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    SubjectRe: [RFC v4 17/17] [media] v4l: Document explicit synchronization behaviour
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    On 10/20/2017 11:50 PM, Gustavo Padovan wrote:
    > From: Gustavo Padovan <gustavo.padovan@collabora.com>
    >
    > 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@collabora.com>
    > ---
    > 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
    > +``V4L2_BUF_FLAG_OUT_FENCE``.
    > +
    > +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?

    Regards,

    Hans

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