I have a Lenovo T530 with nvidia graphics (using nouveau driver) and recently (can't recall exactly when), my backlight controls stopped functioning.
I have entries in /sys/class/backlight (right now it's "thinkpad_screen") with a brightness setting. The reported range is 0-7. Setting values to it changes the value, but not the brightness of the screen.
I tried adding "acpi_backlight=vendor" to my kernel line, which just changed the module from acpi_video0 to thinkpad_screen. I still couldn't change the backlight. I changed acpi_osi to Linux, but that didn't change anything either.
Adding a file to modprobe.d with "options thinkpad_acpi brightness_enable=1" didn't change anything either.
In all situations, xbacklight reports "No outputs have backlight property"
I'm not sure what changed, but I can't change my backlight anymore and all of the common solutions to this for thinkpads don't seem to work.
Anyone have any other ideas?