Ubuntu – How to switch between options in `/sys/class/backlight` to solve brightness problem

acpibrightness

OS : Ubuntu – 18.04 LTS

I have 2 symbolic links in /sys/class/backlight/ directory. 

acpi_video0 -> ../../devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0/
nv_backlight -> ../../devices/pci0000:00/0000:00:01.0/0000:01:00.0/drm/card0/card0-LVDS-1/nv_backlight/

The brightness hotkeys change the value of acpi_video0/brightness. But that doesn't actually change the display's brightness.

But, manually changing the value ofnv_backlight/brightness actually change the display's brightness.

So, linking the hotkeys to nv_backlight instead of acpi_video0 will solve my problem, but I don't know how to do it.

Edit: The pop-up slider progress bar appears on the screen on the operation of the Fn keys for brightness(hotkeys).

Edit:

  1. Input:

    cat /sys/class/backlight/nv_backlight/max_brightness

    Output:

    100

  2. Input: 

    cat /sys/class/backlight/nv_backlight/actual_brightness

    Output(this value changes automatically to the same value, I manually gave into /sys/class/backlight/nv_backlight/brightness):

    10

  3. Input: 

    cat /sys/class/backlight/acpi_video0/max_brightness

    Output:

    15

Best Answer

Bash script redirect-brightness

Your problem is function keys to increase/decrease brightness are updating /sys/class/backlight/acpi_video0/brightness instead of /sys/class/backlight/nv_backlight/brightness

The original request was to intercept the function keys and use them to control the nv_backlight driver. The problem with this is:

  • Tricky udev scripts
  • No pop-up notification slider displaying brightness without complicated Python scripting.

The solution is to use inotify to monitor changes to acpi_video0. Then calculate the brightness percentage and apply that same percentage to nv_backlight.

In the bash script below you need to set:

WatchDriver="/sys/class/backlight/acpi_video0/brightness"
PatchDriver="/sys/class/backlight/nv_backlight/brightness"

Place the script in /usr/local/bin and make it executable using:

chmod a+x /usr/local/bin/redirect-brightness

First run the script from the command line using

redirect-brightness -l

If there is a problem check the log file using:

cat /tmp/redirect-brightness.log

If all works well add redirect-brightness to your startup applications.

redirect-brightness bash 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 >> $LogFile
    # Reverse engineer patch brightness to set
    PatchAct=$(( 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

Original Answer June 11, 2018.

Work In Progress Answer

There are many nVidia users with similar problems in Ubuntu 18.04:

Before writing a script

I can write a script to mimic changes to /acpi_video and populate /nv_backlight but before I do try this:

  • Edit /etc/default/grub.
  • Find quiet splash
  • Add behind splash: acpi_backlight=vendor
  • Save the file
  • Run sudo update-grub

According to this answer it should work for you: Xubuntu 18.04: make Fn brightness keys work with /sys/class/backlight/nv_backlight/

Wayland

Brightness doesn't work under Wayland. Make sure you aren't using it. See: The brightness of laptop screen cannot be adjusted with either the buttons or the slider. Edit

Related Question