Re: [PATCH v2] media: vb2: Fix regression on poll() for RW mode

[Hans Verkuil]

On 04/25/2016 09:59 AM, Ricardo Ribalda Delgado wrote:
> When using a device is read/write mode, vb2 does not handle properly the
> first select/poll operation.
> 
> 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.
> 
> The reintroduced check to the core is enabled by a quirk flag, that
> avoids this check by other subsystems like DVB.
> 
> Reported-by: Dimitrios Katsaros <patcherwork@xxxxxxxxx>
> Cc: Junghak Sung <jh1009.sung@xxxxxxxxxxx>
> Fixes: 49d8ab9feaf2 ("media] media: videobuf2: Separate vb2_poll()")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@xxxxxxxxx>
> ---
> 
> v2: By  Hans Verkuil <hverkuil@xxxxxxxxx> and
>  	Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxxxx>:
> 
> 	Add a quirk bit to enable this behaviour on the core.
>  drivers/media/v4l2-core/videobuf2-core.c | 9 +++++++++
>  drivers/media/v4l2-core/videobuf2-v4l2.c | 9 +--------
>  include/media/videobuf2-core.h           | 4 ++++
>  3 files changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
> index 5d016f496e0e..58eb9be13510 100644
> --- a/drivers/media/v4l2-core/videobuf2-core.c
> +++ b/drivers/media/v4l2-core/videobuf2-core.c
> @@ -2298,6 +2298,15 @@ 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.
> +	 */

This comment should be moved to where the quirk is set in v4l2. This comment shouldn't
refer to vb2 at all. How about:

"If this quirk is set and if QBUF hasn't been called..."

and perhaps at the end of the comment:

"This quirk is set by V4L2 for backwards compatibility reasons."

although I am not completely certain we need that at all here.

> +	if (q->quirk_poll_must_check_waiting_for_buffers &&
> +	    q->waiting_for_buffers && (req_events & (POLLIN | POLLRDNORM)))
> +		return POLLERR;
> +
> +	/*
>  	 * For output streams you can call write() as long as there are fewer
>  	 * buffers queued than there are buffers available.
>  	 */
> diff --git a/drivers/media/v4l2-core/videobuf2-v4l2.c b/drivers/media/v4l2-core/videobuf2-v4l2.c
> index 91f552124050..3e649adf85ef 100644
> --- a/drivers/media/v4l2-core/videobuf2-v4l2.c
> +++ b/drivers/media/v4l2-core/videobuf2-v4l2.c
> @@ -765,6 +765,7 @@ int vb2_queue_init(struct vb2_queue *q)
>  	q->is_output = V4L2_TYPE_IS_OUTPUT(q->type);
>  	q->copy_timestamp = (q->timestamp_flags & V4L2_BUF_FLAG_TIMESTAMP_MASK)
>  			== V4L2_BUF_FLAG_TIMESTAMP_COPY;
> +	q->quirk_poll_must_check_waiting_for_buffers = true;
>  
>  	return vb2_core_queue_init(q);
>  }
> @@ -818,14 +819,6 @@ unsigned int vb2_poll(struct vb2_queue *q, struct file *file, poll_table *wait)
>  			poll_wait(file, &fh->wait, wait);
>  	}
>  
> -	/*
> -	 * 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;
> -
>  	return res | vb2_core_poll(q, file, wait);
>  }
>  EXPORT_SYMBOL_GPL(vb2_poll);
> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> index 8a0f55b6c2ba..3a1620946068 100644
> --- a/include/media/videobuf2-core.h
> +++ b/include/media/videobuf2-core.h
> @@ -400,6 +400,9 @@ struct vb2_buf_ops {
>   * @fileio_read_once:		report EOF after reading the first buffer
>   * @fileio_write_immediately:	queue buffer after each write() call
>   * @allow_zero_bytesused:	allow bytesused == 0 to be passed to the driver
> + * @quirk_poll_must_check_waiting_for_buffers: Return POLLERR at poll when QBUF
> + *              has not been called. This is a vb1 idiom that has been adopted
> + *              also by vb2.
>   * @lock:	pointer to a mutex that protects the vb2_queue struct. The
>   *		driver can set this to a mutex to let the v4l2 core serialize
>   *		the queuing ioctls. If the driver wants to handle locking
> @@ -463,6 +466,7 @@ struct vb2_queue {
>  	unsigned			fileio_read_once:1;
>  	unsigned			fileio_write_immediately:1;
>  	unsigned			allow_zero_bytesused:1;
> +	unsigned		   quirk_poll_must_check_waiting_for_buffers:1;
>  
>  	struct mutex			*lock;
>  	void				*owner;
> 

Regards,

	Hans