1 files changed, 138 insertions, 0 deletions
diff --git a/third_party/rust/libdbus-sys/cross_compile.md b/third_party/rust/libdbus-sys/cross_compile.md
new file mode 100644
index 0000000000..439435f126
--- /dev/null
+++ b/third_party/rust/libdbus-sys/cross_compile.md
@@ -0,0 +1,138 @@
+Cross compiling dbus
+====================
+
+There are (at least) three different approaches to cross compiling dbus. Choose which one fits you best:
+
+ * Using rust-embedded/cross, see below
+ * There is an alternative guide at [issue 184](https://github.com/diwic/dbus-rs/issues/184#issuecomment-520228758), which also contains a [powershell script](https://github.com/diwic/dbus-rs/issues/184#issuecomment-520791888) that set things up for you.
+ * Setting things up manually, see below
+
+The examples below all assume you're trying to compile for Raspberry Pi 2 or 3 running Raspbian. Adjust target triples accordingly if your target is something else.
+
+
+Cross compiling using rust-embedded/cross
+=========================================
+
+Thanks to [jobale](https://github.com/jobale) for providing these instructions
+(taken from [issue 292](https://github.com/diwic/dbus-rs/issues/292)).
+
+Tested on Ubuntu 20.04 | rustc 1.47.0
+
+- Install [rust-embedded/cross](https://github.com/rust-embedded/cross)
+- In your project directory, create a **Cross.toml** file: `touch Cross.toml`
+Add this code in it:
+
+```
+[target.armv7-unknown-linux-gnueabihf]
+image = "rustcross:dbus-armhf"
+
+[build.env]
+passthrough = [
+	"RUSTFLAGS",
+]
+```
+
+
+- In your project directory create a **Dockerfile**: `touch Dockerfile`
+Put this code in it:
+
+```
+# Base image for rapsberrypi 3 target
+FROM rustembedded/cross:armv7-unknown-linux-gnueabihf
+
+# Install libdbus libraries and pkg-config
+RUN dpkg --add-architecture armhf && \
+	    apt-get update && \
+	    apt-get install --assume-yes libdbus-1-dev libdbus-1-dev:armhf pkg-config
+```
+
+For whatever reason, _in the docker image_, armhf libraries are installed in at least 2 locations. That's the reason of all my troubles:
+- /usr/arm-linux-gnueabihf/lib/
+- /usr/lib/arm-linux-gnueabihf/
+
+- Cross needs to know those locations. We pass them to the compiler through a command flag. For convenience I put it in a bash script:
+
+```
+#!/bin/bash
+set -o errexit
+set -o nounset
+set -o pipefail
+set -o xtrace
+
+readonly TARGET_ARCH=armv7-unknown-linux-gnueabihf
+readonly LINK_FLAGS='-L /usr/arm-linux-gnueabihf/lib/ -L /usr/lib/arm-linux-gnueabihf/'
+
+RUSTFLAGS=${LINK_FLAGS} cross build --release --target=${TARGET_ARCH}
+
+```
+
+
+Some more explanations for newcomers like me:
+
+- Cross command act as cargo command (e.g: `cross build` is the same as `cargo build` but for cross-compiling).
+- In Cross.toml,  `passthrough = [ "RUSTFLAGS",]` is what enable us to pass flags/parameters to the compiler in the docker image.
+
+
+
+Setting up cross compiling manually
+===================================
+
+A cross linker
+--------------
+
+Apparently, `rustc` in itself can generate code for many archs but not assemble the generated code into the final executable. Hence you need a cross linker.
+
+**Install it** - here follow whatever guide you have for the target arch. Distributions may also ship with cross toolchains. Example for Ubuntu 18.04:
+
+`sudo apt install gcc-8-multilib-arm-linux-gnueabihf`
+
+**Tell rustc where to find it** - in [.cargo/config](https://doc.rust-lang.org/cargo/reference/config.html) add the following:
+
+```
+[target.armv7-unknown-linux-gnueabihf]
+linker = "arm-linux-gnueabihf-gcc-8"
+```
+
+Target rust std
+---------------
+
+This one's easy, just run rustup:
+
+`rustup target add armv7-unknown-linux-gnueabihf`
+
+
+Target dbus libraries
+---------------------
+
+**Installing the libraries**
+
+Now to the more challenging part. Since we link to a C library `libdbus-1.so`, we also need the target version of that library. However, `libdbus-1.so` in itself links to a systemd library (at least it does so here) which in turn links to other libraries etc, so we need target versions of those libraries too.
+
+Getting an entire rootfs/image is probably the easiest option. The rootfs needs to have `libdbus-1-dev` installed. I e:
+
+ * Boot your target (i e, a raspberry), install `libdbus-1-dev` on it, turn it off and put the SD card into your computer's SD card reader. Mount the partition.
+ * If the above is not an option, you could download an image, mount it at the right offset, like this (ugly hack!) `sudo mount -o loop,offset=50331648 2019-04-08-raspbian-stretch-lite.img /tmp/mnt` and then, to manually make the symlink that `libdbus-1-dev` does for you: `cd /tmp/mnt/usr/lib/arm-linux-gnueabihf && ln -s ../../../lib/arm-linux-gnueabihf/libdbus-1.so.3 libdbus-1.so`.
+ * Or you can use the alternative guide's approach to download, extract and post-process the relevant `.deb` files manually. This might be a preferrable option if an entire image/rootfs would be too large.
+
+**Finding the libraries**
+
+When not cross compiling, finding the right library is done by a `build.rs` script which calls `pkg-config`. This will not work when cross compiling because it will point to the `libdbus-1.so` on the host, not the `libdbus-1.so` of the target.
+Maybe it is possible to teach `pkg-config` how to return the target library instead, but I have not tried this approach. Instead we can override build script altogether and provide the same info manually. This is possible because `libdbus-sys` has a `links = dbus` line.
+
+For the example below we assume that we have mounted a Raspbian rootfs on `/tmp/mnt`, and that the cross linker came with some basic libraries (libc, libpthread etc) that are installed on `/usr/arm-linux-gnueabihf/lib`. Unfortunately, we can't use the basic libraries that are present on the image, because they might contain absolute paths.
+
+And so we add the following to [.cargo/config](https://doc.rust-lang.org/cargo/reference/config.html):
+
+```
+[target.armv7-unknown-linux-gnueabihf.dbus]
+rustc-link-search = ["/usr/arm-linux-gnueabihf/lib", "/tmp/mnt/usr/lib/arm-linux-gnueabihf"]
+rustc-link-lib = ["dbus-1"]
+```
+
+
+Finally
+-------
+
+If we are all set up, you should be able to successfully compile with:
+
+`cargo build --target=armv7-unknown-linux-gnueabihf`