Map scancodes to keycodes

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This page assumes that you have read Keyboard input, which provides wider context.

Mapping scancodes to keycodes is achieved in a layer lower than Xorg and Linux console, which means that changes to this mapping will be effective in both. [1][2][3]

There are two ways of mapping scancodes to keycodes:

The preferred method is to use udev because it uses hardware information (which is a quite reliable source) to choose the keyboard model in a database. It means that if your keyboard model has been found in the database, your keys are recognized out of the box.

Identifying scancodes

You need to know the scancodes of keys you wish to remap. See Keyboard input#Identifying scancodes for details.

Using udev

udev provides a builtin function called hwdb to maintain the hardware database index in /etc/udev/hwdb.bin. The database is compiled from files with .hwdb extension located in directories /usr/lib/udev/hwdb.d/, /run/udev/hwdb.d/ and /etc/udev/hwdb.d/. The default scancodes-to-keycodes mapping file is /usr/lib/udev/hwdb.d/60-keyboard.hwdb. See hwdb(7) for details.

The .hwdb file can apply key mappings to one or more keyboards based on hardware ID glob patterns. You may obtain device identification info by running evemu-describe(1) as the root user. This command is provided by the evemu package.

The evdev: prefix is used to match hardware against a block of mappings. The following hardware matches are supported:

  • Generic input devices (also USB keyboards) identified by the usb kernel modalias: 
    evdev:input:b<bus_id>v<vendor_id>p<product_id>e<version_id>-<modalias>
    where <vendor_id>, <product_id> and <version_id> are the 4-digit hex uppercase vendor, product and version IDs (you can find those by running the lsusb command) and <modalias> is an arbitrary length input-modalias describing the device capabilities. <bus_id> is the 4-digit hex bus id and should be 0003 for usb devices. The possible <bus_id> values are defined in /usr/include/linux/input.h (you can run awk '/BUS_/ {print $2, $3}' /usr/include/linux/input.h to get a list).
  • AT keyboard DMI data matches: 
    evdev:atkbd:dmi:bvn*:bvr*:bd*:svn<vendor>:pn<product>:pvr*
    where <vendor> and <product> are the firmware-provided strings exported by the kernel DMI modalias.
  • Input driver device name and DMI data match: 
    evdev:name:<input device name>:dmi:bvn*:bvr*:bd*:svn<vendor>:pn*
    where <input_device_name> is the name device specified by the driver and <vendor> is the firmware-provided string exported by the kernel DMI modalias.

The format of each line in the block body is KEYBOARD_KEY_<scancode>=<keycode>. The value of <scancode> is hexadecimal, but without the leading 0x (i.e. specify a0 instead of 0xa0), whereas the value of <keycode> is the lower-case keycode name string as listed in /usr/include/linux/input-event-codes.h (see the KEY_<KEYCODE> variables), a sorted list is available at [4]. It is not possible to specify decimal value in <keycode>.

Example for custom hwdb

The example hwdb file will match all AT keyboards: 

/etc/udev/hwdb.d/90-custom-keyboard.hwdb
evdev:atkbd:dmi:bvn*:bvr*:bd*:svn*:pn*:pvr*
 KEYBOARD_KEY_10=suspend
 KEYBOARD_KEY_a0=search

Here is an example of rebinding modifiers on a laptop and USB keyboard: 

/etc/udev/hwdb.d/10-my-modifiers.hwdb
evdev:input:b0003v05AFp8277* # was tested on Kensington Slim Type USB (with old ABI)
 KEYBOARD_KEY_70039=leftalt  # bind capslock to leftalt
 KEYBOARD_KEY_700e2=leftctrl # bind leftalt to leftctrl

evdev:atkbd:dmi:*            # built-in keyboard: match all AT keyboards for now
 KEYBOARD_KEY_3a=leftalt     # bind capslock to leftalt
 KEYBOARD_KEY_38=leftctrl    # bind leftalt to leftctrl

To block the Sleep key, bind it to the "reserved" keyword. Alternatively, you can use "unknown" to map it to the NoSymbol key. For example: 

/etc/udev/hwdb.d/90-block-sleep.hwdb
evdev:input:b0003v03F0p020C* # hp 5308 keyboard controller
 KEYBOARD_KEY_10082=reserved

Updating the Hardware Database Index

After changing the configuration files, the hardware database index, hwdb.bin, needs to be rebuilt.

  • Update hwdb.bin manually by running
# systemd-hwdb update
  • Update automatically on each reboot by commenting out ConditionNeedsUpdate in systemd-hwdb-update.service using a replacement unit file 
/etc/systemd/system/systemd-hwdb-update.service
#  This file is part of systemd.
.
.
#ConditionNeedsUpdate=/etc
.
.

After systemd-hwdb-update.service finished loading systemd-trigger.service will reload the changes from hwdb.bin.

  • Automatically after Systemd upgrade.

On each upgrade of Systemd, the installation script rebuilds hwdb.bin by running udevadm hwdb --update as the root user, so we do not need to care about it.

Reloading the Hardware Database Index

The kernel loads hwdb.bin as part of the boot process, rebooting the system will promise the loading of the updated hwdb.bin.

With udevadm it is possible to load new key mapping from the updated hwdb.bin by running 

# udevadm trigger

Be aware that with udevadm only added or changed key mapping are loaded so if we delete a mapping from the configuration file, rebuild hwdb.bin and run udevadm trigger as the root user, then the deleted mapping still kept by the kernel, at least until a reboot.

Querying the database

You can check that your configuration was loaded either by pressing keys, or by running udevadm info. For the USB keyboard in the above example, this outputs the mapping we configured as follows: 

# udevadm info /dev/input/by-path/*-usb-*-kbd | grep KEYBOARD_KEY
E: KEYBOARD_KEY_70039=leftalt
E: KEYBOARD_KEY_700e2=leftctrl

Using setkeycodes

setkeycodes is a tool to load scancodes-to-keycodes mapping table into Linux kernel. Its usage is: 

# setkeycodes scancode keycode ...

It is possible to specify multiple pairs at once. Scancodes are given in hexadecimal, keycodes in decimal.

Note: Apparently setkeycodes does not work with USB keyboards (Linux 3.14.44-1-lts): 
# setkeycodes 45 30     # bind NumLock (0x45) to KEY_A (30) on AT keyboard
(successful)
# setkeycodes 70053 30  # bind NumLock (0x70053) to KEY_A (30) on USB keyboard
KDSETKEYCODE: Invalid argument
failed to set scancode 620d3 to keycode 31

If using this simple command, changes will be lost after reboot. The changes can be made permanent by creating a new service: 

/etc/systemd/system/setkeycodes.service
[Unit]
Description=Change keycodes at boot

[Service]
Type=oneshot
ExecStart=/usr/bin/setkeycodes [scancode] [keycode] [scancode] [keycode] [...]

[Install]
WantedBy=multi-user.target

and enabling setkeycodes.service.