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Date:   Fri, 3 Nov 2017 08:53:52 +0100
From:   Hans Verkuil <>
To:     Gustavo Padovan <>,
Cc:     Mauro Carvalho Chehab <>,
        Shuah Khan <>,
        Pawel Osciak <>,
        Alexandre Courbot <>,
        Sakari Ailus <>,
        Brian Starkey <>,,
        Gustavo Padovan <>
Subject: Re: [RFC v4 17/17] [media] v4l: Document explicit synchronization

On 10/20/2017 11:50 PM, Gustavo Padovan wrote:
> From: Gustavo Padovan <>
> 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 <>
> ---
>  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

Missing '.' after 'as well'.

To what value is fence_fd set when VIDIOC_QBUF returns? If you don't set the
IN_FENCE flag, what should userspace set fence_fd to? (I recommend 0).

> +will cause ``VIDIOC_QBUF`` to return with error.
> +
> +The fence_fd field (formely the reserved2 field) will be ignored if the

Drop the "(formely the reserved2 field)" part. We're not interested in the
history here.

> +``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
> +
> +At streamoff the out-fences will either signal normally if the drivers waits

drivers -> driver

> +for the operations on the buffers to finish or signal with error if the

error -> an error

> +driver cancels the pending operations.
>  Return Value
>  ============

What should be done if the driver doesn't set ordered_in_driver? How does userspace
know whether in and/or out fences are supported? I'm leaning towards a new capability
flag for QUERYCAPS.

What does VIDIOC_QUERYBUF return w.r.t. the fence flags and fence_fd?



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