On 09/11/2017 03:18 PM, Gustavo Padovan wrote: > 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. It should mean *this* buffer. It's really weird to set this flag for one buffer, only for it to mean 'next' buffer. Keep it simple: the flag just means: send me the output fence fd for this buffer once you have it. If it is not set, then no BUF_QUEUE event is sent. Actually, it could mean one of two things: either if it is not set, then no BUF_QUEUE event is sent, or if it is not set, then the fd in the BUF_QUEUE event is -1. I'm leaning towards the first. I can't see any use-case for sending that event if you are not requesting out fences. Regards, Hans > >> >> 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 >
