[SOLVED] Strange backlight scaling behavior

LRitzdorf · 2022-12-08 19:09:47

Hello! I've been having issues recently (within about the last two weeks) with backlight control on my laptop. The backlight scale seems to be broken in a strange way, such that brightness values are mapped to incorrect actual brightness levels.

First, some background: I'm running standard Arch (stock kernel, no strange modifications) on a Lenovo Legion laptop, with the proprietary Nvidia driver, which were working normally until this issue appeared. I have a backlight device present at:

/sys/class/backlight/nvidia_0/

which is a "proper" backlight, as opposed to xrandr's alpha-channel pseudo-brightness mechanism.

My issue manifests as unexpected brightness levels for certain specified brightnesses. Here's a table of brightness values, as written to "/sys/class/backlight/nvidia_0/brightness":

| Nominal brightness  | Equivalent output    |
| (written to sysfs)  | (estimated manually) |
|---------------------|----------------------|
| 0                   | 0                    |
| 1                   | 45                   |
| 3                   | 70                   |
| 7                   | 90                   |
| 10                  | 100                  |
| 11                  | 11                   |
| 30                  | 30                   |
| 60                  | 60                   |
| 100                 | 100                  |

Essentially, the lower ten levels now correspond to much higher brightnesses, and not even in a linear manner.

Also, the levels read back from "/sys/class/backlight/nvidia_0/brightness" and "/sys/class/backlight/nvidia_0/actual_brightness" match the values written, so I know nothing else is (for example) resetting the backlight without my knowledge.

Has anyone else encountered this? And if so, have you been able to resolve it?

Last edited by LRitzdorf (2022-12-09 19:18:56)

seth · 2022-12-08 22:07:37

within about the last two weeks

The 6.x kernels are around since 2 months, the 525.60.11 driver since 10 days.
Anything suspicious in your pacman log for the relevant time frame?
Do you get the same effect w/
- the LTS kernel
- the 520.56.06-12 driver (matches linux 6.0.10.arch2-1 or you'd have to downgrade and use the DKMS driver, https://archive.archlinux.org/packages/n/ - don't forget nvidia-utils)

What does /sys/class/backlight/nvidia_0/max_brightness report?
Do you also have a backlight device under /sys/class/backlight/acpi_video0 ?

Ceterum censeo: is there a parallel windows installation?

LRitzdorf · 2022-12-08 23:35:57

Mm, it's probably a driver issue, then. I upgraded to kernel 6.x back in mid-October, which is far too early for this issue. I did, however, upgrade to nvidia 525.60.11 (from 520.56.06) on November 30th, which matches my timeframe.
I'll attempt to install 520.56 again, and report back on that in a bit.

Also, I did check /sys/class/backlight/nvidia_0/max_brightness upon first encountering this issue, and it still lists 100 as the maximum value.
The Nvidia backlight device is the only one I have; there's no acpi_video0 on my system.

I do actually have a dual-boot Windows install — is there something I should look at from there?

Last edited by LRitzdorf (2022-12-09 00:20:40)

LRitzdorf · 2022-12-08 23:47:08

All right, the 520.56.06 driver appears to have fixed the issue. Thanks for the troubleshooting tips!

I've linked back to this thread from https://github.com/NVIDIA/open-gpu-kern … issues/409 (though I'm using the proprietary driver). Is there anywhere else I should go to report this?

Last edited by LRitzdorf (2022-12-09 00:23:52)

Arch Linux

#1 2022-12-08 19:09:47

[SOLVED] Strange backlight scaling behavior

#2 2022-12-08 22:07:37

Re: [SOLVED] Strange backlight scaling behavior

#3 2022-12-08 23:35:57

Re: [SOLVED] Strange backlight scaling behavior

#4 2022-12-08 23:47:08

Re: [SOLVED] Strange backlight scaling behavior

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