From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756211AbdHYLPR (ORCPT ); Fri, 25 Aug 2017 07:15:17 -0400 Received: from ec2-52-27-115-49.us-west-2.compute.amazonaws.com ([52.27.115.49]:49825 "EHLO osg.samsung.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754727AbdHYLPP (ORCPT ); Fri, 25 Aug 2017 07:15:15 -0400 Date: Fri, 25 Aug 2017 08:15:03 -0300 From: Mauro Carvalho Chehab To: Hans Verkuil Cc: Hans Verkuil , Linux Doc Mailing List , Linux Media Mailing List , Mauro Carvalho Chehab , Mauro Carvalho Chehab , linux-kernel@vger.kernel.org, Jonathan Corbet , Laurent Pinchart , Sakari Ailus , Hans Verkuil Subject: Re: [PATCH 2/3] media: videodev2: add a flag for vdev-centric devices Message-ID: <20170825081503.13e4df80@vento.lan> In-Reply-To: References: <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> <20170825073517.1112d618@vento.lan> <7d5f952b-028d-0770-0f37-39ab011ec740@cisco.com> <20170825075044.7ffe3232@vento.lan> Organization: Samsung X-Mailer: Claws Mail 3.14.1 (GTK+ 2.24.31; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Em Fri, 25 Aug 2017 12:56:30 +0200 Hans Verkuil escreveu: > On 25/08/17 12:50, Mauro Carvalho Chehab wrote: > > Em Fri, 25 Aug 2017 12:42:51 +0200 > > Hans Verkuil escreveu: > > > >> On 08/25/2017 12:35 PM, Mauro Carvalho Chehab wrote: > >>> Em Fri, 25 Aug 2017 12:13:53 +0200 > >>> Hans Verkuil escreveu: > >>> > >>>> On 08/25/2017 12:06 PM, Mauro Carvalho Chehab wrote: > >>>>> Em Fri, 25 Aug 2017 11:44:27 +0200 > >>>>> Hans Verkuil escreveu: > >>>>> > >>>>>> On 08/25/2017 11:40 AM, Mauro Carvalho Chehab wrote: > >>>>>>> From: Mauro Carvalho Chehab > >>>>>>> > >>>>>>> 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 > >>>>>>> Signed-off-by: Mauro Carvalho Chehab > >>>>>>> --- > >>>>>>> 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 `. > >>>>>>> > >>>>>>> +.. 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