Re: [RFC v4 17/17] [media] v4l: Document explicit synchronization behaviour

[Brian Starkey]

On Fri, Oct 20, 2017 at 07:50:12PM -0200, Gustavo Padovan wrote:
> From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>
>
> Add section to VIDIOC_QBUF about it
>
> v3:
> 	- make the out_fence refer to the current buffer (Hans)
> 	- Note what happens when the IN_FENCE is not set (Hans)
>
> v2:
> 	- mention that fences are files (Hans)
> 	- rework for the new API
>
> Signed-off-by: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>
> ---
> Documentation/media/uapi/v4l/vidioc-qbuf.rst | 31 ++++++++++++++++++++++++++++
> 1 file changed, 31 insertions(+)
>
> diff --git a/Documentation/media/uapi/v4l/vidioc-qbuf.rst b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> index 9e448a4aa3aa..a65a50578bad 100644
> --- a/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> +++ b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> @@ -118,6 +118,37 @@ immediately with an ``EAGAIN`` error code when no buffer is available.
> The struct :c:type:`v4l2_buffer` structure is specified in
> :ref:`buffer`.
>
> +Explicit Synchronization
> +------------------------
> +
> +Explicit Synchronization allows us to control the synchronization of
> +shared buffers from userspace by passing fences to the kernel and/or
> +receiving them from it. Fences passed to the kernel are named in-fences and
> +the kernel should wait on them to signal before using the buffer, i.e., queueing
> +it to the driver. On the other side, the kernel can create out-fences for the
> +buffers it queues to the drivers. Out-fences signal when the driver is
> +finished with buffer, i.e., the buffer is ready. The fences are represented
> +as a file and passed as a file descriptor to userspace.
> +
> +The in-fences are communicated to the kernel at the ``VIDIOC_QBUF`` ioctl
> +using the ``V4L2_BUF_FLAG_IN_FENCE`` buffer
> +flags and the `fence_fd` field. If an in-fence needs to be passed to the kernel,
> +`fence_fd` should be set to the fence file descriptor number and the
> +``V4L2_BUF_FLAG_IN_FENCE`` should be set as well Setting one but not the other
> +will cause ``VIDIOC_QBUF`` to return with error.

nit: full-stop before "Setting".

Also, depending what is decided about passing in a -1 in-fence, the
wording might want to include that behaviour.

> +
> +The fence_fd field (formely the reserved2 field) will be ignored if the
> +``V4L2_BUF_FLAG_IN_FENCE`` is not set.
> +
> +To get an out-fence back from V4L2 the ``V4L2_BUF_FLAG_OUT_FENCE`` flag should
> +be set to ask for a fence to be attached to the buffer. To become aware of
> +the out-fence created one should listen for the ``V4L2_EVENT_OUT_FENCE`` event.
> +An event will be triggered for every buffer queued to the V4L2 driver with the
> +``V4L2_BUF_FLAG_OUT_FENCE``.

Is it worth mentioning the immediate out-fence behaviour for ordered
queues? And/or clarifying that otherwise the V4L2_EVENT_OUT_FENCE
event will not be sent until all in-fence(s) on the buffer (or
previously queued ones) have signalled.

Cheers,
-Brian

> +
> +At streamoff the out-fences will either signal normally if the drivers waits
> +for the operations on the buffers to finish or signal with error if the
> +driver cancels the pending operations.
>
> Return Value
> ============
> --
> 2.13.6