I saw many threads about this but sadly none helped.... so here comes back the brightness problem on Ubuntu 21.10.

Problem

On Ubuntu 21.10 brightness control stopped working, brightness is now always at 100%.

It used to work (and pissed me off because brightness fell down everytime I plugged in AC adaptor), and it's broken for some days/weeks maximum. It could be since upgrade to Ubuntu 21.10 from 21.04, or after a classic package upgrade... I sadly haven't noticed the exact day.

What I tried

1. Using the brightness slider : no effect

2. Using the keyboard : no effect (I see a hover window with a slider moving, but no change on brightness)

3. Switching on/off the night mode : no effect (it 6 a.m. here and my eyes a crying because of the strong white light)

4. Changing default grub options to add acpi_backlight=vendor acpi_osi=linux : this made the problem worse as the previously not working brightness slider completely disappeared. I switched back to default.

5. Installed brightnessctl : command execution is OK and displays different % of brightness, but screen actually remains at full brightness

6. Installed xbacklight : same as 2.

7. Installed ppa:apandada1/brightness-controller : nothing changes but I see errors in the terminal : xrandr: Failed to get size of gamma for output default

When executing this command manually I see the same output :

$ xrandr 
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 1920 x 1080, current 1920 x 1080, maximum 1920 x 1080
default connected primary 1920x1080+0+0 0mm x 0mm
   1920x1080     77.00* 

My configuration

Huawei Matebook 14, AMD Ryzen 5

$ lspci|grep VGA
03:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Picasso (rev c2)

Other threads found, but not helping

• On Ubuntu 16.04
• On Ubuntu 18.04
• On Ubuntu 18.XX
• On Ubuntu 20.04

My guesses

I suspect some display driver problem, as I also noticed some display artefacts lately. When scrolling a web page some parts are scrolling faster than the others, and after 1-2 seconds the display corrects everything.

It's better with an image (I used gimp to simulate what I see) :

Edit : My laptop screen also stays black after closing/reopening the lid, instead of coming back to life... I have to force shut down / restart.

I am not using any additionnal grapics driver.

Any ideas ?

I found the solution thanks to another website linking to another thread. When I upgraded to Ubuntu 21.10 I had to add nomodeset to grub parameters, otherwise Ubuntu kept getting stuck at boot.

Removing this parameter now has fixed the brightness, and boot is still OK. I guess some bug has been fixed since my upgrade :)