On Wed, 28 Jul 2021 15:31:41 +0200 Christian König <christian.koe...@amd.com> wrote:
> Am 28.07.21 um 15:24 schrieb Michel Dänzer: > > On 2021-07-28 3:13 p.m., Christian König wrote: > >> Am 28.07.21 um 15:08 schrieb Michel Dänzer: > >>> On 2021-07-28 1:36 p.m., Christian König wrote: > >>>> At least AMD hardware is already capable of flipping frames on GPU > >>>> events like finishing rendering (or uploading etc). > >>>> > >>>> By waiting in userspace on the CPU before send the frame to the hardware > >>>> you are completely killing of such features. > >>>> > >>>> For composing use cases that makes sense, but certainly not for full > >>>> screen applications as far as I can see. > >>> Even for fullscreen, the current KMS API only allows queuing a single > >>> page flip per CRTC, with no way to cancel or otherwise modify it. > >>> Therefore, a Wayland compositor has to set a deadline for the next > >>> refresh cycle, and when the deadline passes, it has to select the best > >>> buffer available for the fullscreen surface. To make sure the flip will > >>> not miss the next refresh cycle, the compositor has to pick an idle > >>> buffer. If it picks a non-idle buffer, and the pending rendering does not > >>> finish in time for vertical blank, the flip will be delayed by at least > >>> one refresh cycle, which results in visible stuttering. > >>> > >>> (Until the deadline passes, the Wayland compositor can't even know if a > >>> previously fullscreen surface will still be fullscreen for the next > >>> refresh cycle) > >> Well then let's extend the KMS API instead of hacking together workarounds > >> in userspace. > > That's indeed a possible solution for the fullscreen / direct scanout case. > > > > Not for the general compositing case though, since a compositor does not > > want to composite multiple output frames per display refresh cycle, so it > > has to make sure the one frame hits the target. > > Yeah, that's true as well. > > At least as long as nobody invents a mechanism to do this decision on > the GPU instead. That would mean putting the whole window manager into the GPU. Thanks, pq
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