On 03/09/2018 09:49 AM, Gustavo Padovan wrote: > From: Gustavo Padovan <gustavo.pado...@collabora.com> > > 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.pado...@collabora.com> > --- > 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'. 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 after -> afterwards will be also block -> will also be blocked > +that fence signal. signal -> signals > + > +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. are -> is > + > +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. on -> in > + > +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 input argument -> an input argument and > +have both in-fence ond out-fence. have both an in-fence and an 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 remove 'the' > +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 rest sure -> rest assured But I'd rephrase it as: be certain > +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 > Regards, Hans