Lowering screen max brightness with Ubuntu 18.04

Question

I'm trying to set a limit for maximum screen brightness on my Thinkpad X250 running Ubuntu 18.04.

A bit of context : I changed the original screen with a new one (before installing Ubuntu), the brightness controls work perfectly, but I noticed that, over 55-60% brightness, changes are almost unperceivable for human eye, but it drains a lot more battery and it starts running really hot. I'm planning to give the computer to my family with young and unexperienced users, so I would like to set a limit which does not require user action at each startup, and could work for all users without worrying that an update via the software manager could erase the settings.

What I tried : I tried to directly edit /sys/class/backlight/intel_backlight/max_brightness but it refuses to be edited (even after using chmod to change permissions). I saw some anwsers telling to add a command like "echo MAX_VALUE > /sys/class/backlight/intel_backlight/max_brightness" in /etc/rc.local, but I don't have this file. Note that I don't have *acpi_** files or folder in /sys/class/backlight/.

What would be the correct way to set an all-user permanent brightness limit ?

Answer 1

I wrote a script to monitor /sys/class/backlight/*/brightness and redirect it: How to switch between options in `/sys/class/backlight` to solve brightness problem?.

It can be quickly modified to your needs by adding three lines:

    # Modification for: https://askubuntu.com/questions/1051171/lowering-screen-max-brightness-with-ubuntu-18-04
    # If brightness > 50, set to 50
    [[ WatchPer -gt 50 ]] && WatchPer=50

Modified redirected-brightness script

#!/bin/bash

# NAME: redirect-brightness
# PATH: /usr/local/bin
# DESC: Redirect to correct driver when Ubuntu is adjusting the wrong
#       /sys/class/DRIVER_NAME/brightness

# DATE: June 13, 2018. Modified June 14, 2018.

# NOTE: Written for Ubuntu question:
#       https://askubuntu.com/q/1045624/307523

WatchDriver="/sys/class/backlight/intel_backlight"
PatchDriver="/sys/class/backlight/intel_backlight"

# Must be running as sudo
if [[ $(id -u) != 0 ]]; then
    echo >&2 "Root access required. Use: 'sudo redirect-brightness'"
    exit 1
fi

# inotifywait required
type inotifywait >/dev/null 2>&1 || \
    { echo >&2 "'inotifywait' required but it's not installed.  Aborting."; \
      echo >&2 "Use 'sudo apt install inotify-tools' to install it.'"; \
      exit 1; }

# Was right watch driver directory name setup correctly?
if [[ ! -d $WatchDriver ]]; then
    echo >&2 "Watch directory: '$WatchDriver'"; \
    echo >&2 "does not exist. Did you spell it correctly? Aborting.'"; \
    exit 1;
fi

# Was right patch driver directory name setup correctly?
if [[ ! -d $PatchDriver ]]; then
    echo >&2 "Redirect to directory: '$PatchDriver'"; \
    echo >&2 "does not exist. Did you spell it correctly? Aborting.'"; \
    exit 1;
fi

# Get maximum brightness values
WatchMax=$(cat $WatchDriver/max_brightness)
PatchMax=$(cat $PatchDriver/max_brightness)

# PARM: 1="-l" or "--log-file" then write each step to log file.
fLogFile=false
if [[ $1 == "-l" ]] || [[ $1 == "--log-file" ]]; then
    fLogFile=true
    LogFile=/tmp/redirect-brightness.log
    echo redirect-brightness LOG FILE > $LogFile
    echo WatchMax: $WatchMax PatchMax: $PatchMax >> $LogFile
fi

SetBrightness () {
    # Calculate watch current percentage
    WatchAct=$(cat $WatchDriver/actual_brightness)
    WatchPer=$(( WatchAct * 100 / WatchMax ))
    [[ $fLogFile == true ]] && echo WatchAct: $WatchAct WatchPer: $WatchPer >> $LogFile
    # Reverse engineer patch brightness to set
    # Modification for: https://askubuntu.com/questions/1051171/lowering-screen-max-brightness-with-ubuntu-18-04
    # If brightness > 50, set to 50
    [[ WatchPer -gt 50 ]] && WatchPer=50
    PatchAct=$(( PatchMax * WatchPer / 100 ))
    echo $PatchAct | sudo tee $PatchDriver/brightness
    [[ $fLogFile == true ]] && echo PatchAct: $PatchAct >> $LogFile
}

# When machine boots, set brightness to last saved value
SetBrightness

# Wait forever for user to press Fn keys adjusting brightness up/down.
while (true); do
    inotifywait --event modify $WatchDriver/actual_brightness
    [[ $fLogFile == true ]] && \
        echo "Processing modify event in $WatchDriver/actual_brightness" >> $LogFile
    SetBrightness
done

Answer 2

This cannot be done the way you want to do this as this is a Linux Kernel Read-Only value:

There is no code that will read the value of max_brightness and act upon it: the kernel just publishes whatever the maximum value for your hardware is.

If you would try to force the kernel into still accepting writes, it will throw an Input/output error.

Maybe there is a way to:

• Intercept the keypresses individually and then refuse to go higher, but that would mean posting a new question and referring back here as to why you'd want to do that.
• ask Linus Torvalds himself to attend to your bug, but there's very little chance in him solving this in the near future... ;-)