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[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 at 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 at 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