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

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Date:	Fri, 22 Apr 2016 12:21:57 -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 16:56:00 +0200
Hans Verkuil <hverkuil@...all.nl> escreveu:

> On 04/22/2016 04:48 PM, Mauro Carvalho Chehab wrote:
> > Em Fri, 22 Apr 2016 16:31:28 +0200
> > Hans Verkuil <hverkuil@...all.nl> escreveu:
> >   
> >> On 04/22/2016 04:21 PM, Mauro Carvalho Chehab wrote:  
> >>> 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.    
> >>
> >> How about 'quirk_poll_must_check_waiting_for_buffers'? Something with 'quirk' in the
> >> name is a good idea.  
> > 
> > works for me, provided that we add the field as a flag. So it would be like:
> > 
> > #define QUIRK_POLL_MUST_CHECK_WAITING_FOR_BUFFERS 0
> > 
> >  	if (test_bit(q->quirk, QUIRK_POLL_MUST_CHECK_WAITING_FOR_BUFFERS) &&
> > 	    q->waiting_for_buffers && (req_events & (POLLIN | POLLRDNORM)))  
> 
> Why should it be a flag? What is wrong with a bitfield?
> 
> Just curious what the reasoning is for that. I don't see any obvious
> advantage of a flag over a bitfield.

Huh? Flags are implemented as bitfields. See the above code: it is
using test_bit() for the new q->quirk flags/bitfield.

Regards,
Mauro