Re: [PATCH v3 01/15] [media] v4l: Document explicit synchronization behaviour

[Gustavo Padovan <gustavo@padovan.org>]

2017-09-11 Hans Verkuil <hverkuil@xs4all.nl>:

> On 09/11/2017 03:18 PM, Gustavo Padovan wrote:
> > 2017-09-11 Hans Verkuil <hverkuil@xs4all.nl>:
> > 
> >> 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@collabora.com>
> >>>>
> >>>> 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@collabora.com>
> >>>> ---
> >>>>  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?

Gustavo