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

[Mauro Carvalho Chehab]

Em Fri, 22 Apr 2016 16:31:28 +0200
Hans Verkuil <hverkuil@xxxxxxxxx> escreveu:

> On 04/22/2016 04:21 PM, Mauro Carvalho Chehab wrote:
> > Em Fri, 22 Apr 2016 14:37:07 +0200
> > Hans Verkuil <hverkuil@xxxxxxxxx> escreveu:
> >   
> >> On 04/22/2016 02:31 PM, Mauro Carvalho Chehab wrote:  
> >>> Em Fri, 22 Apr 2016 11:19:09 +0200
> >>> Hans Verkuil <hverkuil@xxxxxxxxx> 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@xxxxxxxxx>
> >>>>> Cc: Junghak Sung <jh1009.sung@xxxxxxxxxxx>
> >>>>> Cc: stable@xxxxxxxxxxxxxxx
> >>>>> Fixes: 49d8ab9feaf2 ("media] media: videobuf2: Separate vb2_poll()")
> >>>>> Signed-off-by: Ricardo Ribalda Delgado <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.    
> >>>
> >>> 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)))

> 
> >   
> >>>
> >>> 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 really don't like this. This has a much larger impact on vb2 core then adding
> a simple quirk flag.
> 
> Regards,
> 
> 	Hans


-- 
Thanks,
Mauro