Bug 88941

what graphics are you using?
please attach the lspci output.

Created attachment 159361 [details]
lspci -vvnn output

Please attach your acpidump:
# acpidump > acpidump.txt

Please show me:
# ls /sys/class/backlight

And for each directory there, please test if manually set a value to brightness works:
# cd /sys/class/backlight/acpi_videoX
# cat max_brightness
10
# echo 2 > brightness
# echo 5 > brightness
...

# cd /sys/class/backlight/intel_backlight
# cat max_brightness
5000
# echo 1000 > brightness
# echo 2000 > brightness
...

Created attachment 159451 [details]
acpidump

Created attachment 159461 [details]
backlight folders

Created attachment 159471 [details]
max brightness

I am able to change it manually and form GUI menu, but not with hotkeys

Please dump the below memory range:
0xDAB0EE18, size 0x16D
You can get it by:
$ sudo dd if=/dev/mem of=memory.dump bs=1 count=$((0x16D)) skip=$((0xDAB0EE18))

Please also attach: /sys/kernel/debug/dri/0/i915_opregion

Created attachment 159541 [details]
memory dump

Created attachment 159551 [details]
i915 opregion

Created attachment 159751 [details]
Work around firmware that provides 8+ video output devices

Looks like a similar bug:
https://lkml.org/lkml/2014/2/11/1032

Please test this work around patch.

I have to compile the whole kernel or there is a method to compile only the i915 driver and change it in actual kernel?

I have applied the patch with 
patch -p1 < patch.patch

now I go to driver's folder and type make, but says 
make: *** No targets. Stop.

I type
gcc intel_opregion.c

and get
intel_opregion.c:30:24: fatal error: linux/acpi.h: No such file or directory

ok, I build intel_opregion.ko then I used sudo modprobe intel_opregion.ko, but nothing happens. With sudo insmod intel_opregion.ko I get "Unknown symbol in module"

Have I made something wrong?

Ok, I achieved building the module and installing it. Fix actually works, but with debug options in kernel load. I'm going to test without them.

It works without debug options, but minimum brightness is null, so if I press lower brightness key too much, my screen goes black until I press higher brightness key

Please report back what's the effect of adding "acpi_osi=" to your kernel cmdline option. You do not need to apply any patch with this cmdline option, just use a normal kernel.

That fix the problem of the minimum brightness

What about the other hotkeys? Do those hotkeys always work or they start to work with the newly added cmdline option?

Oh I just realized you have been using acpi_osi=. So with v3.18-rc7 the hotkey for backlight control stops working, what about the final v3.18? Can you please try that kernel version? I have a patch in the final v3.18 that may affect the result:

commit 35d0565b95547ec12d025dc9b1394f22968d113d
Author: Aaron Lu <aaron.lu@intel.com>
Date:   Mon Dec 1 02:09:18 2014 +0100

    ACPI / video: update condition to check if device is in _DOD list

ok, trying. But what i meant was that keys work without debugging options, but the minimum turns off the screen. With acpi_osi= the minimum is fixed. I'll report results in a moment

Keys are blocked with 3.18 vanilla

If I press the key for disabling touchpad, it doesn't switch off/on the touchpad, but write a p. I suppose this is not normal

The problem appears to with 3.18-rc7

Created attachment 160011 [details]
dmesg output with patch

I think this output is different from the last one.

In 3.18 I dont have any strange output in dmesg, only in 3.18-rc7

With acpi_osi= in your kernel cmdline, what's the output of:
# ls /sys/class/backlight

Presumably acpi_osi= has been used to make those hotkeys work, since which kernel version that cmdline option stops working? And when it stops working, what's the output of:
# ls /sys/class/backlight

Also, when it doesn't work, please run acpi_listen and then press the backlight hotkey, is there any output?

KERNEL VERSION  || WORKS?  || OUTPUT of ls /sys/class/backlight
--------------------------------------------------------------
3.13.0          ||  YES    || intel_backlight acpi_video0
3.14.26         ||  YES    || intel_backlight acpi_video0
3.15.10         ||  YES    || intel_backlight acpi_video0
3.16.7          ||  YES    || intel_backlight acpi_video0
3.17.6          ||  YES    || intel_backlight acpi_video0
--------------------------------------------------------------
3.18.rc7        ||  NO     || intel_backlight
3.18.0          ||  NO     || intel_backlight

with 3.18.0 there isn't any output when I use acpi_listen. With 3.17 , there is

*** This bug has been marked as a duplicate of bug 70241 ***