[PATCH v2 1/4] media: v4l: dev-decoder: Add source change V4L2_EVENT_SRC_CH_COLORSPACE

[Hans Verkuil]

On 08/04/2025 08:34, Ming Qian(OSS) wrote:
> Hi Hans,
> 
> On 2025/4/7 17:54, Hans Verkuil wrote:
>> On 17/01/2025 07:19, Ming Qian wrote:
>>> Add a new source change V4L2_EVENT_SRC_CH_COLORSPACE that
>>> indicates colorspace change in the stream.
>>> The change V4L2_EVENT_SRC_CH_RESOLUTION will always affect
>>> the allocation, but V4L2_EVENT_SRC_CH_COLORSPACE won't.
>>>
>>> Signed-off-by: Ming Qian <ming.qian at oss.nxp.com>
>>> ---
>>>   Documentation/userspace-api/media/v4l/vidioc-dqevent.rst | 9 +++++++++
>>>   .../userspace-api/media/videodev2.h.rst.exceptions       | 1 +
>>>   include/uapi/linux/videodev2.h                           | 1 +
>>>   3 files changed, 11 insertions(+)
>>>
>>> diff --git a/Documentation/userspace-api/media/v4l/vidioc-dqevent.rst b/Documentation/userspace-api/media/v4l/vidioc-dqevent.rst
>>> index 8db103760930..91e6b86c976d 100644
>>> --- a/Documentation/userspace-api/media/v4l/vidioc-dqevent.rst
>>> +++ b/Documentation/userspace-api/media/v4l/vidioc-dqevent.rst
>>> @@ -369,6 +369,15 @@ call.
>>>   	loss of signal and so restarting streaming I/O is required in order for
>>>   	the hardware to synchronize to the video signal.
>>>   
>>> +    * - ``V4L2_EVENT_SRC_CH_COLORSPACE``
>>> +      - 0x0002
>>> +      - This event gets triggered when a colorsapce change is detected at
>>
>> colorsapce -> colorspace
>>
> 
> Will fix in v3
> 
>>> +	an input. This can come from a video decoder. Applications will query
>>
>> It can also come from a video receiver. E.g. an HDMI source changes colorspace
>> signaling, but not the resolution.
>>
>>> +	the new colorspace information (if any, the signal may also have been
>>> +	lost)
>>
>> Missing . at the end. Also, if the signal is lost, then that is a CH_RESOLUTION
>> change, not CH_COLORSPACE.
>>
> OK, will fix in v3
>>> +
>>> +	For stateful decoders follow the guidelines in :ref:`decoder`.
>>
>> I think this should emphasize that if CH_COLORSPACE is set, but not CH_RESOLUTION,
>> then only the colorspace changed and there is no need to reallocate buffers.
>>
> 
> OK, will add in v3
> 
>> I also wonder if the description of CH_RESOLUTION should be enhanced to explain
>> that this might also imply a colorspace change. I'm not sure what existing codec
>> drivers do if there is a colorspace change but no resolution change.
> 
> I think there is no uniform behavior at the moment, it depends on the
> behavior of the decoder. Maybe most decoders ignore this.

Can you try to do a quick analysis of this? Don't spend too much time on this,
but it is helpful to have an idea of how existing codecs handle this.

Regards,

	Hans

> 
>>
>> I'm a bit concerned about backwards compatibility issues: if a userspace application
>> doesn't understand this new flag and just honors CH_RESOLUTION, then it would
>> never react to just a colorspace change.
>>
>> Nicolas, does gstreamer look at these flags?
> 
> I checked the gstreamer code, it does check this flag:
> 
> if (event.type == V4L2_EVENT_SOURCE_CHANGE &&
>      (event.u.src_change.changes & V4L2_EVENT_SRC_CH_RESOLUTION)) {
>    GST_DEBUG_OBJECT (v4l2object->dbg_obj,
>        "Can't streamon capture as the resolution have changed.");
>    ret = GST_V4L2_FLOW_RESOLUTION_CHANGE;
> }
> 
> Currently the gstreamer can't handle the CH_COLORSPACE flag.
> 
> Thanks,
> Ming
> 
>>
>> Regards,
>>
>> 	Hans
>>
>>> +
>>>   Return Value
>>>   ============
>>>   
>>> diff --git a/Documentation/userspace-api/media/videodev2.h.rst.exceptions b/Documentation/userspace-api/media/videodev2.h.rst.exceptions
>>> index 35d3456cc812..ac47c6d9448b 100644
>>> --- a/Documentation/userspace-api/media/videodev2.h.rst.exceptions
>>> +++ b/Documentation/userspace-api/media/videodev2.h.rst.exceptions
>>> @@ -526,6 +526,7 @@ replace define V4L2_EVENT_CTRL_CH_RANGE ctrl-changes-flags
>>>   replace define V4L2_EVENT_CTRL_CH_DIMENSIONS ctrl-changes-flags
>>>   
>>>   replace define V4L2_EVENT_SRC_CH_RESOLUTION src-changes-flags
>>> +replace define V4L2_EVENT_SRC_CH_COLORSPACE src-changes-flags
>>>   
>>>   replace define V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ :c:type:`v4l2_event_motion_det`
>>>   
>>> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
>>> index c8cb2796130f..242242c8e57b 100644
>>> --- a/include/uapi/linux/videodev2.h
>>> +++ b/include/uapi/linux/videodev2.h
>>> @@ -2559,6 +2559,7 @@ struct v4l2_event_frame_sync {
>>>   };
>>>   
>>>   #define V4L2_EVENT_SRC_CH_RESOLUTION		(1 << 0)
>>> +#define V4L2_EVENT_SRC_CH_COLORSPACE		(1 << 1)
>>>   
>>>   struct v4l2_event_src_change {
>>>   	__u32 changes;
>>
>