Custom Image Builder for the Raspberry Pi 4 for ROS2 + PREEMPT_RT

This is a custom image builder for the Raspberry Pi 4. Some features:

• Customize the official Ubuntu server image for the Raspberry Pi by mounting it locally (via loop device) and chrooting into it (via systemd-nspawn and qemu-user-static).
  • I can't locate how Canonical generate the official Ubuntu images for the Raspberry Pi, so I had to resort to this method.
  • The default customization in this repo is made for ROS2 with PREEMPT_RT applied.
  • More information about how this works is below.
• With two stages of setup scripts, executing in lock step both inside and outside the chroot, we can cross compile code (via something like CMake toolchain) on the host and copy it into the chroot for making the final image.
• One thing we all hate while building images is to waste a lot of time. These scripts are designed to hopefully not waste your time. It has several features for this:
  • The build process is divided into resumable steps. This means there is no need (in most cases) to restart the build from scratch if you make a mistake. You can experiment with the image as it builds either by deliberately pausing the build process after a certain step or be forced to pause because there are some typos in the build scripts.
  • Nice logs that aids with debugging of the build, should things go wrong.
  • Use tools like pv to display progress when applicable.
• The features of the actual RT image is difficult to document without becoming out of date quickly. Please take a look at focal-rt-ros2/ros2/rootfs/setup/phase1.sh for the setup script that runs against the Ubuntu image and focal-rt-ros2/rootfs for files that gets overlaid on top of the Ubuntu image. That said, some basics are:
  • Installed PREEMPT_RT kernel.
  • Pinned CPU frequency and performance governor.
  • Removed some unnecessary services like snapd and fwupd to save resources.
  • Installed ROS2 galactic from apt.
  • Installed some misc. tooling (like vcgencmd).

Todos

• Setup /etc/security/limits.conf (or maybe limits.conf.d)
• Optionally configure isolcpus and nohz_full for the kernel.
• Fix the issue with LINUX_RT_VERSION and LINUX_RT_VERSION_ACTUALLY (see vars.sh).
• Use a sha256 checksum to ensure downloaded image and kernel are "secure".
• Add overclocking support

How to use

System requirements

Why not docker? Unfortunately, the current setup doesn't work in Docker, as I used systemd-nspawn to make setting up and executing commands in a chroot easier (mainly so I can save some time figuring out the various bind mounts I need, to shutdown the container correctly if a command fails, and to force quit a container if something goes really wrong by pressing ^] 3 times). This tool also rely on loop devices, which are not namespaced and thus not readily usable in Docker without privileged access. It may be possible to use Docker later by changing this code, but for now it's not possible (the code will also likely be uglier as nspawn can't be easily used in docker?).

Thus, you'll need a Linux machine with root and the following tools installed: cut, grep, parted, pv, rsync, truncate, wget, systemd-nspawn, and qemu-aarch64-static.

To build the focal-rt-ros2 image, you'll also need: zip.

For Ubuntu, you can simply run:

$ sudo apt install parted pv rsync wget systemd-container qemu-user-static make zip

To run

$ make focal-rt-ros2

This will build the image to build/ubuntu-20.04.3-rt-ros2-galactic-arm64+raspi.img. You can then dd this to a SD card.

You can see a demo of this in CI. CI builds quite slowly. On my computer this whole process only takes a few minutes.

Debugging build failures if you changed stuff

This is covered in How it works.

How it works

Note: the best way to understand the details is to start reading from builder/main.sh and tracing every function call. Most functions are defined in the order they are called in builder/core.sh and thus the code should read somewhat linearly.

The builder defined in builder/ is relatively generic and can theoretically be used for a number of situations where a vendor provides you with a .img file and you need to customize it. It follows these major steps:

1. Reads a custom vars.sh to populate all the variables needed to build.
2. Download the (ubuntu) image from the vendor.
3. Extract the image via a custom bash function defined in vars.sh to handle different compression algorithms.
4. Increase the size of the image and resize the file system, as the vendor image may be too small to install a large number of images.
5. Setup a loop device.
6. Mount the file systems in the loop device so they're accessible by the host.
7. Copy things like resolv.conf and qemu-user-static into the mounted FS.
8. Copy the rootfs overlay defined via ROOTFS_OVERLAY into the mounted FS.

• If you have setup scripts running inside the chroot, or other files supporting the setup, put it here and copy it in.

9. Run the host-side phase1 setup script (setup_script_phase1_outside_chroot).

• Variables exported by main.sh and vars.sh are usable in this scripts, as well as the other user-defined scripts below.
• For the RT setup, the RT kernel is downloaded from Github in this step.

10. Run the chroot-side phase1 setup script (setup_script_phase1_inside_chroot). This runs the script inside the mounted FS via a chroot in the target architecture via qemu-user-static.

• This is where you install things like the ROS2 packages.

11. Run the host-side phase2 setup script (setup_script_phase2_outside_chroot).

• If you want to cross compile, this is your chance, as the rootfs is readily available and mounted. In CMake, you can set your CMAKE_FIND_ROOT_PATH and CMAKE_SYSROOT to the path to the mounted FS.

12. Run the chroot-side phase2 setup script (setup_script_phase2_inside_chroot). This also runs the script inside the mounted FS.

• Usually this is when you remove any setup files you copied in the chroot and perform some cleanup. resolv.conf and other files setup by the builder will be cleaned by the builder, tho

13. Cleanup the chroot by removing resolv.conf and qemu-user-static.
14. Unmount everything and get rid of the loop back device.

To customize this process, look at the vars.sh file(s) in this repo. There should be extensive comments there.

In the future, maybe it's better to figure out how Canonical generate their official Ubuntu images. However, I can't find how they built their images when I looked. It is likely that a number of steps here will be needed anyway, so the existing structure is not that bad of an idea.

How interrupt and resume works

Each step defined in the code is resumable (see main.sh). If they fail, the builder won't clean everything up to give the developer a chance to debug and possibly fix things manually before continuing. The workflow I take is as follows:

1. Edit the build scripts. Run them.
2. Encounter an error.
3. Manually go into the chroot via systemd-nspawn.
4. Figure out the right commands to run and change the script.
5. Change cache/session.txt to make sure I can resume from the right spot (by removing and adding steps into the file, see below).
6. Run the builder again (just make, or builder/main.sh ...).

While this is not perfect, it still saves a lot of time, as you don't always have to restart from the beginning when encountering an error. The way this works is via two files saved in cache:

• cache/session.txt: contains a list of steps executed, one per line. You can freely change this file if you know what you're doing to selectively execute/skip steps when working with this system. See builder/main.sh for the steps (run_step <step_name>).
  • If this file is removed, then the builder will restart from scratch.
• cache/session-loop-device.txt: This saves the loop device the chroot is mounted to, since the loop devices may be different each time we run this.

There's also a PAUSE_AFTER variable that can be set in vars.sh to instruct the builder to stop after a particular step. This allow you to do some interactive experimentation, which also speeds things up.

To get into the chroot to experiment, run the command:

$ sudo systemd-nspawn -D /tmp/rpi4-image-build/ bash # the path is whatever CHROOT_PATH is

Interrupt and resume if you change the setup scripts for inside the chroot

Sometimes you will change the scripts running inside the chroot and then resume. You'll find this doesn't work, because the script you're changing is not copied into the chroot. To get around this problem and resume, simply delete the step copy_files_to_chroot in cache/session.txt and rerun the builder. The builder will then copy all the files into the chroot again and continue from where it failed.

How to reset your host system if something horribly goes wrong

• Try running ./scripts/cleanup.sh
• Try running the commands in umount_everything manually (see builder/core.sh).
  • Can do this by adding every step in builder/main.sh into cache/session.txt except the umount everything step.
  • TODO: I should create simpler command to run this step only.
• If that doesn't work, try restarting your computer :(.