I haven’t needed to downgrade my kernel for a while. I was using Ubuntu’s 14.04 LTS (long time support) support for a few years, and it was so stable that I just run the following with no problems.

sudo apt-get update
sudo apt-get upgrade

But recently I got a new laptop with the latest chipsets and peripherals and decided to upgrade to the new Ubuntu Xenial 16.04 LTS while I’m at it.

I’m sad to say that the combination of new distro with bleeding edge hardware has twice given me a problem.

I’m sure it’ll be stable soon but in the mean time I’ve had to revert to a previous kernel twice now, so this blog will help me remember what to do if it happens again.

The symptoms are dramatic. After a re-boot my laptop had no network drivers and no graphics drivers, the display reverts to an emergency emulated mode which has very slow mouse response times and it looks huge, like a “my first computer” children’s toy!

Not what I want from my brand new hardware.

I don’t have time or want to fix all the drivers, so the quickest fix is to revert to a previous kernel that was working, remove the broken one, which has the effect of blacklisting it then try the next kernel when it comes out.

Howto

Boot from previous kernel

1. Hold the shift key when you see the Grub screen, to get to the grub options.
2. you may have better luck holding the shift key all the time through the boot if you have a fast system.
3. Choose Advanced options for Ubuntu
4. Make a note of the current kernel numbers (top of the list) you’ll need them later to remove it.
5. Use the arrow key to pick a previous kernel and boot into it, hit enter.

Remove the dodgy kernel

This will remove the broken kernel and drivers, and lets the package manager know that you don’t want it again if you do an update. You should remove the specific broken kernel and it’s headers, don’t remove the super package linux-generic this is the package that Ubuntu uses to upgrade the kernel and headers when they become available. If you remove it you wont get kernel updates automatically and will have to specifically run apt-get to get them.

# use the kernel numbers from previous step to confirm that the broken kernel has been installed
# eg if the currently broken kernel was linux-image-4.4.0-64-generic it should show up in the following command.
dpkg -l | grep linux-image

# remove the broken kernel
sudo apt-get purge linux-image-4.4.0-64-generic

# remove its headers too
sudo apt-get purge linux-headers-4.4.0-64-generic

Reboot

On reboot you should boot into the previous safe version, hold shift down on boot and confirm that the broken kernel is not a choice in the grub advanced settings.

Because we haven’t removed the linux-generic package itself Ubuntu will still attempt to get a new kernel when one is available, just not the one you specifically purged.

That’s it.