How linux maps keyboard scancodes to kernel keycodes?

chomwitt · 10-07-2020, 05:00 AM

In /usr/share/X11/xkb/keycodes/evdev
there is a mapping from :

Quote:

evdev scancodes to something resembling xfree86 keycodes

e.g:
<AC01> = 38;
Trying

Code:

$ sudo xev -event keyboard
state 0x0, keycode 38 (keysym 0x61, a),

and pressing keycap 'A' i do get keycode: 38

But what is <AC01> ? Where can i find what it means?

Also trying to display usb related values (are those called still scancodes)
i get:

Code:

$ sudo evtest /dev/input/by-id/usb-Logitech_USB_Receiver-if01-event-kbd 

Event: time 1601988267.325606, -------------- SYN_REPORT ------------
Event: time 1601988267.453604, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70004
Event: time 1601988267.453604, type 1 (EV_KEY), code 30 (KEY_A), value 0

So usb keyboard sent : (MSC_SCAN), value 70004
linux transforms that keycode to 30? or 38? and that value is used later
from Xserver to map it to keysyms.

But where does that mapping from keyboard sent scancode to linux keycode is happening ?

pan64 · 10-07-2020, 05:14 AM

probably I would start here: https://wiki.archlinux.org/index.php/Keyboard_input

DavidMcCann · 10-07-2020, 11:10 AM

<AC01> means the first key (01) in the third row up (C) of the alphanumeric area (A). So why does it use that?

If you plug a Mac keyboard into a PC using Windows, it doesn't work. That's because Windows expects a keyboard where the first key in the home row has the code 38, but the Mac uses 8. If you plug it into a PC running Linux it does work. Linux doesn't just check that you have a keyboard available somewhere, but it also checks what sort it is. To enable it to use a wide range, it defines the key codes as places on the keyboard in one file (like evdev) and sets the use of the places in another, like /usr/share/X11/xkb/symbols/latin. e.g.

Code:

<AC01> = 38;
key <AC01> {[a, A, ae, AE]};

Once you've loged in it knows what language you're using and can combine the two files (and some others) to make the source code for a keyboard driver, which it then compiles.

Code:

key 38 {[a, A, ae, AE]};