linux-kernel - Re: [PATCH] media: videobuf2: add V4L2_BUF_FLAG

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Date:   Mon, 04 Jul 2022 11:52:54 -0400
From:   Nicolas Dufresne <nicolas@...fresne.ca>
To:     Ming Qian <ming.qian@....com>, mchehab@...nel.org,
        hverkuil-cisco@...all.nl
Cc:     shawnguo@...nel.org, robh+dt@...nel.org, s.hauer@...gutronix.de,
        kernel@...gutronix.de, festevam@...il.com, linux-imx@....com,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] media: videobuf2: add V4L2_BUF_FLAG_CODECCONFIG flag

Le mardi 28 juin 2022 à 10:19 +0800, Ming Qian a écrit :
> By setting the V4L2_BUF_FLAG_CODECCONFIG flag,
> user-space should be able to hint decoder
> the vb2 only contains codec config header,
> but does not contain any frame data.
> It's only used for parsing header, and can't be decoded.

This is copied from OMX specification. I think we we import this, we should at
least refer to the original.

> 
> Current, it's usually used by android.
> 
> Signed-off-by: Ming Qian <ming.qian@....com>
> ---
>  Documentation/userspace-api/media/v4l/buffer.rst | 9 +++++++++
>  include/uapi/linux/videodev2.h                   | 2 ++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/Documentation/userspace-api/media/v4l/buffer.rst b/Documentation/userspace-api/media/v4l/buffer.rst
> index 4638ec64db00..acdc4556f4f4 100644
> --- a/Documentation/userspace-api/media/v4l/buffer.rst
> +++ b/Documentation/userspace-api/media/v4l/buffer.rst
> @@ -607,6 +607,15 @@ Buffer Flags
>  	the format. Any subsequent call to the
>  	:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl will not block anymore,
>  	but return an ``EPIPE`` error code.
> +    * .. _`V4L2-BUF-FLAG-CODECCONFIG`:
> +
> +      - ``V4L2_BUF_FLAG_CODECCONFIG``
> +      - 0x00200000
> +      - This flag may be set when the buffer only contains codec config
> +    header, but does not contain any frame data. Usually the codec config
> +    header is merged to the next idr frame, with the flag
> +    ``V4L2_BUF_FLAG_KEYFRAME``, but there is still some scenes that will
> +    split the header and queue it separately.

I think the documentation is clear. Now, if a driver uses this, will existing
userland (perhaps good to check GStreamer, FFMPEG and Chromium ?) will break ?
So we need existing driver to do this when flagged to, and just copy/append when
the userland didn't opt-in that feature ?

>      * .. _`V4L2-BUF-FLAG-REQUEST-FD`:
>  
>        - ``V4L2_BUF_FLAG_REQUEST_FD``
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index 5311ac4fde35..8708ef257710 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -1131,6 +1131,8 @@ static inline __u64 v4l2_timeval_to_ns(const struct timeval *tv)
>  #define V4L2_BUF_FLAG_TSTAMP_SRC_SOE		0x00010000
>  /* mem2mem encoder/decoder */
>  #define V4L2_BUF_FLAG_LAST			0x00100000
> +/* Buffer only contains codec header */
> +#define V4L2_BUF_FLAG_CODECCONFIG		0x00200000
>  /* request_fd is valid */
>  #define V4L2_BUF_FLAG_REQUEST_FD		0x00800000
>