Re: [PATCH v8 01/17] videodev2.h: add V4L2_CTRL_FLAG_DYNAMIC_ARRAY

Hans Verkuil
2022-06-15T09:33:54Z

Hi Benjamin,

On 6/14/22 10:35, Benjamin Gaignard wrote:
> From: Hans Verkuil <hverkuil-cisco@xs4all.nl>
>
> Add a new flag that indicates that this control is a dynamically sized
> array. Also document this flag.
>
> Currently dynamically sized arrays are limited to one dimensional arrays,
> but that might change in the future if there is a need for it.
>
> The initial use-case of dynamic arrays are stateless codecs. A frame
> can be divided in many slices, so you want to provide an array containing
> slice information for each slice. Typically the number of slices is small,
> but the standard allow for hundreds or thousands of slices. Dynamic arrays
> are a good solution since sizing the array for the worst case would waste
> substantial amounts of memory.
>
> Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
> ---
> .../userspace-api/media/v4l/vidioc-queryctrl.rst | 8 ++++++++
> include/uapi/linux/videodev2.h | 1 +
> 2 files changed, 9 insertions(+)
>
> diff --git a/Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst b/Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst
> index 88f630252d98..a20dfa2a933b 100644
> --- a/Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst
> +++ b/Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst
> @@ -625,6 +625,14 @@ See also the examples in :ref:`control`.
> ``V4L2_CTRL_FLAG_GRABBED`` flag when buffers are allocated or
> streaming is in progress since most drivers do not support changing
> the format in that case.
> + * - ``V4L2_CTRL_FLAG_DYNAMIC_ARRAY``
> + - 0x0800
> + - This control is a dynamically sized 1-dimensional array. It
> + behaves the same as a regular array, except that the number
> + of elements as reported by the ``elems`` field is between 1 and
> + ``dims[0]``. So setting the control with a differently sized
> + array will change the ``elems`` field when the control is
> + queried afterwards.

I am proposing a change to the dynamic array implementation: initially
dynamic array controls start off as empty arrays (0 elements). This also
allows userspace to set a dynamic array control to an empty array.

It probably would also make sense to add a min_dyn_elems to set the minimum
allowed number of elements for a dynamic array. This would most likely be
either 0 or 1.

In the context of this HEVC series, does it help to allow empty dynamic arrays?

For V4L2_CID_STATELESS_HEVC_ENTRY_POINT_OFFSETS in particular it looks like
it would make sense since I think (correct me if I am wrong) there can be 0
entry point offsets. So with empty arrays supported would the field
num_entry_point_offsets still be needed?

If you want to test, then let me know and I mail a very simple patch adding
support for empty arrays (not yet min_dyn_elems, though).

Regards,

Hans

>
> Return Value
> ============
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index 5311ac4fde35..9018aa984db3 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -1890,6 +1890,7 @@ struct v4l2_querymenu {
> #define V4L2_CTRL_FLAG_HAS_PAYLOAD 0x0100
> #define V4L2_CTRL_FLAG_EXECUTE_ON_WRITE 0x0200
> #define V4L2_CTRL_FLAG_MODIFY_LAYOUT 0x0400
> +#define V4L2_CTRL_FLAG_DYNAMIC_ARRAY 0x0800
>
> /* Query flags, to be ORed with the control ID */
> #define V4L2_CTRL_FLAG_NEXT_CTRL 0x80000000