If you are having trouble with your laptop display brightness on Linux and your main GPU is Nvidia, this kernel parameter might fix it!
Note: this will only resolve issues with Nvidia gpus; it won’t have any impact on Intel or AMD gpus at all! You are free to try it (it won’t hurt anything) but it won’t do a thing.
The kernel param is nvidia.NVreg_RegistryDwords=EnableBrightnessControl=1
Here’s how I found the solution: https://forums.developer.nvidia.com/t/brightness-not-working-ubuntu-20-04-lenovo-legion-5-15imh05/178058/17