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

Hans Verkuil
2013-11-22T08:43:56Z

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;
>> };