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. The fence_fd field will be ignored if the +``V4L2_BUF_FLAG_IN_FENCE`` is not set. + +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 +that fence signal. + +If the in-fence signals with an error the buffer will be marked with +``V4L2_BUF_FLAG_ERROR`` when returned to userspace at ``VIDIOC_DQBUF``. +Even with the error the order of dequeueing the buffers are preserved. + +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. The out-fence fd is +sent to userspace as a ``VIDIOC_QBUF`` return argument on the `fence_fd` field. + +Note the the same `fence_fd` field is used for both sending the in-fence as +input argument to receive the out-fence as a return argument. A buffer can +have both in-fence ond out-fence. + +At streamoff the out-fences will either signal normally if the driver waits +for the operations on the buffers to finish or signal with an error if the +driver cancels the pending operations. Buffers with in-fences won't be queued +to the driver if their fences signal. They will be cleaned up. + +The ``V4L2_FMT_FLAG_UNORDERED`` flag in ``VIDIOC_ENUM_FMT`` tells userspace +that the when using this format the order in which buffers are dequeued can +be different from the order in which they were queued. + +Ordering is important to fences because it can optimize the pipeline with +other drivers like a DRM/KMS display driver. For example, if a capture from the +camera is happening in an orderly manner one can send the capture buffer +out-fence to the DRM/KMS driver and rest sure that the buffers will be shown on +the screen at the correct order. If an ordered queue can not be set then such +arrangements with other drivers may not be possible. Return Value ============ diff --git a/Documentation/media/uapi/v4l/vidioc-querybuf.rst b/Documentation/media/uapi/v4l/vidioc-querybuf.rst index dd54747fabc9..9aa3358bee0b 100644 --- a/Documentation/media/uapi/v4l/vidioc-querybuf.rst +++ b/Documentation/media/uapi/v4l/vidioc-querybuf.rst @@ -44,7 +44,7 @@ and the ``index`` field. Valid index numbers range from zero to the number of buffers allocated with :ref:`VIDIOC_REQBUFS` (struct :c:type:`v4l2_requestbuffers` ``count``) minus -one. The ``reserved`` and ``reserved2`` fields must be set to 0. When +one. 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 an array of struct :c:type:`v4l2_plane` and the ``length`` field has to be set @@ -64,8 +64,14 @@ elements will be used instead and the ``length`` field of struct array elements. The driver may or may not set the remaining fields and flags, they are meaningless in this context. -The struct :c:type:`v4l2_buffer` structure is specified in -:ref:`buffer`. +When using in-fences, the ``V4L2_BUF_FLAG_IN_FENCE`` will be set if the +in-fence didn't signal at the time of the +:ref:`VIDIOC_QUERYBUF`. The ``V4L2_BUF_FLAG_OUT_FENCE`` will be set if +the user asked for an out-fence for the buffer and the ``fence_fd`` +field will be set to the out-fence fd. In case ``V4L2_BUF_FLAG_OUT_FENCE`` is +not set ``fence_fd`` will be set to 0 for backward compatibility. + +The struct :c:type:`v4l2_buffer` structure is specified in :ref:`buffer`. Return Value -- 2.14.3
