Gentoo on Raspberry Pi 3B - Cross Compilation



In my previous post, I have installed Gentoo on my Raspberry Pi 3B. However, compiling packages from source codes (that is how Gentoo works) requires a lot of hardware resources, it seems impractial to do that on a Raspberry Pi 3B. Fortunately, it is possible to transfer the heavy compilation tasks to a powerful AMD64 system (such as a Gentoo desktop/server with more compute cores and more memory) using crossdev - a set of bash scripts that utilize emerge to provide a system integrated cross-compilation capability.

Steps for Cross Compilation

The following steps will create a cross build environment on a powerful AMD64 machine for compiling packages for the Raspberry Pi 3B.

Setup Cross-toolchain

Install the crossdev:

$ sudo emerge -av sys-devel/crossdev

Install toolchains for the Raspberry Pi 3B (32-bit mode):

$ sudo crossdev --stable -t armv7a-unknown-linux-gnueabihf

The above command will create a cross build environment under the directory "/usr/armv7a-unknown-linux-gnueabihf", which is a minimal root system:

$ ls /usr/armv7a-unknown-linux-gnueabihf/
bin  etc  lib  run  sbin  sys-include  tmp  usr  var

After that we can cross compile packages and install them into the cross build environment:

$ sudo armv7a-unknown-linux-gnueabihf-emerge -av <PACKAGE>

But before compiling any packages, we need to dive into the cross build environment and configure the portage system.

Configure portage/make.conf

Set FLAGS-related variables (just copied from the "make.conf" file in the Raspberry Pi 3B):

$ sudo nano /usr/armv7a-unknown-linux-gnuabihf/etc/portage/make.conf
        COMMON_FLAGS="-O2 -pipe -march=armv7-a -mfpu=vfpv3-d16 -mfloat-abi=hard"

Disable the introspection USE flag, because there are some existing issues for the gobject-introspection package to be cross compiled:

$ sudo nano /usr/armv7a-unknown-linux-gnuabihf/etc/portage/make.conf

Configure portage/make.profile

The default "make.profile" in the cross build environment, created by the crossdev, is linked to "gentoo/profiles/embedded". Here we set it to "gentoo/profiles/default/linux/arm/17.0/armv7a" to keep it same as the one in the Raspberry Pi 3B:

$ sudo rm /usr/armv7a-unknown-linux-gnueabihf/etc/portage/make.profile
$ sudo ln -s /var/db/repos/gentoo/profiles/default/linux/arm/17.0/armv7a /usr/armv7a-unknown-linux-gnueabihf/etc/portage/make.profile

Finally, we can safely cross compile packages for the Raspberry Pi 3B, or even update the entire system:

$ sudo armv7a-unknown-linux-gnueabihf-emerge -av --update --newuse --deep @world

Thanks for reading :)