Kernel Drivers – How to Debug Input from Input-Device (/dev/input/event*)

Quickly searching, it appears that there is a Linux Joystick Mapper on Sourceforge which claims to do what you want.

I believe it is built on top of uinput which is probably what you'll want to use if you have to build your own. There wrappers to that in various languages, so you won't necessarily have to work in C. Python appears to definitely by possible.

As it turns out the kernel did pick it up, but kept complaining that it's not recognised.

For anyone else having this issue, or wants to map a key that's not read by the OS, read on.

Open a terminal and run dmesg | grep -A 1 -i setkeycodes. This will give you multiple entries like this:

[    9.307463] atkbd serio0: Unknown key pressed (translated set 2, code 0xbe on isa0060/serio0).
[    9.307476] atkbd serio0: Use 'setkeycodes e03e <keycode>' to make it known.

What we are interested is the hexadecimal value after "setkeycodes", in this case this is e03e. If you have multiple of these, you can run tail -f /var/log/kern.log. Once you've done so, you can tap the button you're looking for, and this will give you a the same line as above, and again, we only need the hexadecimal value. Make a note of this.

Now run xmodmap -pke | less and find the appropriate mapping. In my case, I needed to map this to toggle my touch pad, which means I was interested in the following line:

keycode 199 = XF86TouchpadToggle NoSymbol XF86TouchpadToggle

If you can't find whatever you're interested in, read @Gilles answer too, as you can define custom mappings too, then read on (if the kernel reads it, you won't need to add it to xorg.conf.d)

Now I ran the following command: sudo setkeycodes [hexadecimal] [keycode], so in my case that became: setkeycodes e03e 199.

Now you can use the following line to test if it worked and/or you have the correct mapping:

xev | grep -A2 --line-buffered '^KeyRelease' | sed -n '/keycode /s/^.*keycode \([0-9]*\).* (.*, \(.*\)).*$/\1 \2/p'

When you run this command, you need to focus on the newly opened window (xev) and check the console output. In my case it read as following:

207 NoSymbol

This was obviously wrong, as I requested keycode 199, so it's mapped to XF86TouchpadToggle. I checked xmodmap -pke again, and noticed that keycode 207 is actually mapped to NoSymbol, and I noticed that there was an offset difference of 8, so I tried the setkeycodes command again, but the key is mapped to keycode 191.

sudo setkeycodes e03e 191

This worked perfectly.

EDIT -- the solution I provided to have to working on start up does not. I will figure this out tomorrow and update this answer. For now I suppose you can run this on start up manually.