that sort of call than I 
>>>>>>>> am, so
>>>>>>>> I'll take your advice.
>>>>>>>>
>>>>>>>> Implementing a DSI host op function may mean an update to a 
>>>>>>>> number of
>>>>>>>> existing DSI host drivers,
>>>>>>>
>>>>>>> Why? You just add new op to mipi_dsi_host_ops and create appropriate
>>>>>>> helper, which in case of NULL will perform default action, either:
>>>>>>> - return -ENOSYS,
>>>>>>> - try to emulate by calling 
>>>>>>> mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET)
>>>>>>>
>>>>>>> The latter is just wild guess, but I suspect it could work (even 
>>>>>>> now) as
>>>>>>> an alternate way of entering into stop state.
>>>>>>
>>>>>> If we're looking at fixing bridges to use the new DSI state call and
>>>>>> then initialise in "pre_enable" (ie before video is started), then
>>>>>> making that change to the bridge will require support for the DSI
>>>>>> state call or risk regressions.
>>>>>
>>>>> I think there is misunderstanding somewhere, I do not know where, 
>>>>> yet :)
>>>>> My idea is as follow:
>>>>> 1. Add dsi_host.set_state() op to dsi host accompanied with helper, 
>>>>> lets
>>>>> call it mipi_dsi_host_set_state for now.
>>>>> 2. Implement dsi_host.set_state in DSI host you are working with.
>>>>> 3. In body of your bridge pre_enable you call 
>>>>> mipi_dsi_host_set_state in
>>>>> proper place.
>>>>>
>>>>> 1 and 2 do not introduce any regression.
>>>>> 3 can introduce regression only if your bridge is used with another 
>>>>> dsi
>>>>> host but only on such platforms.
>>>>>
>>>>> Why do you think we need to 'fix' it everywhere?
>>>>
>>>> I'm looking at sn65dsi83 which is already working against other
>>>> platforms (I don't know which). If it is fixed to request the DSI
>>>> state and initialise in pre_enable, then those platforms need updating
>>>> too.
>>>> If mipi_dsi_device_transfer(MIPI_DSI_NULL_PACKET) can be used to wake
>>>> the host up then that would be great, but I don't have access to those
>>>> platforms to test.
>>>>
>>>>>> Some may have been getting away with it by making the initialisation
>>>>>> call in "enable" whilst video is active - that sounds like the issue
>>>>>> Marek was trying to log in his patch to SN65DSI83.
>>>>>>
>>>>>>>> but it would be cleaner. It's also what
>>>>>>>> Andrzej has suggested.
>>>>>>>>
>>>>>>>> Thinking it through, a function that requests clock lane 
>>>>>>>> frequency and
>>>>>>>> state (ULPS or LP-11 predominantly), and data lane state (again 
>>>>>>>> ULPS
>>>>>>>> or LP-11) should allow the required behaviour for most of the 
>>>>>>>> bridges
>>>>>>>> I'm aware of. Most want either LP-11, or HS clock at a known
>>>>>>>> frequency.
>>>>>>>
>>>>>>> LP-11 is quite reasonable, but regarding frequency I am not sure who
>>>>>>> should manage it? DSI device datasheets known to me usually 
>>>>>>> mentioned
>>>>>>> max frequency, and it is covered by mipi_dsi_device.hs_rate. Min
>>>>>>> frequency is usually determined by amount of data we need to 
>>>>>>> transfer
>>>>>>> per second.
>>>>>>> Do you need to set frequency from device, or just to know actual 
>>>>>>> frequency?
>>>>>>> In former case (if it is really necessary !!!) you should request
>>>>>>> frequency range anyway, as host usually is able to set only discrete
>>>>>>> number of frequencies. But it need to be clearly specified how 
>>>>>>> and when
>>>>>>> this op can be used - more ops, more questions about interaction 
>>>>>>> between
>>>>>>> them.
>>>>>>> In latter case another mipi host op should be quite easy to 
>>>>>>> implement.
>>>>>>
>>>>>> If a burst mode frequency is specified in struct mipi_dsi_device
>>>>>> hs_rate then there is no issue.
>>>>>
>>>>> hs_rate should be called rather max_hs_rate as it describes only top
>>>>> limit of HS clock, decision which clock to choose is on DSI host side.
>>>>
>>>> Indeed, I've just realised that one.
>>>>
>>>>>> If not, then you'll generally want to use the pixel clock,
>>>>>
>>>>> Probably not, often we want to use rate higher than pixel clock
>>>>> (time-compression in DSI spec), to leave time gaps for LP mode.
>>>>
>>>> Only if MIPI_DSI_MODE_VIDEO_BURST is set in the mode_flags, otherwise
>>>> you should be running at the pixel clock. (Slight assumption as there
>>>> is no definition of what MIPI_DSI_MODE_VIDEO_BURST really means in the
>>>> documentation).
>>>
>>> Specification describes it clearly [1], grep for 'burst'.
>>>
>>> [1]: https://hifpga.com/upfiles/15552304546301142.pdf
>>
>> The MIPI DSI spec defines the concept of burst, it doesn't define the
>> Linux kernel implementation of it.
>>
>> In a Linux DSI host driver:
>> - How should a DSI host driver choose the link frequency to enable
>> "blocks of pixel data can be transferred in a shorter time"? hs_rate
>> defines the max rate, and the pixel clock the minimum. Pick any number
>> in between?
>> - How does a DSI device driver get told the link frequency? Many
>> bridge chips run their PLLs off the DSI HS clock, so need to know how
>> to convert that into a desired pixel clock.
>> - 8.11.4 "Following HS pixel data transmission, the bus may stay in HS
>> Mode for sending blanking packets or go to Low Power Mode, during
>> which it may remain idle, i.e. the host processor remains in LP-11
>> state, or LP transmission may take place in either direction. ".
>> Option in the DSI spec. How is that mode chosen by the DSI device
>> driver for the host driver to implement?
>> - 8.11.1 "To enable PHY synchronization the host processor should
>> periodically end HS transmission and drive the Data Lanes to the LP
>> state. This transition should take place at least once per frame;
>> shown as LPM in the figures in this section." Another option within
>> the DSI spec as to whether it is once per frame or more frequently.
>> How does a DSI device driver signal which mode is required to the DSI
>> host driver? (And I've just had a device that is fussy about this
>> one).
> 
> I think all these questions are rather to hw platform team, if they 
> confirm that hw works well with specific clock and other hw settings 
> developer should obey it. This is probably why almost nobody is using 
> for example hs_rate constraint - developer receives straightforward 
> instruction with fixed set of parameters (or just set of magic numbers 
> without explanation) to program on both DSI host and device - no place 
> for hesitation :)
> As I understand your bridge is re-used in multiple configurations thus 
> you need more intelligence in the driver. Am I right? I think the best 
> would be to just start implementing missing bits.
> 
>>
>> I know we all hate writing documentation, but saying the parent
>> specification defines how it should be implemented on a system isn't
>> true, and developing/updating a DSI driver results in these sorts of
>> questions.
>>
>>>>>> and the DSI
>>>>>> host hasn't necessarily been told the video mode at the point the
>>>>>> bridge/panel requests a new DSI state.
>>>>>
>>>>> pre_enable and enable ops are called after modeset, so video mode 
>>>>> is known.
>>>>
>>>> Thanks, I wasn't aware of that. It didn't help that our DSI driver
>>>> didn't implement a mode_set function as it did all the work in
>>>> pre_enable having looked back at the crtc for the mode.
>>>>
>>>> It looks like there is a slight quirk that there is an encoder
>>>> atomic_mode_set, but not a bridge one. If we're implementing the
>>>> atomic API is it correct to update our bridge's internal state (ie the
>>>> hs_rate) from a (atomic_)mode_set? That's a different discussion
>>>> though.
>>>
>>> It shouldn't matter, the mode should be stored in atomic state. In
>>> pre-enable you should have all required info: videomode and constraints.
>>
>> Sorry, perhaps I didn't describe my scenario well enough.
>> Atomic DSI host bridge driver. Atomic DSI bridge/panel driver.
>>
>> mode_set gets called on all nodes.
>> pre_enables are called starting at the furthest end from the encoder,
>> so the bridge/panel driver pre_enable gets called before the DSI host
>> pre_enable. The bridge/panel wants to power up the DSI host using the
>> new API. The DSI host bridge hasn't had the pre_enable called yet, and
>> currently we're not proposing an atomic_set_state. At that point, what
>> parameters should the DSI host driver be using to determine the link
>> frequency as it doesn't have a state?
> 
> First of all it has access to device's hs_rate, so it can use it.
> But it has also access to crtc state at bridge.encoder->crtc->state. 
> Which should be already set by mode_set - any pre_enable is called after 
> mode_set.
> 
>>
>> Perhaps we should be proposing an atomic_set_state which takes a
>> struct drm_atomic_state *. How would that work if called from a
>> non-atomic bridge driver though?
>>
>>>>
>>>>>>
>>>>>>>> Giving the option for setting back into ULPS also allows
>>>>>>>> for power saving/standby mechanisms should the need arise.
>>>>>>>
>>>>>>> Another op :)
>>>>>>
>>>>>> Does it need to be, or can it be one op as
>>>>>> enum mipi_dsi_lane_state{
>>>>>>     DSI_ULPS,
>>>>>>     DSI_LP11,
>>>>>>     DSI_HS,
>>>>>> };
>>>>>>
>>>>>> int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
>>>>>>           enum mipi_dsi_lane_state clock_lane_state,
>>>>>>           u64 clock_lane_freq,
>>>>>>           enum mipi_dsi_lane_state data_lane_state)
>>>>>
>>>>> I still do not know why do you want to set freq explicitly, ie why DSI
>>>>> host could not choose it, based on bridge constraints, DSI host
>>>>> constraints and requested video mode.
>>>>
>>>> I'm partly trying to piece together what information is already around
>>>> in order to be able to make a good decision on the frequency.
>>>> If we have (max_)hs_rate defined and a mode, then we can make a
>>>> decision within the DSI host driver.
>>>>
>>>> Adding a link-frequencies DT property would allow the system
>>>> configuration to be defined rather than some random heuristics.
>>>
>>> Exynos DSI used samsung,burst-clock-frequency property, I am not sure of
>>> others, but it was rather copy/paste solution from vendor code. Maybe it
>>> is OK this way.
>>
>> Just for interest's sake I had a quick look for how other drivers 
>> configure it.
>> hs_rate is only used by ST-Ericsson MCDE and OMAP.
>> iMX8 nwl-dsi, Synopsys dw-mipi-dsi, hisilicon dw_drm_dsi, mtk_dsi, and
>> msm/dsi_host all set magic flags in the hardware based on
>> MIPI_DSI_MODE_VIDEO_BURST, and appear to have no reporting of the
>> frequency used.
>> Exynos appears to require vendor-specific DT property
>> "samsung,burst-clock-frequency".
>>
>> It would be nice to have a standardised and documented approach.
>>
>> It's obviously not valid for me to reuse a Samsung specific property
>> on a non-Samsung platform, and duplicating it as a different
>> vendor-specific property would imply that it shouldn't be
>> vendor-specific.
>>  From V4L2 there is already the defined link-frequencies DT property to
>> set CSI2 HS rates, so it would seem reasonable for that to be used for
>> DSI as well.
> 
> Sounds OK.
> 
>>
>>>>
>>>> But otherwise I do now agree with you that we probably don't need an
>>>> explicit frequency from the bridge/panel driver.
>>>>
>>>> There is still the situation documented in [1] that the transfer
>>>> function "can be called no matter the state the host is in", so that
>>>> would include as part of attach when we don't have a video mode. Any
>>>> transfer without the MIPI_DSI_MSG_USE_LPM flag is then at an undefined
>>>> HS speed. There are likely very few cases where that matters, so a
>>>> WARN from the DSI host driver is probably sufficient for now.
>>>
>>> But we have already constraints from both players - host and device, so
>>> it should not be an issue to use max allowed speed.
>>
>> For devices that set MIPI_DSI_MODE_VIDEO_BURST we can if hs_rate is
>> set or we otherwise assume 500MHz.
> 
> Maybe better would be fix drivers to provide real limit.
> 
> 
>> This is an issue for devices without MIPI_DSI_MODE_VIDEO_BURST set
>> where they are expecting their pixel rate. Under these conditions we
>> don't know what that pixel rate
>>>>
>>>> [1] 
>>>> https://www.kernel.org/doc/html/latest/gpu/drm-kms-helpers.html?highlight=dsi#c.mipi_dsi_host_ops 
>>>>
>>>>
>>>>> Regarding different modes for clock and data lanes there exists DSI 
>>>>> mode
>>>>> flag MIPI_DSI_CLOCK_NON_CONTINUOUS which determines if host in LP-11
>>>>> data lanes state can also put clock lane into LP-11. Is there need for
>>>>> sth more?
>>>>> I'd like to avoid possibility of nonsense(?) configurations like clock
>>>>> in LP-11 and data lanes in HS.
>>>>
>>>> Having had a first pass at implementing this, then I certainly agree
>>>> with altering the potential states I'd previously suggested.
>>>>       DSI_STANDBY,
>>>
>>> Specification uses LP11 or STOP state to describe LP-11, so I would
>>> prefer to stick to one of it.
>>
>> Sorry, I was meaning standby as in power off the PHY. DSI_OFF is
>> probably more accurate.
>> An LP-11 / STOP state would be DSI_ACTIVE.
>>
>>>>       DSI_ULPS,
>>>
>>> I am not sure but I hope this is LP-00.
>>
>> It is, but also it needs the relevant escape sequence to enter/exit
>> it. You can't simply just switch to LP-00.
>> I'm looking more at link states than physical states.
>>
>>>>       DSI_ACTIVE,
>>>
>>> I am not sure about this one, this would mean start video transmission
>>> in HS, as there is no HS stop state. It does not look like desirable in
>>> pre-enable state. 'HS idle' state is just LP-11.
>>>
>>>> Seems more sensible now
>>>> For the clock lane, as you say, MIPI_DSI_CLOCK_NON_CONTINUOUS
>>>> determines whether DSI_ACTIVE means LP-11 or HS.
>>>> The data lanes follow the clock lane in state, so DSI_ACTIVE means
>>>> LP-11, sending LP commands, or HS if video is actively being
>>>> transmitted.
>>>
>>> I think you are messing up in above sentence :)
>>> For definition look for 'continuous clock' phrase in spec above [1].
>>
>> Not sure I agree with you based on my clarification of what I meant by
>> DSI_STANDBY.
>>
>>>> Having just DSI_ACTIVE also avoids the nonsense you rightly raise of
>>>> LP clock but HS data.
>>>>
>>>> So it can be condensed down to a single state.
>>>> int mipi_dsi_set_state(struct mipi_dsi_device *dsi,
>>>>           enum mipi_dsi_lane_state state);
>>>> Does that seem reasonable?
>>>
>>> Yes, but if we stay with two states I wonder if it wouldn't be better to
>>> use two separate ops, just matter of taste.
>>
>> Well there is the question of whether ULPS is actually useful or not
>> over powering down the PHY, but by making it a single op taking an
>> enum means that even if we ignore it now then it could be added.


It could be added as separate ops as well, no difference.
The only difference is in style IMO. If you provide 'muxing' op (ie with 
parameter 'state'), which will be always demuxed in implementation, it 
will be redundant:
int set_state(state) {
	switch(enum mipi_dsi_lane_state state) {
	case state1: return set_state1();
	case state2: return set_state2();
	default: return ENO...;
}

More straightforward would be provide separate ops.


>> I am aware of DSI devices that will switch to standby when they see a
>> ULPS entry sequence, so being able to request that from disable would
>> be reasonable.

The main question is if we really need ulps. If yes the next question is 
where do you want to use it? In disabled state? Ie before pre-enable, 
and after post-disable? Or maybe in command mode if there is only needed 
partial display update, between long idles?
In both cases the question is who should decide about entering it, my 
1st intuition is that should be rather host - no need for ops.
And entering ULPS on DSI interface should be probable part of putting 
whole pipeline (not only DSI) into some sleep state. Too many question 
to answer without real world example.

Regards
Andrzej

>>
>> If I step back and asked you for a proposal of an API that allows a
>> DSI device to ask the bridge to enable itself based on the existing
>> flags and parameters, what would you have it look like
> 
>>
>>>>
>>>> I guess the other thing to do is document what is meant by each flag.
>>>> Currently the documentation is rather lacking in the intended
>>>> behaviour, which is where I started with this thread.
>>>
>>> Yes, at least in case spec is not clear about it.
>>
>> Again, the spec does not determine the implementation details under a
>> specific operating system.
>>
>> I suspect I'm in the minority where we have a relatively open platform
>> and more and more random non-mainline kernel developers are coming
>> along and wanting to add this or that DSI device. If it isn't
>> documented somewhere as to how to configure this stuff then they go
>> down the totally wrong path, and both the platform and the DRM
>> subsystem look bad.
> 
> This is classic issue - it is hard to write framework/docs for things 
> you cannot test/implement. It is left for devs who need it :)
> 
> 
>> When you have drivers merged into the mainline kernel such as [1]
>> which set mode_flags to just MIPI_DSI_MODE_VIDEO_BURST (presumably
>> command mode but higher link frequency?), or [2] which sets just
>> MIPI_DSI_MODE_VIDEO_SYNC_PULSE (command mode with sync pulses?), it
>> would appear that it isn't clear to experienced developers who are
>> submitting DSI device drivers as to what these flags mean, nor which
>> are valid in various combinations. If they get it wrong, nor is it
>> picked up during review, then relative newbies have no hope.
>>
>> Laurent acknowledged at the start of this thread that a lot of stuff
>> isn't defined. I (or others working on our behalf) can put the work
>> into document it, but that documentation needs to be correct.
>>
>> Thanks
>>    Dave
>>
>> [1] 
>> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/panel/panel-feiyang-fy07024di26a30d.c#L226 
>>
>> I can't find a datasheet for this one, but it seems unlikely.
> 
> http://files.pine64.org/doc/datasheet/pine64/FY07024DI26A30-D_feiyang_LCD_panel.pdf 
> 
> 
> 
> Regards
> Andrzej
> 
> 
>> [2] 
>> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/panel/panel-ilitek-ili9881c.c#L673 
>>
>> I'm fairly certain that ILI9881 doesn't support command mode.
>>
>>> Regards
>>> Andrzej
>>>
>>>
>>>>
>>>> Thanks.
>>>>     Dave
>>>>
>>>>> Regards
>>>>> Andrzej
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>>      Dave
>>>>>>
>>>>>>> Regards
>>>>>>> Andrzej
>>>>>>>
>>>>>>>
>>>>>>>> Does it need a way to pass back the actual DSI frequency being 
>>>>>>>> used,
>>>>>>>> in a similar vein to mode_fixup? That allows for the bridge to 
>>>>>>>> request
>>>>>>>> the display clock, but the burst mode link frequency to be returned
>>>>>>>> (I'm assuming that's a property of the DSI host only, and not the
>>>>>>>> bridge).
>>>>>>>>
>>>>>>>> I'm having a discussion with someone who wants to run SN65DSI85 
>>>>>>>> in the
>>>>>>>> two independent LVDS display mode. That requires a DSI HS clock on
>>>>>>>> DSI-A even if only panel B is active, so with this extra 
>>>>>>>> function that
>>>>>>>> would be achievable as well.
>>>>>>>>
>>>>>>>> Thoughts?
>>>>>>>>
>>>>>>>>>>>>> If the panel/bridge can set a flag that can be checked at 
>>>>>>>>>>>>> this point
>>>>>>>>>>>>> for whether an early clock is required or not, I think that 
>>>>>>>>>>>>> allows us
>>>>>>>>>>>>> to comply with the requirements for a large number of 
>>>>>>>>>>>>> panels/bridges
>>>>>>>>>>>>> (LP-11 vs HS config for clock and or data lanes before 
>>>>>>>>>>>>> pre_enable is
>>>>>>>>>>>>> called).
>>>>>>>>>>>>>
>>>>>>>>>>>>> pre_enable retains the current behaviour (initialise the 
>>>>>>>>>>>>> chain from
>>>>>>>>>>>>> sink to source).
>>>>>>>>>>>>> enable then actually starts sending video and enabling 
>>>>>>>>>>>>> outputs.
>>>>>>>>>>>>
>>>>>>>>>>>> Flags indeed seem like a more contained option. Another one 
>>>>>>>>>>>> could be to
>>>>>>>>>>>> have a mipi_dsi_host to (for example) power up the clock 
>>>>>>>>>>>> lane that would
>>>>>>>>>>>> be called by default before the bridge's enable, or at the 
>>>>>>>>>>>> downstream
>>>>>>>>>>>> bridge driver discretion before that.
>>>>>>>>>>>
>>>>>>>>>>> Which driver will that call?
>>>>>>>>>>
>>>>>>>>>> The parent DSI Host
>>>>>>>>>>
>>>>>>>>>>> An extreme example perhaps, but Toshiba make the TC358860 eDP 
>>>>>>>>>>> to DSI
>>>>>>>>>>> bridge chip[1]. So the encoder will be eDP, but the DSI 
>>>>>>>>>>> config needs
>>>>>>>>>>> to go to that bridge. Does that happen automatically within the
>>>>>>>>>>> framework? I guess so as the bridge will have called
>>>>>>>>>>> mipi_dsi_host_register for the DSI sink to attach to.
>>>>>>>>>>
>>>>>>>>>> In that case, whatever sink would be connected to the bridge 
>>>>>>>>>> would call
>>>>>>>>>> the bridge clock enable hook if it needs it in its pre_enable, 
>>>>>>>>>> or it
>>>>>>>>>> would be called automatically before enable if it doesn't
>>>>>>>>>>
>>>>>>>>>> Would that help?
>>>>>>>>>
>>>>>>>>> Sounds good to me, in theory at least (let's see what issues 
>>>>>>>>> we'll run
>>>>>>>>> into in practice :-)).
>>>>>>>>>
>>>>>>>>> Has anyone given it a try, or is planning to ?
>>>>>>>>>
>>>>>>>>>>> Perhaps a new mipi_dsi_host function to configure the PHY is the
>>>>>>>>>>> easier solution. If it can allow the sink to request whatever
>>>>>>>>>>> combination of states from clock and data lanes that it 
>>>>>>>>>>> fancies, then
>>>>>>>>>>> it can be as explicit as required for the initialisation 
>>>>>>>>>>> sequence, and
>>>>>>>>>>> the host driver does its best to comply with the requests.
>>>>>>>>>>
>>>>>>>>>> I don't know, I'm not really fond in general of solutions that 
>>>>>>>>>> try to
>>>>>>>>>> cover any single case if we don't need it and / or have today 
>>>>>>>>>> an issue
>>>>>>>>>> with this. I'd rather have something that works for the 
>>>>>>>>>> particular
>>>>>>>>>> bridge we were discussing, see if it applies to other bridges 
>>>>>>>>>> and modify
>>>>>>>>>> it if it doesn't until it works for all our cases. Trying to 
>>>>>>>>>> reason in
>>>>>>>>>> all possible cases tend to lead to solutions that are 
>>>>>>>>>> difficult to
>>>>>>>>>> maintain and usually over-engineered.
>>>>>>>>>
>>>>>>>>> A DSI host clock enable operation or a DSI host PHY configuration
>>>>>>>>> operation both fit in the same place in the grand scheme of 
>>>>>>>>> things, so I
>>>>>>>>> don't mind either. We should be able to easily move from a more 
>>>>>>>>> specific
>>>>>>>>> operation to a more generic one if the need arises.
>>>>>>>>>
>>>>>>>>>>> I'd have a slight query over when and how the host would drop 
>>>>>>>>>>> to ULPS
>>>>>>>>>>> or power off. It probably shouldn't be in post_disable as the 
>>>>>>>>>>> sink
>>>>>>>>>>> hasn't had a chance to finalise everything in its post_disable.
>>>>>>>>>>>
>>>>>>>>>>> Perhaps pm_runtime with autosuspend is the right call there?
>>>>>>>>>>
>>>>>>>>>> pm_runtime semantics mean that once the device is suspended, 
>>>>>>>>>> its power
>>>>>>>>>> domains, regulators, clocks, etc. are all shut down, so it 
>>>>>>>>>> doesn't
>>>>>>>>>> really fit the low power state expected by DSI
>>>>>>>>>>
>>>>>>>>>>> [1] 
>>>>>>>>>>> https://toshiba.semicon-storage.com/ap-en/semiconductor/product/interface-bridge-ics-for-mobile-peripheral-devices/display-interface-bridge-ics/detail.TC358860XBG.html 
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>>> When I discussed this briefly with Maxime there was a 
>>>>>>>>>>>>> suggestion of
>>>>>>>>>>>>> using pm_runtime to be able to power up the pipeline as a 
>>>>>>>>>>>>> whole. If
>>>>>>>>>>>>> the bridge driver can use pm_runtime to power up the PHY when
>>>>>>>>>>>>> required, then that may solve the issue, however I know too 
>>>>>>>>>>>>> little of
>>>>>>>>>>>>> the details to say whether that is actually practical.
>>>>>>>>>>>>
>>>>>>>>>>>> I'm not sure it was about this topic in particular. If I 
>>>>>>>>>>>> remember well
>>>>>>>>>>>> our discussion, this was about the vc4 driver that tries to 
>>>>>>>>>>>> circumvent
>>>>>>>>>>>> the framework and call the pre_enable and enable hooks 
>>>>>>>>>>>> itself because it
>>>>>>>>>>>> wasn't properly powered before and thus any DCS-related call 
>>>>>>>>>>>> by the
>>>>>>>>>>>> downstream bridge or panel would end up creating a CPU stall.
>>>>>>>>>>>>
>>>>>>>>>>>> I suggested to use runtime_pm in the DCS related calls to 
>>>>>>>>>>>> make sure the
>>>>>>>>>>>> device is powered because there's no relation between the 
>>>>>>>>>>>> state of the
>>>>>>>>>>>> downstream bridge or panel and whether it can send DCS 
>>>>>>>>>>>> commands or not.
>>>>>>>>>>>> For all we know, it could do it at probe time.
>>>>>>>>>>>
>>>>>>>>>>> pm_runtime is all a bit of a magic black box to me.
>>>>>>>>>>>
>>>>>>>>>>> We had discussed shifting to using pm_runtime from DCS (and 
>>>>>>>>>>> enable)
>>>>>>>>>>> calls to power up the PHY on demand, and that's what I 
>>>>>>>>>>> implemented.
>>>>>>>>>>> However Laurent flagged up that using
>>>>>>>>>>> dsi->encoder->crtc->state->adjusted_mode to get the HS clock 
>>>>>>>>>>> info
>>>>>>>>>>> required to send a HS DCS command from that call is 
>>>>>>>>>>> deprecated, so how
>>>>>>>>>>> do we specify the clock rate to use at that point?
>>>>>>>>>>
>>>>>>>>>> I guess the most sensible would be to have a custom bridge 
>>>>>>>>>> state, and
>>>>>>>>>> add a pointer to the current bridge state in struct 
>>>>>>>>>> drm_bridge. Then, as
>>>>>>>>>> long as you have a bridge pointer you have a way to get the 
>>>>>>>>>> current
>>>>>>>>>> state associated to it, and since we already have 
>>>>>>>>>> atomic_duplicate_state
>>>>>>>>>> / atomic_destroy_state we can create our own structure around 
>>>>>>>>>> it storing
>>>>>>>>>> whatever we want.
>>>>>>>>>
>>>>>>>>> That's a good point. It would only be needed if we use runtime 
>>>>>>>>> PM to
>>>>>>>>> work around the initialization sequence issue, not if we 
>>>>>>>>> implement a DSI
>>>>>>>>> host clock enable/disable operation, right ?
>>>>>>>>
>>>>>>>> Obviously this only works with atomic bridges otherwise you have no
>>>>>>>> state, but I assume all bridges should be heading that route now.
>>>>>>>>
>>>>>>>>>>>>>>> host_transfer calls can supposedly be made at any time, 
>>>>>>>>>>>>>>> however unless
>>>>>>>>>>>>>>> MIPI_DSI_MSG_USE_LPM is set in the message then we're 
>>>>>>>>>>>>>>> meant to send it
>>>>>>>>>>>>>>> in high speed mode. If this is before a mode has been 
>>>>>>>>>>>>>>> set, what
>>>>>>>>>>>>>>> defines the link frequency parameters at this point? 
>>>>>>>>>>>>>>> Adopting a random
>>>>>>>>>>>>>>> default sounds like a good way to get undefined behaviour.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> DSI burst mode needs to set the DSI link frequency 
>>>>>>>>>>>>>>> independently of
>>>>>>>>>>>>>>> the display mode. How is that meant to be configured? I 
>>>>>>>>>>>>>>> would have
>>>>>>>>>>>>>>> expected it to come from DT due to link frequency often 
>>>>>>>>>>>>>>> being chosen
>>>>>>>>>>>>>>> based on EMC restrictions, but I don't see such a thing 
>>>>>>>>>>>>>>> in any
>>>>>>>>>>>>>>> binding.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Undefined too. DSI support was added to DRM without any 
>>>>>>>>>>>>>> design effort,
>>>>>>>>>>>>>> it's more a hack than a real solution. The issue with 
>>>>>>>>>>>>>> devices that can
>>>>>>>>>>>>>> be controlled over both DSI and I2C is completely 
>>>>>>>>>>>>>> unhandled. So far
>>>>>>>>>>>>>> nobody has really cared about implementing DSI right as 
>>>>>>>>>>>>>> far as I can
>>>>>>>>>>>>>> tell.
>>>>>>>>>>>>>
>>>>>>>>>>>>> :-(
>>>>>>>>>>>>>
>>>>>>>>>>>>> Thinking aloud, does having the option to set a burst link 
>>>>>>>>>>>>> frequency
>>>>>>>>>>>>> from DT (or ACPI) have any issue for other platforms?
>>>>>>>>>>>>> Looking at the handling of MIPI_DSI_MODE_VIDEO_BURST in the 
>>>>>>>>>>>>> various
>>>>>>>>>>>>> drivers, all except stm/dw_mipi_dsi-stm.c appear to take it 
>>>>>>>>>>>>> as a "use
>>>>>>>>>>>>> all the defined timings, but drop to LP during blanking" 
>>>>>>>>>>>>> option. The
>>>>>>>>>>>>> link frequency has therefore remained a property of the
>>>>>>>>>>>>> display/bridge.
>>>>>>>>>>>>> dw_mipi_dsi-stm.c cranks the PLL up by 20%, but I haven't 
>>>>>>>>>>>>> followed
>>>>>>>>>>>>> through the full detail of the parameters it computes from 
>>>>>>>>>>>>> there.
>>>>>>>>>>>>
>>>>>>>>>>>> I don't see anything wrong with using link-frequency from 
>>>>>>>>>>>> the DT to
>>>>>>>>>>>> setup the burst frequency. It's what v4l2 has been using for 
>>>>>>>>>>>> a while
>>>>>>>>>>>> without any known (to me) drawback, and we're using the same 
>>>>>>>>>>>> of-graph
>>>>>>>>>>>> bindings, so it shouldn't be too controversial there.
>>>>>>>>>
>>>>>>>>> How would that frequency we picked in practice ? Do panels 
>>>>>>>>> typically
>>>>>>>>> support a range of HS frequencies for DCS HS transfers ?
>>>>>>>>
>>>>>>>> I was thinking more of bridges where they often run a PLL off the
>>>>>>>> incoming DSI clock, and then have a FIFO with potentially different
>>>>>>>> clock rates and timings on input and output. SN65DSI83 supports 
>>>>>>>> that,
>>>>>>>> as do the Toshiba bridge chips I'm currently looking at.
>>>>>>>> You need to know the DSI link frequency to configure the PLL
>>>>>>>> correctly, but then the bridge output timing is a different matter.
>>>>>>>>
>>>>>>>>>>> OK, that sounds like a vague plan.
>>>>>>>>>>>
>>>>>>>>>>>>> DSI and I2C controlled devices is yet another issue that I 
>>>>>>>>>>>>> haven't
>>>>>>>>>>>>> even looked at.
>>>>>>>>>>>>> I think it's more that vc4 wants to ignore DSI should the 
>>>>>>>>>>>>> DSI host
>>>>>>>>>>>>> node be enabled in DT, but there's no panel bound to it. 
>>>>>>>>>>>>> One could say
>>>>>>>>>>>>> that is a DT error and tough luck, but from a user's 
>>>>>>>>>>>>> perspective that
>>>>>>>>>>>>> is a bit harsh.
>>>>>>>>>>>>
>>>>>>>>>>>> I guess the larger "issue" is that the tree in the DT is 
>>>>>>>>>>>> done following
>>>>>>>>>>>> the "control" bus, and Linux likes to tie the life cycle of 
>>>>>>>>>>>> a given
>>>>>>>>>>>> device to its parent bus. Both these decisions make sense, 
>>>>>>>>>>>> but they
>>>>>>>>>>>> interact in a weird way in some occurrences (like this one, 
>>>>>>>>>>>> or Allwinner
>>>>>>>>>>>> has an Ethernet PHY controlled through MMIO which end up in 
>>>>>>>>>>>> the same
>>>>>>>>>>>> case).
>>>>>>>>>>>>
>>>>>>>>>>>> I wonder if using device links here could help though.
>>>>>>>>>>>
>>>>>>>>>>> I really don't know about that one.
>>>>>>>>>>
>>>>>>>>>> It's a piece of infrastructure that was created at first (I 
>>>>>>>>>> think?) to
>>>>>>>>>> model the power dependency between devices that don't have a 
>>>>>>>>>> parent /
>>>>>>>>>> child relationship. For example, if you use DMA, you probably 
>>>>>>>>>> want to
>>>>>>>>>> keep the IOMMU powered as long as you are, but it is in a 
>>>>>>>>>> completely
>>>>>>>>>> separate branch of the "device tree" (not one from the DTB, 
>>>>>>>>>> the one that
>>>>>>>>>> linux DM creates).
>>>>>>>>>>
>>>>>>>>>> It was later expanded to also cover probe order and make sure 
>>>>>>>>>> a supplier
>>>>>>>>>> would probe before its consumer, effectively making EPROBE_DEFER
>>>>>>>>>> obsolete.
>>>>>>>>>>
>>>>>>>>>> The second part is still fairly new, but I think we can solve 
>>>>>>>>>> this by
>>>>>>>>>> adding a device link between the DSI host and whatever is at 
>>>>>>>>>> the end of
>>>>>>>>>> the OF-Graph endpoint.
>>>>>>>>>>
>>>>>>>>>>>>>>> As a follow on, bridge devices can support burst mode (eg 
>>>>>>>>>>>>>>> TI's
>>>>>>>>>>>>>>> SN65DSI83 that's just been merged), so it needs to know 
>>>>>>>>>>>>>>> the desired
>>>>>>>>>>>>>>> panel timings for the output side of the bridge, but the 
>>>>>>>>>>>>>>> DSI link
>>>>>>>>>>>>>>> timings to set up the bridge's PLL. What's the correct 
>>>>>>>>>>>>>>> way for
>>>>>>>>>>>>>>> signalling that? drm_crtc_state->adjusted_mode vs
>>>>>>>>>>>>>>> drm_crtc_state->mode? Except mode is userspace's request, 
>>>>>>>>>>>>>>> not what has
>>>>>>>>>>>>>>> been validated/updated by the panel/bridge.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> adjusted_mode is also a bit of a hack, it solves very 
>>>>>>>>>>>>>> specific issues,
>>>>>>>>>>>>>> and its design assumes a single encoder in the chain with 
>>>>>>>>>>>>>> no extra
>>>>>>>>>>>>>> bridges. We should instead add modes to the bridge state, 
>>>>>>>>>>>>>> and negotiate
>>>>>>>>>>>>>> modes along the pipeline the same way we negotiate formats.
>>>>>>>>>>>>>
>>>>>>>>>>>>> So as I understand it we already have format negotiation 
>>>>>>>>>>>>> between
>>>>>>>>>>>>> bridges via atomic_get_output_bus_fmts and 
>>>>>>>>>>>>> atomic_get_input_bus_fmts,
>>>>>>>>>>>>> so is it possible to extend that to modes?
>>>>>>>>>>>>> Are you thinking bridge state that is owned by the 
>>>>>>>>>>>>> framework, or by
>>>>>>>>>>>>> the individual bridge drivers?
>>>>>>>>>>>>
>>>>>>>>>>>> atomic_check is made for that. I guess we could improve its 
>>>>>>>>>>>> call
>>>>>>>>>>>> sequence to each time a mode is modified along the bridge 
>>>>>>>>>>>> list we
>>>>>>>>>>>> restart the sequence until all components agree (or reject 
>>>>>>>>>>>> it entirely
>>>>>>>>>>>> if they can't), but I don't really see why we would need yet 
>>>>>>>>>>>> another
>>>>>>>>>>>> hook.
>>>>>>>>>
>>>>>>>>> Isn't this what atomic_get_output_bus_fmts() and
>>>>>>>>> atomic_get_input_bus_fmts() implement ?
>>>>>>>>
>>>>>>>> Those negotiate a single u32 bus format between nodes, not a 
>>>>>>>> complete timing.
>>>>>>>>
>>>>>>>>>>> Why do all nodes in the bridge list need to agree? Adjacent 
>>>>>>>>>>> nodes need
>>>>>>>>>>> to agree, but they then also need to retain that agreed timing
>>>>>>>>>>> somewhere.
>>>>>>>>>>
>>>>>>>>>> We might have mutually exclusive requirements though? Let's 
>>>>>>>>>> use the
>>>>>>>>>> example of the VC4 HDMI driver that can't have odd horizontal 
>>>>>>>>>> timings,
>>>>>>>>>> and assume it's a constraint of our DSI driver instead.
>>>>>>>>>>
>>>>>>>>>> Then, we have a DSI->LVDS bridge, a LVDS->RGB bridge and a 
>>>>>>>>>> panel (which
>>>>>>>>>> is a bit ridiculous, but whatever). If the LVDS->RGB bridge 
>>>>>>>>>> can't have
>>>>>>>>>> even horizontal timings, then you just can't display it, even 
>>>>>>>>>> though
>>>>>>>>>> they are not adjacent (unless the bridge in the middle can 
>>>>>>>>>> modify the
>>>>>>>>>> timings between the input and output, but that's not always 
>>>>>>>>>> possible).
>>>>>>>>>>
>>>>>>>>>> Similarly, if for the RGB panel we need to increase a bit some 
>>>>>>>>>> timings
>>>>>>>>>> to accommodate for a larger pixel clock and end up above what 
>>>>>>>>>> the DSI
>>>>>>>>>> host can provide, we're also done.
>>>>>>>>>
>>>>>>>>> The hard part will be to figure out a good heuristics to 
>>>>>>>>> perform the
>>>>>>>>> negotiation without going back and forth (at least not in a way 
>>>>>>>>> that
>>>>>>>>> would require too many iterations, and certainly avoiding infinite
>>>>>>>>> loops). That will be an interesting problem to solve, but maybe 
>>>>>>>>> we'll be
>>>>>>>>> lucky and a simple approach will work for the use cases we have to
>>>>>>>>> support today.
>>>>>>>>
>>>>>>>> One to kick into the long grass possibly then.
>>>>>>>>
>>>>>>>> For burst mode and bridges retiming things, generally I think it 
>>>>>>>> works
>>>>>>>> if you consider a single pass through a mode_fixup equivalent 
>>>>>>>> starting
>>>>>>>> at the panel.
>>>>>>>> The panel advertises what it wants.
>>>>>>>> The closest bridge (eg DSI83) fixup amends panel's requested mode
>>>>>>>> based on any constraints that it has, stores a copy, and passes 
>>>>>>>> that
>>>>>>>> down the chain.
>>>>>>>> The next bridge (eg vc4_dsi) amends the mode based on 
>>>>>>>> restrictions it
>>>>>>>> has (eg burst mode link frequency). That's the end of the chain in
>>>>>>>> this case, so that's the mode that the crtc needs to be programmed
>>>>>>>> with.
>>>>>>>>
>>>>>>>> The modes then need to be passed back up the chain so that eg DSI83
>>>>>>>> knows the link frequency in use, and can therefore configure the 
>>>>>>>> PLL
>>>>>>>> appropriately.
>>>>>>>>
>>>>>>>> The bit it would be nice to fix is that burst mode has effectively
>>>>>>>> increased the horizontal front porch, but that could be fixed 
>>>>>>>> within
>>>>>>>> the bridge so that the panel gets the timing it requested.
>>>>>>>>
>>>>>>>>>>> Taking SN65DSI8[3|4|5] as an example, it supports burst mode, 
>>>>>>>>>>> and the
>>>>>>>>>>> DSI frequency and timings are permitted to be different from 
>>>>>>>>>>> that
>>>>>>>>>>> which it uses on the LVDS side. The LVDS panel and LVDS side 
>>>>>>>>>>> of DSI83
>>>>>>>>>>> need to agree over the format, and the DSI host and DSI side 
>>>>>>>>>>> of DSI83
>>>>>>>>>>> need to agree, but they may be two different timings.
>>>>>>>>>>> Register 0x0B (DSI_CLK_DIVIDER & REFCLK_MULTIPLIER) allows 
>>>>>>>>>>> you to
>>>>>>>>>>> configure the LVDS rate compared to the DSI rate (the driver 
>>>>>>>>>>> currently
>>>>>>>>>>> goes for 1:1), and registers 0x20 to 0x34 allow you to set 
>>>>>>>>>>> the number
>>>>>>>>>>> of active pixel and blanking on the LVDS side (again 
>>>>>>>>>>> currently just
>>>>>>>>>>> copied across).
>>>>>>>>>>>
>>>>>>>>>>> The way I'm seeing burst mode as having been interpreted at 
>>>>>>>>>>> present is
>>>>>>>>>>> that it's largely just a flag to say "drop to LP mode between 
>>>>>>>>>>> lines".
>>>>>>>>>>> The timing that needs to be passed to the crtc is therefore 
>>>>>>>>>>> going to
>>>>>>>>>>> be based on the DSI link rate (converted to pixels) with 
>>>>>>>>>>> increased
>>>>>>>>>>> blanking periods.
>>>>>>>>>>>
>>>>>>>>>>> I guess there are similarities with Media Controller and V4L2 
>>>>>>>>>>> here. A
>>>>>>>>>>> typical chain there could be:
>>>>>>>>>>>      sensor -> scaler -> crop -> CSI-2 receiver.
>>>>>>>>>>> The format on each of those links may be different, but the 
>>>>>>>>>>> chain as a
>>>>>>>>>>> whole needs to be valid. Media Controller largely relies on 
>>>>>>>>>>> userspace
>>>>>>>>>>> to configure all links, but with a DRM chain that isn't 
>>>>>>>>>>> really an
>>>>>>>>>>> option as it's expected that the display chain configures 
>>>>>>>>>>> itself.
>>>>>>>>>>
>>>>>>>>>> Also, the userspace has no concept of media sub-devices in 
>>>>>>>>>> DRM, so it
>>>>>>>>>> just sets the mode on the whole DRM/KMS device, unlike what 
>>>>>>>>>> v4l2 does.
>>>>>>>>>> In v4l2, afaik, if you ended up with the above scenarios it 
>>>>>>>>>> would just
>>>>>>>>>> be rejected when you set the format on the link, letting the 
>>>>>>>>>> userspace
>>>>>>>>>> figure it out. We can't really do that here
>>>>>>>>>
>>>>>>>>> I wonder how long we'll be able to keep userspace out of the 
>>>>>>>>> picture to
>>>>>>>>> configure the internals of the pipeline. I don't want to be the 
>>>>>>>>> first
>>>>>>>>> person who will have a use case that requires this.
>>>>>>>>
>>>>>>>> I suspect none of us want to be the first one to hit this scenario!
>>>>>>>>
>>>>>>>> As I've just posted on the other thread about SN65DSI83, I've hit a
>>>>>>>> major stumbling block with the current design where vc4_dsi (and
>>>>>>>> Exynos) breaks the bridge_chain so that it gets called first and 
>>>>>>>> can
>>>>>>>> do initialisation, and then it calls down the chain.
>>>>>>>> Having now converted to a DSI bridge driver, it works fine with
>>>>>>>> connected non-atomic bridges, but fails with atomic bridges.
>>>>>>>> drm_atomic_add_encoder_bridges adds the state for all the 
>>>>>>>> bridges the
>>>>>>>> framework is aware of. With the split chain it misses adding the 
>>>>>>>> state
>>>>>>>> of our "hidden" bridges, and we can't add the extra state from our
>>>>>>>> atomic_duplicate_state call as we don't have the state to add to 
>>>>>>>> (we
>>>>>>>> just return our state to be added).
>>>>>>>> This bumps up the priority for us of finding a suitable solution 
>>>>>>>> for
>>>>>>>> this initialisation issue, so I'll start looking at how feasible 
>>>>>>>> a new
>>>>>>>> DSI host function is.
>>>>>>>>
>>>>>>>> Thanks
>>>>>>>>       Dave
>>>>>>>>
>>>>>>>
>>>>>
>>>
>