Date:   Tue, 21 Feb 2023 10:38:41 +0200
From:   Pekka Paalanen <ppaalanen@gmail.com>
To:     Christian =?UTF-8?B?S8O2bmln?= <christian.koenig@amd.com>
Cc:     Rob Clark <robdclark@gmail.com>, dri-devel@lists.freedesktop.org,
        freedreno@lists.freedesktop.org, Daniel Vetter <daniel@ffwll.ch>,
        Christian =?UTF-8?B?S8O2bmln?= <ckoenig.leichtzumerken@gmail.com>,
        Michel =?UTF-8?B?RMOkbnplcg==?= <michel@daenzer.net>,
        Tvrtko Ursulin <tvrtko.ursulin@intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        Simon Ser <contact@emersion.fr>,
        Rob Clark <robdclark@chromium.org>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Gustavo Padovan <gustavo@padovan.org>,
        "open list:SYNC FILE FRAMEWORK" <linux-media@vger.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 06/14] dma-buf/sync_file: Support (E)POLLPRI
Message-ID: <20230221103841.0d501f01@eldfell>
In-Reply-To: <2e6e9581-6de8-6aca-3e73-946fbc6ad2a3@amd.com>
References: <20230218211608.1630586-1-robdclark@gmail.com>
        <20230218211608.1630586-7-robdclark@gmail.com>
        <2e6e9581-6de8-6aca-3e73-946fbc6ad2a3@amd.com>
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On Mon, 20 Feb 2023 09:31:41 +0100
Christian K=C3=B6nig <christian.koenig@amd.com> wrote:

> Am 18.02.23 um 22:15 schrieb Rob Clark:
> > From: Rob Clark <robdclark@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@chromium.org> =20
>=20
> The code looks clean, but the different poll flags and their meaning are=
=20
> certainly not my field of expertise.


A good question. epoll_ctl manual refers to poll(2) which says:

       POLLPRI
              There is some exceptional condition on the file descriptor.  =
Possibilities include:

              =E2=80=A2 There is out-of-band data on a TCP socket (see tcp(=
7)).

              =E2=80=A2 A pseudoterminal master in packet mode has seen a s=
tate change on the slave (see ioctl_tty(2)).

              =E2=80=A2 A cgroup.events file has been modified (see cgroups=
(7)).

It seems to be about selecting what events will trigger the poll,
more than how (fast) to poll. At least it is not documented to be
ignored in 'events', so I guess it should work.


Thanks,
pq

> Feel free to add Acked-by: Christian K=C3=B6nig <christian.koenig@amd.com=
>,=20
> somebody with more background in this should probably take a look as well.
>=20
> Regards,
> Christian.
>=20
> > ---
> >   drivers/dma-buf/sync_file.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> >
> > diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
> > index fb6ca1032885..c30b2085ee0a 100644
> > --- a/drivers/dma-buf/sync_file.c
> > +++ b/drivers/dma-buf/sync_file.c
> > @@ -192,6 +192,14 @@ static __poll_t sync_file_poll(struct file *file, =
poll_table *wait)
> >   {
> >   	struct sync_file *sync_file =3D file->private_data;
> >  =20
> > +	/*
> > +	 * The POLLPRI/EPOLLPRI flag can be used to signal that
> > +	 * userspace wants the fence to signal ASAP, express this
> > +	 * as an immediate deadline.
> > +	 */
> > +	if (poll_requested_events(wait) & EPOLLPRI)
> > +		dma_fence_set_deadline(sync_file->fence, ktime_get());
> > +
> >   	poll_wait(file, &sync_file->wq, wait);
> >  =20
> >   	if (list_empty(&sync_file->cb.node) && =20
>=20


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