[PATCH v2 10/41] drm/modes: Add a function to generate analog display modes

[Mateusz Kwiatkowski]

Hi Maxime,

W dniu 5.09.2022 o 15:37, Maxime Ripard pisze:
>>> +    vfp = vfp_min + (porches_rem / 2);
>>> +    vbp = porches - vfp;
>>
>> Relative position of the vertical sync within the VBI effectively moves the
>> image up and down. Adding that (porches_rem / 2) moves the image up off center
>> by that many pixels. I'd keep the VFP always at minimum to keep the image
>> centered.
>
> And you would increase the back porch only then?

Well, increasing vbp only gives a centered image with the default 480i/576i
resolutions. However, only ever changing vbp will cause the image to be always
at the bottom of the screen when the active line count is decreased (e.g.
setting the resolution to 720x480 but for 50Hz "PAL" - like many game consoles
did back in the day).

I believe that the perfect solution would:

- Use the canonical / standard-defined blanking line counts for the standard
  vertical resolutions (480/486/576)
- Increase vfp and vbp from there by the same number if a smaller number of
  active lines is specified, so that the resulting image is centered
- Likewise, decrease vfp and vbp by the same number if the active line number
  is larger and there is still leeway (this should allow for seamless handling
  of 480i vs. 486i for 60 Hz "NTSC")
- If even more active lines are specified, once the limit for vfp is hit, then
  decrease vbp only - the resulting image will definitely be off-center, but
  there's no other way

I hope this makes sense for you as well.

Best regards,
Mateusz Kwiatkowski