Thank you very much generix for your advice.
These are the 2 experiments that I performed after firstly uninstalling xbacklight:
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I installed (Alternatives to) the package “acpilight”
sudo eopkg install acpilight-1.1-1-1-x86_64.eopkg
Nothing changes, brightness control still doesn’t work.
Then I followed the instructions from the “acpilight” webpage:
Setup an “udev” rule to allow users in the “video” group to set the display brightness. To do so, place a file in /etc/udev/rules.d/90-backlight.rules containing:
SUBSYSTEM=="backlight", ACTION=="add", \
RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness", \
RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"
Nothing changes, brightness control still doesn’t work.
Before performing the second experiment, I uninstalled acpilight package and deleted its udev rule.
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Here’s the simple reason why I think that I don’t have the suggested acpi daemon (acpid) installed in my Solus: Solus Development Portal: “acpid” - closed, wontfix:
https://dev.getsol.us/T5271
I added a kernel boot parameter “nvidia.NVreg_EnableBacklightHandler=1” in this way:
$ sudo su
$ mkdir -p /etc/kernel
$ echo "nvidia.NVreg_EnableBacklightHandler=1" > /etc/kernel/cmdline
$ clr-boot-manager update
$ exit
Magically, the brightness control finally works, both by using the slider within Pover options and by using the keyboard FN keys.
My question is: one year after Nvidia started experimenting with this option, when it will be proclaimed as a stable and in what way it will be applied (automatic job or it will still required user’s manual intervention)?
Unfortunately, once again the new brightness setting isn’t remembered after restart. Since Aaron Plattner said that remembering and restoring the backlight brightness on restart isn’t the driver’s job, I will ask Solus developers for help…