V4l2loopback troubleshooting

Autonomous Machines Jetson & Embedded Systems Jetson Nano
1

Hi, I’m trying to install v4l2loopback devices, but when modprobing no /dev/videoX device is created, and modprobe also doesnt give any error.

Here’s a full log from the terminal:

drakorg@dev-jp43:~$ sudo apt list -i | grep -i v4l2loopback

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

drakorg@dev-jp43:~$ sudo apt install v4l2loopback-dkms
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
  libavdevice57 libtext-charwidth-perl libtext-wrapi18n-perl module-assistant v4l-utils
Use 'sudo apt autoremove' to remove them.
Suggested packages:
  v4l2loopback-utils
The following NEW packages will be installed:
  v4l2loopback-dkms
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 23,1 kB of archives.
After this operation, 112 kB of additional disk space will be used.
Get:1 http://ports.ubuntu.com/ubuntu-ports bionic/universe arm64 v4l2loopback-dkms all 0.10.0-1ubuntu1 [23,1 kB]
Fetched 23,1 kB in 1s (34,3 kB/s)
debconf: delaying package configuration, since apt-utils is not installed
Selecting previously unselected package v4l2loopback-dkms.
(Reading database ... 171277 files and directories currently installed.)
Preparing to unpack .../v4l2loopback-dkms_0.10.0-1ubuntu1_all.deb ...
Unpacking v4l2loopback-dkms (0.10.0-1ubuntu1) ...
Setting up v4l2loopback-dkms (0.10.0-1ubuntu1) ...
Loading new v4l2loopback-0.10.0 DKMS files...
It is likely that 4.9.140-tegra belongs to a chroot's host
Building for 4.15.0-20-generic and 4.9.140-tegra
Building initial module for 4.15.0-20-generic
Done.

v4l2loopback:
Running module version sanity check.
 - Original module
   - No original module exists within this kernel
 - Installation
   - Installing to /lib/modules/4.15.0-20-generic/updates/dkms/

depmod...

DKMS: install completed.
Building initial module for 4.9.140-tegra
Done.

v4l2loopback:
Running module version sanity check.
 - Original module
   - No original module exists within this kernel
 - Installation
   - Installing to /lib/modules/4.9.140-tegra/updates/dkms/

depmod...

DKMS: install completed.
drakorg@dev-jp43:~$ sudo modprobe v4l2loopback devices=8
drakorg@dev-jp43:~$ ls -la /dev/video*
ls: cannot access '/dev/video*': No such file or directory
drakorg@dev-jp43:~$

As you can see modprobe doesnt create the devices … and I have no clue on where to start digging.
For the record, /dev/video0 had been created successfully before, and I could use it with no problem, until a moment where it started to give an error (ioctl write error something), so I sudo apt removed it, hoping I would be able to recreate them, but it didn’t work.

I also see on the logs that it refers to two versions, 4.9 and 4.15 and that it says something about one of them probably belonging to a chroot environment, but I’ve no clue what that means, or if it’s normal.

If anyone has any pointer on where to start looking I’d really appreciate it.
The workaround if I don’t manage to make it work again, reinstall from scratch, but if I can avoid that, great.
Thank you.
Regards.
Eduardo

2

One thing I’ve noticed, is that the modprobe autocomplete does not suggest the v4l2loopback module as it would on previous occasions, it now just autocomplets to v4l2-tpg … as if the v4l2loopback is not being recognized or something.

4

Hi,
There are discussions in the topics:
Jetpack 4.2.1 : building v4l2loopback fails
Get two video streams of different resolutions from a single camera with NVIDIA Gstreamer - #7 by Honey_Patouceul

Please check and give it a try.

5

you may also try some options from the list below

sudo su
cd /usr/src/linux-headers-4.9.140-tegra-ubuntu18.04_aarch64/kernel-4.9
## dropped in latter release##make modules_prepare
mkdir v4l2loopback
git clone https://github.com/umlaeute/v4l2loopback.git v4l2loopback
cd v4l2loopback && git checkout -b v0.10.0
make
## if the sequence above fails - adopt the line: make -C /lib/modules/4.9.140-tegra/build M=`$pwd` modules
## make -C /lib/modules/`uname -r`-tegra/build M=/usr/src/v4l2loopback modules_install

make install
apt-get install -y v4l2loopback-dkms v4l2loopback-utils
modprobe v4l2loopback devices=1 video_nr=2 exclusive_caps=1
echo options v4l2loopback devices=1 video_nr=2 exclusive_caps=1 > /etc/modprobe.d/v4l2loopback.conf
echo v4l2loopback > /etc/modules
update-initramfs -u
6

Ok, I think I finally nailed it. It may have had something to do with the fact that a /dev/video0 was always there and I didn’t recall configuring anything to make v4l2loopback autostart, so I took that /dev/video0 as a device that just shouldn’t be there. Because of that and since it didn’t go away with modprobe -r v4l2loopback, i tried to manually delete it, which I did, but it would still come back after a reboot. In the meantime I uninstalled and installed every possible combination of v4l2loopback and tried everything to get rid of that /dev/video0 that was stuck there, just to realize that it wasn’t created by v4l2loopback, but it was the representation of a cam I had connected.

After realizing that /dev/video0 was fine, I made peace with it and focused on making v4l2loopack work on the rest of the devices. The solution was to do a final modprobe -r v4l2loopback. After that it was like the module started to work as expected, and all video1 to video8 appeared after a final modprobe v4l2loopback devices=8.

I then automated with the steps you mentioned in your script (beware that you used a > to /etc/modules where that would trash whatever modules you may have had) … a >> would have been better.

In any case I finally got it working, so thanks to everyone.
Best regards,
Eduardo

8

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