On 25/08/17 12:50, Mauro Carvalho Chehab wrote: > Em Fri, 25 Aug 2017 12:42:51 +0200 > Hans Verkuil <hansverk@xxxxxxxxx> escreveu: > >> 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. > > 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) 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. Regards, Hans -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
