On 09/07/2017 08:42 PM, Gustavo Padovan wrote: > From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx> > > Add section to VIDIOC_QBUF about it > > 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 1f3612637200..fae0b1431672 100644 > --- a/Documentation/media/uapi/v4l/vidioc-qbuf.rst > +++ b/Documentation/media/uapi/v4l/vidioc-qbuf.rst > @@ -117,6 +117,37 @@ 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 them to signal before using the buffer, i.e., queueing wait them -> wait on them (do you wait 'on' a fence or 'for' a fence? I think it's 'on' but I'm not 100% sure) > +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 Start a new sentence here: ...drivers. Out-fences... > +finished with buffer, that is the buffer is ready. The fence are represented s/that is/i.e/ s/The fence/The fences/ > +by file and passed as file descriptor to userspace. s/by file/as a file/ s/as file/as a file/ > + > +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. Failure to set both will s/Failure to set both/Setting one but not the other/ > +cause ``VIDIOC_QBUF`` to return with error. > + > +To get a out-fence back from V4L2 the ``V4L2_BUF_FLAG_OUT_FENCE`` flag should > +be set to notify it that the next queued buffer should have a fence attached to > +it. That means the out-fence may not be associated with the buffer in the > +current ``VIDIOC_QBUF`` ioctl call because the ordering in which videobuf2 core > +queues the buffers to the drivers can't be guaranteed. To become aware of the > +of the next queued buffer and the out-fence attached to it the > +``V4L2_EVENT_BUF_QUEUED`` event should be used. It will trigger an event > +for every buffer queued to the V4L2 driver. This makes no sense. Setting this flag means IMHO that when *this* buffer is queued up to the driver, then it should send the BUF_QUEUED event with an out fence. I.e. it signals that userspace wants to have the out-fence. The requirement w.r.t. ordering is that the BUF_QUEUED events have to be in order, but that is something that the driver can ensure in the case it is doing internal re-ordering. This requirement is something that needs to be documented here, BTW. Anyway, the flag shouldn't refer to some 'next buffer', since that's very confusing. > + > +At streamoff the out-fences will either signal normally if the drivers wait s/drivers wait/driver waits/ > +for the operations on the buffers to finish or signal with error if the > +driver cancel the pending operations. s/cancel/cancels/ Thinking with my evil hat on: What happens if the application dequeues the buffer (VIDIOC_DQBUF) before dequeuing the BUF_QUEUED event? Or if the application doesn't call VIDIOC_DQEVENT at all? Should any pending BUF_QUEUED event with an out fence be removed from the event queue if the application calls DQBUF on the corresponding buffer? Regards, Hans > > Return Value > ============ >
