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

[Marek Olšák]

Jason, both memory-based signalling as well as interrupt-based signalling
to the CPU would be supported by amdgpu. External devices don't need to
support memory-based sync objects. The only limitation is that they can't
convert amdgpu sync objects to dma_fence.

The sad thing is that "external -> amdgpu" dependencies are really
"external <-> amdgpu" dependencies due to mutually-exclusive access
required by non-explicitly-sync'd buffers, so amdgpu-amdgpu interop is the
only interop that would initially work with those buffers. Explicitly
sync'd buffers also won't work if other drivers convert explicit fences to
dma_fence. Thus, both implicit sync and explicit sync might not work with
other drivers at all. The only interop that would initially work is
explicit fences with memory-based waiting and signalling on the external
device to keep the kernel out of the picture.

Marek


On Tue, Apr 27, 2021 at 3:41 PM Jason Ekstrand <jason at jlekstrand.net> wrote:

> Trying to figure out which e-mail in this mess is the right one to reply
> to....
>
> On Tue, Apr 27, 2021 at 12:31 PM Lucas Stach <l.stach at pengutronix.de>
> wrote:
> >
> > Hi,
> >
> > Am Dienstag, dem 27.04.2021 um 09:26 -0400 schrieb Marek Olšák:
> > > Ok. So that would only make the following use cases broken for now:
> > > - amd render -> external gpu
>
> Assuming said external GPU doesn't support memory fences.  If we do
> amdgpu and i915 at the same time, that covers basically most of the
> external GPU use-cases.  Of course, we'd want to convert nouveau as
> well for the rest.
>
> > > - amd video encode -> network device
> >
> > FWIW, "only" breaking amd render -> external gpu will make us pretty
> > unhappy, as we have some cases where we are combining an AMD APU with a
> > FPGA based graphics card. I can't go into the specifics of this use-
> > case too much but basically the AMD graphics is rendering content that
> > gets composited on top of a live video pipeline running through the
> > FPGA.
>
> I think it's worth taking a step back and asking what's being here
> before we freak out too much.  If we do go this route, it doesn't mean
> that your FPGA use-case can't work, it just means it won't work
> out-of-the box anymore.  You'll have to separate execution and memory
> dependencies inside your FPGA driver.  That's still not great but it's
> not as bad as you maybe made it sound.
>
> > > What about the case when we get a buffer from an external device and
> > > we're supposed to make it "busy" when we are using it, and the
> > > external device wants to wait until we stop using it? Is it something
> > > that can happen, thus turning "external -> amd" into "external <->
> > > amd"?
> >
> > Zero-copy texture sampling from a video input certainly appreciates
> > this very much. Trying to pass the render fence through the various
> > layers of userspace to be able to tell when the video input can reuse a
> > buffer is a great experience in yak shaving. Allowing the video input
> > to reuse the buffer as soon as the read dma_fence from the GPU is
> > signaled is much more straight forward.
>
> Oh, it's definitely worse than that.  Every window system interaction
> is bi-directional.  The X server has to wait on the client before
> compositing from it and the client has to wait on X before re-using
> that back-buffer.  Of course, we can break that later dependency by
> doing a full CPU wait but that's going to mean either more latency or
> reserving more back buffers.  There's no good clean way to claim that
> any of this is one-directional.
>
> --Jason
>
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