Re: [PATCH v8 01/17] videodev2.h: add V4L2_CTRL_FLAG_DYNAMIC_ARRAY
From: Benjamin Gaignard
Date: Wed Jun 15 2022 - 06:18:28 EST
Le 15/06/2022 à 11:33, Hans Verkuil a écrit :
Hi Benjamin,
On 6/14/22 10:35, Benjamin Gaignard wrote:
From: Hans Verkuil <hverkuil-cisco@xxxxxxxxx>
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 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?
yes it possible to have 0 entry point offsets in bitstream but in this case
I think the stack won't use V4L2_CID_STATELESS_HEVC_ENTRY_POINT_OFFSETS because
it have not data to send.
I have introduce num_entry_point_offsets field because most of the drivers
only need this information and not the full offsets array so I think it is needed.
For now none of the drivers use V4L2_CID_STATELESS_HEVC_ENTRY_POINT_OFFSETS
control, it is here to be future proof.
Regards,
Benjamin
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