I have same problem with the NVIDIA backlight control, the baclight slider and sun OSC icon show and response to Fn-K / Fn-P but the backlight not change. Tried many way but not success. There are 2 things you may interesting:
- It is reflect with NVIDIA's pwm-info parameter, I tried to remove and the backlight slider / sun OSC gone, tried with W520 but same problem, I think it can be calculate / export from video rom file.
- You may try something in T420 topic