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Turn a $30 USB switch into a full-featured KVM

This utility watches for USB device connect/disconnect events and switches monitor inputs via DDC/CI. This turns a simple USB switch into a full-fledged KVM solution: press one button on your USB switch and all your monitors connect to a different input.

It is supposed to be installed on all computers that could be connected to these monitors, since the app only switches monitors "one way" and relies on itself running on the other computers to switch it "the other way" as needed.

Platforms supported

The app should function on MacOS, Windows, and Linux. Most of the code is in Rust, with the exception of DDC support on MacOS, which is done via statically-linked Swift library.

Configuration

The configuration is pretty similar on all platforms:

On MacOS: the configuration file is expected in ~/Library/Preferences/display-switch.ini On Windows: the configuration file is expected in %APPDATA%\display-switch\display-switch.ini On Linux: the configuration file is expected in $XDG_CONFIG_HOME/display-switch/display-switch.ini or ~/.config/display-switch/display-switch.ini

Configuration file settings:

  usb_device = "1050:0407"
  on_usb_connect = "Hdmi1"
  on_usb_disconnect = "Hdmi2"

usb_device is which USB device to watch (vendor id / device id in hex), and on_usb_connect is which monitor input to switch to, when this device is connected. Supported values are Hdmi1, Hdmi2, DisplayPort1, DisplayPort2, Dvi1. If your monitor has an USB-C port, it's usually reported as DisplayPort2. Input can also be specified as a "raw" decimal or hexadecimal value: on_usb_connect = 0x10

The optional on_usb_disconnect settings allows to switch in the other direction when the USB device is disconnected. Note that the preferred way is to have this app installed on both computers. Switching "away" is problematic: if the other computer has put the monitors to sleep, they will switch immediately back to the original input.

Different inputs on different monitors

display-switch supports per-monitor configuration: add one or more monitor-specific configuration sections to set monitor-specific inputs. For example:

on_usb_connect = "DisplayPort2"
on_usb_disconnect = "Hdmi1"

[monitor1]
monitor_id = "len"
on_usb_connect = "DisplayPort1"

[monitor2]
monitor_id = "dell"
on_usb_connect = "hdmi2"

monitor_id specifies a case-insensitive substring to match against the monitor ID. For example, 'len' would match LEN P27u-10 S/N 1144206897 monitor ID. If more than one section has a match, a first one will be used. on_usb_connect and on_usb_disconnect, if defined, take precedence over global defaults.

USB Device IDs

To locate the ID of your USB device ID on Windows:

  1. Open Device Manager
  2. Locate the USB device, view the properties
  3. Switch to the Details tab and select Hardware IDs in the Property dropdown
  4. You should see a value similar to HID\VID_046D&PID_C52B&MI_00 (the exact values will differ) - the USB device ID is a combination of the Vendor ID and the Product ID - for example, in this case it would be 046D:C52B

To locate the ID of your USB device ID on MacOS, open a terminal and run the following:

brew install lsusb

$ lsusb > a
<switch the usb dock here>
$ lsusb > b
$ opendiff a b

In the command output, the highlighted lines show you which USB IDs are most relevant.

Logging

  • On MacOS: the log file is written to /Users/USERNAME/Library/Logs/display-switch/display-switch.log
  • On Windows: the log file is written to %LOCALAPPDATA%\display-switch\display-switch.log
  • On Linux: The log file is written to $XDG_DATA_HOME/display-switch/display-switch.log or ~/.local/share/display-switch/display-switch.log

Building from source

Windows

Install Rust, then do cargo build --release

MacOS

Install Xcode, install Rust, then do cargo build --release

Running on startup

Windows

Copy display_switch.exe from target\release (where it was built in the previous step) to %APPDATA%\Microsoft\Windows\Start Menu\Programs\Startup (replace Username with your Windows user name).

MacOS

  # Get your INI file in order! (see above)
  cp target/release/display_switch /usr/local/bin
  cp dev.haim.display-switch.daemon.plist ~/Library/LaunchAgents/
  launchctl load ~/Library/LaunchAgents/dev.haim.display-switch.daemon.plist

Linux

Copy built executable:

  cp target/release/display_switch /usr/local/bin

i2c needs root access, so run as root. Create a systemd unit file /etc/systemd/system/display-switch.service with contents

[Unit]
Description=Display switch via USB switch

[Service]
ExecStart=/usr/local/bin/display_switch
Type=simple
Restart=always
User=root

[Install]
WantedBy=multi-user.target

Create the config file at /root/.config/display-switch/display-switch.ini. Then enable the service with

systemctl daemon-reload
systemctl enable display-switch.service
systemctl start display-switch.service