On 09/11/2017 03:34 PM, Gustavo Padovan wrote: > 2017-09-11 Hans Verkuil <hverkuil@xxxxxxxxx>: > >> 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. > > We could go with the first one but in this case it is better to rename it to > V4L2_EVENT_OUT_FENCE or something like this, isn't it? I was thinking the same thing. That would be a better name, yes. Regards, Hans
