[Bug 53153] [gm45] Backlight shuts off during boot when KMS is enabled

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=53153

--- Comment #36 from Jani Nikula <jani.nikula at intel.com> ---
(In reply to comment #33)
> Okay, so I just tried adjusting the backlight by changing the value in
> '/sys/class/backlight/acpi_backlight0/backlight', and it had no effect. 

(In reply to comment #34)
> Trying to change the brightness with the wmi hotkeys DOES send a value to
> '/sys/class/backlight/acpi_video0', but the screen does not change. Each
> time I press Fn+[RightArrow] the value will change in this sequence:
> [0,2,4,6,8,9].
> 
> The ACPI backlight control does not work, so we're forced to use the drm to
> adjust it instead.

Right. Therefore you should use acpi_backlight=vendor kernel parameter to
disable the broken backlight interface /sys/class/backlight/acpi_video0.

Looking at acer-wmi, it's not obvious to me whether this should bring up a
platform backlight interface /sys/class/backlight/acer-wmi or similar. Please
check this and try whether this interface works or not. Also check what
acer-wmi says in dmesg.

If it works, it should be preferred over /sys/class/backlight/intel_backlight.

> The drm backlight control is buggy - the max brightness is the minimum and
> vice versa. So when the drm kicks in during boot and tries to set the
> brightness to max, the backlight shuts off. 
> 
> The parameter i915.invert_brightness=1 solves this problem.
>
> So the issue here is really twofold. There is a buggy ACPI which fails to
> set the brightness, and a buggy drm which sees brightness values as their
> inverse.

Yes. If acer-wmi backlight does not exist or work, it seems you need to set
both acpi_backlight=vendor and i915.invert_brightness=1. 

> I was looking at dmesg and I noticed it always returns this line:
> [   11.089157] acer_wmi: Brightness must be controlled by acpi video driver

This means acer-wmi *thinks* ACPI should work, so it does not create a
backlight interface of its own. See above.

> So I did some research on the acer_wmi module, and it turns out I need to
> enable the backlight subsystem in my kernel config.

If you have anything under /sys/class/backlight, you already have
CONFIG_BACKLIGHT_CLASS_DEVICE=y. Or do you mean something else?

In all of the above, please use the /sys/class/backlight interfaces directly,
and worry about the hotkeys afterwards.

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