Re:

Re:

[Linus Walleij]

On Wed, Feb 26, 2020 at 12:57 PM Ville Syrjälä
<ville.syrjala@linux.intel.com> wrote:
> On Tue, Feb 25, 2020 at 10:52:25PM +0100, Linus Walleij wrote:

> > I have long suspected that a whole bunch of the "simple" displays
> > are not simple but contains a display controller and memory.
> > That means that the speed over the link to the display and
> > actual refresh rate on the actual display is asymmetric because
> > well we are just updating a RAM, the resolution just limits how
> > much data we are sending, the clock limits the speed on the
> > bus over to the RAM on the other side.
>
> IMO even in command mode mode->clock should probably be the actual
> dotclock used by the display. If there's another clock for the bus
> speed/etc. it should be stored somewhere else.

Good point. For the DSI panels we have the field hs_rate
for the HS clock in struct mipi_dsi_device which is based
on exactly this reasoning. And that is what I actually use for
setting the HS clock.

The problem is however that we in many cases have so
substandard documentation of these panels that we have
absolutely no idea about the dotclock. Maybe we should
just set it to 0 in these cases?

Yours,
Linus Walleij

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Re:

[Ville Syrjälä]

On Wed, Feb 26, 2020 at 01:08:06PM +0100, Linus Walleij wrote:
> On Wed, Feb 26, 2020 at 12:57 PM Ville Syrjälä
> <ville.syrjala@linux.intel.com> wrote:
> > On Tue, Feb 25, 2020 at 10:52:25PM +0100, Linus Walleij wrote:
> 
> > > I have long suspected that a whole bunch of the "simple" displays
> > > are not simple but contains a display controller and memory.
> > > That means that the speed over the link to the display and
> > > actual refresh rate on the actual display is asymmetric because
> > > well we are just updating a RAM, the resolution just limits how
> > > much data we are sending, the clock limits the speed on the
> > > bus over to the RAM on the other side.
> >
> > IMO even in command mode mode->clock should probably be the actual
> > dotclock used by the display. If there's another clock for the bus
> > speed/etc. it should be stored somewhere else.
> 
> Good point. For the DSI panels we have the field hs_rate
> for the HS clock in struct mipi_dsi_device which is based
> on exactly this reasoning. And that is what I actually use for
> setting the HS clock.
> 
> The problem is however that we in many cases have so
> substandard documentation of these panels that we have
> absolutely no idea about the dotclock. Maybe we should
> just set it to 0 in these cases?

Don't you always have a TE interrupt or something like that
available? Could just measure it from that if no better
information is available?

-- 
Ville Syrjälä
Intel

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Re:

[Linus Walleij]

On Wed, Feb 26, 2020 at 3:34 PM Ville Syrjälä
<ville.syrjala@linux.intel.com> wrote:
> On Wed, Feb 26, 2020 at 01:08:06PM +0100, Linus Walleij wrote:
> > On Wed, Feb 26, 2020 at 12:57 PM Ville Syrjälä
> > <ville.syrjala@linux.intel.com> wrote:
> > > On Tue, Feb 25, 2020 at 10:52:25PM +0100, Linus Walleij wrote:
> >
> > > > I have long suspected that a whole bunch of the "simple" displays
> > > > are not simple but contains a display controller and memory.
> > > > That means that the speed over the link to the display and
> > > > actual refresh rate on the actual display is asymmetric because
> > > > well we are just updating a RAM, the resolution just limits how
> > > > much data we are sending, the clock limits the speed on the
> > > > bus over to the RAM on the other side.
> > >
> > > IMO even in command mode mode->clock should probably be the actual
> > > dotclock used by the display. If there's another clock for the bus
> > > speed/etc. it should be stored somewhere else.
> >
> > Good point. For the DSI panels we have the field hs_rate
> > for the HS clock in struct mipi_dsi_device which is based
> > on exactly this reasoning. And that is what I actually use for
> > setting the HS clock.
> >
> > The problem is however that we in many cases have so
> > substandard documentation of these panels that we have
> > absolutely no idea about the dotclock. Maybe we should
> > just set it to 0 in these cases?
>
> Don't you always have a TE interrupt or something like that
> available? Could just measure it from that if no better
> information is available?

Yes and I did exactly that, so that is why this comment is in
the driver:

static const struct drm_display_mode sony_acx424akp_cmd_mode = {
(...)
        /*
         * Some desired refresh rate, experiments at the maximum "pixel"
         * clock speed (HS clock 420 MHz) yields around 117Hz.
         */
        .vrefresh = 60,

I got a review comment at the time saying 117 Hz was weird.
We didn't reach a proper conclusion on this:
https://lore.kernel.org/dri-devel/CACRpkdYW3YNPSNMY3A44GQn8DqK-n9TLvr7uipF7LM_DHZ5=Lg@mail.gmail.com/

Thierry wasn't sure if 60Hz was good or not, so I just had to
go with something.

We could calculate the resulting pixel clock for ~117 Hz with
this resolution and put that in the clock field but ... don't know
what is the best?

Yours,
Linus Walleij

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Re:

[Ville Syrjälä]

On Wed, Feb 26, 2020 at 03:56:36PM +0100, Linus Walleij wrote:
> On Wed, Feb 26, 2020 at 3:34 PM Ville Syrjälä
> <ville.syrjala@linux.intel.com> wrote:
> > On Wed, Feb 26, 2020 at 01:08:06PM +0100, Linus Walleij wrote:
> > > On Wed, Feb 26, 2020 at 12:57 PM Ville Syrjälä
> > > <ville.syrjala@linux.intel.com> wrote:
> > > > On Tue, Feb 25, 2020 at 10:52:25PM +0100, Linus Walleij wrote:
> > >
> > > > > I have long suspected that a whole bunch of the "simple" displays
> > > > > are not simple but contains a display controller and memory.
> > > > > That means that the speed over the link to the display and
> > > > > actual refresh rate on the actual display is asymmetric because
> > > > > well we are just updating a RAM, the resolution just limits how
> > > > > much data we are sending, the clock limits the speed on the
> > > > > bus over to the RAM on the other side.
> > > >
> > > > IMO even in command mode mode->clock should probably be the actual
> > > > dotclock used by the display. If there's another clock for the bus
> > > > speed/etc. it should be stored somewhere else.
> > >
> > > Good point. For the DSI panels we have the field hs_rate
> > > for the HS clock in struct mipi_dsi_device which is based
> > > on exactly this reasoning. And that is what I actually use for
> > > setting the HS clock.
> > >
> > > The problem is however that we in many cases have so
> > > substandard documentation of these panels that we have
> > > absolutely no idea about the dotclock. Maybe we should
> > > just set it to 0 in these cases?
> >
> > Don't you always have a TE interrupt or something like that
> > available? Could just measure it from that if no better
> > information is available?
> 
> Yes and I did exactly that, so that is why this comment is in
> the driver:
> 
> static const struct drm_display_mode sony_acx424akp_cmd_mode = {
> (...)
>         /*
>          * Some desired refresh rate, experiments at the maximum "pixel"
>          * clock speed (HS clock 420 MHz) yields around 117Hz.
>          */
>         .vrefresh = 60,
> 
> I got a review comment at the time saying 117 Hz was weird.
> We didn't reach a proper conclusion on this:
> https://lore.kernel.org/dri-devel/CACRpkdYW3YNPSNMY3A44GQn8DqK-n9TLvr7uipF7LM_DHZ5=Lg@mail.gmail.com/
> 
> Thierry wasn't sure if 60Hz was good or not, so I just had to
> go with something.
> 
> We could calculate the resulting pixel clock for ~117 Hz with
> this resolution and put that in the clock field but ... don't know
> what is the best?

I would vote for that approach.

-- 
Ville Syrjälä
Intel

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