The module loads but not creating any device

[happyme531]

• v4l2loopback version: 0.12.7 (0.12.7-397-g9ba7e29)
• kernel version: 5.10.160-rk35xx
• Distribution (+version): Debian GNU/Linux 11 (bullseye)

Steps to reproduce:

1. clone the v4l2loopback repo
2. make, sudo make install, sudo depmod -a
3. sudo modprobe v4l2loopback

Observed Results:

The module loaded: (dmesg)

[57666.139808] v4l2loopback driver version 0.12.7 (0.12.7-397-g9ba7e29) loaded

but no new /dev/videoX created and /dev/v4l2loopback is not exist

user@hinlink-h88k ~/v4l2loopback (main)> ls /dev/v4l2loopback
ls:  '/dev/v4l2loopback': no such file or directory
user@hinlink-h88k ~/v4l2loopback (main) [2]> ls /dev/video1
ls:  '/dev/video1':  no such file or directory

(/dev/video0 is the platform's video device and not related to v4l2loopback)

user@hinlink-h88k ~/v4l2loopback (main) [2]> ls /dev/
ashmem           gpiochip0    initctl@      mmcblk1boot0  rtc@     tty17  tty33  tty5   tty9      vcsa4
autofs           gpiochip1    input/        mmcblk1boot1  rtc0     tty18  tty34  tty50  ttyS2     vcsa5
block/           gpiochip2    kmsg          mmcblk1p1     shm/     tty19  tty35  tty51  ubi_ctrl  vcsa6
btrfs-control    gpiochip3    kvm           mmcblk1rpmb   snd/     tty2   tty36  tty52  uhid      vcsu
bus/             gpiochip4    log@          mpp_service   stderr@  tty20  tty37  tty53  uinput    vcsu1
char/            gpiochip5    loop0         mqueue/       stdin@   tty21  tty38  tty54  urandom   vcsu2
console          hdmirx_hdcp  loop1         net/          stdout@  tty22  tty39  tty55  v4l/      vcsu3
cpu_dma_latency  hugepages/   loop2         null          sw_sync  tty23  tty4   tty56  vcs       vcsu4
crypto           hwrng        loop3         port          tty      tty24  tty40  tty57  vcs1      vcsu5
cuse             i2c-0        loop4         ppp           tty0     tty25  tty41  tty58  vcs2      vcsu6
disk/            i2c-10       loop5         ptmx          tty1     tty26  tty42  tty59  vcs3      vendor_storage
dma_heap/        i2c-11       loop6         ptp0          tty10    tty27  tty43  tty6   vcs4      vhci
dri/             i2c-12       loop7         pts/          tty11    tty28  tty44  tty60  vcs5      video0
drm_dp_aux0      i2c-2        loop-control  ram0          tty12    tty29  tty45  tty61  vcs6      zero
drm_dp_aux1      i2c-6        mali0         random        tty13    tty3   tty46  tty62  vcsa      zram0
fd@              i2c-7        mapper/       rfkill        tty14    tty30  tty47  tty63  vcsa1
full             i2c-9        mem           rga           tty15    tty31  tty48  tty7   vcsa2
fuse             iio:device0  mmcblk1       rkspi-dev2    tty16    tty32  tty49  tty8   vcsa3

However there are something in the /sys/devices/virtual/video4linux/

user@hinlink-h88k ~/v4l2loopback (main)> ll /sys/devices/virtual/video4linux/
总用量 0
drwxr-xr-x 3 root root 0 Jul  7 17:45 video2/
user@hinlink-h88k ~/v4l2loopback (main)> ll /sys/devices/virtual/video4linux/video2/
总用量 0
-r--r--r-- 1 root root 4.0K Jul  7 17:52 buffers
-r--r--r-- 1 root root 4.0K Jul  7 17:45 dev
-rw-r--r-- 1 root root 4.0K Jul  7 17:52 dev_debug
-rw-r--r-- 1 root root 4.0K Jul  7 17:52 format
-r--r--r-- 1 root root 4.0K Jul  7 17:45 index
-rw-r--r-- 1 root root 4.0K Jul  7 17:52 max_openers
-r--r--r-- 1 root root 4.0K Jul  7 17:52 name
drwxr-xr-x 2 root root    0 Jul  7 17:52 power/
-r--r--r-- 1 root root 4.0K Jul  7 17:52 state
lrwxrwxrwx 1 root root    0 Jul  7 17:52 subsystem -> ../../../../class/video4linux/
-rw-r--r-- 1 root root 4.0K Jul  7 17:45 uevent

Expected Results:

The control device and video device are created for use.

Note

The kernel is Rockchip's vendor kernel. Vendor kernel is generally considered buggy so it could be actually a kernel-related issue.
Kernel source is: https://github.com/armbian/linux-rockchip/tree/rk-5.10-rkr4
Used config is: https://github.com/happyme531/armbian-h88k-images/blob/master/config

[umlaeute]

well: the mere existence of /sys/devices/virtual/video4linux/video2/ shows that v4l2loopback (or some other kernel module that provides a similar/same "virtual" interface) is loaded and working (to some extent).

i cannot currently test on a linux-5 kernel.

in any case you could try loading the module with debug=2 or some other high number to see whether it gives some additional information.

[hackermondev]

I'm experiencing this too but /sys/devices/virtual/video4linux/video2/ doesn't even exist for me.

I don't know if it's related but I have successfully loaded v4l2loopback already in the past and I removed it with modprobe -r v4l2loopback.

[umlaeute]

@hackermondev if it doesn't exist than either

• your test is too narrow (/sys/devices/virtual/video4linux/video2/ is created only if v4l2loopback created /dev/video2; if you only created a single device, that might as well be /sys/devices/virtual/video4linux/video0/; or some other device, if the given video-device already exists)
• the module is not loading at all
• (some very old module is loaded)
• your kernel lacks sysfs support

a more robust test would be something like:

have_v4l2loopback=no
for f in  /sys/devices/virtual/video4linux/video*/name; do
  if [ -e "${f}" ]; then
    have_v4l2loopback=yes
    break
  fi
done
echo "v4l2loopback support: ${have_v4l2loopback}"

of course it always helps if you specify the

• kernel version
• v4l2loopback version
• module parameters you are passing when loading the module
• info about pre-existing video-devices

[hackermondev]

I fixed the issue, thanks for the help.

[umlaeute]

@hackermondev it would be great if you could also say how you fixed the issue

[hackermondev]

Yeah, sorry. I'm not sure 100% what fixed the issue but here's what I did:

• I uninstalled the v4l2loopback-dkms apt module I had installed (Not sure why I installed that when I was also using it from the source)
• I used modprobe to remove some other modules that referenced v4l2loopback

lsmod | grep v4l2loopback

 v4l2loopback           49152  0
videodev              225280  1 v4l2loopback

Then, I ran sudo modprobe -r v4l2loopback and sudo modprobe -r videodev.

• I rebuilt everything from the source, reran sudo make install and sudo modprobe v4l2loopback exclusive_caps=1

[umlaeute]

I uninstalled the v4l2loopback-dkms apt module I had installed (Not sure why I installed that when I was also using it from the source)

probably because it's mentioned somewhere, that DKMS will help you with signing modules.

[umlaeute]

closing, as it has been resolved for the OP, and has become unreproducible