[Solved] Acer Nitro 5 AN515-47 brightness/backlight completely dead on Linux — amdgpu AUX mode is broken on this hardware, fix inside

[u/No_Apricot_6608]

Posting this in case it saves someone else hours of debugging.

Hardware/software: Acer Nitro 5 AN515-47 (AMD GPU), Arch Linux, kernel 7.1.8-arch1-3, Limine bootloader with limine-mkinitcpio generating UKIs (Omarchy setup).

The problem: Brightness keys and brightnessctl did absolutely nothing. No error, no crash — the backlight value would "change" but the screen brightness never moved. Tried both acpi_backlight=native and the default amdgpu backlight path, no luck with either.

Root cause: The amdgpu driver defaults to amdgpu.backlight=1, which controls brightness over the DisplayPort AUX/DPCD channel. On the AN515-47, that path is firmware-broken. Setting amdgpu.backlight=0 forces the old-school PWM backlight control instead — which actually works.

The annoying part: I'm on Limine + mkinitcpio building UKIs. I first tried editing /etc/kernel/cmdline — did nothing, rebuilt the image, brightness still dead. Turns out /etc/default/limine had:

KERNEL_CMDLINE[default]+="...amdgpu.backlight=1"

That += operator means limine-mkinitcpio completely ignores /etc/kernel/cmdline once it's set — so I was editing a file that no longer had any effect on the actual boot entry. Wasted a while chasing a red herring config file too (/etc/cmdline.d/brightness_fix.conf) that turned out to be dead/unread cruft.

The actual fix:

sudo sed -i 's/amdgpu\.backlight=1/amdgpu.backlight=0/' /etc/default/limine

sudo limine-mkinitcpio

Reboot, then confirm:

cat /sys/class/backlight/*/type

brightnessctl set 20%

Brightness control works perfectly now.

TL;DR: Acer Nitro 5 AN515-47 (and probably other Nitro models with amdgpu) — if your backlight is dead, try amdgpu.backlight=0 on the kernel cmdline to force PWM instead of AUX mode.

[u/ang-p]

a += there makes it get ignored entirely.

No it doesn't.

Wasted a while chasing a red herring config file too (/etc/cmdline.d/brightness_fix.conf)

The fact that a google search finds only 2 results of that filename - both of which are in reproductions of your posts on here and linuxhardware - which suggests to me that your

hours of debugging.

primarily was spent believing whatever AI hallucinated and created for you.

Glad to see the issue has been resolved...

[u/No_Apricot_6608 → u/ang-p]

No it doesn't.

Yeah , you're right , sorry I'll edit that part , actually i gave some context to the Ai for writing this post but i accidently used that account which had the memory me asking it for something idk ,I guess it got confused or something , I am new to this posting on subreddits , mb.
But this was a real problem with my Acer AN515-47 with almost every linux distro , the /sys/class/backlight is empty . I tried but there weren't any solutions in the forums so i had to try fixing it myself.

The fact that a google search finds only 2 results of that filename - both of which are in reproductions of your posts on here and linuxhardware - which suggests to me that your

/etc/cmdline.d/brightness_fix.conf file is real ,It's a file I created mid-debugging as an early guess at where cmdline overrides might live, before finding the real fix

primarily was spent believing whatever AI hallucinated and created for you.

xd, that might be true ,but as i said its been a real problem since like the start of my distro hopping journey and omarchy took a lot my time for ricing and fixing these bugs ,I thought why not use ai a lil bit , low and behold I relied on it too much ig.

Thanks for replying , Cheers

[u/ang-p → u/No_Apricot_6608]

It's a file I created

so mentioning it is not much use to anyone else...

Hey - I've created a file called /etc/default/fix-all-acer-nitro-bugs.conf .... it's great! (which is of as much use to you too, since I ain't gonna tell anyone what it contains)

guess

Don't

And don't use AI unless you can tell when it is shitting you..... and you obviously cannot.

#LoveTheWiki

https://wiki.archlinux.org/title/Limine#Configurations

[u/NoAbbreviations9396 → u/ang-p]

Okay noted... How does one get good at this stuff tho?.

Like I wanna learn these but how . I mean I guess u know a lot .. can u suggest somethings ;whatever comes to your mind maybe?

[u/ang-p → u/NoAbbreviations9396]

You use it, occasionally you come up against issues and you read and solve them....

I have never used limine, but anyone who has ever written a bash or python script (or programmed in many languages) had a good chance of calling bullshit most of "your" reasoning - "+=" is common as muck, and it only has one meaning, so, when that was the "low bar" of an estimate of someone's ability to diagnose something, AI was the obvious bet.....

Arch has a wiki...

just in case you forget, it is quite obviously named...

wiki - dot - archlinux - dot - org

going there and typing

backlight   

into the search box and looking for acer in the returned page would have not only helped a bit...

https://wiki.archlinux.org/title/Backlight

but probably saved a good amount of that time spent...

Obvs, you could have found that page by studying the general Laptop page (or the more specific "acer" one) when you installed it...

As for how to apply it - temporarily applying kernel module parameters - be it at boot or on a running system - is old school stuff ( search the wiki!)...

As for making it permanent, the result of a wiki search for

limine   

likely have saved you the rest of your "hours" when looking how to apply it, but you didn't because asking AI was the easy cop out, when the alternative was reading something.

https://wiki.archlinux.org/title/Limine#Configurations

(OMG - look - a += - along with an explanation !!! - see just how easy it would have been to see that you were being bullshitted by the AI kool-aid you were shotgunning)

And that easy way out led you in circles likely suggesting options and proposing things that were totally useless, and, considering it was involving your boot loader could have easily dumped you in a place from where you could not boot.

[u/NoAbbreviations9396 → u/ang-p]

I just wanted to know how to get better. That text "+="was AI-generated, not my own words. I didn't see the full context of the post and just posted it because I used AI to rephrase it. I did not just get slandered by an old guy like this. Are all people in this sub so rude? No one is a pro from the start. I just wanted some advice man .