Re: [PATCH 2/3] media: videodev2: add a flag for vdev-centric devices

Mauro Carvalho Chehab
2017-08-25T10:35:27Z

Em Fri, 25 Aug 2017 12:13:53 +0200
Hans Verkuil <hansverk@cisco.com> escreveu:

> On 08/25/2017 12:06 PM, Mauro Carvalho Chehab wrote:
> > Em Fri, 25 Aug 2017 11:44:27 +0200
> > Hans Verkuil <hansverk@cisco.com> escreveu:
> >
> >> On 08/25/2017 11:40 AM, Mauro Carvalho Chehab wrote:
> >>> From: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
> >>>
> >>> As both vdev-centric and mc-centric devices may implement the
> >>> same APIs, we need a flag to allow userspace to distinguish
> >>> between them.
> >>>
> >>> Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
> >>> Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
> >>> ---
> >>> Documentation/media/uapi/v4l/open.rst | 6 ++++++
> >>> Documentation/media/uapi/v4l/vidioc-querycap.rst | 4 ++++
> >>> include/uapi/linux/videodev2.h | 2 ++
> >>> 3 files changed, 12 insertions(+)
> >>>
> >>> diff --git a/Documentation/media/uapi/v4l/open.rst b/Documentation/media/uapi/v4l/open.rst
> >>> index a72d142897c0..eb3f0ec57edb 100644
> >>> --- a/Documentation/media/uapi/v4l/open.rst
> >>> +++ b/Documentation/media/uapi/v4l/open.rst
> >>> @@ -33,6 +33,12 @@ For **vdev-centric** control, the device and their corresponding hardware
> >>> pipelines are controlled via the **V4L2 device** node. They may optionally
> >>> expose via the :ref:`media controller API <media_controller>`.
> >>>
> >>> +.. note::
> >>> +
> >>> + **vdev-centric** devices should report V4L2_VDEV_CENTERED
> >>
> >> You mean CENTRIC, not CENTERED.
> >
> > Yeah, true. I'll fix it.
> >
> >> But I would change this to MC_CENTRIC: the vast majority of drivers are VDEV centric,
> >> so it makes a lot more sense to keep that as the default and only set the cap for
> >> MC-centric drivers.
> >
> > I actually focused it on what an userspace application would do.
> >
> > An specialized application for a given hardware will likely just
> > ignore whatever flag is added, and use vdev, mc and subdev APIs
> > as it pleases. So, those applications don't need any flag at all.
> >
> > However, a generic application needs a flag to allow them to check
> > if a given hardware can be controlled by the traditional way
> > to control the device (e. g. if it accepts vdev-centric type of
> > hardware control).
> >
> > It is an old desire (since when MC was designed) to allow that
> > generic V4L2 apps to also work with MC-centric hardware somehow.
>
> No, not true. The desire is that they can use the MC to find the
> various device nodes (video, radio, vbi, rc, cec, ...). But they
> remain vdev-centric. vdev vs mc centric has nothing to do with the
> presence of the MC. It's how they are controlled.

No, that's not I'm talking about. I'm talking about libv4l plugin
(or whatever) that would allow a generic app to work with a mc-centric
device. That's there for a long time (since when we were reviewing
the MC patches back in 2009 or 2010).

>
> Regarding userspace applications: they can't check for a VDEV_CENTRIC
> cap since we never had any. I.e., if they do:
>
> if (!(caps & VDEV_CENTRIC))
> /* unsupported device */
>
> then they would fail for older kernels that do not set this flag.
>
> But this works:
>
> if (caps & MC_CENTRIC)
> /* unsupported device */
>
> So this really needs to be an MC_CENTRIC capability.

That won't work. The test should take into account the API version
too.

Assuming that such flag would be added for version 4.15, with a VDEV_CENTRIC,
the check would be:

/*
* There's no need to check version here: libv4l may override it
* to support a mc-centric device even for older versions of the
* Kernel
*/
if (caps & V4L2_CAP_VDEV_CENTRIC)
is_supported = true;

/*
* For API version lower than 4.15, there's no way to know for
* sure if the device is vdev-centric or not. So, either additional
* tests are needed, or it would assume vdev-centric and output
* some note about that.
*/
if (version < KERNEL_VERSION(4, 15, 0))
maybe_supported = true;

Thanks,
Mauro