HDMI Aspect Ratio

[Rob Clark]

On Wed, Oct 18, 2017 at 4:06 PM, Lloyd Atkinson <latkinso at codeaurora.org> wrote:
> Hi folks,
>
> We're looking at 4K HDMI mode support, and noticed aspect ratio support
> is in flux.
>
> Aspect ratio parsing was added to the mode:
> https://patchwork.kernel.org/patch/9271401/
>
> But later reverted:
> https://patchwork.kernel.org/patch/9410765/
>
> We're finding that since aspect ratio information isn't supported, modes
> conflict, and the userspace isn't able to see the full list of modes.
>
> Similarly, there's a case where two modes differ only by aspect_ratio
> and vrefresh. In drm_edid.c, within drm_display_mode_from_vic_index,
> newmode->vrefresh is forcibly being cleared to 0. Since refresh rate is
> forced to 0, and aspect ratio isn’t considered in general, the modes
> collide and we can’t advertise one of the modes.
>
> https://github.com/torvalds/linux/blob/v4.14-rc4/drivers/gpu/drm/drm_edid.c#L3153
>
> Have there been any follow-up discussions on this topic?
>
> What is the reason to clear the vrefresh within
> drm_display_mode_from_vic_index?

+Ville who might remember the history..

I don't remember if we found a userspace app that was broken, or if
reverting was an "abundance of caution" thing (which is equally valid,
it is basically impossible to track down and test "all of
userspace"..)

Adding a cap to let userspace opt-in to the new behaviour seems like a
sensible approach, and one that we've used before.

BR,
-R

> Thanks,
> Lloyd Atkinson
>
> --
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