For the purpose of downgrading from 5.10.17 (raspberrypi-kernel_1.20210303-1) back to the last of 5.4 kernel (raspberrypi-kernel_1.20210108-1), to
investigate driver breakage, I did:
wget -O kernel-headers_armhf.deb http://archive.raspberrypi.org/debian/pool/main/r/raspberrypi-firmware/raspberrypi-kernel-headers_1.20210108-1_armhf.deb
wget -O kernel_armhf.deb http://archive.raspberrypi.org/debian/pool/main/r/raspberrypi-firmware/raspberrypi-kernel_1.20210108-1_armhf.deb
sudo dpkg -i kernel-headers_armhf.deb kernel_armhf.deb
sudo apt-mark hold raspberrypi-kernel-headers raspberrypi-kernel
sudo apt-mark showhold
The last step (sudo apt-mark showhold) is for verification. The sudo apt-mark hold step stops them from getting auto-upgraded.
For switching to other kernel versions, refer to the table in Raspbian Kernel Releases.
To unhold (again, repeating the first step of verifying current holds), and go back to the latest:
sudo apt-mark showhold
sudo apt-mark unhold raspberrypi-kernel-headers raspberrypi-kernel
sudo apt-get install raspberrypi-kernel-headers raspberrypi-kernel
Note (Another Tested Alternative): To downgrade to the last of the 4.19 (raspberrypi-kernel_1.20200601-1), replace the first two steps with:
wget -O kernel-headers_armhf.deb http://archive.raspberrypi.org/debian/pool/main/r/raspberrypi-firmware/raspberrypi-kernel-headers_1.20200601-1_armhf.deb
wget -O kernel_armhf.deb http://archive.raspberrypi.org/debian/pool/main/r/raspberrypi-firmware/raspberrypi-kernel_1.20200601-1_armhf.deb
Ubuntu Pi Server is different from both Raspbian and Ubuntu x86/x86_64 Desktop in its boot process. It allows multiple kernel packages to be installed, and it does not automatically downgrade when one installs an earlier kernel package.
What happens is that it copies the highest version available from /boot into /boot/firmware. To downgrade, one simply copies and over-write the pair of
/boot/initrd.img-x.y.z-NNNN-raspi to /boot/firmware/initrd.img, and /boot/vmlinuz-x.y.z-NNN-raspi to /boot/firmware/vmlinuz` .
For example, here is my current listing of /boot . 5.4.0-1032-raspi is the latest from Ubuntu, and it was copied into /boot/firmware when first installed.
I am able to switch between 5.4.0-1030 and 5.8.0-1013 (both built by me to work around respeaker bug #251) by copying the corresponding pair into /boot/firmware.
ubuntu@ubuntu:~$ ls -l /boot
total 144851
-rw------- 1 root root 4265721 Mar 12 03:01 System.map-5.4.0-1030-raspi
-rw------- 1 root root 4167287 Mar 19 20:05 System.map-5.4.0-1032-raspi
-rw------- 1 root root 5021435 Feb 15 12:02 System.map-5.8.0-1013-raspi
-rw-r--r-- 1 root root 220924 Mar 12 03:01 config-5.4.0-1030-raspi
-rw-r--r-- 1 root root 219032 Mar 19 20:05 config-5.4.0-1032-raspi
-rw-r--r-- 1 root root 230013 Feb 15 12:02 config-5.8.0-1013-raspi
lrwxrwxrwx 1 root root 43 Apr 7 21:26 dtb -> dtbs/5.8.0-1013-raspi/./bcm2711-rpi-4-b.dtb
lrwxrwxrwx 1 root root 43 Apr 7 20:15 dtb-5.4.0-1030-raspi -> dtbs/5.4.0-1030-raspi/./bcm2711-rpi-4-b.dtb
lrwxrwxrwx 1 root root 43 Apr 7 20:05 dtb-5.4.0-1032-raspi -> dtbs/5.4.0-1032-raspi/./bcm2711-rpi-4-b.dtb
lrwxrwxrwx 1 root root 43 Apr 7 21:26 dtb-5.8.0-1013-raspi -> dtbs/5.8.0-1013-raspi/./bcm2711-rpi-4-b.dtb
drwxr-xr-x 11 root root 4096 Apr 7 21:26 dtbs
drwxr-xr-x 3 root root 6656 Jan 1 1970 firmware
lrwxrwxrwx 1 root root 27 Apr 7 21:25 initrd.img -> initrd.img-5.8.0-1013-raspi
-rw-r--r-- 1 root root 30253278 Apr 7 20:15 initrd.img-5.4.0-1030-raspi
-rw-r--r-- 1 root root 29954806 Apr 7 20:05 initrd.img-5.4.0-1032-raspi
-rw-r--r-- 1 root root 31926938 Apr 7 21:26 initrd.img-5.8.0-1013-raspi
lrwxrwxrwx 1 root root 27 Apr 7 21:25 initrd.img.old -> initrd.img-5.4.0-1030-raspi
lrwxrwxrwx 1 root root 24 Apr 7 21:25 vmlinuz -> vmlinuz-5.8.0-1013-raspi
-rw------- 1 root root 8567401 Mar 12 03:01 vmlinuz-5.4.0-1030-raspi
-rw------- 1 root root 8392836 Mar 19 20:05 vmlinuz-5.4.0-1032-raspi
-rw------- 1 root root 25047552 Feb 15 12:02 vmlinuz-5.8.0-1013-raspi
lrwxrwxrwx 1 root root 24 Apr 7 21:25 vmlinuz.old -> vmlinuz-5.4.0-1030-raspi
The backup vmlinuz.bak and initrd.img.bak in /boot/firmware/ are 5.4.0-1030 when it was the highest version.
ubuntu@ubuntu:~$ ls -l /boot/firmware/vm*
-rw-r--r--. 1 root root 25047552 Apr 7 21:26 /boot/firmware/vmlinuz
-rw-r--r--. 1 root root 8567401 Mar 12 02:01 /boot/firmware/vmlinuz.bak
ubuntu@ubuntu:~$ ls -l /boot/firmware/init*
-rw-r--r--. 1 root root 31926938 Apr 7 21:26 /boot/firmware/initrd.img
-rw-r--r--. 1 root root 30253278 Apr 7 20:15 /boot/firmware/initrd.img.bak