Ok, after each Nvidia driver upgrade… the brightness section in the Xorg config file gets wiped out, automatically recreated, and we loose brightness control in X11.
Solution :

1. sudo mcedit /etc/X11/xorg.conf
2. Find the line section « Device » (Nvidia) and ensure last line has got the following

   Option "RegistryDwords" "EnableBrightnessControl=1"