Re: [PATCH v9 15/15] v4l: Document explicit synchronization behavior

[Hans Verkuil]

On 04/05/18 22:06, Ezequiel Garcia wrote:
> From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>
> 
> Add section to VIDIOC_QBUF and VIDIOC_QUERY_BUF about it
> 
> v8: amend querybuf documentation.
> 
> v7: minor issues and English improvements (Hans Verkuil)
> 
> 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 <gustavo.padovan@xxxxxxxxxxxxx>
> Signed-off-by: Ezequiel Garcia <ezequiel@xxxxxxxxxxxxx>
> ---
>  Documentation/media/uapi/v4l/vidioc-qbuf.rst     | 54 +++++++++++++++++++++++-
>  Documentation/media/uapi/v4l/vidioc-querybuf.rst | 12 ++++--
>  2 files changed, 62 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/media/uapi/v4l/vidioc-qbuf.rst b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> index 9e448a4aa3aa..88a51b8bec7e 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 +118,58 @@ 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 othe will cause ``VIDIOC_QBUF`` to return

othe -> other

> +with an error.
> +
> +The videobuf2-core will guarantee that all buffers queued with an in-fence will

'It is guaranteed that'

No need to refer to internal frameworks in userspace documentation.

> +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

s/at videobuf2//
s/when/while/

> +waiting on a fence to signal all buffers queued afterwards will also be blocked
> +until that fence 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 is preserved.
> +
> +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 in the `fence_fd` field.
> +
> +Note the same `fence_fd` field is used for both sending the in-fence as
> +at input argument and to receive the out-fence as a return argument. A buffer can
> +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

s/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

orderly -> ordered?

> +out-fence to the DRM/KMS driver and rest sure that the buffers will be shown on

s/sure/assured/

> +the screen at the correct order. If an ordered queue can not be set then such

s/at/in/

'If the queue is not ordered, then...'

> +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..cda73b43c334 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`. Similarly, the ``V4L2_BUF_FLAG_OUT_FENCE`` will be
> +set if there's a pending out-fence for the buffer. Note that the ``fence_fd``
> +field is not set, because the file descriptor only makes sense
> +for the process that received the out-fence in the :ref:`VIDIOC_QBUF` response.
> +
> +The struct :c:type:`v4l2_buffer` structure is specified in :ref:`buffer`.
>  
>  
>  Return Value
> 

Regards,

	Hans