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

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Message-ID: <20170825070632.28580858@vento.lan>
Date:   Fri, 25 Aug 2017 07:06:32 -0300
From:   Mauro Carvalho Chehab <mchehab@...pensource.com>
To:     Hans Verkuil <hansverk@...co.com>
Cc:     Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Mauro Carvalho Chehab <mchehab@...radead.org>,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Hans Verkuil <hans.verkuil@...co.com>
Subject: Re: [PATCH 2/3] media: videodev2: add a flag for vdev-centric
 devices

Em Fri, 25 Aug 2017 11:44:27 +0200
Hans Verkuil <hansverk@...co.com> escreveu:

> On 08/25/2017 11:40 AM, Mauro Carvalho Chehab wrote:
> > From: Mauro Carvalho Chehab <mchehab@....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@....samsung.com>
> > Signed-off-by: Mauro Carvalho Chehab <mchehab@...pensource.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.

At the moment we add that (either in Kernelspace, as proposed for
iMX6 [1] or via libv4l), a mc-centric hardware can also be 
vdev-centric.

[1] one alternative proposed for iMX6 driver, would be to enable
    vdev-centric control only for hardware with a single camera
    slot, like those cheap RPi3-camera compatible hardware, by
    using some info at the DT.

> 
> Regards,
> 
> 	Hans
> 
> > +   :c:type:`v4l2_capability` flag (see :ref:`VIDIOC_QUERYCAP`).
> > +
> > +
> >  For **MC-centric** control, before using the V4L2 device, it is required to
> >  set the hardware pipelines via the
> >  :ref:`media controller API <media_controller>`. For those devices, the
> > diff --git a/Documentation/media/uapi/v4l/vidioc-querycap.rst b/Documentation/media/uapi/v4l/vidioc-querycap.rst
> > index 12e0d9a63cd8..4856821b7608 100644
> > --- a/Documentation/media/uapi/v4l/vidioc-querycap.rst
> > +++ b/Documentation/media/uapi/v4l/vidioc-querycap.rst
> > @@ -252,6 +252,10 @@ specification the ioctl returns an ``EINVAL`` error code.
> >      * - ``V4L2_CAP_TOUCH``
> >        - 0x10000000
> >        - This is a touch device.
> > +    * - ``V4L2_VDEV_CENTERED``
> > +      - 0x20000000
> > +      - This is controlled via V4L2 device nodes (radio, video, vbi,
> > +        sdr
> >      * - ``V4L2_CAP_DEVICE_CAPS``
> >        - 0x80000000
> >        - The driver fills the ``device_caps`` field. This capability can
> > diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> > index 45cf7359822c..d89090d99042 100644
> > --- a/include/uapi/linux/videodev2.h
> > +++ b/include/uapi/linux/videodev2.h
> > @@ -460,6 +460,8 @@ struct v4l2_capability {
> >  
> >  #define V4L2_CAP_TOUCH                  0x10000000  /* Is a touch device */
> >  
> > +#define V4L2_CAP_VDEV_CENTERED          0x20000000  /* V4L2 Device is controlled via V4L2 device devnode */
> > +
> >  #define V4L2_CAP_DEVICE_CAPS            0x80000000  /* sets device capabilities field */
> >  
> >  /*
> >   
> 



Thanks,
Mauro