This seems to be a recurring question, but none of the solutions I found here and there seem to work for me. So here are the facts:
- Toshiba Satellite P855-30D
- Nvidia 640M
- BIOS 1.50
- Ubuntu 13.04 64 bits
- Always at full backlight
I've tried the 4 variants given here: http://ubuntuforums.org/showthread.php?t=2139397&p=12621502&viewfull=1#post12621502 with no luck. Some of them make /sys/class/backlight/* empty. But even when it is populated, whatever I put in /sys/class/backlight/acpi_video0/brightness is ignored.
The brightness keys are fn-F2 and fn-F3 on this model. fn-F2 opens the Power Statistics, fn-F3 crashes X. Other function keys work fine.
Edit after upgrading the BIOS to latest 6.70 :
GRUB_CMDLINE_LINUX=""
/sys/class/backlight/ is populated
/sys/class/backlight/acpi_video0/max_brightness:100
changing /sys/class/backlight/acpi_video0/brightness has no effect
GRUB_CMDLINE_LINUX="acpi_osi=Linux acpi_backlight=vendor"
/sys/class/backlight/ is empty
No param to change
GRUB_CMDLINE_LINUX="acpi_osi="
/sys/class/backlight/ is populated
/sys/class/backlight/acpi_video0/max_brightness:7
changing /sys/class/backlight/acpi_video0/brightness has no effect
GRUB_CMDLINE_LINUX="acpi_osi=\"!Windows 2012\""
/sys/class/backlight/ is populated
/sys/class/backlight/acpi_video0/max_brightness:7
changing /sys/class/backlight/acpi_video0/brightness has no effect
Opened a bug https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1199062 since conventional solutions are not working.
/etc/default/grubyou also ransudo update-gruband rebooted? That's a common mistake that people forget is necessary if they do anything to the GRUB parameters.