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

[Hans Verkuil]

On 08/25/2017 12:35 PM, Mauro Carvalho Chehab wrote:
> Em Fri, 25 Aug 2017 12:13:53 +0200
> Hans Verkuil <hansverk@xxxxxxxxx> escreveu:
> 
>> On 08/25/2017 12:06 PM, Mauro Carvalho Chehab wrote:
>>> Em Fri, 25 Aug 2017 11:44:27 +0200
>>> Hans Verkuil <hansverk@xxxxxxxxx> escreveu:
>>>   
>>>> On 08/25/2017 11:40 AM, Mauro Carvalho Chehab wrote:  
>>>>> From: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxxxx>
>>>>>
>>>>> 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@xxxxxxxxxxxxxxx>
>>>>> Signed-off-by: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxxxxx>
>>>>> ---
>>>>>  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.

Regards,

	Hans