Howdy!
Could you please open a terminal then type in sudo nano /etc/default/grub please?
Once in the editor, use the arrows key on your keyboard to navigate toward the line where it says GRUB_CMDLINE_LINUX_DEFAULT=
Then add acpi_backlight=vendor to the end, but befor the last " (quotation mark)!
Hit CTRL + S to save, then CTRL + X to exit the editor.
Finally, punch in sudo update-grub
Reboot!
Have a wonderful day! 
Heyy
Thanks for your help
This is my grub now:
GRUB_DEFAULT=saved
GRUB_TIMEOUT=10
GRUB_TIMEOUT_STYLE=hidden
GRUB_DISTRIBUTOR="Manjaro"
GRUB_CMDLINE_LINUX_DEFAULT="quiet apparmor=1 security=apparmor resume=UUID=d2ac6ecc-cb0e-4f82-b617-1792c08b94b3 udev.log_priority=3 acpi_backlight=vendor"
GRUB_CMDLINE_LINUX=""
# If you want to enable the save default function, uncomment the following
# line, and set GRUB_DEFAULT to saved.
GRUB_SAVEDEFAULT=true
# Preload both GPT and MBR modules so that they are not missed
GRUB_PRELOAD_MODULES="part_gpt part_msdos"
# Uncomment to enable booting from LUKS encrypted devices
#GRUB_ENABLE_CRYPTODISK=y
# Uncomment to use basic console
GRUB_TERMINAL_INPUT=console
# Uncomment to disable graphical terminal
#GRUB_TERMINAL_OUTPUT=console
# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command 'videoinfo'
GRUB_GFXMODE=auto
# Uncomment to allow the kernel use the same resolution used by grub
GRUB_GFXPAYLOAD_LINUX=keep
# Uncomment if you want GRUB to pass to the Linux kernel the old parameter
# format "root=/dev/xxx" instead of "root=/dev/disk/by-uuid/xxx"
#GRUB_DISABLE_LINUX_UUID=true
# Uncomment to disable generation of recovery mode menu entries
GRUB_DISABLE_RECOVERY=true
# Uncomment and set to the desired menu colors. Used by normal and wallpaper
# modes only. Entries specified as foreground/background.
GRUB_COLOR_NORMAL="light-gray/black"
GRUB_COLOR_HIGHLIGHT="green/black"
# Uncomment one of them for the gfx desired, a image background or a gfxtheme
#GRUB_BACKGROUND="/usr/share/grub/background.png"
GRUB_THEME="/usr/share/grub/themes/manjaro/theme.txt"
# Uncomment to get a beep at GRUB start
#GRUB_INIT_TUNE="480 440 1"
The problem still exists
Howdy!
Could you please provide me the output of ls /sys/class/backlight please?
Thank you!
Heyyy
Yes of course
amdgpu_bl0
Wow that’s the exact thing I see in error when I boot my laptop
And also these are inside amdgpu_bl0 :
-r--r--r-- 1 root root 4096 Oct 5 14:25 actual_brightness
-rw-r--r-- 1 root root 4096 Oct 5 14:25 bl_power
-rw-r--r-- 1 root root 4096 Oct 5 14:04 brightness
lrwxrwxrwx 1 root root 0 Oct 5 14:25 device -> ../../../0000:05:00.0
-r--r--r-- 1 root root 4096 Oct 5 13:36 max_brightness
drwxr-xr-x 2 root root 0 Oct 5 14:25 power
-r--r--r-- 1 root root 4096 Oct 5 14:25 scale
lrwxrwxrwx 1 root root 0 Oct 5 13:36 subsystem -> ../../../../../../class/backlight
-r--r--r-- 1 root root 4096 Oct 5 13:36 type
-rw-r--r-- 1 root root 4096 Oct 5 13:36 uevent
Thank you!
Instead of acpi_backlight=vendor in your grub configuration, please change it to acpi_backlight=amdgpu_bl0 !
May also mask the acpi_video0 by imploring sudo systemctl mask acpi_video0
Hope that may help!
Ok now it got worse 
Away from the brightness set to 100% every time, the errors I see when booting are:
Failed to start Load/Save #tness of backlight:amdgpu_bl0
Failed to start Load/Save #tness of backlight:acpi_video0
I changed acpi_backlight to native based on this question and the error Failed to start Load/Save #tness of backlight:acpi_video0 vanished.
But I still see amdgpu_bl0
Oh no
Might as well try to mask amdgpu as well, see if it does something…
sudo systemctl mask amdgpu_bl0
I run it and restarted but still no result
Also my brightness problem is still there 
Hmmm 
Lets try to sudo systemctl unmask acpi_video0 , then set acpi_backlight=video0 in grub …
Again, those two errors showed up
What’s the problem?
Have you ran sudo update-grub , may I ask?
Yes I ran it and restarted 
Alright, sudo systemctl unmask amdgpu_b10 then add acpi_osi=Linux acpi_backlight=vendor to your grub config.
Unfortunately, the problem still exists 
I mean both error on boot and brightness issue
Okay, could you please roll back to kernel57, see if thats the issue?
Ok now my kernel is:
5.7.19-2-MANJARO
And the error on boot disappeared(Thanks!!)
But still, my brightness is set to 100%
Hm, then lets remove acpi_osi=Linux from grub, and change to acpi_backlight=native 
Still no chance
I guess grub is getting angry with me
Hm, getting out of ideas😬
Try to completely remove the acpi backlight string in grub…
And now the error Failed to start Load/Save #tness of backlight:acpi_video0 shows up again
