Re: [PATCH v5 09/10] media: v4l2: Add DELETE_BUFS ioctl

[Hans Verkuil <hverkuil-cisco@xs4all.nl>]

On 24/08/2023 11:21, Benjamin Gaignard wrote:
> VIDIOC_DELETE_BUFS ioctl allows to delete buffers from a queue.
> The number of buffers to delete in given by count field of
> struct v4l2_delete_buffers and the range start at the index
> specified in the same structure.
> 
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
> ---
>  .../userspace-api/media/v4l/user-func.rst     |  1 +
>  .../media/v4l/vidioc-delete-bufs.rst          | 73 +++++++++++++++++++
>  .../media/common/videobuf2/videobuf2-core.c   | 34 +++++++++
>  .../media/common/videobuf2/videobuf2-v4l2.c   | 16 ++++
>  drivers/media/v4l2-core/v4l2-dev.c            |  1 +
>  drivers/media/v4l2-core/v4l2-ioctl.c          | 17 +++++
>  include/media/v4l2-ioctl.h                    |  4 +
>  include/media/videobuf2-core.h                |  9 +++
>  include/media/videobuf2-v4l2.h                | 11 +++
>  include/uapi/linux/videodev2.h                | 17 +++++
>  10 files changed, 183 insertions(+)
>  create mode 100644 Documentation/userspace-api/media/v4l/vidioc-delete-bufs.rst
> 
> diff --git a/Documentation/userspace-api/media/v4l/user-func.rst b/Documentation/userspace-api/media/v4l/user-func.rst
> index 15ff0bf7bbe6..3fd567695477 100644
> --- a/Documentation/userspace-api/media/v4l/user-func.rst
> +++ b/Documentation/userspace-api/media/v4l/user-func.rst
> @@ -17,6 +17,7 @@ Function Reference
>      vidioc-dbg-g-chip-info
>      vidioc-dbg-g-register
>      vidioc-decoder-cmd
> +    vidioc-delete-bufs
>      vidioc-dqevent
>      vidioc-dv-timings-cap
>      vidioc-encoder-cmd
> diff --git a/Documentation/userspace-api/media/v4l/vidioc-delete-bufs.rst b/Documentation/userspace-api/media/v4l/vidioc-delete-bufs.rst
> new file mode 100644
> index 000000000000..a55fe6331fc8
> --- /dev/null
> +++ b/Documentation/userspace-api/media/v4l/vidioc-delete-bufs.rst
> @@ -0,0 +1,73 @@
> +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
> +.. c:namespace:: V4L
> +
> +.. _VIDIOC_DELETE_BUFS:
> +
> +************************
> +ioctl VIDIOC_DELETE_BUFS
> +************************
> +
> +Name
> +====
> +
> +VIDIOC_DELETE_BUFS - Deletes buffers from a queue
> +
> +Synopsis
> +========
> +
> +.. c:macro:: VIDIOC_DELETE_BUFs
> +
> +``int ioctl(int fd, VIDIOC_DELETE_BUFs, struct v4l2_delete_buffers *argp)``
> +
> +Arguments
> +=========
> +
> +``fd``
> +    File descriptor returned by :c:func:`open()`.
> +
> +``argp``
> +    Pointer to struct :c:type:`v4l2_delete_buffers`.
> +
> +Description
> +===========
> +
> +Applications can optionally call the :ref:`VIDIOC_DELETE_BUFS` ioctl to
> +delete buffers from a queue.
> +
> +.. c:type:: v4l2_delete_buffers
> +
> +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
> +
> +.. flat-table:: struct v4l2_delete_buffers
> +    :header-rows:  0
> +    :stub-columns: 0
> +    :widths:       1 1 2
> +
> +    * - __u32
> +      - ``index``
> +      - The starting buffer index to delete.
> +    * - __u32
> +      - ``count``
> +      - The number of buffers to be deleted.
> +    * - __u32
> +      - ``type``
> +      - Type of the stream or buffers, this is the same as the struct
> +	:c:type:`v4l2_format` ``type`` field. See
> +	:c:type:`v4l2_buf_type` for valid values.
> +    * - __u32
> +      - ``reserved``\ [13]
> +      - A place holder for future extensions. Drivers and applications
> +	must set the array to zero.
> +
> +Return Value
> +============
> +
> +On success 0 is returned, on error -1 and the ``errno`` variable is set
> +appropriately. The generic error codes are described at the
> +:ref:`Generic Error Codes <gen-errors>` chapter.
> +
> +EBUSY
> +    File I/O is in progress.
> +
> +EINVAL
> +    The buffer ``index`` doesn't exist in the queue.
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index 70e36389b704..3d915b4c33b2 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -1634,6 +1634,40 @@ int vb2_core_prepare_buf(struct vb2_queue *q, unsigned int index, void *pb)
>  }
>  EXPORT_SYMBOL_GPL(vb2_core_prepare_buf);
>  
> +int vb2_core_delete_buf(struct vb2_queue *q, unsigned int index)

I didn't realize this the first time I reviewed this, but I would
suggest that you change this to

vb2_core_delete_bufs(struct vb2_queue *q, unsigned int index, unsigned int count)

This is consistent with vb2_core_create_bufs, and I strongly suspect it will
help with locking (mmap_lock), since you don't need to lock for every buffer you
delete, you can do it up front.

And you can move the index/count validation to the core as well, that will help
if e.g. dvb would want to use this ioctl in the future.

Regards,

	Hans