Having trouble understanding scancodes (and their udev mapping)

Question

First off, I know that scancodes are basically the raw byte interpretation of keyboard presses which I can display for example with showkey --scancodes.
However why does showkey display at least two bytes for each key that I press (is it for press/release?) and for some keys even 4 and more? And how can I dump that down to a single byte which I can specify in a hwdb database for udev?
For example, this is the output for Fn+Brightness Up/Down:
showkey --scancodes
kb mode was UNICODE
[ if you are trying this under X, it might not work
since the X server is also reading /dev/console ]
press any key (program terminates 10s after last keypress)...
0x9c
0x1c 0x9c # pressed Enter
0xe0 0x54 0xe0 0xd4 # pressed Fn+Brightness Up
0xe0 0x4c 0xe0 0xcc # pressed Fn+Brightness Down
The according mappings for brightness up/down in the udev database for my keyboard model are:
KEYBOARD_KEY_88=!brightnessup # Fn+Up
KEYBOARD_KEY_89=!brightnessdown # Fn+Down
So assuming 0x54/0xd4 and 0x4c/0xcc are my "relevant" bytes, how do they lead to the scancodes 0x88 and 0x89 in the hwdb?

Answer 1

Yes thats what I assumed, however what I dont understand is which one do I need in case I want to overwrite or add a new key in a (custom) udev hwdb file.
The wiki doesn't mention anything specific about the difference between key press and key release and in the example I provided I was wondering how these scancodes for press/release of a key combination from showkey -s (like 0xe0 0x54 0xe0 0xd4) led to the specified value (0x88) in the udev hwdb file, since the wiki suggest that one should use showkey -s[ to figure out the scancode of a key.
Last edited by Triver (2014-11-23 16:13:07)