Reference : https://superuser.com/questions/79822/how-to-swap-the-fn-use-of-function-keys-on-an-apple-keyboard-in-linux

echo X > /sys/module/hid_apple/parameters/fnmode

if X == 0 : Fn key disabled

X == 1 : Fn key pressed by default

X == 2 : Fn key released by default

From /drivers/hid/hid-apple.c