Score:1

Ubuntu

v4l2loopback module failing on Ubuntu 20.04

Luís de Sousa

1/2/23, 8:50 AM

I am trying a new web camera on a workstation that hitherto had no video capture devices attached. It should be plug-and-play but none of the programmes is able to find the camera once connected.

First I checked whether the kernel itself was identifying the camera:

$ lsusb
[...]
Bus 001 Device 007: ID 046d:082d Logitech, Inc. HD Pro Webcam C920
[...]

So there is a camera, it must be a software thing. Checking connected devices with video for Linux gives one more clue:

$ v4l2-ctl --list-devices
Cannot open device /dev/video0, exiting.

Looked like the kernel module was disabled, so I tried to start it up:

$ sudo modprobe v4l2loopback
modprobe: FATAL: Module v4l2loopback not found in directory /lib/modules/5.4.0-81-generic

Odd, the module was not even installed. I then pulled it from the packages and gave it another try:

$ sudo aptitude install v4l2loopback-dkms
[...]

$ sudo modprobe v4l2loopback
modprobe: ERROR: could not insert 'v4l2loopback': Unknown symbol in module, or unknown parameter (see dmesg)

And this is what I found among the messages:

$ dmesg
[...]
[1271004.074955] v4l2loopback: Unknown symbol video_ioctl2 (err -2)
[1271004.075027] v4l2loopback: Unknown symbol v4l2_ctrl_handler_init_class (err -2)
[1271004.075105] v4l2loopback: Unknown symbol video_devdata (err -2)
[1271004.075167] v4l2loopback: Unknown symbol v4l2_ctrl_new_custom (err -2)
[1271004.075221] v4l2loopback: Unknown symbol video_unregister_device (err -2)
[1271004.075276] v4l2loopback: Unknown symbol video_device_alloc (err -2)
[1271004.075336] v4l2loopback: Unknown symbol v4l2_device_register (err -2)
[1271004.075410] v4l2loopback: Unknown symbol __video_register_device (err -2)
[1271004.075463] v4l2loopback: Unknown symbol v4l2_ctrl_handler_free (err -2)
[1271004.075513] v4l2loopback: Unknown symbol v4l2_device_unregister (err -2)
[1271004.075562] v4l2loopback: Unknown symbol video_device_release (err -2)

I believe this means some software dependencies are missing. But which? There is a discussion at GitHub in a cloud context, but the solution is too general (install all extra package modules available).

1512

1 + 3

usb

kernel

webcam

v4l

David

1/2/23, 8:57 AM

According to Logitech, the C920S HD Pro works very well with standard VOIP programs, such as Skype, Discord, and others. Thanks to Linux kernel developers, most Logitech webcam works very well, and there is no requirement to install any third-party drivers to get it running. BUT this comment is from May 2020 and may not be true with 20.04

Luís de Sousa

1/2/23, 8:58 AM

This camera works well with other Ubuntu computers, but not this one.

David

1/2/23, 8:59 AM

I think you just proved the point not with 20.04

Score:1

Ubuntu

Luís de Sousa

1/2/23, 9:16 AM

After some more searching I noticed something odd, none of the extra kernel modules was actually installed on this system:

$ aptitude search linux-modules-extra*
[...]
p   linux-modules-extra-5.4.0-70-generic                                                              - Linux kernel extra modules for version 5.4.0 on 64 bit x86 SMP
p   linux-modules-extra-5.4.0-71-generic                                                              - Linux kernel extra modules for version 5.4.0 on 64 bit x86 SMP
p   linux-modules-extra-5.4.0-72-generic                                                              - Linux kernel extra modules for version 5.4.0 on 64 bit x86 SMP
p   linux-modules-extra-5.4.0-73-generic                                                              - Linux kernel extra modules for version 5.4.0 on 64 bit x86 SMP
p   linux-modules-extra-5.4.0-74-generic                                                              - Linux kernel extra modules for version 5.4.0 on 64 bit x86 SMP
p   linux-modules-extra-5.4.0-77-generic                                                              - Linux kernel extra modules for version 5.4.0 on 64 bit x86 SMP
p   linux-modules-extra-5.4.0-80-generic                                                              - Linux kernel extra modules for version 5.4.0 on 64 bit x86 SMP
c   linux-modules-extra-5.4.0-81-generic                                                              - Linux kernel extra modules for version 5.4.0 on 64 bit x86 SMP

That being an obvious culprit, I installed the package and gave it another try:

$ sudo aptitude install linux-modules-extra-5.4.0-81-generic
[...]

$ sudo modprobe v4l2loopback

$ v4l2-ctl --list-devices
Dummy video device (0x0000) (platform:v4l2loopback-000):
    /dev/video0

Albeit that brought the module back to life, the web camara remains invisible to the software. But that is another question.

0 + 0

Gerd

4/6/23, 9:21 AM

Did you try to call `v4l2-ctl --list-devices` without loading `v4l2loopback`. On my system (which is Linux Mint 20.2) the `v4l2loopback` module was only needed when I wanted to create a virtual device for my webcam (Logitech HD Pro Webca 920) but nor for normal operation.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: v4l2loopback module failing on Ubuntu 20.04

TH: โมดูล v4l2loopback ล้มเหลวบน Ubuntu 20.04

RO: Modulul v4l2loopback eșuează pe Ubuntu 20.04

RU: Сбой модуля v4l2loopback в Ubuntu 20.04

VI: mô-đun v4l2loopback không thành công trên Ubuntu 20.04

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