[arch-general] Nvidia backlight control - acpi_video0/brightness changes - display doesn't?

[David C. Rankin]

All,

  Laptop: HP 8760w, Quadro 3000M, nvidia-340xx packages.

  The driver works perfectly (no errors in Xorg.0.log), but I have no way to
control the screen brightness. I've followed the suggestions in:

https://wiki.archlinux.org/index.php/NVIDIA/Tips_and_tricks

  but I still have no backlight control. Using the laptop function keys, the
values in /sys/class/backlight/acpi_video0/brightness are properly set, e.g.

$ cat /sys/class/backlight/acpi_video0/brightness
8

(now tap the [func][F9], brightness lower, twice)

$ cat /sys/class/backlight/acpi_video0/brightness
6

(actual_brightness value tracks brightness)

$ cat /sys/class/backlight/acpi_video0/max_brightness
20

  This works with OR without /etc/X11/xorg.conf.d/20-nvidia.conf:

        Option "RegistryDwords" "EnableBrightnessControl=1"

  Does anyone have any suggestions on anything else to try before I start
loading the aur nvidia-bl or nvidiabl packages? I don't mind adding more
software, but if there are a few more tweaks to try before going that route, I
would like to try them first. Anybody get backlight working with a similar
model laptop?

-- 
David C. Rankin, J.D.,P.E.