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

[Hans Verkuil <hverkuil@xs4all.nl>]

On 11/21/2013 08:09 PM, Laurent Pinchart wrote:
> Hi Hans,
> 
> Thank you for the patch.
> 
> 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.

Regards,

	Hans

> 
>> +	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;
>> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
>> index bd8218b..2d88897 100644
>> --- a/include/media/videobuf2-core.h
>> +++ b/include/media/videobuf2-core.h
>> @@ -250,10 +250,13 @@ struct vb2_buffer {
>>   *			receive buffers with @buf_queue callback before
>>   *			@start_streaming is called; the driver gets the number
>>   *			of already queued buffers in count parameter; driver
>> - *			can return an error if hardware fails or not enough
>> - *			buffers has been queued, in such case all buffers that
>> - *			have been already given by the @buf_queue callback are
>> - *			invalidated.
>> + *			can return an error if hardware fails, in that case all
>> + *			buffers that have been already given by the @buf_queue
>> + *			callback are invalidated.
>> + *			If there were not enough queued buffers to start
>> + *			streaming, then this callback returns -ENODATA, and the
>> + *			vb2 core will retry calling @start_streaming when a new
>> + *			buffer is queued.
>>   * @stop_streaming:	called when 'streaming' state must be disabled; driver
>>   *			should stop any DMA transactions or wait until they
>>   *			finish and give back all buffers it got from buf_queue()
>> @@ -321,6 +324,9 @@ struct v4l2_fh;
>>   * @done_wq:	waitqueue for processes waiting for buffers ready to be
>> dequeued * @alloc_ctx:	memory type/allocator-specific contexts for each
>> plane * @streaming:	current streaming state
>> + * @retry_start_streaming: start_streaming() was called, but there were not
>> enough + *		buffers queued. If set, then retry calling start_streaming 
> when
>> + *		queuing a new buffer.
>>   * @fileio:	file io emulator internal data, used only if emulator is 
> active
>> */
>>  struct vb2_queue {
>> @@ -353,6 +359,7 @@ struct vb2_queue {
>>  	unsigned int			plane_sizes[VIDEO_MAX_PLANES];
>>
>>  	unsigned int			streaming:1;
>> +	unsigned int			retry_start_streaming:1;
>>
>>  	struct vb2_fileio_data		*fileio;
>>  };