[Mesa-dev] [RFC] Linux Graphics Next: Explicit fences everywhere and no BO fences - initial proposal

Daniel Vetter daniel at ffwll.ch
Wed Apr 28 10:05:47 UTC 2021

On Tue, Apr 27, 2021 at 02:01:20PM -0400, Alex Deucher wrote:
> On Tue, Apr 27, 2021 at 1:35 PM Simon Ser <contact at emersion.fr> wrote:
> >
> > On Tuesday, April 27th, 2021 at 7:31 PM, Lucas Stach <l.stach at pengutronix.de> wrote:
> >
> > > > Ok. So that would only make the following use cases broken for now:
> > > >
> > > > - amd render -> external gpu
> > > > - amd video encode -> network device
> > >
> > > FWIW, "only" breaking amd render -> external gpu will make us pretty
> > > unhappy
> >
> > I concur. I have quite a few users with a multi-GPU setup involving
> > AMD hardware.
> >
> > Note, if this brokenness can't be avoided, I'd prefer a to get a clear
> > error, and not bad results on screen because nothing is synchronized
> > anymore.
> It's an upcoming requirement for windows[1], so you are likely to
> start seeing this across all GPU vendors that support windows.  I
> think the timing depends on how quickly the legacy hardware support
> sticks around for each vendor.

Yeah but hw scheduling doesn't mean the hw has to be constructed to not
support isolating the ringbuffer at all.

E.g. even if the hw loses the bit to put the ringbuffer outside of the
userspace gpu vm, if you have pagetables I'm seriously hoping you have r/o
pte flags. Otherwise the entire "share address space with cpu side,
seamlessly" thing is out of the window.

And with that r/o bit on the ringbuffer you can once more force submit
through kernel space, and all the legacy dma_fence based stuff keeps
working. And we don't have to invent some horrendous userspace fence based
implicit sync mechanism in the kernel, but can instead do this transition
properly with drm_syncobj timeline explicit sync and protocol reving.

At least I think you'd have to work extra hard to create a gpu which
cannot possibly be intercepted by the kernel, even when it's designed to
support userspace direct submit only.

Or are your hw engineers more creative here and we're screwed?
Daniel Vetter
Software Engineer, Intel Corporation

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