Knoppix on a Raspberry Pi is an ongoing idea I started several years ago. The Raspberry Pi is a relatively cheap, yet widely available single-board computer, which is also used in embedded and mobile projects. Although many distributions are available for the Pi, creating a new one with some Knoppix technology could be a perfect base for projects.

Because of the Knoppix default hardware setting to boot from the internal SD card reader, the boot procedure already has many similarities with existing distributions. However, Raspbian (the Debian derivate) for Raspberry Pi increased complexity and became a "too many parts to tune" problem. Also, I'm not happy with Debian's move from SysVinit to systemd in Raspbian, which has caused some stuck-at-boot problems in Raspbian-based projects for me.

Therefore, I decided to give the "build-from-scratch" approach a try (Table 1) instead of basing Knoppix on Debian, as in previous versions.

Yocto and OpenEmbedded

Yocto is a framework for creating custom Linux distributions for specialized embedded hardware, as well as for standard boards. Yocto is based on OpenEmbedded, a build framework for embedded Linux [1].

Yocto [2] provides a sample distribution called "Poky," which is filled with templates for the so-called meta-layers. These are directories containing build recipes for software components, which can be thought of as stacked on top of each other, so you have a base layer for the hardware-dependent parts and features, and incremental layers for graphical environments and user software.

Optimally, all changes toward the standard meta-directories that come with the Yocto distribution would be placed into one or more self-created directories with your own recipes, add-ons, and override rules.

Striving to "guarantee 100% reproducible builds," Yocto first downloads, compiles, and installs a completely new development environment from original sources for the host and target architectures, so you have the same version of compilers and libraries on each build system, with no errors attributable to different versions or features. Still, to run Yocto scripts, the host distribution needs to meet some requirements:

• Git 1.8.3.1 or greater
• tar 1.24 or greater
• Python 3.4.0 or greater.

For Debian as the hosting distribution, the following installation and update command is provided in the documentation [3]:

sudo apt-get install gawk wget git-core diffstat unzip texinfo gcc-multilib build-essential chrpath socat cpio python python3 python3-pip python3-pexpect xz-utils debianutils iputils-ping

After these packages are installed, Yocto is supposed to be able to build all software packages and images included in the Poky sample distribution. The integration hierarchy of Yocto is shown in Figure 1.

Figure 1: Yocto integration hierarchy.

Layer Cake

The Yocto framework itself is rather small after installation; uncompressed, it is less than 100MB. During the build process, sources are downloaded via git or http, unpacked, and compiled, which fills up the build/tmp directory with a tremendous amount of data (Table 2).

When first using Yocto, I found it somewhat unusual that all the seemingly important build stuff, including compilers and supporting binaries as well as the final generated SD card image, resided in a directory named tmp. Even more remarkable is that when removing the tmp directory and its contents completely, rebuilding the project takes less than a tenth of the original time, because most files will be restored quickly from the sstate-cache.

Setting Up Yocto

Before starting the build process, an initial setup needs to be done. I'm using 32-bit userspace binaries on a 64-bit kernel, which is a special setup that Yocto does not yet handle well. For this reason, I start a new shell that changes the output of uname -m from x86_64 to i686, which ensures Yocto will download and compile all supporting tools for the correct host architecture:

setarch i386 /bin/bash

Afterward, paths and the environment need to be configured in the poky directory for the current shell:

cd yocto/poky
source oe-init-build-env

These commands have to be repeated whenever I restart the build procedure from a new shell.

The resulting architecture and software choices, as well as the preferred configurations, can be changed by modifying some variables in build/conf/local.conf (Listing 1).