Comment 28 for bug 121833

I have the same problem on my laptop (hp dv2000): the backlight is set to 100% if the value written to /sys/class/backlight/acpi_video0/brightness is not among the values provided in /proc/acpi/video/VGA/LCD/brightness:
 levels: 100 60 20 28 36 44 52 60 68 76 84 92 100

I think that the kernel has to patched either to return error when setting invalid values, or to set the brightness to the closest valid value.

I am attaching a patch for the second option. It is working for me.