It seems that I can change keyboard brightness by writing to a file /sys/..../brightness, but the owner is root, so I need sudo, which is inconvenient. So, I tried to give my user account the write permission to that file.
I tried setacl -m u:myuser:w /sys/.../brightness as root but I got “Operation not supported”. I tried setting ACL on a regular file in my home directory, and that worked. Both my home directory and the / are on one Btrfs partition, and the mounting options are the same in the fstab file.
Why can’t I set ACL on that brightness file? Is that because it is not a real file (I guess it is some sort of virtual file)? Or did I do something wrong?