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

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Date:   Thu, 28 Jul 2022 10:34:19 -0700
From:   Abhinav Kumar <quic_abhinavk@...cinc.com>
To:     Rob Clark <robdclark@...il.com>,
        Doug Anderson <dianders@...omium.org>
CC:     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

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@...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.

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.

>>>>
>>>> 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

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@...cinc.com>

I would also wait to see if others have different thought about this.

>>
>> -Doug