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

[Mauro Carvalho Chehab <mchehab@s-opensource.com>]

Em Fri, 25 Aug 2017 12:56:30 +0200
Hans Verkuil <hverkuil@xs4all.nl> escreveu:

> On 25/08/17 12:50, Mauro Carvalho Chehab wrote:
> > Em Fri, 25 Aug 2017 12:42:51 +0200
> > Hans Verkuil <hansverk@cisco.com> escreveu:
> >   
> >> On 08/25/2017 12:35 PM, Mauro Carvalho Chehab wrote:  
> >>> 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).    
> >>
> >> So? Such a plugin would obviously remove the MC_CENTRIC cap. Which makes
> >> perfect sense.
> >>
> >> There are a lot of userspace applications that do not use libv4l. It's
> >> optional, not required, to use that library. We cannot design our API with
> >> the assumption that this library will be used.
> >>  
> >>>     
> >>>>
> >>>> 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;    
> >>
> >>
> >> 	is_supported = true;
> >> 	if (caps & V4L2_CAP_MC_CENTRIC)
> >> 		is_supported = false;
> >>  	if (version < KERNEL_VERSION(4, 15, 0))
> >>  		maybe_supported = true;
> >>
> >> I don't see the difference. BTW, no application will ever do that version check.
> >> It doesn't help them in any way to know that it 'may' be supported.  
> > 
> > Yeah, this can work. The only drawback is that, if we end by
> > implementing vdev compatible support is that such drivers will
> > have to clean the V4L2_CAP_MC_CENTRIC flag.  
> 
> You mean implementing vdev compatible support in libv4l? (Just making sure
> I understand you correctly)

Yes, either there or at the Kernel, as it seems we'll never have it
there, as nobody is working on it anymore.

> In that case it doesn't matter if the libv4l code would set the VDEV_CENTRIC flag
> or remove the MC_CENTRIC flag. That makes no difference, or course.

True, but the text will have to be clear that a MC_CENTRIC device is a
device that can't be controlled by a V4L2-centric application.

Thanks,
Mauro