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
