Hi Hans,
Le jeudi 10 juin 2021 à 13:36 +0200, Hans Verkuil a écrit :
> 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@xxxxxxxxx>
> ---
> .../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 07e54029e1e9..a1a0dbff4a6f 100644
> --- a/Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst
> +++ b/Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst
> @@ -601,6 +601,14 @@ 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
The minimum of 1 item is indeed logical for slices (or tiles in AV1), but I
think we should remove this limitation so it won't get in the way in other
applications. There is otherwise no way (other then creating a "invalid entry"
value) to signal that the array is now empty. The driver could erroneously endup
using ancient values.
Perhaps this can be waved later when we need it ? Would that be backward
compatible ?
> + array will change the ``elems`` field when the control is
> + queried afterwards.
>
> Return Value
> ============
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index 9260791b8438..7222fc855d6b 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -1869,6 +1869,7 @@ 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
