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

[Hans Verkuil]

On 04/22/2016 11:47 AM, Ricardo Ribalda Delgado wrote:
> Thanks for your review!
> 
> I will fix it that way. Do you mind to wait until Monday for a patch?

No problem.

	Hans

> 
> Regards
> 
> On 22 Apr 2016 11:19, "Hans Verkuil" <hverkuil@xxxxxxxxx <mailto:hverkuil@xxxxxxxxx>> wrote:
> 
>     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@xxxxxxxxx <mailto:patcherwork@xxxxxxxxx>>
>     > Cc: Junghak Sung <jh1009.sung@xxxxxxxxxxx <mailto:jh1009.sung@xxxxxxxxxxx>>
>     > Cc: stable@xxxxxxxxxxxxxxx <mailto:stable@xxxxxxxxxxxxxxx>
>     > Fixes: 49d8ab9feaf2 ("media] media: videobuf2: Separate vb2_poll()")
>     > Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@xxxxxxxxx <mailto:ricardo.ribalda@xxxxxxxxx>>
>     > ---
>     >  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.
> 
>     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);
>     >
>