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

["Ming Qian(OSS)" <ming.qian@oss.nxp.com>]

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@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.

> 
> 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;
>