Brightness Control
supports an arbitrary number of displays!
Brightness Controller changes gamma and not hardware setting and as such is equivalent to xrandr.
Display Data Channel
Enable the display to communicate its supported display modes to the adapter and that enable the computer host to adjust monitor parameters, such as brightness and contrast.
Need to be member of i2c group to be usefull (or sudo)
ddcutil / Doc - a Linux program for querying and changing monitor settings, such as brightness and color levels. - Display Brightness Slider for Gnome Shell - Brightness slider for all the monitors detected by ddcutil
DC/CI kernel module allows any tool which can use the kernel’s backlight system (/sys/class/backlight) to drive the backlight on a DDC/CI monitor.
- xbacklight
see also