[PATCH v5 2/3] videobuf2: Add min_reqbufs_allocation field to vb2_queue structure

[Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxxxxx>]

Add 'min_reqbufs_allocation' field in vb2_queue structure so drivers
can specificy the minimum number of buffers to allocate when calling
VIDIOC_REQBUFS.
If used this minimum should be higher than the minimum number of
queued buffers needed to start streaming.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxxxxx>
---
version 5:
- Add WARN_ON() vb2_core_queue_init()
- Check min_reqbufs_allocation value in vb2_core_queue_init()
  so it is at least equal to min_queued_buffers + 1.
- Rework min_reqbufs_allocation documentation.

 drivers/media/common/videobuf2/videobuf2-core.c | 10 ++++++++--
 include/media/videobuf2-core.h                  | 13 +++++++++++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
index 8912dff5bde3..a183edf11586 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 + 1);
+	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 +2521,12 @@ int vb2_core_queue_init(struct vb2_queue *q)
 	if (WARN_ON(q->supports_requests && q->min_queued_buffers))
 		return -EINVAL;
 
+	q->min_reqbufs_allocation = max_t(unsigned int,
+					  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);
diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
index f9a00b6c3c46..2ed57749a280 100644
--- a/include/media/videobuf2-core.h
+++ b/include/media/videobuf2-core.h
@@ -553,6 +553,18 @@ struct vb2_buf_ops {
  *		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. Drivers can set this if there has to
+ *		be a certain number of buffers available for the hardware to
+ *		work effectively. If set, then @min_reqbufs_allocation must be
+ *		larger than @min_queued_buffers + 1.
+ *		This field is only used by VIDIOC_REQBUFS. This allows calling
+ *		that ioctl with a buffer count of 1 and it will be automatically
+ *		adjusted to a workable buffer count. VIDIOC_CREATE_BUFS will not
+ *		modify the requested buffer count.
+ *		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];
 
-- 
2.40.1