---
title: "Keyboard Mapping"
description: "Changing keyboard mapping at the hwdb level"
author: "Thurstylark"
date: 2021-9-25
draft: false
---


References:
- https://wiki.archlinux.org/index.php/Map_scancodes_to_keycodes
- https://github.com/Earnestly/pkgbuilds/blob/master/system-config/hwdb-capslock-as-control.hwdb

Maping keys is pretty simple, and if it's done at the udev level, it's universal, so it doesn't require any X or Wayland specific configuration.

As an example, I map my compose key (menu key) to right meta (aka super, logo, windows, apple, or command key), and CapsLock to left Control.

## Get Scancode

First, get the scancode(s) of the key(s) you wish to change. Easiest way is to use `evteest(1)`. For each keypress, you'll get information similar to the following:

```
Event: time 1496690003.603531, -------------- SYN_REPORT ------------
Event: time 1496690003.723467, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70065
Event: time 1496690003.723467, type 1 (EV_KEY), code 127 (KEY_COMPOSE), value 0
```

The code you will need to use in the udev hwdb config file is the `MSC_SCAN` value (in this case: 70065).

## Get Hardware ID

Next you will need the hardware Vendor and Product ids from `lsusb(1)`. These are two sets of four hexidecimal digits separated by a colon (:) printed right before the device name.

```
Bus 001 Device 005: ID 258a:1006 <keyboard name here>
```

In this example, the Vendor ID is `258a`, and the Product ID is `1006`.

## Create udev hwdb Configuration

Then create `/etc/udev/hwdb.d/61-custom-keyboard.hwdb` with the following contents:

```
evdev:input:b0003v258Ap1006*
 KEYBOARD_KEY_70065=rightmeta
 KEYBOARD_KEY_70039=leftctrl
```

A few notes on the format of this file:
- The format for the device identifying line for usb devices is `evdev:input:b0003v<Vendor ID>p<ProductID>*`. 
	- If `<Product ID>` and `<Vendor ID>` contain letters, they must be capitalized
	- These scancodes can be quite device-specific, so it is wise to be specific to *at least* the vendor and product IDs
	- There are methods for identifying atk keyboards as well. Check the Arch Wiki page listed in the references above for more info
- Defining a key to change is done with `KEYBOARD_KEY_<scancode>=<new keycode>`
	- `<scancode>` is the code we pulled from evtest earlier
	- `<new keycode>` is the keycode we want to emit when the key is pressed. Names of keycodes are listed in `/usr/include/linux/input-event-codes.h`, and should be all lowercased in your udev config.
- The filename `61-custom-keyboard.hwdb` is somewhat arbitrary, but make sure that you order your file *after* the existing `60-keyboard.hwdb`.

## Update Hardware Database Index

After changing the config files, the hwdb needs to be rebuilt:

```
# systemd-hwdb update
```

To start using this new hwdb, either reboot or tell udev to reload:

```
# udevadm trigger
```

Note that this will only work for adding or modifying existing key mappings. Deleted key mappings are kept in the kernel until a reboot.

## Testing the New Mappings

A simple test can be run before trying the keys directly:

```
# udevadm info /dev/input/by-path/*-usb-*-kbd | grep KEYBOARD_KEY
```
