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DeadLink404

Member

Registered: 2017-11-24

Posts: 40

[solved] max backlight brightness too high

After finally finding a solution to adjust screen brightness
setting /sys/class/backlight/intel_backlight/brightness manually works now finally.
(I needed the i915.enable_dpcd_backlight=1 kernel paramter.)

And now I get

cat /sys/class/backlight/intel_backlight/max_brightness
65535

But the actual max_brightness is 255. 
As Setting it to 256 sets the lcd brightness to 0 (classic overflow). 256 is the same as 1 and so on.
Since the keyboard shortcuts depend on the right max_brightness they behave funky, as it adds 3277 to the brightness,
thus stepping over mutiples of 255 repeatedly resulting in flicker and the resulting brightness is fairly random afterwards
(3277 mod 255 =217 so it goes from 0 to 217 to 179 and so on)

Is there a way to manually set max_brightness to 255?

Last edited by DeadLink404 (2018-05-21 09:11:18)

NoSuck

Member

Registered: 2015-03-04

Posts: 157

Website

Re: [solved] max backlight brightness too high

Given that you have a device in /sys/class/backlight, i.e. hardware brightness controls, is there a reason you're not using xbacklight -set?

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ewaller

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From: Pasadena, CA

Registered: 2009-07-13

Posts: 19,804

Re: [solved] max backlight brightness too high

So, set your brightness steps to 257.  (Only half kidding)

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DeadLink404

Member

Registered: 2017-11-24

Posts: 40

Re: [solved] max backlight brightness too high

Given that you have a device in /sys/class/backlight, i.e. hardware brightness controls, is there a reason you're not using xbacklight -set?

xbacklight sets brightness relative to the wrong max brigthness as well.
So full brightness would be around 0.39 in that case.
I now have a script using xbacklight that only increments if it is below 0.39 which is a decent workaround.

So, set your brightness steps to 257.  (Only half kidding)

How would I go about doing that?

Last edited by DeadLink404 (2018-05-20 21:50:16)

NoSuck

Member

Registered: 2015-03-04

Posts: 157

Website

Re: [solved] max backlight brightness too high

Are you sure that i915.enable_dpcd_backlight=1 is the right kernel parameter for your hardware?

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DeadLink404

Member

Registered: 2017-11-24

Posts: 40

Re: [solved] max backlight brightness too high

Are you sure that i915.enable_dpcd_backlight=1 is the right kernel parameter for your hardware?

I am fairly certain as the backlight is uncontrolable otherwise.

However because of the lspci lockup issue I had
acpi_osi="!Windows 2015" as kernel parameter. Using the acpi_osi=! acpi_osi="Windows 2009" alternative instead makes the backlight behave correctly.

Going to try a few other combinations but this is working for now.

Edit: I think I figured it out, the acpi_video0 interface does not have the problem that the intel one has.
For acpi_osi="!Windows 2015" I need acpi_backlight=video
to get the acpi_video0 interface and then set

option "Backlight" "acpi_video0"

in 20-intel.conf

Anyway the intel_backlight/max_brightness is still broken with that,
but it doesn't get used so whatever, right?

Last edited by DeadLink404 (2018-05-21 09:37:20)