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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 (Intel Macs only), Windows, and Linux.

NOTE: Display Switch is currently not working on M1 Macs: M1 series SoC support in ddc-macos-rs is planned but is not implemented yet.

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, Dvi2, Vga1. 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.

Running external commands

display-switch supports running external commands upon connection or disconnection of USB devices. This configuration can be global (runs every time a configured USB device is connected or disconnected) or per-monitor (runs only when a given monitor is being switched):

usb_device = "1050:0407"
on_usb_connect = "Hdmi1"
on_usb_disconnect = "DisplayPort2"
on_usb_connect_execute = "echo connected"
on_usb_disconnect_execute = "echo disconnected"

[monitor1]
monitor_id="foobar"
on_usb_connect_execute = "echo usb connected, monitor 'foobar' being switched"
on_usb_disconnect_execute = "'c:\\program files\\my app.exe' --parameter"

Notes:

  1. External applications are executed as the same user that started display-switch.
  2. This program supports splitting supplied configuration into application name and parameters, but no other shell features are supported.
  3. If the application path contains spaces, surround the full file path with single quotes.
  4. On Windows, escape the backslashes (replace \ with \, see the example above).

USB Device IDs

Windows

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

MacOS

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.

Linux

To locate the ID of your USB device on Linux, first install lsusb, which your Linux distro should have a package for. (On Debian, Ubuntu and RedHat, the package name is usbutils.) Then, in a terminal, run the following:

$ lsusb > a
<switch the usb dock here>
$ lsusb > b
$ diff -u a b

The diff output will show 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

Linux

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.

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/

Enable read/write access to i2c devices for users in i2c group. Run as root :

groupadd i2c
echo 'KERNEL=="i2c-[0-9]*", GROUP="i2c"' >> /etc/udev/rules.d/10-local_i2c_group.rules
udevadm control --reload-rules && udevadm trigger

Then add your user to the i2c group :

sudo usermod -aG i2c $(whoami)

Create a systemd unit file in your user directory (/home/$USER/.config/systemd/user/display-switch.service) with contents

[Unit]
Description=Display switch via USB switch

[Service]
ExecStart=/usr/local/bin/display_switch
Type=simple
StandardOutput=journal
Restart=always

[Install]
WantedBy=default.target

Create the config file at /home/$USER/.config/display-switch/display-switch.ini. Then enable the service with

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