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

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Date:	Fri, 22 Apr 2016 09:31:41 -0300
From:	Mauro Carvalho Chehab <mchehab@....samsung.com>
To:	Hans Verkuil <hverkuil@...all.nl>
Cc:	Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>,
	Pawel Osciak <pawel@...iak.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	<linux-media@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Junghak Sung <jh1009.sung@...sung.com>,
	<stable@...r.kernel.org>
Subject: Re: [PATCH] media: vb2: Fix regression on poll() for RW mode

Em Fri, 22 Apr 2016 11:19:09 +0200
Hans Verkuil <hverkuil@...all.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@...il.com>
> > Cc: Junghak Sung <jh1009.sung@...sung.com>
> > Cc: stable@...r.kernel.org
> > Fixes: 49d8ab9feaf2 ("media] media: videobuf2: Separate vb2_poll()")
> > Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@...il.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