displaylink-ddc

Control an external monitor over DDC/CI when it is connected through a DisplayLink dock on Linux. Brightness, contrast, colour, input and power, plus an optional service that makes the monitor follow your laptop brightness keys.

Why

ddcutil does not work on most DisplayLink docks. The evdi driver (1.14 and later) does create an I2C bus named DisplayLink I2C Adapter, and DDC/CI commands pass through it. But the EDID read on that bus fails, so ddcutil reports "No monitor detected" and stops before it sends any DDC command.

This tool skips the EDID step. It talks DDC/CI directly to address 0x37 on the DisplayLink bus.

Requirements

• Linux with the DisplayLink driver (evdi 1.14 or later)
• DDC/CI turned on in the monitor menu
• Python 3 (standard library only)
• GNOME, for the brightness sync service only

Tested on Ubuntu with GNOME 46, evdi 1.14.15, a Lenovo ThinkPad Hybrid USB-C dock and a Samsung monitor.

Install

git clone https://github.com/RickyMillar/displaylink-ddc.git
cd displaylink-ddc
./install.sh            # or ./install.sh --no-sync for the command only

The installer:

1. loads the i2c-dev kernel module now and at boot
2. adds a udev rule that gives the logged-in user access to the DisplayLink I2C bus (with the uaccess tag, so you do not need to log out)
3. copies monitor and monitor-brightness-sync to ~/.local/bin
4. enables the monitor-brightness-sync systemd user service

Remove everything with ./uninstall.sh.

Usage

monitor                       show brightness, contrast, RGB gain
monitor brightness 60         set an absolute value
monitor brightness +10        relative change, clamped to 0..max
monitor contrast|red|green|blue [VALUE]
monitor preset [6500k|9300k|user1|user2]
monitor mode [standard|mixed|movie|games]
monitor input [dp1|dp2|hdmi]
monitor power [on|standby|off]
monitor caps                  show the capabilities string of the monitor

The named values for preset, mode and input come from one Samsung monitor. Other monitors can use other values. Run monitor caps and look at the values in brackets after each code, for example 60( 12 0F 10). You can always give a raw value, for example monitor input 0x11.

Caution: Before you change the input, make sure that you can get back to the dock input with the monitor buttons.

Brightness sync

monitor-brightness-sync listens for changes to the GNOME screen brightness (org.gnome.SettingsDaemon.Power.Screen.Brightness on D-Bus). It sets the external monitor to the same percentage. It waits 0.15 s after the last change, so one held key press gives one DDC write.

systemctl --user status monitor-brightness-sync
systemctl --user disable --now monitor-brightness-sync   # turn it off

The sync is one-way. A change with the monitor buttons stays until the next laptop brightness change.

Troubleshooting

Symptom	Check
no 'DisplayLink I2C Adapter' bus found	Is the dock connected? Is evdi 1.14 or later? Run cat /sys/bus/i2c/devices/i2c-*/name.
Permission denied	Run getfacl /dev/i2c-N for the DisplayLink bus. Your user must be in the list. If not, run ./install.sh again.
no valid reply for feature 0x..	The monitor does not support that feature (see monitor caps), or DDC/CI is off in the monitor menu.

License

MIT