Added docker file that cross-compiles for Raspberry Pi Zero

docker/README.md

@@ -0,0 +1,19 @@
+
+# Cross-compiling for RPi Zero in docker
+
+- Note: targets other than RPi Zero aren't (yet) supported
+
+- Build the docker container
+   `docker build docker/ -f docker/pizero.dockerfile -t pizero:local`
+
+- Run cargo build inside the docker container (this will mount the current
+  directory inside the container).
+   ```
+   docker run --rm --mount "type=bind,source=$(pwd),target=/haxo" \
+      --mount "type=bind,source=$HOME/.cargo,target=/cargo" pizero:local \
+      cargo build --target arm-unknown-linux-gnueabihf --release --features midi
+   ```
+
+## References
+- https://capnfabs.net/posts/cross-compiling-rust-apps-linker-shenanigans-multistrap-chroot/
+- https://earthly.dev/blog/cross-compiling-raspberry-pi/

docker/fix-sysroot.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+# Fix broken symlinks in sysroot.
+#
+# Some libraries are installed with absolute paths, but they are now
+# nested within the /sysroot/ folder.
+#
+# Inspired by https://capnfabs.net/posts/cross-compiling-rust-apps-linker-shenanigans-multistrap-chroot/
+
+find /sysroot/ -xtype l | while read link; do
+	fixed=/sysroot$(readlink $link)
+	if [[ -e $fixed ]]; then
+		ln -sf $fixed $link
+	else
+		echo "failed to fix symlink $link" >&2
+		exit 1
+	fi
+done

docker/multistrap-config

@@ -0,0 +1,19 @@
+[General]
+# Target architecture
+arch=armhf
+# Sysroot location
+directory=/sysroot/
+# Unpack the packages
+unpack=true
+# Tidy up afterwards (makes the container smaller)
+cleanup=true
+# Both of these refer to the 'Raspbian' stanza, below
+bootstrap=Raspbian
+aptsources=Raspbian
+
+[Raspbian]
+# Required packages for our build
+packages=libasound2-dev libdbus-1-dev libssl-dev libc6-dev libfluidsynth-dev
+source=http://raspbian.raspberrypi.org/raspbian/
+# Distribution version name
+suite=bullseye

docker/pizero.dockerfile

@@ -0,0 +1,41 @@
+FROM --platform=linux/amd64 rust:1.75-bullseye
+
+WORKDIR /setup
+
+RUN apt-get update && apt-get install multistrap gpg --assume-yes
+
+# This is where we'll create the sysroot for compilation.
+RUN mkdir /sysroot/
+
+# Install raspberry pi keyring.
+# We install it directly into the sysroot so that multistrap can use it.
+RUN mkdir -p /sysroot/etc/apt/trusted.gpg.d
+RUN curl -sL http://archive.raspbian.org/raspbian.public.key | gpg --import - \
+  && gpg --export 9165938D90FDDD2E \
+    > /sysroot/etc/apt/trusted.gpg.d/raspbian-archive-keyring.gpg
+
+# Setup the sysroot.
+COPY multistrap-config .
+RUN multistrap -f ./multistrap-config
+# Fix broken symlinks in the sysroot.
+COPY fix-sysroot.sh .
+RUN ./fix-sysroot.sh
+
+# Download the cross compiler.
+RUN mkdir -p /opt/
+RUN wget -q -O- https://github.com/tttapa/docker-arm-cross-toolchain/releases/latest/download/x-tools-armv6-rpi-linux-gnueabihf.tar.xz \
+  | tar xJ -C /opt
+
+# Add the rust target.
+RUN rustup target add arm-unknown-linux-gnueabihf
+
+# Setup compilation environment variables.
+ENV PKG_CONFIG_LIBDIR_arm_unknown_linux_gnueabihf=/sysroot/usr/lib/arm-linux-gnueabihf/pkgconfig
+ENV PKG_CONFIG_SYSROOT_DIR_arm_unknown_linux_gnueabihf=/sysroot/
+ENV CARGO_HOME=/cargo
+ENV RUSTFLAGS="-C link-arg=--sysroot=/sysroot/ -C linker=/opt/x-tools/armv6-rpi-linux-gnueabihf/bin/armv6-rpi-linux-gnueabihf-gcc"
+
+# Change workdir (this is where the haxo source should get mounted).
+WORKDIR /haxo
+# Prevent git complaining about "detected dubious ownership in repository"
+RUN git config --global --add safe.directory /haxo