linux-kernel - Re: [PATCH 1/1] drm/panel: st7703: Fix vertical refresh rate of XBD599

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Message-ID: <87zg90e6s5.fsf@oltmanns.dev>
Date:   Sun, 26 Feb 2023 16:17:32 +0100
From:   Frank Oltmanns <frank@...manns.dev>
To:     Guido Günther <agx@...xcpu.org>,
        Ondřej Jirman <megous@...ous.com>
Cc:     Purism Kernel Team <kernel@...i.sm>,
        Thierry Reding <thierry.reding@...il.com>,
        Sam Ravnborg <sam@...nborg.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        "open list:DRM PANEL DRIVERS" <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] drm/panel: st7703: Fix vertical refresh rate of XBD599

Hi Ondřej,
hi Guido,

On 2023-02-19 at 13:35:42 +0100, Ondřej Jirman <megous@...ous.com> wrote:

> On Sun, Feb 19, 2023 at 12:45:53PM +0100, Frank Oltmanns wrote:
>> Fix the XBD599 panel’s slight visual stutter by correcting the pixel
>> clock speed so that the panel’s 60Hz vertical refresh rate is met.
>>
>> Set the clock speed using the underlying formula instead of a magic
>> number. To have a consistent procedure for both panels, set the JH057N
>> panel’s clock also as a formula.
>>
>> —
>>  drivers/gpu/drm/panel/panel-sitronix-st7703.c | 4 ++–
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff –git a/drivers/gpu/drm/panel/panel-sitronix-st7703.c b/drivers/gpu/drm/panel/panel-sitronix-st7703.c
>> index 6747ca237ced..cd7d631f7573 100644
>> — a/drivers/gpu/drm/panel/panel-sitronix-st7703.c
>> +++ b/drivers/gpu/drm/panel/panel-sitronix-st7703.c
>> @@ -139,7 +139,7 @@ static const struct drm_display_mode jh057n00900_mode = {
>>  	.vsync_start = 1440 + 20,
>>  	.vsync_end   = 1440 + 20 + 4,
>>  	.vtotal	     = 1440 + 20 + 4 + 12,
>> -	.clock	     = 75276,
>> +	.clock	     = (720 + 90 + 20 + 20) * (1440 + 20 + 4 + 12) * 60 / 1000,
>>  	.flags	     = DRM_MODE_FLAG_NHSYNC | DRM_MODE_FLAG_NVSYNC,
>>  	.width_mm    = 65,
>>  	.height_mm   = 130,
>> @@ -324,7 +324,7 @@ static const struct drm_display_mode xbd599_mode = {
>>  	.vsync_start = 1440 + 18,
>>  	.vsync_end   = 1440 + 18 + 10,
>>  	.vtotal	     = 1440 + 18 + 10 + 17,
>> -	.clock	     = 69000,
>> +	.clock	     = (720 + 40 + 40 + 40) * (1440 + 18 + 10 + 17) * 60 / 1000,
>
> As for pinephone, A64 can’t produce 74.844 MHz precisely, so this will not work.
>
> Better fix is to alter the mode so that clock can be something the only SoC this
> panel is used with can actually produce.
>
> See eg. <https://github.com/megous/linux/commit/dd070679d717e7f34af7558563698240a43981a6>
> which is tested to actually produce 60Hz by measuring the vsync events against
> the CPU timer.

I did some testing using a 60fps video I produced using the following command:
    ffmpeg -f lavfi -i testsrc=duration=10:size=80x50:rate=60 -vf “drawtext=text=%{n}:fontsize=36:r=60:x=(w-tw)/2: y=h-(1*lh):fontcolor=white:box=1:boxcolor=0x00000099” test_80x50.mp4

This 10-second video shows an increasing number on every frame, which makes it easy to spot dropped frames by recording the playback with a camera that can record more than 60fps (I used a 240fps camera).

When playing back that video with your current drm-6.2 branch I get a steady 60fps. But applying either your or my patch to mainline, only helps very little. Frames are being skipped more often than not in both cases.

Therefore, I need to investigate more and retract the patch for now.

The other two patches I sent earlier, however, are far more important for making the pinephone usable on mainline.

Best regards,
  Frank

>
> Your patch will not produce the intended effect.
>
> kind regards,
> 	o.
>
>>  	.flags	     = DRM_MODE_FLAG_NHSYNC | DRM_MODE_FLAG_NVSYNC,
>>  	.width_mm    = 68,
>>  	.height_mm   = 136,
>> –
>> 2.39.1
>>