linux-kernel - Re: [PATCH v7 2/6] [media] v4l: add 'unordered' flag to format description ioctl

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Date:   Fri, 12 Jan 2018 13:05:06 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Gustavo Padovan <gustavo@...ovan.org>, linux-media@...r.kernel.org
Cc:     Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Shuah Khan <shuahkh@....samsung.com>,
        Pawel Osciak <pawel@...iak.com>,
        Alexandre Courbot <acourbot@...omium.org>,
        Sakari Ailus <sakari.ailus@....fi>,
        Brian Starkey <brian.starkey@....com>,
        Thierry Escande <thierry.escande@...labora.com>,
        linux-kernel@...r.kernel.org,
        Gustavo Padovan <gustavo.padovan@...labora.com>
Subject: Re: [PATCH v7 2/6] [media] v4l: add 'unordered' flag to format
 description ioctl

On 01/10/18 17:07, Gustavo Padovan wrote:
> From: Gustavo Padovan <gustavo.padovan@...labora.com>
> 
> For explicit synchronization it important for userspace to know if the
> format being used by the driver can deliver the buffers back to userspace
> in the same order they were queued with QBUF.
> 
> Ordered streams fits nicely in a pipeline with DRM for example, where
> ordered buffer are expected.
> 
> Signed-off-by: Gustavo Padovan <gustavo.padovan@...labora.com>
> ---
>  Documentation/media/uapi/v4l/vidioc-enum-fmt.rst | 3 +++
>  include/uapi/linux/videodev2.h                   | 1 +
>  2 files changed, 4 insertions(+)
> 
> diff --git a/Documentation/media/uapi/v4l/vidioc-enum-fmt.rst b/Documentation/media/uapi/v4l/vidioc-enum-fmt.rst
> index 019c513df217..368115f44fc0 100644
> --- a/Documentation/media/uapi/v4l/vidioc-enum-fmt.rst
> +++ b/Documentation/media/uapi/v4l/vidioc-enum-fmt.rst
> @@ -116,6 +116,9 @@ one until ``EINVAL`` is returned.
>        - This format is not native to the device but emulated through
>  	software (usually libv4l2), where possible try to use a native
>  	format instead for better performance.
> +    * - ``V4L2_FMT_FLAG_UNORDERED``
> +      - 0x0004
> +      - This is a format that doesn't guarantee timely order of frames.

I'd rephrase this:

"This format doesn't guarantee ordered buffer handling. I.e. the order in
which buffers are dequeued with VIDIOC_DQBUF may be different from the order
in which they were queued with VIDIOC_QBUF."

(Use proper links to VIDIOC_(D)QBUF)

I would also like to see an example of a driver that uses this. The cobalt
driver is a candidate for this.

Regards,

	Hans

>  
>  
>  Return Value
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index 982718965180..58894cfe9479 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -716,6 +716,7 @@ struct v4l2_fmtdesc {
>  
>  #define V4L2_FMT_FLAG_COMPRESSED 0x0001
>  #define V4L2_FMT_FLAG_EMULATED   0x0002
> +#define V4L2_FMT_FLAG_UNORDERED  0x0004
>  
>  	/* Frame Size and frame rate enumeration */
>  /*
>