Can't change brightness on Toshiba l755 laptop

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http://askubuntu.com – I have a Toshiba l755 laptop, and I installed 12.04 64bit on it. The default brightness of the laptop is set to maximum value, and I can't change it. If I go to Configuration → Brightness, the brightness doesn't change and neither does pressing the function keys. I tried changing some parameters of grub file, setting GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux acpi_backlight=vendor". When changing the brightness from configuration, the value of /sys/class/backlight/toshiba/brightess changes, but screen remains the same. If I use the key functions, it changes the "actual_bri (HowTos)