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

[Pekka Paalanen]

On Mon, 20 Feb 2023 09:31:41 +0100
Christian König <christian.koenig@xxxxxxx> wrote:

> Am 18.02.23 um 22:15 schrieb Rob Clark:
> > 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>  
> 
> The code looks clean, but the different poll flags and their meaning are 
> 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:

              • There is out-of-band data on a TCP socket (see tcp(7)).

              • A pseudoterminal master in packet mode has seen a state change on the slave (see ioctl_tty(2)).

              • 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 König <christian.koenig@xxxxxxx>, 
> somebody with more background in this should probably take a look as well.
> 
> Regards,
> Christian.
> 
> > ---
> >   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 = file->private_data;
> >   
> > +	/*
> > +	 * 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);
> >   
> >   	if (list_empty(&sync_file->cb.node) &&  
>

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