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

[Hans Verkuil]

On 04/22/2016 04:48 PM, Mauro Carvalho Chehab wrote:
> 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)))

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.

Regards,

	Hans