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: =

Shamelessly referencing the Arch wiki here, although Arch uses system.d for udev a lot of it still applies to Batocera:

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:

SUBSYSTEM=="usb", ATTRS{idVendor}=="YOURVID", ATTRS{idProduct}=="YOURPID", TAG+="uaccess"

Where the values YOURVID and YOURPID are replaced 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.

Udev doesn't strictly need the VID or PID to identify a device, its name attribute can be used instead (however this is more susceptible to being applied to the similar devices connected).

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.

In order to create a udev rule either its name (or other uniquely identifiable attribute) or the VID and PID need to be discovered.

  1. Run the list command for the device in question:
    • For input devices, this can be achieved with evtest or for a more systematic approach, cat /proc/bus/input/devices
    • For USB devices in general: lsusb
    • For devices in general (or if you don't know what subsystem it uses), search through the /dev/ folder in root: ls /dev and then ls /dev/<subsystem>
  2. Once the device's path is discovered, run the following command to list all of its accessible attributes:
    sudo udevadm info --query all --path /sys/block/sdb/sdb5/ --attribute-walk

    where /sys/block/sdb/sdb5/ is the path the device is using, where-ever that may be.

  3. Make an appropriate ##-<any-name-you-want>.rules where ## is a number (udev will apply the rules in this specified order, most of the time this can be 99 with no issues)
  4. If not making a boot udev rule, it can be tested immediately by reloading the udev service:
    udevadm control --reload-rules && udevadm trigger
    • Once this is done, run the following to test if the rule has been parsed and applied correctly:
      udevadm info -q all -n /dev/<subsystem>/<device>

The keyboard can be identified with evtest (in this example, event3):

# evtest 
No device specified, trying to scan all of /dev/input/event*
Available devices:
/dev/input/event0:	Lid Switch
/dev/input/event1:	Sleep Button
/dev/input/event2:	Power Button
/dev/input/event3:	AT Translated Set 2 keyboard
/dev/input/event4:	Video Bus
/dev/input/event5:	Video Bus
/dev/input/event6:	HDA Intel PCH Headphone
/dev/input/event7:	HDA Intel HDMI HDMI/DP,pcm=3
/dev/input/event8:	HDA Intel HDMI HDMI/DP,pcm=7
/dev/input/event9:	HDA Intel HDMI HDMI/DP,pcm=8
/dev/input/event10:	FocalTechPS/2 FocalTech Touchpad
/dev/input/event11:	Asus WMI hotkeys
/dev/input/event12:	8Bitdo  8BitDo N30 Pro 2
/dev/input/event13:	USB2.0 HD UVC WebCam: USB2.0 HD
Select the device event number [0-13]:

Create a udev rule to set this event as a joystick. This can be achieved with this one-line command (where AT Translated Set 2 keyboard is the name of your keyboard):

echo 'SUBSYSTEM=="input", ATTRS{name}=="AT Translated Set 2 keyboard", MODE="0666", ENV{ID_INPUT_JOYSTICK}="1", ENV{ID_INPUT_KEYBOARD}="0"' > /etc/udev/rules.d/99-custom.rules

To make sure it's work, reload the rules and check if the flag is present:

udevadm control --reload-rules && udevadm trigger
udevadm info -q all -n /dev/input/event3

Identify the name attribute of the device. Download this file and then replace the <> respectively:

SUBSYSTEM=="input", ATTRS{name}=="<name of device>", MODE="0666", RUN+="<path/to/script.sh>"

Save the file to /userdata/system/udev/rules.d/.

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: 19 months ago
  • by atari