media: videobuf2: Add min_reqbufs_allocation field to vb2_queue structure

Add 'min_reqbufs_allocation' field in the vb2_queue structure so drivers
can specify the minimum number of buffers to allocate when calling
VIDIOC_REQBUFS.
When initializing the queue, v4l2 core makes sure that the following
constraints are respected:
- the minimum number of buffers to allocate must be at least 2 because
one buffer is used by the hardware while the other is being processed
by userspace.
-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@collabora.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

1 files changed, 14 insertions, 1 deletions

diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
index 667bf9ee1101..4a8b9135cec8 100644
--- a/include/media/videobuf2-core.h
+++ b/include/media/videobuf2-core.h
@@ -549,9 +549,21 @@ 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.
  * @alloc_devs:	&struct device memory type/allocator-specific per-plane device
  */
 /*
@@ -622,6 +634,7 @@ 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];