[PATCH 1/1] drm/panel: st7703: Fix vertical refresh rate of XBD599

[Frank Oltmanns]

Hi Ondřej,
hi all,

Ondřej Jirman <megous at megous.com> writes:
> 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.
>
> Your patch will not produce the intended effect.
>
> kind regards,
> 	o.
>

The TL;DR of my upcoming musings are: Thank you very much for your feedback! Any
recommendations for an informative read about the topic that you or anybody else
has, is greatly appreciated.

How did you measure the vsync events? Were you using vblank interrupts [1]?

I have to admit that I tested only visually and couldn’t spot a difference
between your patch and mine. I’ll need to put more thinking into this, and maybe
you or anyone reading this can help me with that.

My interpretation of the `struct drm_display_mode` documentation [2] was, that
these are logical dimensions/clocks that somewhere down the stack are converted
to their physical/hardware representation.

But now I’ve read the description of the struct’s “crtc_clock” member more
carefully. It says:
“Actual pixel or dot clock in the hardware. This differs from the
logical @clock when e.g. using interlacing, double-clocking, stereo
modes or other fancy stuff that changes the timings and signals
actually sent over the wire.”

So, can I say that if we don’t use “interlacing, double-clocking, stereo modes
or other fancy stuff” that `crtc_clock` will be equal to `clock` and therefore
we have to choose `clock` according to the SoC’s capabilities?

Also, I haven’t found a source about which values to use for the front and back
porch part of the panel and why can you just “arbitrarily” change those. My
assumption is, that those are just extra pixels we can add to make the
dimensions match the ratio of clock and vertical refresh rate. At least that
seems to be, what you did in your patch. But again, no source to back my
assumption about the range the porches can have.

I’ve put the following docs on my “to read and understand” list:
• Allwinner A64 User Manual (to learn more about the SoC’s TCON0 and what
  clock’s the SoC can produce)
• drm-internals.rst
• “Rendering PinePhone’s display” [3], to learn why it produces 69 MHz.
• Your commit message for the PinePhone Pro panel [4] (found on your blog:
  <https://xnux.eu/log/>)

Is there anything else I should add?

Thank you again and best regards,
  Frank

[1] <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/drm_vblank.c>
[2] <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/drm/drm_modes.h#n198>
[3] <https://lupyuen.github.io/articles/de>
[4] <https://github.com/megous/linux/commit/a173b114c9323c718530280b3a918d0925edaa6a>

>>  	.flags	     = DRM_MODE_FLAG_NHSYNC | DRM_MODE_FLAG_NVSYNC,
>>  	.width_mm    = 68,
>>  	.height_mm   = 136,
>> –
>> 2.39.1
>>