From: Gustavo Padovan <gustavo.pado...@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.pado...@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.
+
+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``.
+
+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
