Re: [PATCH] drm: add capability DRM_CAP_ASYNC_UPDATE

From: Daniel Vetter
Date: Fri Dec 21 2018 - 08:26:45 EST

On Fri, Dec 21, 2018 at 10:47:27AM +0100, Michel Dänzer wrote:
> On 2018-12-20 6:16 p.m., Michel Dänzer wrote:
> > On 2018-12-20 6:09 p.m., Daniel Vetter wrote:
> >> On Thu, Dec 20, 2018 at 6:03 PM Alex Deucher <alexdeucher@xxxxxxxxx> wrote:
> >>> On Thu, Dec 20, 2018 at 11:54 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> >>
> >>>> Not sure about the gamma thing since we had opposite bugs on i915
> >>>> about gamma not being vsynced and tearing terribly. Cursor is special
> >>>> since it tends to be too small to notice tearing.
> >>>
> >>> Our cursor hw (and possibly gamma as well Nicholas? Harry?) is double
> >>> buffered, so we can update it any time for the most part and the
> >>> changes won't take affect until the next vupdate period.
> >>
> >> Hm, I guess we could make the gamma update optionally async, and let
> >> drivers deal. The issue is that the current async helper stuff won't
> >> cope with gamma updates (it's aimed at plane updates only, not at crtc
> >> property updates). Or we get userspace to do proper atomic updates. Or
> >> we do some faking in the kernel, e.g. waiting with the gamma update
> >> until the next atomic update happens. But that kinda breaks
> >> ->atomic_check.
> >
> > Would it be possible to merge gamma changes into a pending commit, if
> > there is one, and create a new commit otherwise?
> >
> > Otherwise the atomic API just moves this same problem to be solved by
> > userspace.
> Actually, I'm not sure this is even solvable in a Xorg driver. When it
> gets called to update the gamma LUT:
> 1. If there's a pending atomic commit, it cannot amend that, can it?

Yup, but on the kernel side we kinda have the same problem.

> 2. It has no way of knowing when the next atomic commit would be
> submitted e.g. for a page flip, so it kind of has to create its own
> commit for the gamma update.

Block handler is what I had in mind for the fallback commit, if no other
commit happened meanwhile (which would need to include it).
Daniel Vetter
Software Engineer, Intel Corporation