Include findings from JuliaLang#10917 in the ARM readme.

README.arm.md

@@ -1,9 +1,20 @@
 # Building Julia on ARM
 
 Julia has been compiled on several ARMv7 / Cortex A15 Samsung
-Chromebooks running Ubuntu Linux under Crouton. This is a work in
-progress - several tests are known to fail, and backtraces are not
-available.
+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
 
 In addition to the standard `build-essentials` toolchain the following
 libraries must be installed to build on ARM. On Debian/Ubuntu, use the
@@ -13,6 +24,8 @@ following command:
 sudo apt-get install libblas3gf liblapack3gf libfftw3-dev libgmp3-dev libmpfr-dev libblas-dev liblapack-dev cmake gcc-4.7 g++-4.7 gfortran libgfortran3
 ````
 
+# ARM specific Make.user
+
 Please start from the standard [build
 instructions](README.md#source-download-and-compilation), in
 particular the Linux notes.
@@ -27,53 +40,21 @@ include $(JULIAHOME)/Make.arm
 Then proceed to build as described in the primary README. Just typing
 `make -j 4` at this stage should build julia.
 
-# Installing Crouton for Chromebooks
-
-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)
-
-# Known ARM issues
-
-- This is the list of known issues on ARM: [https://github.com/JuliaLang/julia/labels/arm](https://github.com/JuliaLang/julia/labels/arm)
-
-# Building LLVM on ARM
+If you run in to issues building LLVM, see these notes:
+[http://llvm.org/docs/HowToBuildOnARM.html](http://llvm.org/docs/HowToBuildOnARM.html)
 
-- If you run in to issues building LLVM, see these notes: [http://llvm.org/docs/HowToBuildOnARM.html](http://llvm.org/docs/HowToBuildOnARM.html)
+# Raspberry Pi
 
-# For the Raspberry Pi version 2 (RPi2)
+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).
 
-Success in building Julia has been reported using the following procedure:
+# Chromebook
 
-1.  Follow the steps above to install the `build-essentals` and other packages required for ARM
-2.  Install FFTW and libedit using `sudo apt-get install libfftw3-3 libedit-dev`
-3.  Download the [LLVM 3.6.0 binaries for ARMv7a] (http://llvm.org/releases/3.6.0/clang+llvm-3.6.0-armv7a-linux-gnueabihf.tar.xz) and extract them in a local directory.
-4.  For each file in the extracted `bin`, `include`, and `lib` subdirectories, create symlinks from the corresponding directory under `/usr/local`.
-5.  Using the `Make.arm` configuration below, build Julia using `make -j 2`. (Limiting `make` to 2 simultaneous jobs will reduce the possibility of running out of memory/swap space during the build process.)
-
-Note that the resulting Julia binary may fail various tests but will run code and provide the REPL.
-
-`Make.arm` for RPi2:
-
-```
-override LLVM_ASSERTIONS=1
-LLVM_FLAGS+="--with-cpu=cortex-a9 --with-float=hard --with-abi=aapcs-vfp --with-fpu=neon --enable-targets=arm --enable-optimized --enable-assertions"
-
-override OPENBLAS_DYNAMIC_ARCH=0
-override OPENBLAS_TARGET_ARCH=ARMV7
-override USE_BLAS64=0
-
-override USE_SYSTEM_LIBM=1
-override USE_SYSTEM_FFTW=1
-override USE_SYSTEM_GMP=1
-override USE_SYSTEM_MPFR=1
-override USE_SYSTEM_LAPACK=1
-
-JCFLAGS += -fsigned-char
-
-override USE_SYSTEM_LLVM=1
-override USE_SYSTEM_BLAS=1
-```
+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)