Date: Fri, 25 Aug 2017 07:35:17 -0300
From: Mauro Carvalho Chehab <mchehab@s-opensource.com>
To: Hans Verkuil <hansverk@cisco.com>
Cc: Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Mauro Carvalho Chehab <mchehab@osg.samsung.com>,
        Mauro Carvalho Chehab <mchehab@infradead.org>,
        linux-kernel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
        Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        Hans Verkuil <hans.verkuil@cisco.com>
Subject: Re: [PATCH 2/3] media: videodev2: add a flag for vdev-centric
 devices
Message-ID: <20170825073517.1112d618@vento.lan>
In-Reply-To: <44bdeabc-8899-8f7e-dd26-4284c5b589a1@cisco.com>
References: <cover.1503653839.git.mchehab@s-opensource.com>
        <8d504be517755ee9449a007b5f2de52738c2df63.1503653839.git.mchehab@s-opensource.com>
        <4f771cfa-0e0d-3548-a363-6470b32a6634@cisco.com>
        <20170825070632.28580858@vento.lan>
        <44bdeabc-8899-8f7e-dd26-4284c5b589a1@cisco.com>
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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