Udev hwdb to re-map a key when I don't know the key code

Question

I've written a file /etc/udev/hwdb.d/70-keyboard.hwdb to re-map some of my keyboard keys. There's an additional key that I want to re-map, but I can't figure out what identifier I should use for it.

Ordinarily, I run evtest and press keyboard keys to get an output like the following:

Event: time 1639158329.532434, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7000f
Event: time 1639158329.532434, type 1 (EV_KEY), code 38 (KEY_L), value 1

Then I use the hex "value" from the first line above (containing "EV_MSC") to compose a line like the following in my 70-keyboard.hwdb file:

 KEYBOARD_KEY_7000f=rightctrl

The key that I want to re-map now is my KEY_KBDILLUMDOWN key, and when I run evtest to identify its hex value, I only get one line of output, and it's not the line that contains "EV_MSC":

Event: time 1639158417.741165, type 1 (EV_KEY), code 229 (KEY_KBDILLUMDOWN), value 1

How can I remap this key in udev?

Answer 1

Perhaps this key event is injected by ACPI driver (instead of real key press event) in which case you may handle it by acpid. Try acpi_listen (while acpid is running). Events handlers are at /etc/acpi/events/ which are calling handlers (default is right in /etc/acpi/).

If that is the case you can write a simple evdev wrapper script to generate keypress events as handler to acpi events. See here for examples.