For i915 systems with broken ACPI backlight controls, synchronize the ACPI value with the native control

sync-acpi-intel.sh

#!/bin/bash
# Copyright (c) 2011, Michel Alexandre Salim <salimma@fedoraproject.org>
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# BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
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BACKLIGHT_DIR=/sys/class/backlight
ACPI=${BACKLIGHT_DIR}/acpi_video0
INTEL=${BACKLIGHT_DIR}/intel_backlight

ACPI_MAX=`cat ${ACPI}/max_brightness`
INTEL_MAX=`cat ${INTEL}/max_brightness`
# this is about 500
#MULTIPLIER=$((INTEL_MAX/(ACPI_MAX+1)))
MULTIPLIER=500
MIN_BRIGHTNESS=$((INTEL_MAX-(MULTIPLIER*ACPI_MAX)))

function fib {
    let a=$1
    let b=$2
    let n=$3

    if [ $n == 0 ]; then
        INTEL_BRIGHTNESS=$a
    else
        fib $b $((a + b)) $((n-1))
    fi
}

while inotifywait -e modify ${ACPI}/brightness >/dev/null 2>&1; do
    BRIGHTNESS=`cat ${ACPI}/brightness`
    # specially handle maximum value
    if [ ${BRIGHTNESS} == ${ACPI_MAX} ]; then
        INTEL_BRIGHTNESS=$INTEL_MAX
    else
        # arithmetic
        INTEL_BRIGHTNESS=$((MULTIPLIER*(BRIGHTNESS+1)))
        # geometric
        #INTEL_BRIGHTNESS=$((INTEL_MAX >> (8-BRIGHTNESS)))
        #INTEL_BRIGHTNESS=$((25 << BRIGHTNESS))
        #fibonacci
        #fib 100 200 $(BRIGHTNESS)
  fi
  echo ${INTEL_BRIGHTNESS} > ${INTEL}/brightness
done

Thanks for this, Michel. I found it very useful while seeking a way to get backlight control to work on my new computer! :-) François

Great! Thank you very much.
Note for Fedora users: you may need to install inotify-tools for inotifywait command to work.