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 /sys/class/backlight:

> 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 "acpi_backlight=vendor" or "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I'm asking here because the guides I can find seem mostly quite outdated and I'm at a loss for the next step.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.


It turns out that enabling Optimus actually fixes this problem. I had been avoiding enabling it because in the past, it had poor Linux support and would break the display server.

With Linux Mint 18, kernel 4.4, and nvidia-361, enabling Optimus in the T420 BIOS (in the "Display" section) causes everything to work out-of-the-box on the ThinkPad T420. You can enter the BIOS configuration by holding F10 during boot.

  • it works because Optimus loads the intel driver, and hence /sys/class/backlight is provided by the i915 driver, which works. There is still no nvidia acpi driver. – Tim Richardson Feb 15 '17 at 9:31

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