Ubuntu port of v4l2loopback doesn't behave the same as official port (v0.12.5)

[Brett D]

Apps like OBS, which use v4l2loopback to create virtual cameras, have started losing their ability to create virtual cameras. In the case of OBS, it's able to create a camera one time. When the camera or app is "shutdown" the camera is unable to be created again unless a user manually removes the modprobe rule with "sudo modprobe -r v4l2loopback"

Ubuntu 20.04 comes with "0.12.5-1ubuntu1". That version seems to exhibit this behavior.

If you "downgrade" to "0.12.5-1" from https://packages.ubuntu.com/groovy/all/v4l2loopback-dkms/download the problem is gone.

The author of v4l2loopback indicates this "is a problem with Ubuntu specific modifications" and it should be reported here.

This is documented here:
https://github.com/umlaeute/v4l2loopback/issues/404
https://github.com/obsproject/obs-studio/issues/4808

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: v4l2loopback-dkms 0.12.5-1ubuntu1
ProcVersionSignature: Ubuntu 5.11.0-37.41-generic 5.11.22
Uname: Linux 5.11.0-37-generic x86_64
ApportVersion: 2.20.11-0ubuntu65.3
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: ubuntu:GNOME
Date: Mon Oct 11 08:50:33 2021
InstallationDate: Installed on 2020-05-01 (527 days ago)
InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: v4l2loopback
UpgradeStatus: Upgraded to hirsute on 2021-06-03 (130 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Boris Erdmann (boris-erdmann)]

This bug is still present in v4l2loopback-dkms_0.12.5-1ubuntu3 on impish.

[Bruce Lerner (1-bruce)]

This bug affects me.

[Mark Junker (fubar-coder-k)]

This bug affects me.

[Boris Erdmann (boris-erdmann)]

To those who just wrote "This bug affects me": Please log in and follow this link:
https://bugs.launchpad.net/ubuntu/+source/v4l2loopback/+bug/1946660/+affectsmetoo

[Israel Teixeira de Andrade (israeltandrade)]

This bug affects me.

[You-Sheng Yang (vicamo)]

The first open has:
```
[17047.368576] v4l2-loopback[1992]: opened dev:00000000c093a43d with image:0000000000000000
[17047.368577] /var/lib/dkms/v4l2loopback/0.12.5/build/v4l2loopback.c:1993[v4l2_loopback_open]
[17047.368582] video10: VIDIOC_QUERYCAP: driver=v4l2 loopback, card=OBS Video Source, bus=platform:v4l2loopback-000, version=0x00050f07, capabilities=0x
85200002, device_caps=0x05200002
```

The second open has:
```
[17091.427287] /var/lib/dkms/v4l2loopback/0.12.5/build/v4l2loopback.c:1964[v4l2_loopback_open]
[17091.427292] v4l2-loopback[1992]: opened dev:00000000c093a43d with image:0000000000000000
[17091.427293] /var/lib/dkms/v4l2loopback/0.12.5/build/v4l2loopback.c:1993[v4l2_loopback_open]
[17091.427297] video10: VIDIOC_QUERYCAP: driver=v4l2 loopback, card=OBS Video Source, bus=platform:v4l2loopback-000, version=0x00050f07, capabilities=0x85200000, device_caps=0x05200000
[17091.427304] video10: VIDIOC_G_FMT: error -22: type=vid-out, width=4294966992, height=4294967295, pixelformat=.... little-endian (0x00000000), field=any, bytesperline=1401200640, sizeimage=4288352138, colorspace=-744513136, flags=0xfffffed0, ycbcr_enc=0, quantization=0, xfer_func=0
```

As indicated in the capabilities field returned from the VIDIOC_QUERYCAP call, 0x85200000 means no V4L2_CAP_VIDEO_CAPTURE(0x01) neither V4L2_CAP_VIDEO_OUTPUT(0x02) is set. And since it reports neither, kernel `check_fmt` in "drivers/media/v4l2-core/v4l2-ioctl.c" will evaluate variable is_vid to false, and therefore returns -EINVAL at the next `VIDIOC_G_FMT` call with type=`V4L2_BUF_TYPE_VIDEO_OUTPUT`.

This is a direct result from https://github.com/umlaeute/v4l2loopback/commit/9a77eb6 merged in https://github.com/umlaeute/v4l2loopback/pull/415.

[You-Sheng Yang (vicamo)]

Per inspection result from above comment, the behavior is actually a bug in the upstream v4l2loopback source, function v4l2_loopback_write:

  /* there's at least one writer, so don'stop announcing output capabilities */
  dev->ready_for_output = 0;

Which set `dev->ready_for_output` to zero without correcting its `dev->type`.

[You-Sheng Yang (vicamo)]

Continue discuss on upstream issue https://github.com/umlaeute/v4l2loopback/issues/404. Stay tuned.

[Mark Tompkins (mets5273)]

The issue persists into 22.04

[Martin Wimpress  (flexiondotorg)]

For Ubuntu users, I've made a PPA containing builds of `v4l2loopback` for 20.04 (Focal), 21.10 (Impish), 22.04 (Jammy), 22.10 (Kinetic) that includes backports of the following patches:

 🩹 https://github.com/umlaeute/v4l2loopback/pull/477
 🩹 https://github.com/umlaeute/v4l2loopback/pull/485

The patch above fixes the ability to start/stop the Virtual Camera in OBS many times. It "works for me"™

The PPA is here:

 📦 https://launchpad.net/~flexiondotorg/+archive/ubuntu/v4l2loopback

Feedback welcome. If this fix works widely, I'll work towards landing in the official packages for Ubuntu.

[Boris Erdmann (boris-erdmann)]

Martin,

thanks a lot for providing that. I would like to confirm that it works now, but I was on the downgraded original version 0.12.5-1 before and replacing it with your package leaves me with

$ find /lib/modules/ | grep v4l2loopback.ko
/lib/modules/5.15.0-40-generic/kernel/v4l2loopback/v4l2loopback.ko
/lib/modules/5.15.0-41-generic/kernel/v4l2loopback/v4l2loopback.ko
/lib/modules/5.15.0-40-lowlatency/kernel/v4l2loopback/v4l2loopback.ko
/lib/modules/5.15.0-41-lowlatency/kernel/v4l2loopback/v4l2loopback.ko
/lib/modules/5.15.0-41-lowlatency/updates/dkms/v4l2loopback.ko

How can I be sure that the new module is actually loaded (given that I am currently running 5.15.0-41-lowlatency)? You can also see that the new module was built only for the kernel running at install time.

[Brandon Whaley (redkrieg)]

Martin's PPA version works for me on Jammy. See if your srcversion matches mine maybe?

redkrieg@cortex-1:~$ modinfo v4l2loopback
filename: /lib/modules/5.17.15-76051715-generic/extra/v4l2loopback.ko
license: GPL
author: Vasily Levin, IOhannes m zmoelnig <email address hidden>,Stefan Diewald,Anton Novikovet al.
description: V4L2 loopback video device
srcversion: B6031838219D91DE046F379
depends: videodev
retpoline: Y
name: v4l2loopback
vermagic: 5.17.15-76051715-generic SMP preempt mod_unload modversions
parm: debug:debugging level (higher values == more verbose) (int)
parm: max_buffers:how many buffers should be allocated (int)
parm: max_openers:how many users can open loopback device (int)
parm: devices:how many devices should be created (int)
parm: video_nr:video device numbers (-1=auto, 0=/dev/video0, etc.) (array of int)
parm: card_label:card labels for every device (array of charp)
parm: exclusive_caps:whether to announce OUTPUT/CAPTURE capabilities exclusively or not (array of bool)
parm: max_width:maximum frame width (int)
parm: max_height:maximum frame height (int)
redkrieg@cortex-1:~$ ls -hal /lib/modules/5.17.15-76051715-generic/extra/v4l2loopback.ko
-rw-r--r-- 1 root root 91K Jul 15 10:09 /lib/modules/5.17.15-76051715-generic/extra/v4l2loopback.ko

[Fabien Gagné (fabiengagne)]

I confirm that the issue is in Kubuntu 22.04 LTS as well.