On 03/13/2018 08:33 PM, Hans Verkuil wrote: > On 03/09/2018 09:49 AM, Gustavo Padovan wrote: >> From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx> >> >> Add section to VIDIOC_QBUF and VIDIOC_QUERY_BUF about it >> >> v6: - Close some gaps in the docs (Hans) >> >> v5: >> - Remove V4L2_CAP_ORDERED >> - Add doc about V4L2_FMT_FLAG_UNORDERED >> >> v4: >> - Document ordering behavior for in-fences >> - Document V4L2_CAP_ORDERED capability >> - Remove doc about OUT_FENCE event >> - Document immediate return of out-fence in QBUF >> >> 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 | 55 +++++++++++++++++++++++- >> Documentation/media/uapi/v4l/vidioc-querybuf.rst | 12 ++++-- >> 2 files changed, 63 insertions(+), 4 deletions(-) >> >> diff --git a/Documentation/media/uapi/v4l/vidioc-qbuf.rst b/Documentation/media/uapi/v4l/vidioc-qbuf.rst >> index 9e448a4aa3aa..371d84966e34 100644 >> --- a/Documentation/media/uapi/v4l/vidioc-qbuf.rst >> +++ b/Documentation/media/uapi/v4l/vidioc-qbuf.rst >> @@ -54,7 +54,7 @@ When the buffer is intended for output (``type`` is >> or ``V4L2_BUF_TYPE_VBI_OUTPUT``) applications must also initialize the >> ``bytesused``, ``field`` and ``timestamp`` fields, see :ref:`buffer` >> for details. Applications must also set ``flags`` to 0. The >> -``reserved2`` and ``reserved`` fields must be set to 0. When using the >> +``reserved`` field must be set to 0. When using the >> :ref:`multi-planar API <planar-apis>`, the ``m.planes`` field must >> contain a userspace pointer to a filled-in array of struct >> :c:type:`v4l2_plane` and the ``length`` field must be set >> @@ -118,6 +118,59 @@ 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. 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 flag 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 an error. > > This sentence is confusing since it is not clear what 'one' and 'the other' refer > to. Be specific here. I think it should be 'Setting V4L2_BUF_FLAG_IN_FENCE but not > fence_fd'. Ignore this comment. > > The fence_fd field will be ignored if the >> +``V4L2_BUF_FLAG_IN_FENCE`` is not set. Looking at the code, I don't think this is correct. You get an error if IN_FENCE is not set but fence_fd is. Removing this sentence would fix this and the previous sentence would make a lot more sense. >> + >> +The videobuf2-core will guarantee that all buffers queued with an in-fence will >> +be queued to the drivers in the same order. Fences may signal out of order, so >> +this guarantee at videobuf2 is necessary to not change ordering. So when >> +waiting on a fence to signal all buffers queued after will be also block until > > after -> afterwards > will be also block -> will also be blocked > >> +that fence signal. > > signal -> signals What is missing in the documentation is what happens when you mix in-fence buffers and buffers without an in-fence. Regards, Hans
