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SubjectRe: [PATCH v8 13/13] [media] v4l: Document explicit synchronization behavior
On 03/09/2018 09:49 AM, Gustavo Padovan wrote:
> From: Gustavo Padovan <>
> Add section to VIDIOC_QBUF and VIDIOC_QUERY_BUF about it
> v6: - Close some gaps in the docs (Hans)
> v5:
> - Remove V4L2_CAP_ORDERED
> - Add doc about V4L2_FMT_FLAG_UNORDERED
> v4:
> - Document ordering behavior for in-fences
> - Document V4L2_CAP_ORDERED capability
> - Remove doc about OUT_FENCE event
> - Document immediate return of out-fence in QBUF
> 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 | 55 +++++++++++++++++++++++-
> Documentation/media/uapi/v4l/vidioc-querybuf.rst | 12 ++++--
> 2 files changed, 63 insertions(+), 4 deletions(-)
> diff --git a/Documentation/media/uapi/v4l/vidioc-qbuf.rst b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> index 9e448a4aa3aa..371d84966e34 100644
> --- a/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> +++ b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> @@ -54,7 +54,7 @@ When the buffer is intended for output (``type`` is
> or ``V4L2_BUF_TYPE_VBI_OUTPUT``) applications must also initialize the
> ``bytesused``, ``field`` and ``timestamp`` fields, see :ref:`buffer`
> for details. Applications must also set ``flags`` to 0. The
> -``reserved2`` and ``reserved`` fields must be set to 0. When using the
> +``reserved`` field must be set to 0. When using the
> :ref:`multi-planar API <planar-apis>`, the ``m.planes`` field must
> contain a userspace pointer to a filled-in array of struct
> :c:type:`v4l2_plane` and the ``length`` field must be set
> @@ -118,6 +118,59 @@ 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. 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 flag 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 an error.

This sentence is confusing since it is not clear what 'one' and 'the other' refer
to. Be specific here. I think it should be 'Setting V4L2_BUF_FLAG_IN_FENCE but not

The fence_fd field will be ignored if the
> +``V4L2_BUF_FLAG_IN_FENCE`` is not set.
> +
> +The videobuf2-core will guarantee that all buffers queued with an in-fence will
> +be queued to the drivers in the same order. Fences may signal out of order, so
> +this guarantee at videobuf2 is necessary to not change ordering. So when
> +waiting on a fence to signal all buffers queued after will be also block until

after -> afterwards
will be also block -> will also be blocked

> +that fence signal.

signal -> signals

> +
> +If the in-fence signals with an error the buffer will be marked with
> +``V4L2_BUF_FLAG_ERROR`` when returned to userspace at ``VIDIOC_DQBUF``.
> +Even with the error the order of dequeueing the buffers are preserved.

are -> is

> +
> +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. The out-fence fd is
> +sent to userspace as a ``VIDIOC_QBUF`` return argument on the `fence_fd` field.

on -> in

> +
> +Note the the same `fence_fd` field is used for both sending the in-fence as
> +input argument to receive the out-fence as a return argument. A buffer can

input argument -> an input argument and

> +have both in-fence ond out-fence.

have both an in-fence and an out-fence.

> +
> +At streamoff the out-fences will either signal normally if the driver waits
> +for the operations on the buffers to finish or signal with an error if the
> +driver cancels the pending operations. Buffers with in-fences won't be queued
> +to the driver if their fences signal. They will be cleaned up.
> +
> +The ``V4L2_FMT_FLAG_UNORDERED`` flag in ``VIDIOC_ENUM_FMT`` tells userspace
> +that the when using this format the order in which buffers are dequeued can

remove 'the'

> +be different from the order in which they were queued.
> +
> +Ordering is important to fences because it can optimize the pipeline with
> +other drivers like a DRM/KMS display driver. For example, if a capture from the
> +camera is happening in an orderly manner one can send the capture buffer
> +out-fence to the DRM/KMS driver and rest sure that the buffers will be shown on

rest sure -> rest assured

But I'd rephrase it as: be certain

> +the screen at the correct order. If an ordered queue can not be set then such
> +arrangements with other drivers may not be possible.
> Return Value
> ============
> diff --git a/Documentation/media/uapi/v4l/vidioc-querybuf.rst b/Documentation/media/uapi/v4l/vidioc-querybuf.rst
> index dd54747fabc9..9aa3358bee0b 100644
> --- a/Documentation/media/uapi/v4l/vidioc-querybuf.rst
> +++ b/Documentation/media/uapi/v4l/vidioc-querybuf.rst
> @@ -44,7 +44,7 @@ and the ``index`` field. Valid index numbers range from zero to the
> number of buffers allocated with
> :ref:`VIDIOC_REQBUFS` (struct
> :c:type:`v4l2_requestbuffers` ``count``) minus
> -one. The ``reserved`` and ``reserved2`` fields must be set to 0. When
> +one. The ``reserved`` field must be set to 0. When
> using the :ref:`multi-planar API <planar-apis>`, the ``m.planes``
> field must contain a userspace pointer to an array of struct
> :c:type:`v4l2_plane` and the ``length`` field has to be set
> @@ -64,8 +64,14 @@ elements will be used instead and the ``length`` field of struct
> array elements. The driver may or may not set the remaining fields and
> flags, they are meaningless in this context.
> -The struct :c:type:`v4l2_buffer` structure is specified in
> -:ref:`buffer`.
> +When using in-fences, the ``V4L2_BUF_FLAG_IN_FENCE`` will be set if the
> +in-fence didn't signal at the time of the
> +:ref:`VIDIOC_QUERYBUF`. The ``V4L2_BUF_FLAG_OUT_FENCE`` will be set if
> +the user asked for an out-fence for the buffer and the ``fence_fd``
> +field will be set to the out-fence fd. In case ``V4L2_BUF_FLAG_OUT_FENCE`` is
> +not set ``fence_fd`` will be set to 0 for backward compatibility.
> +
> +The struct :c:type:`v4l2_buffer` structure is specified in :ref:`buffer`.
> Return Value



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