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Date: Wed, 24 Jan 2024 12:31:26 +0100
From: Hans Verkuil <hverkuil@...all.nl>
To: Benjamin Gaignard <benjamin.gaignard@...labora.com>, mchehab@...nel.org
Cc: linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
kernel@...labora.com
Subject: Re: [PATCH v17 1/8] videobuf2: Add min_reqbufs_allocation field to
vb2_queue structure
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@...labora.com>
> ---
> 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];
>
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