Cross-building the Ubuntu Kernel for the Raspberry Pi 4 (arm64)

I recently had to rebuild the Ubuntu kernel for the Raspberry Pi pretty often to apply some patches. Building on the Pi is simple, but takes A LOT of hours and completely saturates the rpi4. Also, it means installing many dev packages, which I do not typically need. I also tried to do it on a development rpi3: took many hours, failed a few times because of insufficient RAM etc… It is a pain.

I therefore decided to try to cross-build it from my machine, which is not a Ubuntu machine at the moment. Quickest and simplest way I found is to use Docker. The result seems to work, so I decided to also provide the tools I created for myself here: https://github.com/carlonluca/docker-rpi-ubuntu-kernel.

Usage

The image only contains the tools and the libs needed to compile. The kernel itself and the script is provided externally. A workspace directory will contain both the kernel code to build and the output debian packages. With the following command the script is executed in the container and you should get your debs:

docker run --rm -it --name builder -v $PWD/workspace:/workspace \
    -v $PWD/build.sh:/workspace/build.sh carlonluca/docker-rpi-ubuntu-kernel:focal \
    /workspace/build.sh

The workspace directory must contain a src directory with the kernel, so you’ll have to create it yourself and then patch your kernel:

mkdir workspace
cd workspace
git clone https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux-raspi/+git/hirsute src
[apply needed patches]

The resulting packages can then be moved to the pi and installed with:

sudo dpkg -i *.deb

Hope this tool can be useful! Bye ;-)