[RFC,v4,17/17,media] v4l: Document explicit synchronization behaviour
Commit Message
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(+)
Comments
On Fri, Oct 20, 2017 at 07:50:12PM -0200, 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
>+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
>
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? 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
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
@@ -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.
+
+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``.
+
+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
============