Re: [RFC v4 17/17] [media] v4l: Document explicit synchronization behaviour

[Gustavo Padovan <gustavo@padovan.org>]

Hi Hans,

2017-11-03 Hans Verkuil <hverkuil@xs4all.nl>:

> On 10/20/2017 11:50 PM, Gustavo Padovan wrote:
> > From: Gustavo Padovan <gustavo.padovan@collabora.com>
> > 
> > Add section to VIDIOC_QBUF about it
> > 
> > v3:
> > 	- make the out_fence refer to the current buffer (Hans)
> > 	- Note what happens when the IN_FENCE is not set (Hans)
> > 
> > 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 9e448a4aa3aa..a65a50578bad 100644
> > --- a/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> > +++ b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> > @@ -118,6 +118,37 @@ 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 on them to signal before using the buffer, i.e., queueing
> > +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
> > +finished with buffer, i.e., the buffer is ready. The fences are represented
> > +as a file and passed as a file descriptor to userspace.
> > +
> > +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 Setting one but not the other
> 
> Missing '.' after 'as well'.
> 
> To what value is fence_fd set when VIDIOC_QBUF returns?

It should be -1 because we will be reusing the fence_fd field to return
the out_fence to userspace in the cases we don't need to use the
OUT_FENCE event. Like GPU drivers does with fences. That is the better
way to send the out fence back that I can think of at the moment.

> If you don't set the
> IN_FENCE flag, what should userspace set fence_fd to? (I recommend 0).

0 is still a valid fd, so the implementation is currently accepting -1
and 0.

> 
> > +will cause ``VIDIOC_QBUF`` to return with error.
> > +
> > +The fence_fd field (formely the reserved2 field) will be ignored if the
> 
> Drop the "(formely the reserved2 field)" part. We're not interested in the
> history here.
> 
> > +``V4L2_BUF_FLAG_IN_FENCE`` is not set.
> > +
> > +To get an out-fence back from V4L2 the ``V4L2_BUF_FLAG_OUT_FENCE`` flag should
> > +be set to ask for a fence to be attached to the buffer. To become aware of
> > +the out-fence created one should listen for the ``V4L2_EVENT_OUT_FENCE`` event.
> > +An event will be triggered for every buffer queued to the V4L2 driver with the
> > +``V4L2_BUF_FLAG_OUT_FENCE``.
> > +
> > +At streamoff the out-fences will either signal normally if the drivers waits
> 
> drivers -> driver
> 
> > +for the operations on the buffers to finish or signal with error if the
> 
> error -> an error
> 
> > +driver cancels the pending operations.
> >  
> >  Return Value
> >  ============
> > 
> 
> What should be done if the driver doesn't set ordered_in_driver? How does userspace
> know whether in and/or out fences are supported? I'm leaning towards a new capability
> flag for QUERYCAPS.

Yep. That is what we agreed last week in Prague.

> What does VIDIOC_QUERYBUF return w.r.t. the fence flags and fence_fd?

It does return the IN_FENCE flag if the fence didn't signal yet and the
OUT_FENCE one if the user set it on QBUF. The fence_fd is set to -1,
because the fd is specific to the pid using it.

Gustavo