[PATCH] drm: document DRM_MODE_PAGE_FLIP_EVENT interactions with atomic

[Daniel Stone]

On Mon, 14 Apr 2025 at 09:24, Simon Ser <contact at emersion.fr> wrote:
> On Friday, January 17th, 2025 at 12:15, Pekka Paalanen <pekka.paalanen at haloniitty.fi> wrote:
> > Wasn't this also conditional on the DRM_CAP_CRTC_IN_VBLANK_EVENT or did
> > userspace really need to count the events even without it?
>
> DRM_CAP_CRTC_IN_VBLANK_EVENT is unconditionally set to 1. It doesn't seem
> like there is any interaction between these two. So yeah, I suppose
> user-space needs to count if they are on kernel < v4.12.
>
> > Nevertheless, should there be a "see also DRM_CAP_CRTC_IN_VBLANK_EVENT"?
>
> This sounds a bit out-of-place to me TBH. It's orthogonal to event delivery
> and it's linked from struct drm_event_vblank already.

Right. One completion event will be delivered per CRTC in any case.
It's just that *cough* someone forgot to include the CRTC ID in the
event in the version of atomic support that first landed, so you get n
events which each refer to _a_ CRTC, but userspace has no idea which.

I don't think we need to refer too much to the caps. Realistically, I
suspect everyone who ever tried to use a kernel which didn't have the
CRTC ID in the completion event with atomic userspace is in this
thread, apart from Maarten.

Cheers,
Daniel