Jyothish's Notes

Adjusting Monitor Brightness in Linux

May 20, 20253 min read

Problem

The kernel backlight interface does not include external monitor support.

What worked

Loading i2c-dev module and running ddcutil worked for me.

As for the ddcci module, it seems to not be updated to support the newest kernel version. For this reason I can’t use my favorite user-space utilities for the job :(.

I’ll describe how I came to learning this (I think it’s good to let an newbie know).

What I did initially

  • I had installed ddcci a few weeks ago, but I didn’t get what I was supposed to do. brightnessctl did not work. Turns out firstly I built it for a newer kernel which I hadn’t compiled, and secondly I never tried modprobe-ing it. So my lack of understanding back then.

Plan and Collecting Data

  • We need to gather information about external monitors using their display information interface (DDC/CI and EDID) and supply the right commands.

  • We use i2c-dev (built-in) and ddcci (external) kernel modules for the same.

  • If we are accessing it by i2c-dev interfaces, the utility to use is ddcutil.

  • If we are using ddcci, we’ll be using user-space utilities like Brightness, brillo, ddccontrol, etc.

  • From linux-i2c Mailing List:

    • A monitor is accessed via device /dev/i2c-n, created by the video device driver.
    • EDID data is found by reading address 0x50.
    • The monitor’s settings are read and written at address 0x37.
    • DDC/CI can be used to communicate with the monitor by address 0x37.

  • From External Monitors | Backlight | Arch Wiki:

    • A generic monitor backlight can be controlled in many possible ways:
      • Vendor hotkey inaccessible to the OS
      • Either by ACPI, graphic or platform driver. In this case backlight control is exposed through /sys/class/backlight, which can be used by user-space utilities.
      • By writing into the graphics card register directly via setpci
      • OLED screens don’t have backlight, so they are either controlled with PWM
    • For external monitors, DDC/CI interface can be used to control the monitors who provide MCCS commands via I2C.
    • ddcutil can be used with the interface /dev/i2c-n
    • Otherwise, ddcci kernel module can be installed, preferably with DKMS, which will expose an interface in /sys/class/, which can be used by user-space utilities.
    • Using i2c-dev and ddcci modules simultaneously may cause the device to appear as busy.

Organizing Notes

Display Information

Interaction Interface

  • The interface is called DDC/CI (Display Data Channel/Command Interface), a bidirectional software interface over I2C hardware serial interface.
    • The commands to control the monitors are specified in the Monitor Control Command Set (MCCS) standard document.

Metadata Format

  • The Extended Display Identification Data (EDID) and Enhanced-EDID are metadata standards published by VESA for display devices to convey their display capabilities to the video source.
    • It is said to be replaced by DisplayID

EDID is used in combination with DCC to collect display information.

Another approach I found

I found another approach here: https://clinta.github.io/external-monitor-brightness/

I initially found this counter-intuitive as it uses both i2c-dev and ddcci modules, and also ddcutil as well as user-space utilities.

Turns out it solves a problem (listed in ddcci-linux issue 7) which required using a udev rules to handle hot-plugged devices, for which a /sys/class/backlight/ device would be created by invoking the I2C interface of the device using the ddcci module by passing the 0x37 address. This was implemented using a systemd service.

Graph View

Backlinks