Re: [PATCH v17 1/8] videobuf2: Add min_reqbufs_allocation field to vb2_queue structure

[Hans Verkuil]

The language needs tightening up a bit:

On 19/01/2024 10:49, Benjamin Gaignard wrote:
> Add 'min_reqbufs_allocation' field in vb2_queue structure so drivers

in -> in the

> can specificy the minimum number of buffers to allocate when calling

specificy -> specify

> VIDIOC_REQBUFS.
> When initializing the queue, v4l2 core makes sure that the following
> constraint are respected:

constraint -> constraints

> - minimum number of buffers to allocate should be at least 2 because

minimum -> the minimum
should -> must

> one buffer is used by the hardware and other processed by userspace.

and other -> while the other is being

> - if a minimum number of buffers before start streaming is specify
> the minimum number of buffers to allocate should be min_queued_buffers
> + 1 to keep one buffer available for userspace.

Replace this with:

- if the driver needs 'min_queued_buffers' in the queue before calling
start_streaming(), then the minimum requirement is 'min_queued_buffers + 1'
to keep at least one buffer available for userspace.

> 
> Simplify __vb2_init_fileio() by using 'min_reqbufs_allocation' directly
> to avoid duplicating the minimum number of buffers to allocate computation.
> 
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxxxxx>
> ---
> version 17:
> - rewrite commit message.
> - simplify __vb2_init_fileio() code.
> - rewrite documentation
> 
>  .../media/common/videobuf2/videobuf2-core.c   | 37 +++++++++++--------
>  include/media/videobuf2-core.h                | 15 +++++++-
>  2 files changed, 36 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index b6bf8f232f48..d74e93d00f58 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -865,7 +865,7 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
>  	/*
>  	 * Make sure the requested values and current defaults are sane.
>  	 */
> -	num_buffers = max_t(unsigned int, *count, q->min_queued_buffers);
> +	num_buffers = max_t(unsigned int, *count, q->min_reqbufs_allocation);
>  	num_buffers = min_t(unsigned int, num_buffers, q->max_num_buffers);
>  	memset(q->alloc_devs, 0, sizeof(q->alloc_devs));
>  	/*
> @@ -917,7 +917,7 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
>  	 * There is no point in continuing if we can't allocate the minimum
>  	 * number of buffers needed by this vb2_queue.
>  	 */
> -	if (allocated_buffers < q->min_queued_buffers)
> +	if (allocated_buffers < q->min_reqbufs_allocation)
>  		ret = -ENOMEM;
>  
>  	/*
> @@ -2521,6 +2521,24 @@ int vb2_core_queue_init(struct vb2_queue *q)
>  	if (WARN_ON(q->supports_requests && q->min_queued_buffers))
>  		return -EINVAL;
>  
> +	/*
> +	 * The minimum requirement should really be 2: one buffer is used

should really be -> is

> +	 * by the hardware and one other processed by userspace.

and one other -> while the other is being

> +	 */
> +	if (q->min_reqbufs_allocation < 2)
> +		q->min_reqbufs_allocation = 2;
> +
> +	/*
> +	 * If the driver needs 'min_queued_buffers' before start streaming

before -> in the queue before calling
start streaming -> calling start_streaming(),

> +	 * then the minimum requirement is min_queued_buffers + 1 to keep

Add quotes around 'min_queued_buffers + 1'

> +	 * at least one buffer available for userspace
> +	 */
> +	if (q->min_reqbufs_allocation < q->min_queued_buffers + 1)
> +		q->min_reqbufs_allocation = q->min_queued_buffers + 1;
> +
> +	if (WARN_ON(q->min_reqbufs_allocation > q->max_num_buffers))
> +		return -EINVAL;
> +
>  	INIT_LIST_HEAD(&q->queued_list);
>  	INIT_LIST_HEAD(&q->done_list);
>  	spin_lock_init(&q->done_lock);
> @@ -2713,7 +2731,6 @@ static int __vb2_init_fileio(struct vb2_queue *q, int read)
>  	struct vb2_fileio_data *fileio;
>  	struct vb2_buffer *vb;
>  	int i, ret;
> -	unsigned int count = 0;
>  
>  	/*
>  	 * Sanity check
> @@ -2734,18 +2751,8 @@ static int __vb2_init_fileio(struct vb2_queue *q, int read)
>  	if (q->streaming || vb2_get_num_buffers(q) > 0)
>  		return -EBUSY;
>  
> -	/*
> -	 * Start with q->min_queued_buffers + 1, driver can increase it in
> -	 * queue_setup()
> -	 *
> -	 * 'min_queued_buffers' buffers need to be queued up before you
> -	 * can start streaming, plus 1 for userspace (or in this case,
> -	 * kernelspace) processing.
> -	 */
> -	count = max(2, q->min_queued_buffers + 1);
> -
>  	dprintk(q, 3, "setting up file io: mode %s, count %d, read_once %d, write_immediately %d\n",
> -		(read) ? "read" : "write", count, q->fileio_read_once,
> +		(read) ? "read" : "write", q->min_reqbufs_allocation, q->fileio_read_once,
>  		q->fileio_write_immediately);
>  
>  	fileio = kzalloc(sizeof(*fileio), GFP_KERNEL);
> @@ -2759,7 +2766,7 @@ static int __vb2_init_fileio(struct vb2_queue *q, int read)
>  	 * Request buffers and use MMAP type to force driver
>  	 * to allocate buffers by itself.
>  	 */
> -	fileio->count = count;
> +	fileio->count = q->min_reqbufs_allocation;
>  	fileio->memory = VB2_MEMORY_MMAP;
>  	fileio->type = q->type;
>  	q->fileio = fileio;
> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> index 56719a26a46c..fe3423ff3807 100644
> --- a/include/media/videobuf2-core.h
> +++ b/include/media/videobuf2-core.h
> @@ -550,9 +550,21 @@ struct vb2_buf_ops {
>   *		@start_streaming can be called. Used when a DMA engine
>   *		cannot be started unless at least this number of buffers
>   *		have been queued into the driver.
> - *		VIDIOC_REQBUFS will ensure at least @min_queued_buffers
> + *		VIDIOC_REQBUFS will ensure at least @min_queued_buffers + 1
>   *		buffers will be allocated. Note that VIDIOC_CREATE_BUFS will not
>   *		modify the requested buffer count.
> + * @min_reqbufs_allocation: the minimum number of buffers to be allocated when
> + *		calling VIDIOC_REQBUFS. Note that VIDIOC_CREATE_BUFS will *not*
> + *		modify the requested buffer count and does not use this field.
> + *		Drivers can set this if there has to be a certain number of
> + *		buffers available for the hardware to work effectively.
> + *		This allows calling VIDIOC_REQBUFS with a buffer count of 1 and
> + *		it will be automatically adjusted to a workable	buffer count.
> + *		If set, then @min_reqbufs_allocation must be larger than
> + *		@min_queued_buffers + 1.
> + *		If this field is > 3, then it is highly recommended that the
> + *		driver implements the V4L2_CID_MIN_BUFFERS_FOR_CAPTURE/OUTPUT
> + *		control.
>   */
>  /*
>   * Private elements (won't appear at the uAPI book):
> @@ -618,6 +630,7 @@ struct vb2_queue {
>  	u32				timestamp_flags;
>  	gfp_t				gfp_flags;
>  	u32				min_queued_buffers;
> +	u32				min_reqbufs_allocation;
>  
>  	struct device			*alloc_devs[VB2_MAX_PLANES];
>