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

[Daniel Stone]

Hi,

On Tue, 20 Apr 2021 at 16:16, Christian König <
ckoenig.leichtzumerken at gmail.com> wrote:

> Am 20.04.21 um 17:07 schrieb Daniel Stone:
>
> If the compositor no longer has a guarantee that the buffer will be ready
> for composition in a reasonable amount of time (which dma_fence gives us,
> and this proposal does not appear to give us), then the compositor isn't
> trying to use the buffer for compositing, it's waiting asynchronously on a
> notification that the fence has signaled before it attempts to use the
> buffer.
>
> Marek's initial suggestion is that the kernel signal the fence, which
> would unblock composition (and presumably show garbage on screen, or at
> best jump back to old content).
>
> My position is that the compositor will know the process has crashed
> anyway - because its socket has been closed - at which point we destroy all
> the client's resources including its windows and buffers regardless.
> Signaling the fence doesn't give us any value here, _unless_ the compositor
> is just blindly waiting for the fence to signal ... which it can't do
> because there's no guarantee the fence will ever signal.
>
>
> Yeah, but that assumes that the compositor has change to not blindly wait
> for the client to finish rendering and as Daniel explained that is rather
> unrealistic.
>
> What we need is a fallback mechanism which signals the fence after a
> timeout and gives a penalty to the one causing the timeout.
>
> That gives us the same functionality we have today with the in software
> scheduler inside the kernel.
>

OK, if that's the case then I think I'm really missing something which
isn't explained in this thread, because I don't understand what the
additional complexity and API change gains us (see my first reply in this
thread).

By way of example - say I have a blind-but-explicit compositor that takes a
drm_syncobj along with a dmabuf with each client presentation request, but
doesn't check syncobj completion, it just imports that into a VkSemaphore +
VkImage and schedules work for the next frame.

Currently, that generates an execbuf ioctl for the composition (ignore KMS
for now) with a sync point to wait on, and the kernel+GPU scheduling
guarantees that the composition work will not begin until the client
rendering work has retired. We have a further guarantee that this work will
complete in reasonable time, for some value of 'reasonable'.

My understanding of this current proposal is that:
* userspace creates a 'present fence' with this new ioctl
* the fence becomes signaled when a value is written to a location in
memory, which is visible through both CPU and GPU mappings of that page
* this 'present fence' is imported as a VkSemaphore (?) and the userspace
Vulkan driver will somehow wait on this value  either before submitting
work or as a possibly-hardware-assisted GPU-side wait (?)
* the kernel's scheduler is thus eliminated from the equation, and every
execbuf is submitted directly to hardware, because either userspace knows
that the fence has already been signaled, or it will issue a GPU-side wait
(?)
* but the kernel is still required to monitor completion of every fence
itself, so it can forcibly complete, or penalise the client (?)

Lastly, let's say we stop ignoring KMS: what happens for the
render-with-GPU-display-on-KMS case? Do we need to do the equivalent of
glFinish() in userspace and only submit the KMS atomic request when the GPU
work has fully retired?

Clarifying those points would be really helpful so this is less of a
strawman. I have some further opinions, but I'm going to wait until I
understand what I'm actually arguing against before I go too far. :) The
last point is very salient though.

Cheers,
Daniel
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