Keyboard bridge

A bridge to run on my Raspberry Pi to transmit keyboard events over to the OTG cable. USB on-the-go or OTG allows it to act as a master rather than a slave, and thusly masquerade as a keyboard.
My use case for this is for this to be used in conjunction with Kanata to handle keyboard layers and layouts. Thusly, even on a locked down computer, I can use another keyboard layout and have all my layers.

Features

• Functions as a keyboard bridge
• Keylogger (if so desired)

Usage and installation

Exiting

Press <Enter> ~ . <Backspace> <Backspace> <Backspace> <Enter> to exit.
Note: You must hold Shift after Enter to get ~, not before.

Prerequisites

• The Raspberry Pi plugged into a computer through a USB C cable with data lines
• A keyboard plugged into the Raspberry Pi that Linux recognizes
• Rust and Cargo (https://rustup.rs/)
• A Linux kernel with libcomposite available as a kernel module (most likely is)

Setup

1. Enable the RPi USB OTG as a keyboard device.

   # In project root
   chmod +x ./enable-rpi-hid.sh
   sudo ./enable-rpi-hid.sh
   sudo reboot

2. Start the bridge once to ensure it works

   # In project root
   chmod +x ./run.sh
   ./run.sh

3. Plug in the keyboard and start typing
4. Exit (see Exiting)

Autostart

1. Enable autologin for your user (sudo raspi-config, 1 System Options -> S5 Boot / Auto Login -> B2 Console Autologin)
2. Add the following to your .bashrc (or any other shell's runcom/autostart file if used)

   if [ "$(tty)" = "/dev/tty1" ] && [ "$TERM" = "linux" ]; then if [ -n "$(pgrep keyboard-bridge)" ]; then :; else
       keyboard-bridge
   fi fi

How it works

This program "grabs" all real keyboards connected in /dev/input, where grabbing means retaining exclusive access so that no key events can be sent to any other programs. Then, it maps the key events taken in to valid keyboard USB events.

The considerations in intercepting /dev/input events are as follows:

• Itercepting the raw input from an actual USB keyboard and just piping the input to the OTG output wouldn't allow for another program to do key interception and remapping
• Using stdin does not allow for special and modifier keys (e.g. page up and control+shift) to be received.
• Using X.Org or Wayland to handle key events would be bloated

However, I found it easier to have my Pi autologin to my user and launch this in .bashrc, especially since there's no need to either hook into Kanata and I don't need the keyboard to send any events.

Key Mime Pi

Thanks to Michael Lynch for creating Key Mime Pi (I like the pun), a tool designed to transfer keypresses between computers over WebSockets with a Flask webpage. His project was excellent in terms of documentation.