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

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Date:	Fri, 22 Apr 2016 18:45:41 +0200
From:	Hans Verkuil <hverkuil@...all.nl>
To:	Mauro Carvalho Chehab <mchehab@....samsung.com>
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

On 04/22/2016 05:21 PM, Mauro Carvalho Chehab wrote:
> 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.

I mean C bitfields like this:

        unsigned                        fileio_read_once:1;
        unsigned                        fileio_write_immediately:1;
        unsigned                        allow_zero_bytesused:1;

This is already used in struct vb2_queue, so my proposal would be to add:

	unsigned			quirk_poll_must_check_waiting_for_buffers:1;

Regards,

	Hans