udev Rules

udev stands for userspace /dev and is a device manager for the Linux kernel. The /dev path is ordinarily where device information is accessed from, udev rules allow you to alter this information in a predictable and permanent manner. For example, what drivers to use/blacklist for a specific device being plugged in.

udev rules are checked when:

  1. Booting up, all udev rules are parsed and a rules database is built in memory
  2. When an event happens, it checks its rule database and performs the appropriate actions.

In a nutshell, udev rules do always have two major parts:

  • Matches (“If the condition(s) matches”…). The operator for matches are == and !=
  • Actions (…“then do this!”). The operator for actions is: =

Batocera supports booting up and event udev rules like with any ordinary operating system. These can be manipulated in the usual /etc/udev/rules.d/ directory. However, changes cannot be permanently stored there due to Batocera's filesystem (overlays can be used, but are forgotten between upgrades).

But if you only need event udev rules, then the /userdata/system/udev/rules.d/ path can be used instead. The path must be initially created manually.

For example, if you wanted to enable the Bluetooth passthrough udev rule of Dolphin for your own Bluetooth dongle (list of compatible adapters for Dolphin, not necessarily Batocera is further down that page), you would use this file:

52-dolphin.rules
SUBSYSTEM=="usb", ATTRS{idVendor}=="YOURVID", ATTRS{idProduct}=="YOURPID", TAG+="uaccess"

Replace the values YOURVID and YOURPID according to your specific device found in the list mentioned above. Then save it and copy it to /userdata/system/udev/rules.d/52-dolphin.rules and reboot Batocera.

If the udev rule does not function in this path, then it is a booting up udev rule and must use the aforementioned /etc/udev/rules.d/ directory.

If you need something activated earlier in the boot process or something more intimate to the system, you can instead use boot scripts.

As an example, let's take the udev rule 52-dolphin.rules from above to dive a little deeper into udev rules:

SUBSYSTEM, ATTRS{idVendor} and ATTRS{idProduct} are all variables that do all have a == between them and their assigned values. This means they are all matches (conditions). On the other hand, the last variable TAG does have a +=, which adds itself to any other TAG action being executed on this specific event (e.g. by an additional udev rule matching the same event).

This udev rule can be read as follows:

If the detected device is a USB device with the vendor ID 'YOURVID' and the product ID 'YOURPID', then, in addition to any other TAG action on this specific event, tag the device with 'uaccess' (which provides access to the /dev/ file for the logged in user (in case of Batocera, mostly root user)).

Once Batocera has booted you can view the existing udev rules for your system in /etc/udev/rules.d/. You can do this from the file manager ([F1] on the system list) and going up one level, or via SSH. You cannot access this location from the network share, as it is outside of the userdata partition.

If you find that your udev rule isn't working after reboot for any reason, you can alternatively try putting your *.rules file directly into /etc/udev/rules.d/ and running batocera-save-overlay afterward. This will call it slightly earlier in the boot process. Remember: Everything saved to the boot partition via batocera-save-overlay will get lost after updating Batocera to a new version.

  • udev_rules.txt
  • Last modified: 13 hours ago
  • by grandmabetty