bctl 0.0.2

service for simultaneously controlling brightness of laptop & external displays

BCTL - brightness control

This is a simple daemon for controlling displays' brightnesses. It consists of a daemon process listening for user requests (e.g. changing brightness) and a client to send commands to the daemon. udev events are monitored for screen (dis)connections. Desktop notifications are shown on brightness change.

Installation

$ pipx install bctl

[!NOTE] This will install the client & daemon executables, but it's user responsibility to launch the daemon process, covered below.

Why?

Main reason for this program is to provide a simple, general-puropse central point for controlling brightness of all the connected screens simultaneously and keeping track of their current brightness levels.

Controlling laptops' internal screen is generally the easiest, as its device object is exposed under /sys/class/backlight/ dir -- it's the external displays that tend to be trickier.

For controlling external screens' brightness there are roughly two main methods, explained below. As the recommended method -- ddcutil -- takes in some cases non-trivial amount of time to execute (up to ~200ms), it can be slightly jarring to change brightness when spamming the key.

As this solution caches set brightness values there's no need to query it from ddcutil, making e.g. desktop notification generation simpler.

Also screen connections & disconnections are kept track of via an udev monitor, and there's an option to force all the screens' brightnesses to be kept in sync.

Managing external displays

ddcci kernel driver

This kernel module should detect the devices and expose 'em under /sys/class/backlight/, just like the laptop's internal display (e.g. /sys/class/backlight/amdgpu_bl0) is by default. This requires installation of ddcci-dkms package and loading ddcci kernel module.

Note as of '25 there are loads of issues w/ this kernel module's auto-detection logic, e.g. see issues 7, 42, 46

Current workaround seems to be manually enabling displays as per this reddit post:

• Before state (no external display devices listed/avail):

$ ls -l /sys/class/backlight
total 0
lrwxrwxrwx 1 root root 0 Sep  7 09:44 amdgpu_bl0 -> ../../devices/pci0000:00/0000:00:08.1/0000:07:00.0/drm/card0/card0-eDP-1/amdgpu_bl0

• Enable manually:

$ echo 'ddcci 0x37' | sudo tee /sys/bus/i2c/devices/i2c-11/new_device

• After (ddcci11 external screen avail):

$ ls -l /sys/class/backlight
total 0
lrwxrwxrwx 1 root root 0 Sep  7 09:44 amdgpu_bl0 -> ../../devices/pci0000:00/0000:00:08.1/0000:07:00.0/drm/card0/card0-eDP-1/amdgpu_bl0
lrwxrwxrwx 1 root root 0 Sep  7 10:41 ddcci11 -> ../../devices/pci0000:00/0000:00:08.1/0000:07:00.0/i2c-11/11-0037/ddcci11/backlight/ddcci11

ddcutil

This is the recommended backend for controlling external displays. Requires i2c kernel module, but as of v1.4 "ddcutil installation should automatically install this file, making manual configuration unnecessary"

Note: arch wiki states:

Using ddcci and i2c-dev simultaneously may result in resource conflicts such as a Device or resource busy error

Meaning it's best to choose one of the options, not both.

Usage

Daemon

As mentioned earlier, a daemon process needs to be started that keeps track of the displays. Easiest way to do so would be utilizing your OS's process manager. An example of a systemd user service file (e.g. ~/.config/systemd/user/bctld.service) would be:

[Unit]
Description=bctld aka brightness control daemon
PartOf=graphical-session.target
StartLimitIntervalSec=90
StartLimitBurst=5

[Service]
Type=simple
ExecStart=%h/.local/bin/bctld
Restart=on-failure
RestartSec=2
RestartPreventExitStatus=100

[Install]
WantedBy=graphical-session.target

Enable & start this unit by running

$ systemctl --user enable --now bctld.service

Client

With daemon running, the client is used to send commands to the daemon. List available commands via bctl --help

Some examples:

• bctl up - bump brightness up by brightness_step config
• bctl down - bump brightness down by brightness_step config
• bctl up 20 - bump brightness up by 20%
• bctl down 20 - bump brightness down by 20%
• bctl delta 20 - bump brightness up by 20%
• bctl delta -- -20 - bump brightness down by 20%
• bctl set +20 - bump brightness up by 20%
• bctl set -- -20 - bump brightness down by 20%
• bctl set 55 - set brightness to 55%
• bctl get - returns current brightness level in %; see the get_strategy config item in config.py to set how differing brightnesses get consolidated into a single int
• bctl setvcp D6 01 - set vcp feature D6 to value 01 for all detected DDC displays; this is simply shortcut for ddcutil setvcp D6 01

The daemon also registers signal handlers for SIGUSR1 & SIGUSR2, so sending said signals to the daemon process allows bumping brightness up and down respectively; e.g.: kill -s SIGUSR1 "$(pgrep -x bctld)" or killall -s SIGUSR1 bctld

Socket

The client and daemon communicate over a unix socket set via socket_path config. If using the provided client is too slow (e.g. for querying brightness), it's possible to talk to the daemon directly over this socket. For instance current brightness can be fetched via following command, which is equivalent to bctl get:

$ socat - UNIX-CONNECT:$XDG_RUNTIME_DIR/bctl/bctld-ipc.sock <<< '["get",0,0]' | jq -re '.[1]'
75

Likewise, for setting brightness you might define a shell function similar to:

# Sets the screens' brightness level
#
# @param {digit|string}   percentage to set the brightness level to (without the percentage sign).
#                         may also prefix with + or - if delta change is wanted.
#
# @returns {void}
set_brightness() {
    local msg

    msg="${1%\%*}"  # strip trailing % if it was (mistakenly) given
    if [[ "$msg" =~ ^[0-9]{1,3}$ ]]; then
        msg="[\"set\",1,$msg]"
    elif [[ "$msg" =~ ^[-+][0-9]{1,3}$ ]]; then
        msg="[\"delta\",1,$((msg))]"
    else
        echo -e "illegal brightness arg provided: [$msg]" 1>&2
        return 1
    fi

    socat - UNIX-CONNECT:$XDG_RUNTIME_DIR/bctl/bctld-ipc.sock <<< "$msg"
}

[!WARNING]
Please note there will be no guarantees about the stability of this api as it's part of internal comms spec.

Configuration

User configuration file is read from $XDG_CONFIG_HOME/bctl/config.json. For full config list see the config.py file that defines the defaults, but the most important ones you might want to be aware of or change are:

Config	Type	Default	Description
msg_consumption_window_sec	float	0.1	event consumption window in seconds
udev_event_debounce_sec	float	3.0	udev event debounce window in seconds
brightness_step	int	5	percentage to bump brightness up or down per change
sync_brightness	bool	False	whether to keep screens' brightnesses in sync
main_display_ctl	str	DDCUTIL	backend for brightness control
internal_display_ctl	str	RAW	backend for controlling internal display
notify.icon.root_dir	str	''	notification icon directory
fatal_exit_code	int	100	error code daemon should exit with when restart shouldn't be attempted. you might want to use this value in systemd unit file w/ RestartPreventExitStatus config

msg_consumption_window_sec

Defines an event consumption window, meaning if say 'brightness up' key is spammed 5x during said window, ddcutil is invoked just once bumping up the brightness by 5x<brightness_step> value, as opposed to running ddcutil 5 times bumping 1x<brightness_step> each time.

main_display_ctl

This config sets the main backend for controlling the brightness. Available options:

• DDCUTIL - controls external displays via ddcutil, requires ddcutil to be on PATH, described above.
• RAW - all displays are controlled via the device interfaces under /sys/class/backlight directory. In order to control external displays using this backend, you'd likely need the installation of ddcci kernel driver, described above.
• BRIGHTNESSCTL - all displays are controlled via brightnessctl program.
• BRILLO - all displays are controlled via brillo program.

internal_display_ctl

This config sets the backend used only for controlling the internal display brightness, as that's not what ddcutil does. Only in effect if main_display_ctl=DDCUTIL and we're running on a laptop. Available options are RAW | BRIGHTNESSCTL | BRILLO

notify.icon.root_dir

Notification icon directory. Icon is chosen based on brightness level, and final used icon will be notify.icon.root_dir + notify.icon.brightness_{full,high,medium,low,off}.

Note either half of final value may be an empty string, so if you want to use single icon for all levels, set icon full path to notify.icon.root_dir and set notify.icon.brightness_{full,high,medium,low,off} values to an empty string.

Troubleshooting

External display (dis)connection not detected

Current implementation relies on listening for drm subsystem change action udev events. Some graphic cards (and/or monitors, unsure) are known to either not emit said events, emit them only sometimes, or emit different ones. Recommend you try debugging it via running $ udevadm monitor that starts listening for udev events, then connect or disconnect your monitor and see what events are printed out. With that info feel free to open an issue.

As a hacky workaround it's also possible to enable periodic polling by setting periodic_init_sec to seconds at which interval display detection should happen. Wouldn't set it to anything lower than 30.

Additionally you may opt out of udev monitoring altoghether (see config.py), and rely on your own custom detection; in that case daemon can be asked to re-initialize its state by sending init command via the client: $ bctl init

See also

• https://github.com/orhun/i3-workspace-brightness