On Fri, Oct 20, 2017 at 07:50:12PM -0200, Gustavo Padovan wrote:
From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> --- 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
