[RFC PATCH] drm/edid: Make 144 Hz not preferred on Sharp LQ140M1JW46

[Doug Anderson]

Hi,

On Thu, Jul 28, 2022 at 10:34 AM Abhinav Kumar
<quic_abhinavk at quicinc.com> wrote:
>
> Hi Rob and Doug
>
> On 7/22/2022 10:36 AM, Rob Clark wrote:
> > On Fri, Jul 22, 2022 at 9:48 AM Doug Anderson <dianders at chromium.org> wrote:
> >>
> >> Hi,
> >>
> >> On Fri, Jul 22, 2022 at 9:37 AM Abhinav Kumar <quic_abhinavk at quicinc.com> wrote:
> >>>
> >>> + sankeerth
> >>>
> >>> Hi Doug
> >>>
> >>> On 7/21/2022 3:23 PM, Douglas Anderson wrote:
> >>>> The Sharp LQ140M1JW46 panel is on the Qualcomm sc7280 CRD reference
> >>>> board. This panel supports 144 Hz and 60 Hz. In the EDID, the 144 Hz
> >>>> mode is listed first and thus is marked preferred. The EDID decode I
> >>>> ran says:
> >>>>
> >>>>     First detailed timing includes the native pixel format and preferred
> >>>>     refresh rate.
> >>>>
> >>>>     ...
> >>>>
> >>>>     Detailed Timing Descriptors:
> >>>>       DTD 1:  1920x1080  143.981 Hz  16:9   166.587 kHz  346.500 MHz
> >>>>                    Hfront   48 Hsync  32 Hback  80 Hpol N
> >>>>                    Vfront    3 Vsync   5 Vback  69 Vpol N
> >>>>       DTD 2:  1920x1080   59.990 Hz  16:9    69.409 kHz  144.370 MHz
> >>>>                    Hfront   48 Hsync  32 Hback  80 Hpol N
> >>>>                    Vfront    3 Vsync   5 Vback  69 Vpol N
> >>>>
> >>>> I'm proposing here that the above is actually a bug and that the 60 Hz
> >>>> mode really should be considered preferred by Linux.
>
> Its a bit tricky to say that this is a bug but I think we can certainly
> add here that for an internal display we would have ideally had the
> lower resolution first to indicate it as default.

Yeah, it gets into the vagueness of the EDID spec in general. As far
as I can find it's really up to the monitor to decide by what means it
chooses the "preferred" refresh rate if the monitor can support many.
Some displays may decide that the normal rate is "preferred" and some
may decide that the high refresh rate is "preferred". Neither display
is "wrong" per say, but it's nice to have some consistency here and to
make it so that otherwise "dumb" userspace will get something
reasonable by default. I'll change it to say:

While the EDID spec appears to allow a display to use any criteria for
picking which refresh mode is "preferred" or "optimal", that vagueness
is a bit annoying. From Linux's point of view let's choose the 60 Hz
one as the default.


> >>>> The argument here is that this is a laptop panel and on a laptop we
> >>>> know power will always be a concern. Presumably even if someone using
> >>>> this panel wanted to use 144 Hz for some use cases they would only do
> >>>> so dynamically and would still want the default to be 60 Hz.
> >>>>
> >>>> Let's change the default to 60 Hz using a standard quirk.
> >>>>
> >>>> Signed-off-by: Douglas Anderson <dianders at chromium.org>
> >>>
> >>> Yes, we were aware that 144Hz was getting picked. We found that while
> >>> debugging the screen corruption issue.
> >>>
> >>> Well, yes power would be less with 60Hz but so will be the performance.
> >>
> >> What performance specifically will be less with 60 Hz? In general the
> >> sc7280 CPU is a bit memory-bandwidth constrained and the LCD refresh
> >> from memory is a non-trivial part of that. Reducing to 60 Hz will
> >> relieve some of the memory bandwidth pressure and will actually allow
> >> tasks on the CPU to run _faster_. I guess the downside is that some
> >> animations might be a little less smooth...
> >
> > I guess he is referring to something that is vblank sync'd running
> > faster than 60fps.
> >
> > but OTOH it is a bit of a waste for fbcon to be using 144Hz.  And
> > there are enough android games that limit themselves to 30fps to save
> > your "phone" battery.  So it seems a lot more sane to default to 60Hz
> > and let userspace that knows it wants more pick the 144Hz rate when
> > needed.
> >
> > BR,
> > -R
>
> Yes i was referring to vblank synced apps.
>
> >
> >>
> >>
> >>> The test teams have been validating with 144Hz so far so we are checking
> >>> internally with the team whether its OKAY to goto 60Hz now since that
> >>> kind of invalidates the testing they have been doing.
> >>
> >> You're worried that the panel itself won't work well at 60 Hz, or
> >> something else about the system won't? The whole system in general
> >> needs to work well with 60 Hz displays and I expect them to be much
> >> more common than 144 Hz displays. Quite honestly if switching to 60 Hz
> >> uncovers a problem that would be a huge benefit of landing this patch
> >> because it would mean we'd find it now rather than down the road when
> >> someone hooks up a different panel.
>
> I was worried that it will invalidate the testing they did so far but
> since you have confirmed that you would prefer 60Hz to be more
> thoroughly tested than 144Hz, I have informed the internal teams of this
> change and given the heads up.
>
> You can have my R-b for this change,
>
> Reviewed-by: Abhinav Kumar <quic_abhinavk at quicinc.com>
>
> I would also wait to see if others have different thought about this.

Thanks! I'll probably wait another week or so then I'll land in
drm-misc-next with your tag.

-Doug