Comment # 70 on bug 899879 from Egbert Eich

(In reply to Freek de Kruijf from comment #67)
> > 1. What you get when you do: 
> >    systemctl list-units | grep backlight
> 
> # systemctl list-units | grep backlight
>  
> sys-devices-pci0000:00-0000:00:02.0-drm-card0-card0\x2deDP\x2d1-
> intel_backlight.device      loaded active plugged  
> /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight
>   systemd-backlight@backlight:intel_backlight.service                       
> loaded active exited    Load/Save Screen Backlight Brightness of
> backlight:intel_backlight
>   system-systemd\x2dbacklight.slice                                         
> loaded active active    system-systemd\x2dbacklight.slice


OK, so there is no ACPI backlight. That's what I was trying to find out.

> > 2. What you get by doing:
> >    ls /var/lib/systemd/backlight/
> 
> # ls /var/lib/systemd/backlight/
> pci-0000:00:02.0:backlight:intel_backlight

Ok. This is the file then where systemd stores the backlight value on shut
down.

> > 3. Determine if you get a somewhat linear behaviour between 0 and 4687.
> 
> No. 0 is black and even 1 is bright. It looks like values above 4687 make a
> difference in brightness.

This is something we need to report to Intel. The non-linear behavior is of
course he root of all your problems.

I need one more info, though:

(In reply to Freek de Kruijf from comment #69)

> >    # dmesg > dmesg.out
> >    Please get me the content of 'dmesg.out'.
> 
> See attached file dmesg.out.gz

Ok, this contains a lot of useful information already.
> > 2. run as root:
> >    # intel_bios_dumper  intel_bios.data
> >    get me the output file 'intel_bios.data'
> 
> # intel_bios_dumper intel_bios.data
> Couldn't read graphics card ROM: Input/output error

Ok, not sure why this happens. We should be able to get the information I'm
missing from the kernel logs as well if you set drm.debug=0x6 (instead of 0x4)
and get me a dmesg again.
Do you have an account on bugs.freedesktop.org? I may open a ticket there for
Intel to look into this. It would be good if I could add you to this in case
Intel needs further information from you.