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

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Date:   Fri, 22 Jul 2022 10:36:44 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Abhinav Kumar <quic_abhinavk@...cinc.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Sankeerth Billakanti <quic_sbillaka@...cinc.com>
Subject: Re: [RFC PATCH] drm/edid: Make 144 Hz not preferred on Sharp LQ140M1JW46

On Fri, Jul 22, 2022 at 9:48 AM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Fri, Jul 22, 2022 at 9:37 AM Abhinav Kumar <quic_abhinavk@...cinc.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.
> > >
> > > 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@...omium.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

>
>
> > 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.
>
> -Doug