Comment 61 for bug 850065

That indicates that the interaction with the bios is okay -- actual_brightness actually fetches the brightness level from the bios, so we can see that the values you write are getting through. But whatever the bios is doing doesn't actually affect the brightness for whatever reason. I'll look at the DSDT some more to see if there are any clues, but right now it looks like your bios is just broken in this respect. You could check to see if there's a bios update for your machine (although it may be difficult to apply unless you're dual-booting Windows).

We can go ahead and try toshiba_acpi, but based on the DSDT I'm pretty sure it isn't going to work either. I've uploaded a dkms package you can install to get a version of toshiba_acpi that should work on your machine. After you install the package and load the module, you should have a new folder /sys/class/backlight/toshiba. Try the same things with adjusting the brightness there and see if it has any effect.

  http://people.canonical.com/~sforshee/lp850065/toshiba-acpi/toshiba-acpi-dkms_0.1_all.deb

It looks like you are using the proprietary nvidia module, and it doesn't appear to be exporting any kind of brightness control (unless it's through their control panel application, but that's not a very good solution anyway). Have you tried the nouveau driver? If it works okay for you then it might give you a working backlight control. It may be worth trying, and if it does work we can get your system using it instead of the ACPI control.

Beyond that, unless I find something in your DSDT, there's probably not anything that can be done. At this point it looks like the brightness control in your bios is just broken.