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

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Date:	Fri, 22 Apr 2016 11:21:36 -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 14:37:07 +0200
Hans Verkuil <hverkuil@...all.nl> escreveu:

> On 04/22/2016 02:31 PM, Mauro Carvalho Chehab wrote:
> > 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;  
> 
> s/check_waiting_for_buffers/v4l2_needs_to_wait_for_buffers/

Better, but still hell of a hack. Maybe we could add a quirks
flag and add a flag like:
	VB2_FLAG_ENABLE_POLLERR_IF_WAITING_BUFFERS_AND_NO_QBUF
(or some better naming, I'm not inspired today...)

Of course, such quirk should be properly documented.

> > 
> > 2) we should not mess the core due to some V4L2 mess.  
> 
> Well, the only other alternative I see is to split vb2_core_poll() into two
> since the check has to happen in the middle. The v4l2 code would call core_poll1(),
> then do the check and afterwards call core_poll2(). And that would really be ugly.

Actually, the first callback would be better called as
vb2_core_poll_check() - or something with similar name.

IMHO, this is the cleaner solution, although it adds an extra cost.


> I would probably NACK that.
> 
> Better ideas are welcome.
> 
> Regards,
> 
> 	Hans


-- 
Thanks,
Mauro