Bug#942219: systemd-backlight at backlight:dell_backlight.service fails at boot for dell d600, d620, d820

[Rado S]

=- Michael Biebl wrote on Wed 16.Oct'19 at 13:06:29 +0200 -=

> >>> # cat /sys/class/backlight/intel_backlight/brightness
> >>> 1500
> > 
> > I get 79560.
> > 
> >> and change it via
> >>> # echo 2000 > /sys/class/backlight/intel_backlight/brightness
> > 
> > Setting it via intel to 66666 makes it darker.
> > 
> >> What's the output you get for your dell_backlight?
> > 
> >>From dell_backlight I get:
> > # cat dell_backlight/brightness
> > 6
> > 
> > But when setting it, it says:
> > 
> > # echo 5 > dell_backlight/brightness
> > -ksh: echo: write to 1 failed [Input/output error]
> > {...}
> > What next?
> 
> Dunno. You'll probably need to ask someone who is familiar with
> Dell hardware. I have no idea why these Dell laptops provide a
> dell_backlight device and what this device supposed to control.
> Apparently it can't be used to change the brightness despite it
> exposing the brightness attribute.

For the record:
the dell value for brightness reflects the scale given in the BIOS
menu: (0-7)

The dell vostro works OK:
despite having an Intel GM45 it has no intel_backlight device at
the same place as the others.
Instead it has acpi_video0 which scales like the dell_backlight
(values 0-7) rather than the intel_backlight (*1000).
At Installation time xorg also decided not to use a gpu specific
driver (xorg-video-intel) with the vostro for the acpi_video0, which
in return made X operate slow compared to the older dells, which
still do use the xorg-video-intel (or radeon).

Which components of the installer decide whether to use xorg-drivers
and which devices to create for backlight handling?

Is this still systemd or again a kernel issue?

Thanks for your support so far,
Rado