Build RT_PREEMPT kernel for Raspberry Pi 4
Introduction
This README describes necessary steps to build and install RT_PREEMPT Linux kernel for the Raspberry Pi4 board. RT Kernel is a part of the ROS2 real-time system setup. Raspberry Pi4 is a reference board used by the ROS 2 real-time community for the development. RT Kernel is configured as described in Kernel configuration section. Kernel is built automatically by the Github action, and the artifacts are located under the RPI4 RT Kernel build. Please follow installation instructions to deploy a new kernel to the RPI4 board.
Raspberry Pi 4 RT Linux kernel
Ubuntu raspi kernel is modified to produce an RT Linux kernel. Ubuntu is a ROS 2 Tier 1 platform and Ubuntu kernel was selected to align to it.
Download ready-to-use RT Kernel deb packages
RT Kernel is configured using configuration parameters from the file. In the case you need to build your own kernel read the description below.
Using GUI
Go to the Action tab, find the latest RPI4 RT Kernel build, go inside the latest workflow run, download, and unzip artifacts called RPI4 RT Kernel deb packages. This archive contains four debian packages. Follow instructions to deploy them on the RPI4.
Using command line
Go to the Developer settings and generate a token to access the repo via Github API. Use this token in conjunction with your Github name to retrieve build artifacts.
$ token=<my_token>
# rertieve all artifacts
$ curl -i -u <my github name>:$token -H "Accept: application/vnd.github.v3+json" https://api.github.com/repos/ros-realtime/linux-real-time-kernel-builder/actions/artifacts | grep archive_download_url
"archive_download_url": "https://api.github.com/repos/ros-realtime/linux-real-time-kernel-builder/actions/artifacts/91829081/zip",
"archive_download_url": "https://api.github.com/repos/ros-realtime/linux-real-time-kernel-builder/actions/artifacts/91534731/zip",
# download the latest one
$ curl -u <my github name>:$token -L -H "Accept: application/vnd.github.v3+json" https://api.github.com/repos/ros-realtime/linux-real-time-kernel-builder/actions/artifacts/91829081/zip --output rpi4_rt_kernel.zip
$ unzip rpi4_rt_kernel.zipRaspberry Pi 4 RT Linux kernel build
Ubuntu 20.04 x86_64 based Dockerfile is developed to cross-compile a new kernel.
Build environment
Docker container comes with cross-compilation tools installed, and a ready-to-build RT Linux kernel:
- ARMv8 cross-compilation tools
- Linux source build dependencies
- Linux source
buildinfo, from where kernel config is copied - Ubuntu
raspiLinux source installed under~/linux_build - RT kernel patch downloaded and applied - the nearest to the recent
raspiUbuntu kernel
It finds the latest raspi linux-image and the closest to it RT patch. If the build arguments specified it will build a corresponding kernel version instead.
Build and run docker container
For the local build:
git clone https://github.com/ros-realtime/linux-real-time-kernel-builder
cd linux-real-time-kernel-builderdocker build [--build-arg UNAME_R=<raspi release>] [--build-arg RT_PATCH=<RT patch>] -t rtwg-image .where:
<raspi release>is in a form of5.4.0-1034-raspi, see Ubuntu raspi Linux kernels<RT patch>is in a form of5.4.106-rt54, see RT patches
docker run -t -i rtwg-image bashKernel configuration
There is a separate kernel configuration fragment.config-fragment introduced to apply ROS2 real-time specific kernel settings. Below is an example:
$ cat .config-fragment
CONFIG_PREEMPT_RT=y
CONFIG_NO_HZ_FULL=y
CONFIG_HZ_1000=y
# CONFIG_AUFS_FS is not setIf you need to reconfigure it, run
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- menuconfigAlternatively, you can modify .config-fragment and then merge your changes in the .config by running
cd $HOME/linux_build/linux-raspi-*
ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- ./scripts/kconfig/merge_config.sh .config $HOME/linux_build/.config-fragmentKernel build
cd $HOME/linux_build/linux-raspi-*
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -j `nproc` deb-pkgYou need 16GB free disk space to build it, it takes a while, and the results are located:
raspi ~/linux_build/linux-raspi-5.4.0 $ ls -la ../*.deb
-rw-r--r-- 1 user user 11430676 May 17 14:40 ../linux-headers-5.4.101-rt53_5.4.101-rt53-1_arm64.deb
-rw-r--r-- 1 user user 487338132 May 17 14:40 ../linux-image-5.4.101-rt53-dbg_5.4.101-rt53-1_arm64.deb
-rw-r--r-- 1 user user 39355940 May 17 14:40 ../linux-image-5.4.101-rt53_5.4.101-rt53-1_arm64.deb
-rw-r--r-- 1 user user 1055272 May 17 14:40 ../linux-libc-dev_5.4.101-rt53-1_arm64.debDeploy new kernel on Raspberry Pi4
Download and install Ubuntu 20.04 server image
Follow these links to download and install Ubuntu 20.04 on your Raspberry Pi4
- Install Ubuntu on a Raspberry Pi
- Download Ubuntu Raspberry Pi server image
- Create an Ubuntu image for a Raspberry Pi on Ubuntu
# initial username and password
ubuntu/ubuntuUpdate your system
After that you need to connect to the Internet and update your system
$ sudo apt-get update && apt-get upgradeInstall Ubuntu Desktop (optional)
Optionally you can install a desktop version
$ sudo apt-get update && apt-get upgrade && apt-get install ubuntu-desktopCopy a new kernel to your system and install it
Assumed you have already copied all *.deb kernel packages to your $HOME directory
cd $HOME
sudo dpkg -i *.debAdjust vmlinuz and initrd.img links
There is an extra step in compare to the x86_64 install because update-initramfs ignores new kernel
sudo ln -s -f /boot/vmlinuz-5.4.101-rt53 /boot/vmlinuz
sudo ln -s -f /boot/vmlinuz-5.4.0-1034-raspi /boot/vmlinuz.old
sudo ln -s -f /boot/initrd.img-5.4.101-rt53 /boot/initrd.img
sudo ln -s -f /boot/initrd.img-5.4.0-1034-raspi /boot/initrd.img.old
cd /boot
sudo cp vmlinuz firmware/vmlinuz
sudo cp vmlinuz firmware/vmlinuz.bak
sudo cp initrd.img firmware/initrd.img
sudo cp initrd.img firmware/initrd.img.bak
sudo rebootAfter reboot you should see a new RT kernel installed
ubuntu@ubuntu:~$ uname -a
Linux ubuntu 5.4.101-rt53 #1 SMP PREEMPT_RT Mon May 17 12:10:16 UTC 2021 aarch64 aarch64 aarch64 GNU/LinuxIntel UP2 board RT kernel build
To build x86_64 Linux kernel, see Building Realtime rt_preempt kernel for ROS 2
Why is LTTng included in the kernel?
LTTng stands for Linux Trace Toolkit: next generation and is an open source toolkit that enables low-level kernel tracing which can be extremely useful when calculating callback times, memory usage and many other key characteristics.
As this repository is within the ros-realtime organization it can be assumed that most users will install ROS 2 on the end system - which then they can use ros2_tracing to trace various things. Since ros2_tracing uses LTTng as its tracer, and since the lttng-modules package is not easily available for the raspberry-pi RT linux kernel we build it into the kernel here as a work around.