RE: [PATCH v6 8/8] drm/msm/dp: Handle eDP mode_valid differently from dp

[Sankeerth Billakanti <sbillaka@qti.qualcomm.com>]

Hi Dmitry,

> > > > > > > On Wed, 30 Mar 2022 at 19:04, Sankeerth Billakanti
> > > > > > > <quic_sbillaka@quicinc.com> wrote:
> > > > > > > >
> > > > > > > > The panel-edp driver modes needs to be validated
> > > > > > > > differently from DP because the link capabilities are not
> > > > > > > > available for EDP by
> > > that time.
> > > > > > > >
> > > > > > > > Signed-off-by: Sankeerth Billakanti
> > > > > > > > <quic_sbillaka@quicinc.com>
> > > > > > >
> > > > > > > This should not be necessary after
> > > > > > >
> > > > >
> > >
> https://patchwork.freedesktop.org/patch/479261/?series=101682&rev=1.
> > > > > > > Could you please check?
> > > > > > >
> > > > > >
> > > > > > The check for DP_MAX_PIXEL_CLK_KHZ is not necessary anymore
> > > > > > but
> > > we
> > > > > > need to return early for eDP because unlike DP, eDP context
> > > > > > will not have the information about the number of lanes and link
> clock.
> > > > > >
> > > > > > So, I will modify the patch to return after the
> > > > > > DP_MAX_PIXEL_CLK_KHZ
> > > > > check if is_eDP is set.
> > > > >
> > > > > I haven't walked through all the relevant code but something you
> > > > > said above sounds strange. You say that for eDP we don't have
> > > > > info about the number of lanes? We _should_.
> > > > >
> > > > > It's certainly possible to have a panel that supports _either_ 1
> > > > > or
> > > > > 2 lanes but then only physically connect 1 lane to it. ...or you
> > > > > could have a panel that supports 2 or 4 lanes and you only connect 1
> lane.
> > > > > See, for instance, ti_sn_bridge_parse_lanes. There we assume 4
> > > > > lanes but if a "data-lanes" property is present then we can use
> > > > > that to know that fewer lanes are physically connected.
> > > > >
> > > > > It's also possible to connect more lanes to a panel than it supports.
> > > > > You could connect 2 lanes to it but then it only supports 1.
> > > > > This case needs to be handled as well...
> > > > >
> > > >
> > > > I was referring to the checks we do for DP in
> > > > dp_bridge_mode_valid. We check if the Link bandwidth can support
> > > > the pixel bandwidth. For an external DP connection, the Initial
> > > > DPCD/EDID read after cable connection will return the sink
> > > > capabilities like link rate, lane count and bpp information that
> > > > are used to we filter out the unsupported
> > > modes from the list of modes from EDID.
> > > >
> > > > For eDP case, the dp driver performs the first dpcd read during
> > > > bridge_enable. The dp_bridge_mode_valid function is executed
> > > > before bridge_enable and hence does not have the full link or the
> > > > sink capabilities information like external DP connection, by then.
> > >
> > > It sounds to me like we should emulate the HPD event for eDP to be
> > > handled earlier than the get_modes()/prepare() calls are attempted.
> > > However this might open another can of worms.
> > >
> >
> > For DP, the HPD handler mainly initiates link training and gets the EDID.
> >
> > Before adding support for a separate eDP panel, we had discussed about
> > this internally and decided to emulate eDP HPD during enable(). Main
> > reason being, eDP power is guaranteed to be on only after
> bridge_enable().
> > So, eDP link training can happen and sustain only after bridge_enable().
> >
> > Emulating HPD before/during get_modes will not have any effect because:
> 
> As we have seen, the term HPD is significantly overloaded. What do you
> want to emulate?
> 

On DP, we use HPD event for link training and EDID read.

I understood that you wanted me to emulate HPD event before get_modes()
but because the panel power is controlled by panel-edp, whatever programming
we do on the sink side will be reset when panel power will be turned off by
the pm_runtime_put_autosuspend() of the panel-edp in panel_edp_get_modes().

> >
> > 1. get_modes() will go to panel's get_modes() function to power on
> > read EDID
> >
> > 2. panel power will be turned off after get_modes(). Panel power off
> > will reset every write transaction in DPCD. anyway invalidating link
> > training
> 
> I tend to agree with Doug here. eDP link power status should be handled by
> the pm_runtime_autosuspend with grace period being high enough to cover
> the timeslot between get_mode() and enable().
> 
> panel-edp already does most of required work.
> 

The eDP controller resources are enabled through the host_init() and the link
resources need to be powered on for doing link training, which needs to happen
in the enable() with generic panel-edp.

> >
> > 3. mode_valid will land in dp driver but panel will not be powered on
> > at that time and we cannot do aux transfers or DPCD read writes.
> 
> Why do we need to perform AUX writes in mode_valid?
> 

I am trying to justify why we cannot have mode_valid() implementation similar to DP for eDP.
The detect() and get_modes() are called from panel bridge and panel-edp.c respectively.
The first eDP specific call which will land in the dp_driver is mode_valid(), in which the
controller cannot perform aux access because the panel will not be powered-on.

As the panel-power and backlight are panel resources, we are not enabling/voting for them
from the DP/eDP controller driver.

> >
> > > > So, we need to proceed with the reported mode for eDP.
> > >
> > > Well... Even if during the first call to get_modes() the DPCD is not
> > > read, during subsequent calls the driver has necessary information,
> > > so it can proceed with all the checks, can't it?
> > >
> >
> > get_modes() currently does not land in DP driver. It gets executed in
> > panel bridge. But the mode_valid() comes to DP driver to check the
> > controller compatibility.
> 
> Yes, this is correct. the DP's mode_valid() knows the hardware limitations
> (max clock speed, amount of lanes, etc) and thus it can decide whether the
> mode is supported by the whole chain or not.
> We should not skip such checks for the eDP.
> 
> 

For eDP, we have no information about the number of lanes or the link rate supported
We only know the max lanes from the data-lanes DT property.

> --
> With best wishes
> Dmitry