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SubjectRe: [RFC PATCH] drm/edid: Make 144 Hz not preferred on Sharp LQ140M1JW46
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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@chromium.org> wrote:
>>
>> Hi,
>>
>> On Fri, Jul 22, 2022 at 9:37 AM Abhinav Kumar <quic_abhinavk@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.

>>>>
>>>> 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@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@quicinc.com>

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

>>
>> -Doug

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