# Building Julia on ARM

Julia has been compiled on several ARMv7 / Cortex A15 Samsung
Chromebooks running Ubuntu Linux under Crouton, Raspberry Pi systems
and Odroid boards. This is a work in progress - several tests are
known to fail, and backtraces are not available.

Most of the build failures in building the Julia system image are due
to LLVM not being able to detect the correct ARM processor type and
features. Experimenting with different `JULIA_CPU_ARCH` settings can
help in such cases. For example, this is needed on Raspberry Pi, as
discussed below.

This is the list of known issues on ARM:
 [https://github.com/JuliaLang/julia/labels/arm](https://github.com/JuliaLang/julia/labels/arm)

# Build dependencies

We recommend using at least Ubuntu 14.04 and gcc 4.8, which is part of the
standard `build-essentials`.

Julia on ARM can be built by simply typing `make`, which will download all
the relevant libraries. This is the *recommended* way, and it will take a
few hours.

OpenBLAS detects the target architecture reasonably well, but in case
it does not, you can force the target architecture should you need to in
`Make.user`.

````
override OPENBLAS_TARGET_ARCH=ARMV7
````

Similarly, one can install other system libraries instead of building them,
should the build be troublesome, by adding the following lines in `Make.user`:

````
override USE_SYSTEM_BLAS=1
override USE_SYSTEM_LAPACK=1
override USE_SYSTEM_LIBM=1
override USE_SYSTEM_FFTW=1
override USE_SYSTEM_GMP=1
override USE_SYSTEM_MPFR=1
override USE_SYSTEM_ARPACK=1
````

The following command will install all the necessary libraries on Ubuntu.

````
sudo apt-get install libblas3gf liblapack3gf libfftw3-dev libgmp3-dev libmpfr-dev libblas-dev liblapack-dev cmake gcc-4.8 g++-4.8 gfortran libgfortran3 m4 libedit-dev
````

# ARM specific build problems

If you run into issues building LLVM, see these notes:
[http://llvm.org/docs/HowToBuildOnARM.html](http://llvm.org/docs/HowToBuildOnARM.html)

# Raspberry Pi

The Raspberry Pi ARM CPU is not correctly detected by LLVM. Before
starting the build, `export JULIA_CPU_ARCH=arm1176jzf-s`. This tells
LLVM that the CPU has VFP support. See the discussion in
[#10917](https://github.com/JuliaLang/julia/issues/10917).

# Raspberry Pi 2

In the case of Raspberry Pi 2, download LLVM binaries from the LLVM website, since building LLVM on our own for some reason does not produce a working build.

1.  Download the [LLVM 3.6.1 binaries for ARMv7a] (http://llvm.org/releases/3.6.1/clang+llvm-3.6.1-armv7a-linux-gnueabihf.tar.xz) and extract them in a local directory.
2.  For each file in the extracted `bin`, `include`, and `lib` subdirectories, create symlinks from the corresponding directory under `/usr/local`.
3. Add the following to `Make.user`:
```
override USE_SYSTEM_LLVM=1
```


# Chromebook

On Chromebooks, you have to first install Crouton.  If you do not have
an Ubuntu chroot running on your Chromebook using Crouton, you can do
so by following these tutorials.

- [Crouton Tutorial 1](http://www.howtogeek.com/162120/how-to-install-ubuntu-linux-on-your-chromebook-with-crouton/)
- [Crouton Tutorial 2](http://lifehacker.com/how-to-install-linux-on-a-chromebook-and-unlock-its-ful-509039343)

These tutorials will end up installing Ubuntu 12.04, and you have to
upgrade to Ubuntu 14.04, or install Ubuntu 14.04 from scratch by
finding appropriate `crouton` help.