Re: [RFC PATCH 4/8] vb2: retry start_streaming in case of insufficient buffers.

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Hans,

On Friday 22 November 2013 09:43:23 Hans Verkuil wrote:
> On 11/21/2013 08:09 PM, Laurent Pinchart wrote:
> > On Thursday 21 November 2013 16:22:02 Hans Verkuil wrote:
> >> From: Hans Verkuil <hans.verkuil@cisco.com>
> >> 
> >> If start_streaming returns -ENODATA, then it will be retried the next
> >> time a buffer is queued. This means applications no longer need to know
> >> how many buffers need to be queued before STREAMON can be called. This is
> >> particularly useful for output stream I/O.
> >> 
> >> If a DMA engine needs at least X buffers before it can start streaming,
> >> then for applications to get a buffer out as soon as possible they need
> >> to know the minimum number of buffers to queue before STREAMON can be
> >> called. You can't just try STREAMON after every buffer since on failure
> >> STREAMON will dequeue all your buffers. (Is that a bug or a feature?
> >> Frankly, I'm not sure).
> >> 
> >> This patch simplifies applications substantially: they can just call
> >> STREAMON at the beginning and then start queuing buffers and the DMA
> >> engine will kick in automagically once enough buffers are available.
> >> 
> >> This also fixes using write() to stream video: the fileio implementation
> >> calls streamon without having any queued buffers, which will fail today
> >> for any driver that requires a minimum number of buffers.
> >> 
> >> Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
> >> ---
> >> 
> >>  drivers/media/v4l2-core/videobuf2-core.c | 66 +++++++++++++++++++-------
> >>  include/media/videobuf2-core.h           | 15 ++++++--
> >>  2 files changed, 64 insertions(+), 17 deletions(-)
> >> 
> >> diff --git a/drivers/media/v4l2-core/videobuf2-core.c
> >> b/drivers/media/v4l2-core/videobuf2-core.c index 9ea3ae9..5bb91f7 100644
> >> --- a/drivers/media/v4l2-core/videobuf2-core.c
> >> +++ b/drivers/media/v4l2-core/videobuf2-core.c
> >> @@ -1332,6 +1332,39 @@ int vb2_prepare_buf(struct vb2_queue *q, struct
> >> v4l2_buffer *b) }
> >> 
> >>  EXPORT_SYMBOL_GPL(vb2_prepare_buf);
> >> 
> >> +/**
> >> + * vb2_start_streaming() - Attempt to start streaming.
> >> + * @q:		videobuf2 queue
> >> + *
> >> + * If there are not enough buffers, then retry_start_streaming is set to
> >> + * true and 0 is returned. The next time a buffer is queued and
> >> + * retry_start_streaming is true, this function will be called again to
> >> + * retry starting the DMA engine.
> >> + */
> >> +static int vb2_start_streaming(struct vb2_queue *q)
> >> +{
> >> +	int ret;
> >> +
> >> +	/* Tell the driver to start streaming */
> >> +	ret = call_qop(q, start_streaming, q, atomic_read(&q->queued_count));
> >> +
> >> +	/*
> >> +	 * If there are not enough buffers queued to start streaming, then
> >> +	 * the start_streaming operation will return -ENODATA and you have to
> >> +	 * retry when the next buffer is queued.
> >> +	 */
> >> +	if (ret == -ENODATA) {
> >> +		dprintk(1, "qbuf: not enough buffers, retry when more buffers are
> >> queued.\n");
> >> +		q->retry_start_streaming = true;
> >> +		return 0;
> >> +	}
> >> +	if (ret)
> >> +		dprintk(1, "qbuf: driver refused to start streaming\n");
> >> +	else
> >> +		q->retry_start_streaming = false;
> >> +	return ret;
> >> +}
> >> +
> >>  static int vb2_internal_qbuf(struct vb2_queue *q, struct v4l2_buffer *b)
> >>  {
> >>  	int ret = vb2_queue_or_prepare_buf(q, b, "qbuf");
> >> @@ -1377,6 +1410,12 @@ static int vb2_internal_qbuf(struct vb2_queue *q,
> >> struct v4l2_buffer *b) /* Fill buffer information for the userspace */
> >> 
> >>  	__fill_v4l2_buffer(vb, b);
> >> 
> >> +	if (q->retry_start_streaming) {
> >> +		ret = vb2_start_streaming(q);
> >> +		if (ret)
> >> +			return ret;
> >> +	}
> >> +
> >>  	dprintk(1, "%s() of buffer %d succeeded\n", __func__, vb-
> >> v4l2_buf.index);
> >> return 0;
> >>  }
> >> 
> >> @@ -1526,7 +1565,8 @@ int vb2_wait_for_all_buffers(struct vb2_queue *q)
> >>  		return -EINVAL;
> >>  	}
> >> 
> >> -	wait_event(q->done_wq, !atomic_read(&q->queued_count));
> >> +	if (!q->retry_start_streaming)
> >> +		wait_event(q->done_wq, !atomic_read(&q->queued_count));
> >>  	return 0;
> >>  
> >>  }
> >>  EXPORT_SYMBOL_GPL(vb2_wait_for_all_buffers);
> >> @@ -1640,6 +1680,9 @@ static void __vb2_queue_cancel(struct vb2_queue *q)
> >>  {
> >>  	unsigned int i;
> >> 
> >> +	if (q->retry_start_streaming)
> >> +		q->retry_start_streaming = q->streaming = 0;
> >> +
> >>  	/*
> >>  	 * Tell driver to stop all transactions and release all queued
> >>  	 * buffers.
> >> @@ -1689,12 +1732,9 @@ static int vb2_internal_streamon(struct vb2_queue
> >> *q, enum v4l2_buf_type type) list_for_each_entry(vb, &q->queued_list,
> >> queued_entry)
> >> 
> >>  		__enqueue_in_driver(vb);
> >> 
> >> -	/*
> >> -	 * Let driver notice that streaming state has been enabled.
> >> -	 */
> >> -	ret = call_qop(q, start_streaming, q, atomic_read(&q->queued_count));
> >> +	/* Tell driver to start streaming. */
> > 
> > Wouldn't it be better to reset q->retry_start_streaming to 0 here instead
> > of in the several other locations ?
> 
> I don't follow. retry_start_streaming is set only in vb2_start_streaming or
> in queue_cancel. I'm not sure what vb2_internal_streamon has to do with
> this.

My point is that the code would be simpler and less error-prone if we reset 
retry_start_streaming in a single location right before starting the stream 
instead of in multiple locations when stopping the stream. There would be less 
chances to introduce a bug when refactoring code in the future in that case.

> >> +	ret = vb2_start_streaming(q);
> >>  	if (ret) {
> >> -		dprintk(1, "streamon: driver refused to start streaming\n");
> >>  		__vb2_queue_cancel(q);
> >>  		return ret;
> >>  	}
> >> @@ -2264,15 +2304,15 @@ static int __vb2_init_fileio(struct vb2_queue *q,
> >> int read) goto err_reqbufs;
> >> 
> >>  			fileio->bufs[i].queued = 1;
> >>  		}
> >> -
> >> -		/*
> >> -		 * Start streaming.
> >> -		 */
> >> -		ret = vb2_streamon(q, q->type);
> >> -		if (ret)
> >> -			goto err_reqbufs;
> >>  	}
> >> 
> >> +	/*
> >> +	 * Start streaming.
> >> +	 */
> >> +	ret = vb2_streamon(q, q->type);
> >> +	if (ret)
> >> +		goto err_reqbufs;
> >> +
> >>  	q->fileio = fileio;
> >>  	
> >>  	return ret;

-- 
Regards,

Laurent Pinchart