Messages in this thread | | | Date | Fri, 27 Oct 2017 11:08:23 +0100 | From | Brian Starkey <> | Subject | Re: [RFC v4 17/17] [media] v4l: Document explicit synchronization behaviour |
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On Fri, Oct 20, 2017 at 07:50:12PM -0200, 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 >+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 >
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