Re: [PATCH] media: vb2: Fix regression on poll() for RW mode

Mauro Carvalho Chehab
2016-04-22T12:31:48Z

Em Fri, 22 Apr 2016 11:19:09 +0200
Hans Verkuil <hverkuil@xs4all.nl> escreveu:

> Hi Ricardo,
>
> On 04/21/2016 11:15 AM, Ricardo Ribalda Delgado wrote:
> > When using a device is read/write mode, vb2 does not handle properly the
> > first select/poll operation. It allways return POLLERR.
> >
> > The reason for this is that when this code has been refactored, some of
> > the operations have changed their order, and now fileio emulator is not
> > started by poll, due to a previous check.
> >
> > Reported-by: Dimitrios Katsaros <patcherwork@gmail.com>
> > Cc: Junghak Sung <jh1009.sung@samsung.com>
> > Cc: stable@vger.kernel.org
> > Fixes: 49d8ab9feaf2 ("media] media: videobuf2: Separate vb2_poll()")
> > Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
> > ---
> > drivers/media/v4l2-core/videobuf2-core.c | 8 ++++++++
> > drivers/media/v4l2-core/videobuf2-v4l2.c | 8 --------
> > 2 files changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
> > index 5d016f496e0e..199c65dbe330 100644
> > --- a/drivers/media/v4l2-core/videobuf2-core.c
> > +++ b/drivers/media/v4l2-core/videobuf2-core.c
> > @@ -2298,6 +2298,14 @@ unsigned int vb2_core_poll(struct vb2_queue *q, struct file *file,
> > return POLLERR;
> >
> > /*
> > + * For compatibility with vb1: if QBUF hasn't been called yet, then
> > + * return POLLERR as well. This only affects capture queues, output
> > + * queues will always initialize waiting_for_buffers to false.
> > + */
> > + if (q->waiting_for_buffers && (req_events & (POLLIN | POLLRDNORM)))
> > + return POLLERR;
>
> The problem I have with this is that this should be specific to V4L2. The only
> reason we do this is that we had to stay backwards compatible with vb1.
>
> This is the reason this code was placed in videobuf2-v4l2.c. But you are correct
> that this causes a regression, and I see no other choice but to put it in core.c.
>
> That said, I would still only honor this when called from v4l2, so I suggest that
> a new flag 'check_waiting_for_buffers' is added that is only set in vb2_queue_init
> in videobuf2-v4l2.c.
>
> So the test above becomes:
>
> if (q->check_waiting_for_buffers && q->waiting_for_buffers &&
> (req_events & (POLLIN | POLLRDNORM)))
>
> It's not ideal, but at least this keeps this v4l2 specific.

I don't like the above approach, for two reasons:

1) it is not obvious that this is V4L2 specific from the code;

2) we should not mess the core due to some V4L2 mess.

>
> Regards,
>
> Hans
>
> > +
> > + /*
> > * For output streams you can call write() as long as there are fewer
> > * buffers queued than there are buffers available.
> > */
> > diff --git a/drivers/media/v4l2-core/videobuf2-v4l2.c b/drivers/media/v4l2-core/videobuf2-v4l2.c
> > index 91f552124050..c9bad9ef2104 100644
> > --- a/drivers/media/v4l2-core/videobuf2-v4l2.c
> > +++ b/drivers/media/v4l2-core/videobuf2-v4l2.c
> > @@ -818,14 +818,6 @@ unsigned int vb2_poll(struct vb2_queue *q, struct file *file, poll_table *wait)
> > poll_wait(file, &fh->wait, wait);
> > }
> >
> > - /*
> > - * For compatibility with vb1: if QBUF hasn't been called yet, then
> > - * return POLLERR as well. This only affects capture queues, output
> > - * queues will always initialize waiting_for_buffers to false.
> > - */
> > - if (q->waiting_for_buffers && (req_events & (POLLIN | POLLRDNORM)))
> > - return POLLERR;
> > -
> > return res | vb2_core_poll(q, file, wait);
> > }
> > EXPORT_SYMBOL_GPL(vb2_poll);
> >
>

--
Thanks,
Mauro