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ikt

Member

Registered: 2023-05-19

Posts: 5

[SOLVED] Display brightness low after unlocking session with lightdm

Hi, I've got a pretty specific issue, and I'm not sure if it's some driver, Xfce, LightDM or a poltergeist.
After unlocking a session that's been locked as "dm-tool lock", the display brightness is turned down.

Checking /sys/class/backlight/acpi_video0/brightness, it's still the same old value as it was before locking. Turning the brightness down properly sets the display brightness to the last value minus one step - and increasing the brightness correctly sets the brightness to the last value plus one step. Echoing the same value that is already in /sys/class/backlight/acpi_video0/brightness to itself also restores the brightness.

I've uninstalled all screensavers to no avail, so that doesn't seem to influence stuff. I've stopped xfce4-power-manager but even there, the display brightness still dropped. The brightness itself is still the same when the dm lock activates.

Here's a demonstration: https://www.youtube.com/watch?v=QWsEM4yh3T0

ThinkPad T400 (no ATi graphics, just the Intel GM45 with whatever drivers come on Mint 21)
Mint 21.1, Xfce 4.16, LightDM 1.30.0
Linux 6.1.0-1010-oem

Lock command through xflock4:
xconf-query -c xfce4-session -p /general/LockCommand -s "dm-tool lock"

I have no clue how to proceed. Who's setting the display brightness? Why isn't /sys/class/backlight/acpi_video0/brightness updated?

Last edited by ikt (2023-05-20 07:39:10)

ToZ

Administrator

From: Canada

Registered: 2011-06-02

Posts: 10,990

Re: [SOLVED] Display brightness low after unlocking session with lightdm

Hello and welcome.

Hi, I've got a pretty specific issue, and I'm not sure if it's some driver, Xfce, LightDM or a poltergeist.
After unlocking a session that's been locked as "dm-tool lock", the display brightness is turned down.

This is strange.

ThinkPad T400 (no ATi graphics, just the Intel GM45 with whatever drivers come on Mint 21)

Exactly which driver are you using?

lspci -vnk | grep -A12 VGA

Also, do you have more than one backlight interface:

ls /sys/class/backlight

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ikt

Member

Registered: 2023-05-19

Posts: 5

Re: [SOLVED] Display brightness low after unlocking session with lightdm

Thanks for the reply! I think I figured it out.

I only had

acpi_video0

in /sys/class/backlight. But apparently, you kinda need `intel_backlight`. I did some further poking and tried to add the kernel flag

acpi_backlight=vendor

- which added thinkpad_screen, but kept acpi_video0.

After some further poking with the kernel flags, it turns out there was a change to the backlight handling for kernel 6.1+.

I reverted to kernel 5.19, and the issue seems to be resolved!

The lspci output, may help improve searchability for this issue for others:

00:02.0 0300: 8086:2a42 (rev 07) (prog-if 00 [VGA controller])
    Subsystem: 17aa:20e4
    Flags: bus master, fast devsel, latency 0, IRQ 16
    Memory at f4400000 (64-bit, non-prefetchable) [size=4M]
    Memory at d0000000 (64-bit, prefetchable) [size=256M]
    I/O ports at 1800 [size=8]
    Expansion ROM at 000c0000 [virtual] [disabled] [size=128K]
    Capabilities: [90] MSI: Enable- Count=1/1 Maskable- 64bit-
    Capabilities: [d0] Power Management version 3
    Kernel driver in use: i915
    Kernel modules: i915

00:02.1 0380: 8086:2a43 (rev 07)

On 5.19, my /sys/class/backlight contains intel_backlight and thinkpad_screen. This does not seem to have adverse effects. acpi_backlight=vendor is still present in the kernel flags.

Last edited by ikt (2023-05-20 07:44:43)