About udev

udev is a device manager for the Linux kernel that gives the system access to various running hardware via device .rules files also known as udev rules.

udev rules are used to allow and manage the access to a specific devices, so without a proper udev rule some devices such as custom controller could not be used by RetroDECK nor by Steam or any other part of the system.

Important directories

/lib/udev/rules.d/

This directory contains the default .rules file shipped by your system.
They should not be edited.

/etc/udev/rules.d/ or /run/udev/rules.d (depending on the system)

This directory contains custom .rules file additions to those shipped in /lib/udev/rules.d/ and the administrator can add more rules into this directory.

If a .rules file exist for the same device under /lib/udev/rules.d/ and /etc/udev/rules.d/ the /etc version will always take preset over the lib version.

Example of a .rules file

The content of a Merlin UTMS modem .rules file.

ATTRS{prod_id2}=="Merlin UMTS Modem", ATTRS{prod_id1}=="Novatel Wireless", SYMLINK+="MerlinUMTS"

A .rules file can also contain more the one devices example multiple 8Bitdo controllers:

# 8Bitdo F30 P1
SUBSYSTEM=="input", ATTRS{name}=="8Bitdo FC30 GamePad", ENV{ID_INPUT_JOYSTICK}="1", TAG+="uaccess"

# 8Bitdo F30 P2
SUBSYSTEM=="input", ATTRS{name}=="8Bitdo FC30 II", ENV{ID_INPUT_JOYSTICK}="1", TAG+="uaccess"

Controller udev projects

Valve's - Steam Devices

The Steam Devices package is usually installed when you install Steam on your system, it contains rules for the most common controllers.
steam-devices github

Game Devices udev

The following project is an effort to combine all game devices into one package but it is still early and several are missing.
game-devices-udev codeberg

Batocera udev

The Batocera project has also combined a list of other controllers that might be missing from the two projects above.
Batocera - Controllers github

Quick tips on udev installation

Installing a udev rule needs administrator root access with sudo and the rules should be put in either the /etc/udev/rules.d/ or /run/udev/rules.d example from above.
- You can copy the .rules from terminal into the directory either from terminal or with sudo access file browser.
- The rules should be in the .rules file format and should be extracted from any .zip .7z .tar or any other compressed format.
The udev rule should be added when the RetroDECK or any other software that you want access to the device is not running.
- After a rule is added you will need to either reload the udevadm from terminal by issuing the following command: sudo udevadm control --reload-rules or just reboot the system.
For SteamOS or other immutable systems udev rules might or might not persistent persist over SteamOS updates (we can't say for certain).

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