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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
thinkpad_screen

and, although my screen is stuck at 100% brightness,

> cat /sys/class/backlight/thinkpad_screen/actual_brightness 
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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I'm asking here because the guides I can find seem mostly quite outdated and I'm at a loss for the next step.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.

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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.

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    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. Feb 15, 2017 at 9:31

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