[PATCH v4 06/14] dma-buf/sync_file: Support (E)POLLPRI

[Rob Clark]

On Wed, Feb 22, 2023 at 1:49 AM Pekka Paalanen <ppaalanen at gmail.com> wrote:
>
> On Tue, 21 Feb 2023 09:53:56 -0800
> Rob Clark <robdclark at gmail.com> wrote:
>
> > On Tue, Feb 21, 2023 at 8:48 AM Luben Tuikov <luben.tuikov at amd.com> wrote:
> > >
> > > On 2023-02-20 11:14, Rob Clark wrote:
> > > > On Mon, Feb 20, 2023 at 12:53 AM Pekka Paalanen <ppaalanen at gmail.com> wrote:
> > > >>
> > > >> On Sat, 18 Feb 2023 13:15:49 -0800
> > > >> Rob Clark <robdclark at gmail.com> wrote:
> > > >>
> > > >>> From: Rob Clark <robdclark at chromium.org>
> > > >>>
> > > >>> Allow userspace to use the EPOLLPRI/POLLPRI flag to indicate an urgent
> > > >>> wait (as opposed to a "housekeeping" wait to know when to cleanup after
> > > >>> some work has completed).  Usermode components of GPU driver stacks
> > > >>> often poll() on fence fd's to know when it is safe to do things like
> > > >>> free or reuse a buffer, but they can also poll() on a fence fd when
> > > >>> waiting to read back results from the GPU.  The EPOLLPRI/POLLPRI flag
> > > >>> lets the kernel differentiate these two cases.
> > > >>>
> > > >>> Signed-off-by: Rob Clark <robdclark at chromium.org>
> > > >>
> > > >> Hi,
> > > >>
> > > >> where would the UAPI documentation of this go?
> > > >> It seems to be missing.
> > > >
> > > > Good question, I am not sure.  The poll() man page has a description,
> > > > but my usage doesn't fit that _exactly_ (but OTOH the description is a
> > > > bit vague).
> > > >
> > > >> If a Wayland compositor is polling application fences to know which
> > > >> client buffer to use in its rendering, should the compositor poll with
> > > >> PRI or not? If a compositor polls with PRI, then all fences from all
> > > >> applications would always be PRI. Would that be harmful somehow or
> > > >> would it be beneficial?
> > > >
> > > > I think a compositor would rather use the deadline ioctl and then poll
> > > > without PRI.  Otherwise you are giving an urgency signal to the fence
> > > > signaller which might not necessarily be needed.
> > > >
> > > > The places where I expect PRI to be useful is more in mesa (things
> > > > like glFinish(), readpix, and other similar sorts of blocking APIs)
> > > Hi,
> > >
> > > Hmm, but then user-space could do the opposite, namely, submit work as usual--never
> > > using the SET_DEADLINE ioctl, and then at the end, poll using (E)POLLPRI. That seems
> > > like a possible usage pattern, unintended--maybe, but possible. Do we want to discourage
> > > this? Wouldn't SET_DEADLINE be enough? I mean, one can call SET_DEADLINE with the current
> > > time, and then wouldn't that be equivalent to (E)POLLPRI?
> >
> > Yeah, (E)POLLPRI isn't strictly needed if we have SET_DEADLINE.  It is
> > slightly more convenient if you want an immediate deadline (single
> > syscall instead of two), but not strictly needed.  OTOH it piggy-backs
> > on existing UABI.
>
> In that case, I would be conservative, and not add the POLLPRI
> semantics. An UAPI addition that is not strictly needed and somewhat
> unclear if it violates any design principles is best not done, until it
> is proven to be beneficial.
>
> Besides, a Wayland compositor does not necessary need to add the fd
> to its main event loop for poll. It could just SET_DEADLINE, and then
> when it renders simply check if the fence passed or not already. Not
> polling means the compositor does not need to wake up at the moment the
> fence signals to just record a flag.

poll(POLLPRI) isn't intended for wayland.. but is a thing I want in
mesa for fence waits.  I _could_ use SET_DEADLINE but it is two
syscalls and correspondingly more code ;-)

> On another matter, if the application uses SET_DEADLINE with one
> timestamp, and the compositor uses SET_DEADLINE on the same thing with
> another timestamp, what should happen?

The expectation is that many deadline hints can be set on a fence.
The fence signaller should track the soonest deadline.

BR,
-R

> Maybe it's a soft-realtime app whose primary goal is not display, and
> it needs the result faster than the window server?
>
> Maybe SET_DEADLINE should set the deadline only to an earlier timestamp
> and never later?
>
>
> Thanks,
> pq