It seems we have reached a new epoch in the Linux backlight stuggle: today, I
upgraded my T420 to Linux Mint 18, and the backlight again stopped working (as
expected). I did the usual, which is to add
"video.use_native_backlight=1" to my
GRUB boot options.
To my dismay, the backlight controls still didn’t work. After trying various drivers, all of the possible ACPI kernel command-line options, and various commands to set the backlight brightness directly from the terminal, it occurred to me that nvidia has been working on Linux support for Optimus, and their new drivers include that support.
As a last resort, I re-enabled Optimus in my T420 BIOS settings, and removed all of my modifications to the stock Linux Mint 18 grub file. To my great surprise and immense relief, the system Just Works. I’m happy to say that it goes to sleep, wakes up, and adjusts backlight brightness with no issues.
At last, after a solid five years of workarounds, my T420 shines as brightly as I want it to shine. Thanks to nvidia for all the vast improvements to their Linux driver, and to all the members of the Linux community who researched and shared their ideas for fixing backlight problems.