Hi Hans, 2017-11-03 Hans Verkuil <hverkuil@xxxxxxxxx>: > 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? It should be -1 because we will be reusing the fence_fd field to return the out_fence to userspace in the cases we don't need to use the OUT_FENCE event. Like GPU drivers does with fences. That is the better way to send the out fence back that I can think of at the moment. > If you don't set the > IN_FENCE flag, what should userspace set fence_fd to? (I recommend 0). 0 is still a valid fd, so the implementation is currently accepting -1 and 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. Yep. That is what we agreed last week in Prague. > What does VIDIOC_QUERYBUF return w.r.t. the fence flags and fence_fd? It does return the IN_FENCE flag if the fence didn't signal yet and the OUT_FENCE one if the user set it on QBUF. The fence_fd is set to -1, because the fd is specific to the pid using it. Gustavo
