I use an HP laptop with an AMD R7-5800U CPU and Ubuntu 18.04, and I can't adjust display brightness with kernel versions 5.4 or 5.10.3.
I also tried other solutions, including adjust the brightness in /sys/class/backlight, and in this path, there is a directory named as api_video0.
When I change the brightness from settings, the brightness in this path is changed while the screen brightness didn't change at all.
82 Answers
I'm having the same problem with the same laptop. I tried 20.04, 20.10, but none of them works. Also the USB-C and hdmi doesn't work either.
This is a problem common with Nvidia cards. If you do have an Nvidia video card, you are going to need to load the driver.
First of all do a uname -a, it will show something like 4.15.0-142-generic, not exactly that, but something like that in the output.
After that dosudo apt install linux-headers-4.15.0-142 linux-headers-4.15.0-142-generic.
It's not going to be that exact version, but the numbers have to match whatever uname -a gave you.
Then you are going to add the graphics driver repository:sudo add-apt-repository ppa:graphics-drivers/ppa
Now you can check what driver is available:sudo ubuntu-drivers devices
It should display a list and some entry is going to say "recommended" next to it. So you can go ahead and install it.sudo ubuntu-drivers autoinstall
Make sure to reboot and you should be all set. All that said, I really do hope that somebody fixes the issue that linux-headers packages are never part of any dependencies even though you can't load the video drivers correctly without them. 16.04 had a huge problem where it lost its mind every time the kernel updated. Every time there was a kernel update, before rebooting had to remember to go in and manually install the correct header set. Otherwise pain was inevitable.
