[Mesa-dev] Linux Graphics Next: Userspace submission update

Daniel Vetter daniel at ffwll.ch
Thu Jun 3 07:47:33 UTC 2021

On Wed, Jun 02, 2021 at 11:16:39PM -0400, Marek Olšák wrote:
> On Wed, Jun 2, 2021 at 2:48 PM Daniel Vetter <daniel at ffwll.ch> wrote:
> > On Wed, Jun 02, 2021 at 05:38:51AM -0400, Marek Olšák wrote:
> > > On Wed, Jun 2, 2021 at 5:34 AM Marek Olšák <maraeo at gmail.com> wrote:
> > >
> > > > Yes, we can't break anything because we don't want to complicate things
> > > > for us. It's pretty much all NAK'd already. We are trying to gather
> > more
> > > > knowledge and then make better decisions.
> > > >
> > > > The idea we are considering is that we'll expose memory-based sync
> > objects
> > > > to userspace for read only, and the kernel or hw will strictly control
> > the
> > > > memory writes to those sync objects. The hole in that idea is that
> > > > userspace can decide not to signal a job, so even if userspace can't
> > > > overwrite memory-based sync object states arbitrarily, it can still
> > decide
> > > > not to signal them, and then a future fence is born.
> > > >
> > >
> > > This would actually be treated as a GPU hang caused by that context, so
> > it
> > > should be fine.
> >
> > This is practically what I proposed already, except your not doing it with
> > dma_fence. And on the memory fence side this also doesn't actually give
> > what you want for that compute model.
> >
> > This seems like a bit a worst of both worlds approach to me? Tons of work
> > in the kernel to hide these not-dma_fence-but-almost, and still pain to
> > actually drive the hardware like it should be for compute or direct
> > display.
> >
> > Also maybe I've missed it, but I didn't see any replies to my suggestion
> > how to fake the entire dma_fence stuff on top of new hw. Would be
> > interesting to know what doesn't work there instead of amd folks going of
> > into internal again and then coming back with another rfc from out of
> > nowhere :-)
> >
> Going internal again is probably a good idea to spare you the long
> discussions and not waste your time, but we haven't talked about the
> dma_fence stuff internally other than acknowledging that it can be solved.
> The compute use case already uses the hw as-is with no inter-process
> sharing, which mostly keeps the kernel out of the picture. It uses glFinish
> to sync with GL.
> The gfx use case needs new hardware logic to support implicit and explicit
> sync. When we propose a solution, it's usually torn apart the next day by
> ourselves.
> Since we are talking about next hw or next next hw, preemption should be
> better.
> user queue = user-mapped ring buffer
> For implicit sync, we will only let userspace lock access to a buffer via a
> user queue, which waits for the per-buffer sequence counter in memory to be
> >= the number assigned by the kernel, and later unlock the access with
> another command, which increments the per-buffer sequence counter in memory
> with atomic_inc regardless of the number assigned by the kernel. The kernel
> counter and the counter in memory can be out-of-sync, and I'll explain why
> it's OK. If a process increments the kernel counter but not the memory
> counter, that's its problem and it's the same as a GPU hang caused by that
> process. If a process increments the memory counter but not the kernel
> counter, the ">=" condition alongside atomic_inc guarantee that signaling n
> will signal n+1, so it will never deadlock but also it will effectively
> disable synchronization. This method of disabling synchronization is
> similar to a process corrupting the buffer, which should be fine. Can you
> find any flaw in it? I can't find any.

Hm maybe I misunderstood what exactly you wanted to do earlier. That kind
of "we let userspace free-wheel whatever it wants, kernel ensures
correctness of the resulting chain of dma_fence with reset the entire
context" is what I proposed too.

Like you say, userspace is allowed to render garbage already.

> The explicit submit can be done by userspace (if there is no
> synchronization), but we plan to use the kernel to do it for implicit sync.
> Essentially, the kernel will receive a buffer list and addresses of wait
> commands in the user queue. It will assign new sequence numbers to all
> buffers and write those numbers into the wait commands, and ring the hw
> doorbell to start execution of that queue.

Yeah for implicit sync I think kernel and using drm/scheduler to sort out
the dma_fence dependencies is probably best. Since you can filter out
which dma_fence you hand to the scheduler for dependency tracking you can
filter out your own ones and let the hw handle those directly (depending
how much your hw can do an all that). On i915 we might do that to be able
to use MI_SEMAPHORE_WAIT/SIGNAL functionality in the hw and fw scheduler.

For buffer tracking with implicit sync I think cleanest is probably to
still keep them wrapped as dma_fence and stuffed into dma_resv, but
conceptually it's the same. If we let every driver reinvent their own
buffer tracking just because the hw works a bit different it'll be a mess.

Wrt wait commands: I'm honestly not sure why you'd do that. Userspace gets
to keep the pieces if it gets it wrong. You do still need to handle
external dma_fence though, hence drm/scheduler frontend to sort these out.
Daniel Vetter
Software Engineer, Intel Corporation

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