On 10/20/2017 11:50 PM, 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 Missing '.' after 'as well'. To what value is fence_fd set when VIDIOC_QBUF returns? If you don't set the IN_FENCE flag, what should userspace set fence_fd to? (I recommend 0). > +will cause ``VIDIOC_QBUF`` to return with error. > + > +The fence_fd field (formely the reserved2 field) will be ignored if the Drop the "(formely the reserved2 field)" part. We're not interested in the history here. > +``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 drivers -> driver > +for the operations on the buffers to finish or signal with error if the error -> an error > +driver cancels the pending operations. > > Return Value > ============ > What should be done if the driver doesn't set ordered_in_driver? How does userspace know whether in and/or out fences are supported? I'm leaning towards a new capability flag for QUERYCAPS. What does VIDIOC_QUERYBUF return w.r.t. the fence flags and fence_fd? Regards, Hans
