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

From: Abhinav Kumar
Date: Thu Jul 28 2022 - 13:34:30 EST

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@xxxxxxxxxxxx> wrote:

Hi,

On Fri, Jul 22, 2022 at 9:37 AM Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx> 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@xxxxxxxxxxxx>

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@xxxxxxxxxxx>

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


-Doug