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
   
   

   
   

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:

• Brightness on laptop MSI GT72VR in Ubuntu 18.04 with NVIDIA GTX 1060 Mobile doesn't work


• computer doesnt wake from suspension and couldnt change the brightness


• Brightness control missing when using Nvidia drivers but re-appears with X.Org


• Xubuntu 18.04: make Fn brightness keys work with /sys/class/backlight/nv_backlight/


• How to alter backlight driver in lubuntu


• Brightness on laptop MSI GT72VR in Ubuntu 18.04 with NVIDIA GTX 1060 Mobile doesn't work


• NVIDIA display issue with brightness control and video playback


• Unable to change the screen brightness


• The brightness of laptop screen cannot be adjusted with either the buttons or the slider. Edit

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