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Date:   Thu, 22 Dec 2022 00:24:44 +0200
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Mauro Carvalho Chehab <mchehab@kernel.org>
Cc:     Ricardo Ribalda <ribalda@chromium.org>,
        Yunke Cao <yunkec@chromium.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        linux-media@vger.kernel.org,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 3/3] media: uvcvideo: Add a unique suffix to camera
 names
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References: <20220920-resend-meta-v4-0-3ac355b66723@chromium.org>
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Hi Mauro,

On Wed, Dec 21, 2022 at 10:02:48PM +0000, Mauro Carvalho Chehab wrote:
> Em Tue, 6 Dec 2022 00:02:22 +0100 Ricardo Ribalda escreveu:
> > On Mon, 5 Dec 2022 at 23:16, Laurent Pinchart wrote:
> > > On Fri, Dec 02, 2022 at 06:08:19PM +0100, Ricardo Ribalda wrote:  
> > > > Some cameras have multiple data inputs (i.e. IR sensor and RGB sensor),
> > > > append a unique number to the device name.
> > > >
> > > > Fixes v4l2-compliance:
> > > >     Media Controller ioctls:
> > > >          fail: v4l2-test-media.cpp(205): v2_entity_names_set.find(key) != v2_entity_names_set.end()
> > > >        test MEDIA_IOC_G_TOPOLOGY: FAIL
> > > >          fail: v4l2-test-media.cpp(394): num_data_links != num_links
> > > >        test MEDIA_IOC_ENUM_ENTITIES/LINKS: FAIL
> > > >
> > > > Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> > > > ---
> > > >  drivers/media/usb/uvc/uvc_driver.c | 3 ++-
> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> > > > index 215fb483efb0..f4032ebb3689 100644
> > > > --- a/drivers/media/usb/uvc/uvc_driver.c
> > > > +++ b/drivers/media/usb/uvc/uvc_driver.c
> > > > @@ -1963,7 +1963,8 @@ int uvc_register_video_device(struct uvc_device *dev,
> > > >               break;
> > > >       }
> > > >
> > > > -     strscpy(vdev->name, dev->name, sizeof(vdev->name));
> > > > +     snprintf(vdev->name, sizeof(vdev->name), "%s %u", dev->name,
> > > > +              stream->header.bTerminalLink);  
> > >
> > > This won't be perfect as the string is not guaranteed to fit in
> > > vdev->name, but I suppose it will help as a quick fix for some devices.
> > > How about the other devices ? Won't they still exhibit the above
> > > v4l2-compliance failure ? Isn't that something that will still affect
> > > Chrome OS devices ?  
> > 
> > We could place the id first... but that will look bad: Eg:
> > 
> > 1- My favorite camera
> > 
> > Another option is to remove the last chars to fit the id. Eg:
> > 
> > My favorite came-1
> > 
> > If you prefer any of those options or have a better idea I can implement that.
> > 
> > > The change should not cause any regression as big as in patch 1/3.
> > > However, unless I'm mistaken users will notice a device name change,
> > > especially when selecting a device in their web browser. Could that be a
> > > problem ?  
> > 
> > I think the only side effect is that the first time that the kernel
> > changes the naming convention, if there are more than one camera on
> > the system, the video conference might pick a different camera.
> > The good news is that the user will be presented with cameras with
> > different names. Now some cameras show very confusing names:
> > 
> > ribalda@alco:~/work/linux$ for a in /dev/video* ; do yavta -l $a| grep
> > "Dell Webcam"; done
> > Device `Dell Webcam WB7022' on `usb-0000:2d:00.0-1.2.3.1' (driver
> > 'uvcvideo') supports video, capture, without mplanes.
> > Device `Dell Webcam WB7022' on `usb-0000:2d:00.0-1.2.3.1' (driver
> > 'uvcvideo') supports meta-data, capture, without mplanes.
> > Device `Dell Webcam WB7022' on `usb-0000:2d:00.0-1.2.3.1' (driver
> > 'uvcvideo') supports video, capture, without mplanes.
> > Device `Dell Webcam WB7022' on `usb-0000:2d:00.0-1.2.3.1' (driver
> > 'uvcvideo') supports meta-data, capture, without mplanes.
> 
> That is bad, as some apps like camorama actually store the user
> preferences (last used resolution and fps), preserving them the next
> time the camera with the same name is used.
> 
> In the specific case of camorama, it uses the by-id filename, which is
> derived from the name, as stored at:
> 
> 	$ ls -la /dev/v4l/by-id/usb-Quanta_HD_Camera_0001-video-index*
> 	lrwxrwxrwx 1 root root 12 dez 21 09:59 /dev/v4l/by-id/usb-Quanta_HD_Camera_0001-video-index0 -> ../../video0
> 	lrwxrwxrwx 1 root root 12 dez 21 09:59 /dev/v4l/by-id/usb-Quanta_HD_Camera_0001-video-index1 -> ../../video1
> 
> With this change, not only the camera name will become bigger (and may
> end losing the index0/index1 data), but it will also lost the stored 
> preferences on apps like camorama, causing regressions.
> 
> It sounds a lot easier to teach udev to change the name on an unique
> way, on machines where you need it.

It's not a udev issue, it's an API compliance problem, entity names in a
media controller graph must be unique, and they are not.

Losing the stored preference is possibly inconvenient in some cases, but
I don't think it's a real blocker.

-- 
Regards,

Laurent Pinchart