Building Custom Linux Input Rules at the evdev Level for Specialized Hardware Macro Arrays

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• Identify the device. Use evtest and lsusb to capture the vendor, product, and event types the hardware emits.
• Write a udev rule. Match by vendor and product ID, then run a helper or set properties such as ID_INPUT_KEYBOARD.
• Add hwdb entries. Map raw scancodes to standard keycodes in /etc/udev/hwdb.d so the kernel translates them.
• Rebuild the hwdb. Run systemd-hwdb update and trigger the device to apply changes without a reboot.
• Test with evtest. Confirm the remapped events appear as the expected keycodes before involving a compositor.
• Handle chording. Macro arrays often send simultaneous presses; decide whether to synthesize a single key or a sequence.
• Keep rules in version control. Input configuration is invisible until it breaks, so track every file.
• Document the scancode map. Future maintenance depends on knowing which raw code maps to which logical key.

Architecture

Input flows from the hardware through the kernel evdev driver, where hwdb remapping occurs, into udev-tagged device nodes, and finally to the compositor or application. Remapping at the evdev level means every consumer sees the corrected events without per-app configuration.

Production configuration

lsusb | grep -i macro
sudo evtest /dev/input/eventX
# capture vendor and product IDs
udevadm info -a -n /dev/input/eventX | grep -iE 'idVendor|idProduct'

# /etc/udev/rules.d/99-macro-array.rules
SUBSYSTEM=="input", ATTRS{idVendor}=="1234", ATTRS{idProduct}=="5678", ENV{ID_INPUT_KEYBOARD}="1"

# /etc/udev/hwdb.d/99-macro-array.hwdb
evdev:input:b0003v1234p5678*
 KEYBOARD_KEY_70004=a
 KEYBOARD_KEY_70005=b
 KEYBOARD_KEY_70006=c

sudo systemd-hwdb update
sudo udevadm trigger /dev/input/eventX
sudo evtest /dev/input/eventX
# confirm remapped keycodes appear

Edge cases from production

• Some macro arrays present as multiple input devices; rules must match each one separately.
• hwdb patterns are case-sensitive and whitespace-sensitive; a trailing space breaks the match silently.
• Compositors may cache the keymap; restart the session after changing evdev rules.
• Devices that change product ID between firmware versions need multiple rule entries.

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