In “Building mjmech dependencies with bazel”, I described my rationale as it were for attempting to build all of the mjmech dependencies within bazel for cross compilation onto the raspberry pi.  mjmech has two big dependencies which were going to cause most of the transitive fallout:

• gstreamer - We use gstreamer to interface with the webcam, format RTSP streams for FPV on the control station, and to render the control station and heads up display.  Granted, not all of gstreamer is used, but we do depend on features that require ffmpeg and X11.
• opencv - The use of opencv had been minimal to non-existant previously, as we hadn’t actually done any computer vision on the robot itself.  However, one of the big motivations for switching to the raspberry pi in the first place was to at least to be able to do active target tracking onboard.

And then there are a few other direct dependencies that are “easy”, if nothing else because they have such few transitive dependencies.

Previously, I set up bazel to be able to cross compile for the raspberry pi using an extracted sysroot.  That sysroot was very minimal, basically just glibc and the kernel headers.  The software used for SMMB has many dependencies beyond that though, including some heavyweight ones such as gstreamer and I needed some solution for building against them.

Options

There were two basic options:

1. Install all the dependencies I cared about on an actual raspberry pi, and extract them into the sysroot.
2. Build all the dependencies I cared about using bazel’s external projects mechanism.

The former would certainly be quicker in the short term, at the expense of needing to check in or otherwise version a very large sysroot.  It would also be annoying to update, as I would need to keep around a physical raspberry pi and continually reset it to zero in order to generate suitably pristine sysroots.

In the previous post, I described the motivation for switching the mjmech build system to bazel.  For that to be useful with Super Mega Microbot, I first had to get a toolchain configured for bazel such that it could produce binaries that would run on the raspberry pi.

All the work in this post is publicly available on github: https://github.com/mjbots/rpi_bazel

Compiler and sysroot

First, I needed to pick the compiler I would be using and how to get the target system libraries available for cross compilation.  In the past I’ve always done the gcc/binutils/gnu everything cross toolchain dance, however here I thought I would try something a bit more reproducible and see if I could make clang work.  Amazingly, a single clang binary supports all possible target types!  clang-6, which can be had through a PPA on Ubuntu 16.04 and natively on Ubuntu 18.04 supports it out of the box.

The first piece I tackled when switching to the Raspberry Pi 3B+ for Super Mega Microbot was building our existing control software.  The software we used for the 2016 Robogames is largely C++ and was built with SCons, https://github.com/mjbots/mjmech/.  For all our previous platforms for both Savage Solder and SMMB, we had just built the software on the device itself, which while a little slow, was certainly convenient and required very little sophistication from the build system.  Raspian is debian based, so this shouldn’t be hard, right?