From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx> Add section to VIDIOC_QBUF about it 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 1f3612637200..fae0b1431672 100644 --- a/Documentation/media/uapi/v4l/vidioc-qbuf.rst +++ b/Documentation/media/uapi/v4l/vidioc-qbuf.rst @@ -117,6 +117,37 @@ 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 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 fence are represented +by file and passed as 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. Failure to set both will +cause ``VIDIOC_QBUF`` to return with error. + +To get a out-fence back from V4L2 the ``V4L2_BUF_FLAG_OUT_FENCE`` flag should +be set to notify it that the next queued buffer should have a fence attached to +it. 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 +of the next queued buffer and the out-fence attached to it the +``V4L2_EVENT_BUF_QUEUED`` event should be used. It will trigger an event +for every buffer queued to the V4L2 driver. + +At streamoff the out-fences will either signal normally if the drivers wait +for the operations on the buffers to finish or signal with error if the +driver cancel the pending operations. Return Value ============ -- 2.13.5
