[Bug 78200] [regression] Backlight too dim to use from kernel 3.14 and systemd error

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=78200

--- Comment #19 from Jani Nikula <jani.nikula at intel.com> ---
(In reply to comment #18)
> Therefore there is no kernel or systemd bug, and I will close this report as
> resolved.
> 
> The problem that led to my confusion was that at some point a kernel update
> changed the max_brightness value from the driver for my system

This is likely because of the fix

commit b6ab66aa5d376583a17137cbb2d3a728f29acae2
Author: Jani Nikula <jani.nikula at intel.com>
Date:   Tue Feb 25 13:11:47 2014 +0200

    drm/i915: use backlight legacy combination mode also for i915gm/i945gm

which would have that effect, and it matches your platform.

I think you trying different kernel versions with and without that patch lead
to systemd saving and then using out of range saved brightness value on 3.13. I
think it's a bug in systemd, while it's not the root cause here. I sent a bug
report in the disguise of a patch to systemd [1] to fix this:

http://mid.gmane.org/1399453261-31967-1-git-send-email-jani.nikula@intel.com

> However using the Fn + Up
> Arrow keys did not reset the value in
> /sys/class/backlight/intel_backlight/brightness despite the screen becoming
> normal brightness, and so this low value persisted when systemd saved the
> current value and restored it at next boot.

This is probably due to ACPI handling the keys. Depending on the BIOS on your
system, it's possible we sync that to the intel_backlight brightness too in
recent kernels.

> Sorry it took me so long to recognise that it was my kde script that was
> causing the symptoms.

No worries. Thanks for following up, and I'm glad you figured it out.

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