Comment 330 for bug 415023

BIOS upgrade to 10G (see #265, #269) fixed it for me too :-)
As others, I can access just 5 levels by hotkeys (seems like 9 levels jumping by 2 each press, hard to prove).
However, after running:

xrandr --output LVDS --set BACKLIGHT_CONTROL combination # no KMS, YMMV

I can access all 9 levels by hotkeys!
What's strange, it keeps working for all BACKLIGHT_CONTROL values, even when I set it back to the original "kernel" value!
So I'm now unable to reproduce the 5 levels situation (will experiment more next time I reboot).

Of the possible BACKLIGHT_CONTROL modes, "kernel" seems best because it allows to read and set the brightness through xrandr:

xrandr -v --verbose | grep -A1 BACKLIGHT
xrandr --output LVDS --set BACKLIGHT 5 # 0..8 inclusive

(again, I'm running with nomodeset => KMS off; xrandr interface details seem to depend on this so YMMV.)

However, with all BACKLIGHT_CONTROL modes, I can read and set the brightness through:

cat /proc/acpi/video/IGD/LCD/brightness
echo 70 | sudo tee /proc/acpi/video/IGD/LCD/brightness # 20..100 inclusive, in steps of 10

The brightness applet is also independent of BACKLIGHT_CONTROL - for all modes, can access all 9 levels but can't read the current level (sometimes its icon says it knows it can't read it, sometimes doesn't - but anyway its level never reflects changes made by hotkeys).

All this leads me to guess the 5-levels issue is still some kind of keystroke-stuffing-triggering-double-adjustment, unrelated to the xrandr interface - but somehow using that xrandr interface once kills this interaction...