Build 64-bit kernel for Raspberry Pi 3, using native tools

04/10/2017 - 18:08
Average: 4 (3 votes)

Raspberry Pi 3 64-bit kernel using native tools

The SoC on the Raspberry Pi 3 supports 64-bit mode, unfortunately raspbian and the foundation does not provide 64-bit support (yet?) even if they are advertising the Pi 3 as a 64-bit platform.

There are 3 ways to get a 64-bit kernel compiled:

  1. Native build on another 64-bit ARM platform
  2. Cross-compilation on another platform
  3. Cross-compilation on the Pi 3 itself

This tutorial will explain how to do option number 3. (And will actually work just fine for option number 2 as they are pretty much the same)

As we need to build the tools we need, that is aarch64 binutils and gcc, we need to install the required build tools first. Start by first installing required dependencies for compilation of these tools:

sudo apt-get install build-essential libgmp-dev libmpfr-dev libmpc-dev bc git-core

Build the cross compilation tools

To be able to cross-compile we need to build a couple of tools ourselves as they are not provided. These are aarch64 versions of binutils (assembler, linker) and gcc (C compiler). We will install these tools in their own prefix or path,

/opt/aarch64

Build and install Binutils

Download the latest binutils, (Tested with 2.29.1 at time of writing, feel free to try any later version when available)

wget https://ftp.gnu.org/gnu/binutils/binutils-2.29.1.tar.bz2

Untar the binutils archive

tar xf binutils-2.29.1.tar.bz2

Configure build and install

cd binutils-2.29.1
./configure --prefix=/opt/aarch64 --target=aarch64-linux-gnu --disable-nls
make -j4
sudo make install

Binutils is now installed, to be able to use it add "/opt/aarch64/bin/" to your path:

export PATH=$PATH:/opt/aarch64/bin/

Build and install GCC

Next up is gcc, the C compiler. We will build a minimal compiler for C only and no userland support. This is enough to get a aarch64 kernel compiled.

Download latest stable gcc (Tested with 6.4.0 at time of writing, feel free to try any later version when available):

wget https://ftp.gnu.org/gnu/gcc/gcc-6.4.0/gcc-6.4.0.tar.xz

Untar the gcc archive

tar xf gcc-6.4.0.tar.xz

Next configure gcc and build gcc. We will configure the build for only a minimal C compiler, that is enough for building a kernel.

mkdir gcc-out
cd gcc-out
../gcc-6.4.0/configure --prefix=/opt/aarch64 --target=aarch64-linux-gnu --with-newlib --without-headers \
 --disable-shared --disable-threads --disable-libssp --disable-decimal-float \
 --disable-libquadmath --disable-libvtv --disable-libgomp --disable-libatomic \
 --enable-languages=c
make all-gcc -j4
sudo make install-gcc

Test that the cross gcc runs ok, run:

aarch64-linux-gnu-gcc -v

It should report something close to this is everything is ok:

Using built-in specs.
COLLECT_GCC=aarch64-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/opt/aarch64/libexec/gcc/aarch64-linux-gnu/6.4.0/lto-wrapper
Target: aarch64-linux-gnu
Configured with: ../gcc-6.4.0/configure --prefix=/opt/aarch64 --target=aarch64-linux-gnu --with-newlib --without-headers --disable-shared --enable-languages=c
Thread model: posix
gcc version 6.4.0 (GCC)

If all is well, then you can continue to building the kernel itself!

Build the Linux kernel

Now you have a toolchain that is able to build 64-bit ARM kernels. So the next step is to download the Raspberry Pi kernel sources, configure them and build a 64-bit kernel and modules.

Download Linux kernel sources

Get the most up to date stable branch kernel sources (4.9 branch at time of writing) directly from the Raspberry Pi GitHub repository with

git clone --depth=1 -b rpi-4.9.y https://github.com/raspberrypi/linux.git
Keeping sources fresh

The Linux kernel is a moving target and evolves almost daily. To keep updated with latest changes, run the following command in the directory you checked out above:

git pull

Configure Linux kernel for 64-bit Raspberry Pi 3

We use output directory for kernel build so we can use the same source tree for other configuration (32-bit for example)

mkdir kernel-out
cd linux
make O=../kernel-out/ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-  bcmrpi3_defconfig

The kernel is now configured with a default configuration for a 64-bit Raspberry Pi kernel.

Advanced configuration option

In case you need to customize the configuration (Add support for some specific hardware, trim out things you don't need or otherwise mess around with the configuration) run

make O=../kernel-out/ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- menuconfig

and adjust the configuration for your needs.

Build the kernel and modules

Next build the kernel and modules with:

make -j4 O=../kernel-out/ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-

If all goes well, as it should, you should now have a 64-bit kernel, modules and device tree built.

Keywords: 
64-bit
Raspberry Pi
Linux
stretch
jessie