Sorry, it is a bit long, as I can only use my own words, not the Linuxian ones..
No problems at all, I always prefer a well-articulated response in place of one that might be missing some important information.
1-Following one of your last suggestions, I tried to echo the content of the sub-folders of /proc/acpi/video/IGPU, namely LCD0, CRT0,DSP0 and HDMI.I had a try only on LCD0 before.
As already told, LCD0 can be echoed and a new value for brightness can be changed in it, although without any effect on the actual screen brightness.
For the 3 other sub-folders, echoing is not possible (error message returned by terminal) , meaning probably they are empty.That had to be done, now it is and this door is shut.
So we stay with the fact that a brightness value can be changed in /proc/acpi/video/IGPU/LCD0, but that this file and/or value is not opened or read by some part of the system which should use it.
Exactly, the problem is just that: how to enable that value to be actually used?
I'll investigate the problem more, but at the moment I really have no other ideas to fix the issue.
However, came back to my mind this:
I: I'm puzzled by the fact you are always speaking about backlight control, which is not the current issue..but brightness itself.
You: Yes sorry, by talking about backlight control I really mean brightness control.
Sorry, the fact is just that my english is far from perfect.
I used the 'backlight' word even when I meant to say 'brightness'.
But it's just my fault with english, nothing more. All the commands we used and files we touched are correct.
So, this sentence:
Of course all this has no interest and should go directly to the wastebin if acpi_brightness does not exist as a symbolic variable and if assigning in a GRUB command line a value to a non-existent symbolic variable produces no error at all and lets the system start..which would be strange in my view.
The 'acpi_brightness' option does not exist, passing a non-existent option on the grub kernel boot line produces just a warning in your kernel log (telling you the option does not exist). The system boot is not stopped, and this is the correct behavior, because a system has to be stopped only on critical/unrecoverable errors.
So, pass an invalid option or do not pass any has the same effect.
Anyway, the 'acpi_backlight' option can be set to 'vendor' or 'video'.
From the official linux documentation:
If set to vendor, prefer vendor specific driver
(e.g. thinkpad_acpi, sony_acpi, etc.) instead
of the ACPI video.ko driver.
Please consider a little donation to keep the 'Linux On My Samsung' project up and running. Thank you!