From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx> Add section to VIDIOC_QBUF about it Signed-off-by: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx> --- Documentation/media/uapi/v4l/vidioc-qbuf.rst | 30 ++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) diff --git a/Documentation/media/uapi/v4l/vidioc-qbuf.rst b/Documentation/media/uapi/v4l/vidioc-qbuf.rst index 1f3612637200..6bd960d3972b 100644 --- a/Documentation/media/uapi/v4l/vidioc-qbuf.rst +++ b/Documentation/media/uapi/v4l/vidioc-qbuf.rst @@ -117,6 +117,36 @@ 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 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, that is the buffer is ready. + +The in-fences and out-fences are communicated at the ``VIDIOC_QBUF`` ioctl +using the ``V4L2_BUF_FLAG_IN_FENCE`` and ``V4L2_BUF_FLAG_OUT_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. + +To get a out-fence back from V4L2 the ``V4L2_BUF_FLAG_OUT_FENCE`` flag should +be set and the `fence_fd` field will be returned with the out-fence file +descriptor related to the next buffer to be queued internally to the V4L2 +driver. That means the out-fence may not be associated with the buffer in the +current ``VIDIOC_QBUF`` ioctl call because the ordering in which videobuf2 core +queues the buffers to the drivers can't be guaranteed. To become aware of the +buffer with which the out-fence will be attached the ``V4L2_EVENT_BUF_QUEUED`` +should be used. It will trigger an event for every buffer queued to the V4L2 +driver. + +Note that the `fence_fd` field is both an input and output argument here with +different meaning on each direction. As input argument it carries an in-fence +and as output argument it carries an out-fence. Return Value ============ -- 2.13.5
