Repeated kernel warnings/stacktraces with kernel 5.4 and newer

The 20.10 version (0.12.5-1) was reworked considerably and I was told by someone else that it would not exhibit those warnings.

Test build uploaded to: https://launchpad.net/~smb/+archive/ubuntu/focal/+packages

The attached debdiff contains the proposed adjustments to the DKMS package.

The kernel check was added in [1] v5.4-rc1 by:

Author: Hans Verkuil <email address hidden>
Date: Tue Jul 23 04:21:25 2019 -0400

    media: v4l2-dev/ioctl: require non-zero device_caps, verify sane querycap results

    Now that all V4L2 drivers set device_caps in struct video_device, we can add
    a check for this to ensure all future drivers fill this in.

    Also verify that when the querycap ioctl is called the driver didn't mess
    with the device_caps value and that capabilities is a superset of device_caps.

    Signed-off-by: Hans Verkuil <email address hidden>
    Reviewed-by: Sakari Ailus <email address hidden>
    [<email address hidden>: fix too-long line]
    Signed-off-by: Mauro Carvalho Chehab <email address hidden>

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3c1350501c21db8e3b1a38d9e97db29694305c3b

Thanks for the updated debdiff as requested, LGTM now.
Some minor cleanup 0.12.3-1ubuntu1 -> 0.12.3-1ubuntu0.1

Further I think we should add DKMS rebuild with the Focal kernel as test-step explicitly (it will be done on install of the upgrade anyway, but to be formally correct)

FYI: I was on the receiving end of a video call using this updated driver - I can confirm it works.

Reviewed, made some minor fixups (version as mentioned), extended the SRU template n the description and sponsored into Focal-proposed

Hello Stefan, or anyone else affected,

Accepted v4l2loopback into focal-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/v4l2loopback/0.12.3-1ubuntu0.1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-focal to verification-done-focal. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-focal. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

$ dpkg -l|grep v4l2loop
ii v4l2loopback-dkms 0.12.3-1ubuntu0.1 all Source for the v4l2loopback driver (DKMS)
ii v4l2loopback-utils 0.12.3-1ubuntu0.1 all Commandline utilities for the for the v4l2-loopback module

$ dkms status|grep v4l2
v4l2loopback, 0.12.3, 5.4.0-51-generic, x86_64: installed
v4l2loopback, 0.12.3, 5.4.0-52-generic, x86_64: installed

$ modprobe v4l2loopback exclusive_caps=1
$ dmesg|grep v4l2-ioctl
<nothing>

I already was using the updated code, so I repeated the steps in a VM with the current version of the driver and got the following in dmesg:

[ 471.604470] WARNING: CPU: 1 PID: 18911 at drivers/media/v4l2-core/v4l2-ioctl.c:1064 v4l_querycap+0x8b/0xa0 [videodev]
[ 471.604602] WARNING: CPU: 1 PID: 18911 at drivers/media/v4l2-core/v4l2-ioctl.c:1069 v4l_querycap+0x8f/0xa0 [videodev]

The verification of the Stable Release Update for v4l2loopback has completed successfully and the package is now being released to -updates. Subsequently, the Ubuntu Stable Release Updates Team is being unsubscribed and will not receive messages about this bug report. In the event that you encounter a regression using the package from -updates please report a new bug using ubuntu-bug and tag the bug report regression-update so we can easily find any regressions.

This bug was fixed in the package v4l2loopback - 0.12.3-1ubuntu0.1

---------------
v4l2loopback (0.12.3-1ubuntu0.1) focal; urgency=medium

  * Fix kernel warnings triggered by vidioc_querycap (LP: #1899633).

 -- Stefan Bader <email address hidden> Tue, 13 Oct 2020 15:07:55 +0200

This fix has rendered v4l2loopback obsolete. Indeed, the device is no longer detected by any applications (e.g. VLC, Firefox, etc.).
I had to revert to the previous version to be able to use it again.

The last fix also broke v4l2loopback for me. Most applications seem to check the V4L2_CAP_VIDEO_CAPTURE flag (eg Firefox: https://hg.mozilla.org/releases/mozilla-release/file/aac9479c6a32ce3b95556da24f3b00c00e145506/media/webrtc/trunk/webrtc/modules/video_capture/linux/device_info_linux.cc#l386), which is now unset by the fix. This means apps such as Firefox do not detect virtual cameras created by v4l2loopback with this patch.

Would loading the new module with exclusive_caps=1 work as a quick work-around. I guess I missed adding the caps explicitly in the query function. So that relies on thing being set as device caps.

No, looking quickly at the code that should be set in the capabilities returned by the query capabilities call. But not in the static device_caps. And if that is added back, one cannot drop that flag for exclusive_caps functionality without triggering repeated kernel warnings.

Can confirm, this change prevents Zoom from listing my v4l2lsink from OBS. I had to downgrade to the "base" package so that my co-workers can continue to see my stupid effects and overlays.

Right now, the only app that appears to work with the new module is Chromium. Which was where I was testing because that seemed more demanding. Will be working on the regression on bug 1901491.

Confirmed that with v4l2loopback-dkms 0.12.3-1ubuntu0.1, configured devices are not correctly advertised. This breaks Zoom, Cheese, etc. Compiling v4l2loopback from upstream source works correctly.