Comment 5 for bug 1389215

Discovered another behaviour. If you play with the brightness controls under Xorg, nothing changes. However, suspend the computer and resume it, and the brightness will be setup to the last setup you left when playing with the brightness controls. Thus, the value in the kernel persists.

You can also test this with the CLI:

# cd /sys/class/backlight/acpi_video0
# echo 10 >brightness # Nothing happens
# cat actual_brightness
10
# echo 5 >brightness # Nothing happens
# cat actual_brightness
5

<suspend computer>
<resume computer>

Now the brightness level is set to 5 and the screen finds itself at that level.

In the backend, the controls are changing the brightness levels, it is just that during an Xorg Session with NVIDIA proprietary drivers, the screen's real brightness is not updated. As if the hardware signal to the screen is never sent, except when in the console or upon resume.