[PATCH] drm: rework description of primary and cursor planes

[Simon Ser]

On Thursday, December 10th, 2020 at 4:56 PM, Daniel Vetter <daniel at ffwll.ch> wrote:

> Huh so crtc are registered forward and planes backward? I guess adding
> amd people. And yeah sounds like defacto you can't figure out which
> primary plane goes to which crtc, and we just take whatever goes.
> Maybe that stricter approach with more guarantees just doesn't work,
> ship sailed already :-/

Yeah. Even if we fixed the amdgpu driver and added the check, user-space still
couldn't have the guarantee that it can associate the n-th primary plane with
the n-th CRTC, because it might run with an old kernel.

If we really wanted to allow user-space to discover the internal
->{primary,cursor} pointers, I think we should just expose a new property. That
way, the uAPI would be a lot more explicit and a lot less guessing. The cost is
that it wouldn't work on older kernels, but with amdgpu user-space can't rely
on the implicit rule you've suggested anyways.