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

[Ming Qian(OSS)]

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@xxxxxxxxxxx>
> > ---
> >   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;