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

[John Cox]

Hi

>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?

I'd assumed that the control simply wouldn't be set if empty, but yes I
think it makes sense to allow empty otherwise there would be no way of
clearing the control if it was ever set?

>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 - assuming that the entry_point array is still 1D (I think that was
what was decided) then still you need it to work out which entry points
belong to which slice.

Regards

JC

>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