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turtles
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PostPosted: Thu Mar 19, 2015 7:37 pm    Post subject: What makes backlight automatically dim? Reply with quote

Greetings all my lcd backlight automatically dims to about half brightness if I unplug the power adapter.
I am curious what is doing the dimming?

I have been playing around with laptop-mode-tools on my laptop and get some odd behavior.
When I edit the file
Code:
 /etc/laptop-mode/conf.d/lcd-brightness.conf

And set
Code:
CONTROL_BRIGHTNESS=0

or if I stop laptop-mode-tools
my lcd backlight still automatically dims to about half brightness if I unplug the power adapter.
however
Code:
cat /sys/class/backlight/acpi_video0/brightness

reports "0" which is all the way dim
and
Code:
cat /sys/class/backlight/intel_backlight/brightness

reports
"976" which is all the way bright

Is this some sort of BIOS or hardware level control or is it some other linux process?
I am thinking its hardware or BIOS since the files are not changing.
However should ACPI not update those files with the actual brightness level?

I have a recent Intel chipset:
Code:

00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core processor Graphics Controller (rev 09) (prog-if 00 [VGA controller])
        Subsystem: Dell Device 0549
        Flags: bus master, fast devsel, latency 0, IRQ 40
        Memory at f6400000 (64-bit, non-prefetchable) [size=4M]
        Memory at e0000000 (64-bit, prefetchable) [size=256M]
        I/O ports at f000 [size=64]
        Expansion ROM at <unassigned> [disabled]
        Capabilities: [90] MSI: Enable+ Count=1/1 Maskable- 64bit-
        Capabilities: [d0] Power Management version 2
        Capabilities: [a4] PCI Advanced Features
        Kernel driver in use: i915


So now I start laptop-mode and enable
Code:
CONTROL_BRIGHTNESS=1

I wrote 3 scripts to see what its doing

BATT_BRIGHTNESS_COMMAND="/usr/local/bin/lcd_on_battery.sh"
LM_AC_BRIGHTNESS_COMMAND="/usr/local/bin/lcd_full.sh"
NOLM_AC_BRIGHTNESS_COMMAND="/usr/local/bin/lcd_full_nolm.sh"
BRIGHTNESS_OUTPUT="/dev/null"

Each script logs when it was called and does a
Code:
echo '976' > /sys/class/backlight/intel_backlight/brightness

to set the lcd backlight to full.
When I plug in the power I see in the log the script LM_AC_BRIGHTNESS_COMMAND="/usr/local/bin/lcd_full.sh"
never gets called. Instead NOLM_AC_BRIGHTNESS_COMMAND="/usr/local/bin/lcd_full_nolm.sh" is called.
Is this the expected behavior of CONTROL_BRIGHTNESS=1 ?

Now when I unplug AC power the lcd backlight dims then
BATT_BRIGHTNESS_COMMAND="/usr/local/bin/lcd_on_battery.sh" does get called and it brightens again.

Its not really a problem since my scripts solve the problem, however I am curious
why does the kernel (3.6.11) not know the correct status of the hardware?
Thoughts?
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szatox
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PostPosted: Thu Mar 19, 2015 7:53 pm    Post subject: Reply with quote

I'd start with your DE's power saving features :roll:
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turtles
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PostPosted: Thu Mar 19, 2015 9:06 pm    Post subject: Reply with quote

Thanks though I really dont use a DE.
I have a very minimal openbox setup with tint2.
Sometimes I use JWM as well.

I did boot into the BIOS and found a setting for lcd brightness that was set to half on battery.
The question still stands:
Why are BIOS changes to a hardware state like brightness are not picked up by ACPI?
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