linux-kernel - Re: [PATCH v2 1/4] media: v4l: dev-decoder: Add source change V4L2_EVENT_SRC_CH

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Message-ID: <3e5f003a-f689-4f5a-ac75-6bf95379637b@xs4all.nl>
Date: Mon, 7 Apr 2025 11:54:04 +0200
From: Hans Verkuil <hverkuil-cisco@...all.nl>
To: Ming Qian <ming.qian@....nxp.com>, mchehab@...nel.org
Cc: nicolas@...fresne.ca, shawnguo@...nel.org, robh+dt@...nel.org,
 s.hauer@...gutronix.de, kernel@...gutronix.de, festevam@...il.com,
 linux-imx@....com, xiahong.bao@....com, eagle.zhou@....com,
 tao.jiang_2@....com, imx@...ts.linux.dev, linux-media@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/4] media: v4l: dev-decoder: Add source change
 V4L2_EVENT_SRC_CH_COLORSPACE

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@....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

> +	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.

> +
> +	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.

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'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?

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;