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

[Gustavo Padovan]

Hi Hans,

2017-11-03 Hans Verkuil :

> On 10/20/2017 11:50 PM, Gustavo Padovan wrote:
> > From: Gustavo Padovan 
> > 
> > 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 
> > ---
> >  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

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

[Hans Verkuil]

On 10/20/2017 11:50 PM, Gustavo Padovan wrote:
> From: Gustavo Padovan 
> 
> 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 
> ---
>  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? If you don't set the
IN_FENCE flag, what should userspace set fence_fd to? (I recommend 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.

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

Regards,

Hans

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

[Brian Starkey]

On Fri, Oct 20, 2017 at 07:50:12PM -0200, Gustavo Padovan wrote:

From: Gustavo Padovan 

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 
---
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
+will cause ``VIDIOC_QBUF`` to return with error.


nit: full-stop before "Setting".

Also, depending what is decided about passing in a -1 in-fence, the
wording might want to include that behaviour.


+
+The fence_fd field (formely the reserved2 field) will be ignored if the
+``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``.


Is it worth mentioning the immediate out-fence behaviour for ordered
queues? And/or clarifying that otherwise the V4L2_EVENT_OUT_FENCE
event will not be sent until all in-fence(s) on the buffer (or
previously queued ones) have signalled.

Cheers,
-Brian


+
+At streamoff the out-fences will either signal normally if the drivers waits
+for the operations on the buffers to finish or signal with error if the
+driver cancels the pending operations.

Return Value

--
2.13.6