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Is there any way to find out who created a device node? If it has been udev or the kernel itself?

In the specific case of mine, I have two webcams. Nodes created for them are /dev/video0 and /dev/video1. Unfortunately this is not a constant assignment. Now I thought that would be the case when using udev, but when I create a udev rule it seems to me that video0 and video1 nodes are already created prior to udev being able to assign a more constant node like /dev/videoLogitech.

I need some suggestions regarding which log-file or which command might help me to find out about what happens here.

Rui F Ribeiro
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1 Answers1

6

If you want to check order of actions use:

udevadm monitor --property

by doing this you get a listing of actions. When you add your camera one of the first entries is likely to be something like:

KERNEL[110935.814197] add      /devices/.../video4linux/video1 (video4linux)
ACTION=add
DEVNAME=/dev/video1  <<<<---- devname

As pointed out by @goldilocks:

Unfortunately, if you are using systemd, it's version of udev has a complex "persistent naming scheme" that's hard coded into udevd, so you are right, just parsing the rules.d stuff could leave a lot of mysteries.

Also from man udev for systemd:

The following keys can get values assigned:

NAME The name to use for a network interface. See systemd.link(5) for a higher-level mechanism for setting the interface name. The name of a device node cannot be changed by udev, only additional symlinks can be created.

Naming and debugging

Your best option is likely to use something like:

udevadm test $(udevadm info -q path -n video1)

It gives you a "test run" on your rule(s) and report any errors.

After adding/changing a rule, remember to reload with:

udevadm control --reload-rules

When it comes to naming a devices you likely get something like:

NAME="smile2thecamera" ignored, kernel device nodes can not be renamed; 
      please fix it in /etc/udev/rules.d/83-webcam.rules:6

You could instead use a SYMLINK (if that suffice) as in:

KERNEL=="video[0-9]", .... SYMLINK+="video-logitech"

Should give you something like this on test:

creating link '/dev/video-logitech' to '/dev/video1'
creating symlink '/dev/video-logitech' to 'video1'

DEVLINKS=.... /dev/video-logitech
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