2017-09-11 Hans Verkuil <hverkuil@xxxxxxxxx>: > On 09/11/2017 12:50 PM, Hans Verkuil wrote: > > 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. > > Just ignore this comment. I assume v4 will implement it like this. What approach do you mean by "like this". I'm confused now. :) In fact, I was in doubt between these two different approaches here. Should the flag mean *this* or the *next* buffer? The buffers can still be reordered at the videobuf2 level, because they might be waiting on in-fences and the fences may signal out of order. Then I went for the *next* buffer approach because we don't know that buffer for sure. But now thinking on this again we shouldn't have problems with the *this* buffer approach also. > > Regards, > > Hans > > > > >> + > >> +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? Good catch, we need to clean that up. Gustavo
