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

[Rob Clark]

On Tue, Feb 21, 2023 at 8:48 AM Luben Tuikov <luben.tuikov@xxxxxxx> wrote:
>
> On 2023-02-20 11:14, Rob Clark wrote:
> > On Mon, Feb 20, 2023 at 12:53 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> >>
> >> On Sat, 18 Feb 2023 13:15:49 -0800
> >> Rob Clark <robdclark@xxxxxxxxx> wrote:
> >>
> >>> From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >>>
> >>> 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@xxxxxxxxxxxx>
> >>
> >> 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.

BR,
-R