evdevremapkeys

A daemon to remap key events on linux input devices

Motivation

The remapping of input key events is an problem, and one that has been solved at many levels over the years. On a traditional X11 desktop, the usual way to do this is with xbindkeys; it's simple and effective and you shouldn't try and write something different.

However, with the shift to Wayland, we have a problem. Wayland obviously isn't X11 so any X11 based remapping utility isn't going to work. Wayland compositors typically use libinput to manage input events, but while libinput supports remapping conceptually, it does not expose any mechanism to configure it. This is left as an exercise to the compositor and neither Weston nor Mutter expose remapping.

So where does this leave us? If we are to provide a remapping mechanism that is not dependent on the compositor, it must run below libinput, which means it must work with the linux input subsystem. And so, here we are.

Technical approach

There's only one real sane approach to doing event remapping at the input subsystem level: Read events from physical input devices, and then generate new input events on a virtual device managed through uinput.

One legitimate question is whether the virtual device attempts to fully replicate the original physical device, just with remapped events, or whether it's a dedicate device that only emits the new events which leaving the physical device free to send events directly to other clients.

Depending on your exact use-case, you might be able to leave the original physical device as-is, but for me, it turned out that I had to swallow the original events because they will be picked up by libinput and then trigger actions in my desktop environment.

To avoid this, you have to take a grab on the physical device, so no other client receives events, and then forward all un-modified events through your virtual device. It's annoying but unavoidable - you can't hide individual events from other clients.

Why not evmapd

There is an existing project called evmapd which is, ostensibly, exactly what we're looking for - a daemon that will take input events from one device, and then generate new events on a different uinput based device. I made a serious attempt at using it but ultimately found it too limiting to rely on:

• It doesn't support 1:N mappings, which I particularly care about
  • eg: Mapping a mouse button to a key combination like Super+A
• It relies on an obscure and hard to obtain library for command line argument handling (libcfg+)
• It's written in C, which I will always respect, but which doesn't add much value to this kind of program
• It's doesn't look actively maintained; it solved whatever problem the author originally had, and that was it.

Having said all that, it has a set of capabilities to handle remappings that are not from keys to other keys - particuarly creating virtual joystick axes from key events. If you care about those capabilities, you should use evmapd. I'm not going to expand beyond key to key remappings.

Requirements

• Python >= 3.4 (for asyncio)
• python-daemon >= 2.1.2
• Python evdev binding >= 0.7.0
• pyxdg > 0.25
• PyYAML >= 3.12