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

[Michael Biebl]

Am 16.10.19 um 05:10 schrieb Rado S:
> =- Michael Biebl wrote on Tue 15.Oct'19 at 13:33:15 +0200 -=
> 
>> I can get the current brightness via
>>> # 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]
> 
> This is what both links look like:
> 
> 0 lrwxrwxrwx 1 root root 0 Oct 16 05:01 dell_backlight -> ../../devices/platform/dell-laptop/backlight/dell_backlight/
> 0 lrwxrwxrwx 1 root root 0 Oct 16 04:54 intel_backlight -> ../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/
> 
> dell_backlight/:
> total 0
> 0 -r--r--r-- 1 root root 4096 Oct 16 05:03 actual_brightness
> 0 -rw-r--r-- 1 root root 4096 Oct 16 05:03 bl_power
> 0 -rw-r--r-- 1 root root 4096 Oct 16 05:04 brightness
> 0 lrwxrwxrwx 1 root root    0 Oct 16 05:03 device -> ../../../dell-laptop/
> 0 -r--r--r-- 1 root root 4096 Oct 16 05:03 max_brightness
> 0 drwxr-xr-x 2 root root    0 Oct 16 05:03 power/
> 0 lrwxrwxrwx 1 root root    0 Oct 16 04:52 subsystem -> ../../../../../class/backlight/
> 0 -r--r--r-- 1 root root 4096 Oct 16 05:03 type
> 0 -rw-r--r-- 1 root root 4096 Oct 16 05:03 uevent
> 
> intel_backlight/:
> total 0
> 0 -r--r--r-- 1 root root 4096 Oct 16 04:54 actual_brightness
> 0 -rw-r--r-- 1 root root 4096 Oct 16 04:54 bl_power
> 0 -rw-r--r-- 1 root root 4096 Oct 16 04:54 brightness
> 0 lrwxrwxrwx 1 root root    0 Oct 16 04:54 device -> ../../card0-LVDS-1/
> 0 -r--r--r-- 1 root root 4096 Oct 16 04:54 max_brightness
> 0 drwxr-xr-x 2 root root    0 Oct 16 04:54 power/
> 0 lrwxrwxrwx 1 root root    0 Oct 16 04:54 subsystem -> ../../../../../../../class/backlight/
> 0 -r--r--r-- 1 root root 4096 Oct 16 04:54 type
> 0 -rw-r--r-- 1 root root 4096 Oct 16 04:54 uevent
> 
> 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.
-- 
Why is it that all of the instruments seeking intelligent life in the
universe are pointed away from Earth?

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