On Thu, May 20, 2021 at 10:13:38AM +0200, Michel Dänzer wrote: > On 2021-05-20 9:55 a.m., Daniel Vetter wrote: > > On Wed, May 19, 2021 at 5:48 PM Michel Dänzer <mic...@daenzer.net> wrote: > >> > >> On 2021-05-19 5:21 p.m., Jason Ekstrand wrote: > >>> On Wed, May 19, 2021 at 5:52 AM Michel Dänzer <mic...@daenzer.net> wrote: > >>>> > >>>> On 2021-05-19 12:06 a.m., Jason Ekstrand wrote: > >>>>> On Tue, May 18, 2021 at 4:17 PM Daniel Vetter <dan...@ffwll.ch> wrote: > >>>>>> > >>>>>> On Tue, May 18, 2021 at 7:40 PM Christian König > >>>>>> <ckoenig.leichtzumer...@gmail.com> wrote: > >>>>>>> > >>>>>>> Am 18.05.21 um 18:48 schrieb Daniel Vetter: > >>>>>>>> On Tue, May 18, 2021 at 2:49 PM Christian König > >>>>>>>> <ckoenig.leichtzumer...@gmail.com> wrote: > >>>>>>>> > >>>>>>>>> And as long as we are all inside amdgpu we also don't have any > >>>>>>>>> oversync, > >>>>>>>>> the issue only happens when we share dma-bufs with i915 (radeon and > >>>>>>>>> AFAIK nouveau does the right thing as well). > >>>>>>>> Yeah because then you can't use the amdgpu dma_resv model anymore and > >>>>>>>> have to use the one atomic helpers use. Which is also the one that > >>>>>>>> e.g. Jason is threathening to bake in as uapi with his dma_buf ioctl, > >>>>>>>> so as soon as that lands and someone starts using it, something has > >>>>>>>> to > >>>>>>>> adapt _anytime_ you have a dma-buf hanging around. Not just when it's > >>>>>>>> shared with another device. > >>>>>>> > >>>>>>> Yeah, and that is exactly the reason why I will NAK this uAPI change. > >>>>>>> > >>>>>>> This doesn't works for amdgpu at all for the reasons outlined above. > >>>>>> > >>>>>> Uh that's really not how uapi works. "my driver is right, everyone > >>>>>> else is wrong" is not how cross driver contracts are defined. If that > >>>>>> means a perf impact until you've fixed your rules, that's on you. > >>>>>> > >>>>>> Also you're a few years too late with nacking this, it's already uapi > >>>>>> in the form of the dma-buf poll() support. > >>>>> > >>>>> ^^ My fancy new ioctl doesn't expose anything that isn't already > >>>>> there. It just lets you take a snap-shot of a wait instead of doing > >>>>> an active wait which might end up with more fences added depending on > >>>>> interrupts and retries. The dma-buf poll waits on all fences for > >>>>> POLLOUT and only the exclusive fence for POLLIN. It's already uAPI. > >>>> > >>>> Note that the dma-buf poll support could be useful to Wayland > >>>> compositors for the same purpose as Jason's new ioctl (only using client > >>>> buffers which have finished drawing for an output frame, to avoid > >>>> missing a refresh cycle due to client drawing), *if* it didn't work > >>>> differently with amdgpu. > >>>> > >>>> Am I understanding correctly that Jason's new ioctl would also work > >>>> differently with amdgpu as things stand currently? If so, that would be > >>>> a real bummer and might hinder adoption of the ioctl by Wayland > >>>> compositors. > >>> > >>> My new ioctl has identical semantics to poll(). It just lets you take > >>> a snapshot in time to wait on later instead of waiting on whatever > >>> happens to be set right now. IMO, having identical semantics to > >>> poll() isn't something we want to change. > >> > >> Agreed. > >> > >> I'd argue then that making amdgpu poll semantics match those of other > >> drivers is a pre-requisite for the new ioctl, otherwise it seems unlikely > >> that the ioctl will be widely adopted. > > > > This seems backwards, because that means useful improvements in all > > other drivers are stalled until amdgpu is fixed. > > > > I think we need agreement on what the rules are, reasonable plan to > > get there, and then that should be enough to unblock work in the wider > > community. Holding the community at large hostage because one driver > > is different is really not great. > > I think we're in violent agreement. :) The point I was trying to make is > that amdgpu really needs to be fixed to be consistent with other drivers > ASAP.
It's not that easy at all. I think best case we're looking at about a one year plan to get this into shape, taking into account usual release/distro update latencies. Best case. But also it's not a really big issue, since this shouldn't stop compositors from using poll on dma-buf fd or the sync_file stuff from Jason: The use-case for this in compositors is to avoid a single client stalling the entire desktop. If a driver lies by not setting the exclusive fence when expected, you simply don't get this stall avoidance benefit of misbehaving clients. But also this needs a gpu scheduler and higher priority for the compositor (or a lot of hw planes so you can composite with them alone), so it's all fairly academic issue. Iow amdgpu being different on these wont cause any actual issues I think. The only case that does break is when the compositor does an mmap on the dma-buf fd and relies on poll to indicate when the rendering is done. Not even sure amdgpu supports mmap on dma-buf or not. That's the only case I could think of which would result in actual corruption anywhere, and not just stalls when no one expects them. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
