I've just installed Linux Mint 18 on a clean partition on my Thinkpad T420. It uses an NVS4200M graphics chipset, which is an Optimus-based card. However, I have disabled Optimus in the BIOS, and forced the system to use the discrete graphics card, so from the operating-system level, I appear to have a discrete graphics card.
The system installs cleanly, and my first change was to select
nvidia-361 as the graphics driver using the Driver Manager. After a reboot, the system still works. I can verify that the kernel module is loaded. However, my brightness keys do not work.
Here are the contents of
> ls /sys/class/backlight
and, although my screen is stuck at 100% brightness,
> cat /sys/class/backlight/thinkpad_screen/actual_brightness
I've been using various flavors of Ubuntu for almost 5 years on this computer, and at various points in the past I've needed to change my GRUB command line to include
"video.use_native_backlight=1". However, neither of these seems to bring back my brightness control.
I'm asking here because the guides I can find seem mostly quite outdated and I'm at a loss for the next step. Thanks in advance for any ideas.
Edit: I should add that the backlight controls work until the Nvidia spash screen appears, then they stop affecting the screen. I see the brightness indicator increasing and decreasing in Cinnamon, but the screen brightness stays at the level to which it was set right when the Nvidia splash appeared.