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SubjectRe: [RFC v5 01/11] [media] v4l: add V4L2_CAP_ORDERED to the uapi
Em Wed, 15 Nov 2017 15:10:47 -0200
Gustavo Padovan <gustavo@padovan.org> escreveu:

> From: Gustavo Padovan <gustavo.padovan@collabora.com>
>
> When using explicit synchronization userspace needs to know if
> the queue can deliver everything back in the same order, so we added
> a new capability that drivers can use to report that they are capable
> of keeping ordering.
>
> In videobuf2 core when using fences we also make sure to keep the ordering
> of buffers, so if the driver guarantees it too the whole pipeline inside
> V4L2 will be ordered and the V4L2_CAP_ORDERED should be used.
>
> Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
> ---
> Documentation/media/uapi/v4l/vidioc-querycap.rst | 3 +++
> include/uapi/linux/videodev2.h | 1 +
> 2 files changed, 4 insertions(+)
>
> diff --git a/Documentation/media/uapi/v4l/vidioc-querycap.rst b/Documentation/media/uapi/v4l/vidioc-querycap.rst
> index 66fb1b3d6e6e..ed3daa814da9 100644
> --- a/Documentation/media/uapi/v4l/vidioc-querycap.rst
> +++ b/Documentation/media/uapi/v4l/vidioc-querycap.rst
> @@ -254,6 +254,9 @@ specification the ioctl returns an ``EINVAL`` error code.
> * - ``V4L2_CAP_TOUCH``
> - 0x10000000
> - This is a touch device.
> + * - ``V4L2_CAP_ORDERED``
> + - 0x20000000
> + - The device queue is ordered.
> * - ``V4L2_CAP_DEVICE_CAPS``
> - 0x80000000
> - The driver fills the ``device_caps`` field. This capability can
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index 185d6a0acc06..cd6fc1387f47 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -459,6 +459,7 @@ struct v4l2_capability {
> #define V4L2_CAP_STREAMING 0x04000000 /* streaming I/O ioctls */
>
> #define V4L2_CAP_TOUCH 0x10000000 /* Is a touch device */
> +#define V4L2_CAP_ORDERED 0x20000000 /* Is the device queue ordered */

I guess we discussed that at the Linux Media summit.
The problem of making it a global flag is that drivers may support
ordered formats only for some of the formats. E. g., a driver that
delivers both MPEG and RGB output formats may deliver ordered
buffers for RGB, and unordered ones for MPEG.

So, instead of doing a global format at v4l2_capability, it is probably
better to use the flags field at struct v4l2_fmtdesc.

That would allow userspace to know in advance what formats support
it, by calling VIDIOC_ENUM_FMT.

Regards,
Mauro

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