[Bug 75791] i915: backlight turns off during boot

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=75791

--- Comment #10 from Jani Nikula <jani.nikula at intel.com> ---
(In reply to mcy from comment #9)
> (In reply to Jani Nikula from comment #8)
> > Please attach /sys/kernel/debug/dri/0/i915_opregion
> 
> Hum, /sys/kernel/debug is empty. Do I have to add another boot parameter to
> the kernel or something? I tried adding "debug" but then it won't boot (it
> stays blocked right before the moment it should ask for my LUKS passphrase).

Either debugfs is not mounted (fixed by 'sudo mount -t debugfs none
/sys/kernel/debug') or you don't have permissions.

> $ ls -l /sys/class/backlight
> total 0
> lrwxrwxrwx 1 root root 0 mai 12 13:14 intel_backlight ->
> ../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight
> 
> (It is the exact same output with 3.14.3.)

Apparently it's not there from 3.15-rc2 onwards, because:
[drm:intel_panel_setup_backlight] native backlight control not available per
VBT

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