I installed Kubuntu 20.04 to my new machine with CPU AMD Ryzen 5 4500U， unfortunately, the GPU failed to function properly to adjust the display lightness.
Friends guided me that the kernel 5.4 released earlier than AMD Ryzen 5， so updating kernel to latest 5.8 might be a decent solution.
I followed the instruction How to Install Linux Kernel 5.8 in Ubuntu
cd /tmp/
wget -c https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.8/amd64/linux-headers-5.8.0-050800_5.8.0-050800.202008022230_all.deb
wget -c https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.8/amd64/linux-headers-5.8.0-050800-generic_5.8.0-050800.202008022230_amd64.deb
wget -c https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.8/amd64/linux-image-unsigned-5.8.0-050800-generic_5.8.0-050800.202008022230_amd64.deb
wget -c https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.8/amd64/linux-modules-5.8.0-050800-generic_5.8.0-050800.202008022230_amd64.deb
sudo dpkg -i *.deb
However, after restart it report error:
error: /boot/vmlinuz-5.8.0-050800-generic has invalid signature.
error: you need to load the kernel first
Press any key to continue...
I re-installed the kernel 5.8 more than twice, it report such the identical error.
How could solve the problem?
