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[Linus Walleij]

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?

Yours,
Linus Walleij