Tools and distributions for embedded Linux development

The deployment of Linux on the desktop and in the server room is well served by the general-purpose distribution. In the embedded world things are very different: although Linux is used widely, the concept of the general-purpose distribution is much less in evidence. Many vendors rely on forked board support packages or home-grown builds to create their systems, effectively creating their own customized distribution in the process. While embedded platforms represent a challenge to the traditional Linux distribution, there is no shortage of community projects to support the development of embedded Linux systems.

What makes embedded systems different?

The primary challenge presented by embedded systems is their diversity. Embedded device hardware tends to vary both in terms of design and capability. Even between generations of the same product, it is not unusual for a major system component such as the CPU to change completely. In addition, most embedded devices contain a fair amount of custom, closed-source software which is not packaged by a distribution. The combined effect of this hardware and software diversity is that most embedded devices are unique in one way or another, meaning it is difficult to design an embedded distribution which is simultaneously general enough to be useful and targeted enough to be efficient across a range of embedded platforms.

To make matters worse, embedded Linux development often takes place behind closed doors. In some cases this is due to a lack of hardware to support wider community involvement, since development platforms for many embedded devices will only exist within the labs of the companies manufacturing them. In other cases, manufacturers of embedded Linux devices rely on keeping their designs as secret as possible in order to keep ahead of the competition. This secrecy tends to stifle the community engagement that helps fuel the development of many desktop and server distributions.

In addition to the closed nature of many embedded projects, embedded Linux distributions face a challenge of a lack of perceived utility within developer communities. Given the heavily customized nature of much of the software within a given device, developers tend to view any external software as a starting point rather than a completed product. Rather than a distribution being a central provider, it is simply another component in a larger project.

Meeting the challenge

Despite the challenges of the embedded Linux ecosystem, embedded Linux tools and distributions can offer real value to developers working within that space. Various approaches are being developed by a number of embedded Linux projects, generating a suite of tools which may be brought to bear on a particular project according to its needs.

Dispensing with tools: the DIY approach

If complete control over configuration is important to an embedded developer, then the most efficacious approach is to build the whole system manually. This would typically involve hand-crafting a cross-compilation toolchain targeting the embedded device, then downloading and building the individual packages required to make up the software stack. Although a somewhat arduous process, the DIY approach to embedded Linux offers several key benefits, including the ability to fine-tune packages to requirements, an enforced understanding of the precise contents of the embedded root file system, and the ability to pick and choose system components in order to meet project goals.

The basics of putting together a DIY embedded Linux system are illustrated in Karim Yaghmour's book Building Embedded Linux Systems. In addition, the Cross Linux From Scratch guide covers the creation a completely DIY cross-compiled Linux installation in greater detail.

Meta-distributions and configurable build architectures

The DIY approach to an embedded distribution offers the greatest control and flexibility, but it does require a large investment of time and no small amount of expertise to get a basic system up and running. To help mitigate this, a number of configurable build tools exist which simplify the process. This approach combines the flexibility of selecting, configuring, and building sources by hand with the convenience of a framework which hides much of that detail until a developer needs it. At the time of this writing, several different build architectures are in active development.

Buildroot is perhaps the simplest of the build framework projects. Buildroot provides a menu-driven configuration similar to that used by the Linux kernel, and an automated build based around makefiles. It can be used to build anything from a cross-compilation toolchain right up to a complete root filesystem for the device. It also provides a simple mechanism for developers to patch software packages as they are built. Since its initial release in 2005, Buildroot has achieved considerable popularity in a range of applications, which can include anything from chip manufacturer board support packages to embedded firmware for rugged displays.

OpenEmbedded is a more complex system which utilizes a custom build tool, Bitbake, to provide information on how to build individual packages. Bitbake is derived from Gentoo's portage system, and many of the concepts utilized in Bitbake will be familiar to Gentoo users. OpenEmbedded provides Bitbake build recipes for many packages, from cross-compilation tools to the Qt framework and X. It is used as the build tool for a number of Linux distributions, including Ångström and Openmoko, and is also used by Gumstix to create firmware for their devices.

In contrast, Scratchbox takes a different approach from both OpenEmbedded and Buildroot. Rather than providing build instructions for the cross-compilation of packages, Scratchbox provides a chrooted environment in which the default build tools produce cross-compiled code. In this environment, packages may be built as though they were being built natively, thereby avoiding much of the hassle of configuring packages for cross-compilation. In addition to providing a compilation environment, Scratchbox also integrates Qemu to provide an emulated platform for running cross-compiled code. This could be a useful feature early in the development of a new embedded device when the actual hardware is not yet available. Scratchbox has been used by the Maemo project, which is used in Nokia's Internet Tablet products—and the N900 phone.

Finally, Firmware Linux marries the simplicity of Buildroot to the native compilation approach of Scratchbox. Starting life in 2001 as a means of automating a Linux From Scratch build, Firmware Linux has developed into a general set of scripts for the creation of a native development environment for the target hardware. Similar to Scratchbox, Firmware Linux supports the use of Qemu as an emulation environment for running target binaries and compiling code.

Miniature desktop distributions

Automated build environments allow developers to build complete embedded systems from source. At the same time, a number of projects are developing distributions which approach embedded software from the opposite direction, by slimming down general-purpose distributions to fit the embedded space.

Emdebian aims to make Debian suitable for embedded devices. So far, this effort has taken two complementary paths. Firstly, Emdebian Crush provides a heavily modified set of packages based around a Busybox root filesystem to provide the smallest possible installation of Debian. At the time of this writing, Crush supports ARM platforms only. Parallel to working on Crush, the Emdebian team has been developing Emdebian Grip, a low-fat install which tracks the development of the forthcoming stable release of Debian. Emdebian Grip has been used in Toby Churchill's SL40 Lightwriter, and some of Digi's ARM-based embedded modules, as well as various silicon provider board support packages.

Similar to Emdebian, Gentoo Embedded (aka Embedded Gentoo) works to make the Gentoo distribution more suitable for embedded devices. This brings Gentoo's portage package management tool to bear on the embedded space, and also provides a handbook which guides users through the process of putting together a Gentoo Embedded system; a process which will be generally familiar to users of the Gentoo handbook for desktop and server systems. Readers may be familiar with Gentoo Embedded through its use in Google's Chromium OS project, where it has been employed as a cross-compilation tool to build AMD64 binaries on an x86 host.

Commercial offerings

Alongside freely available open source embedded Linux tools and distributions, there are a number of commercial embedded Linux distributions available. Just as with desktop Linux, a commercial agreement with a vendor may offer various incentives to reassure pointy-haired bosses: guaranteed integration support, a quicker time-to-market, enhanced development tools, etc. MontaVista Linux is one of the most prominent offerings in this space, however a number of other commercial embedded distributions exist including Wind River Linux and BlueCat Linux.

Complete software stacks

Recent initiatives have seen the development of complete, application-specific software stacks for embedded platforms. Examples include Google's Android and Nokia's Maemo project, which are geared toward mobile phones and internet tablets; or one of the many network router firmware projects (e.g. FreeWRT, Tomato, or OpenWrt). These projects can offer a complete solution for certain developments, although it is more likely that they are employed as a foundation to build upon. For example, Android is in use in the Archos 5 internet tablet and Barnes and Noble's "nook" e-book reader, and Maemo has been merged with Intel's Moblin to form MeeGo.

Many paths to enlightenment

Although the nature of embedded devices makes a general-purpose Linux distribution difficult to develop there are nonetheless a range of projects which support embedded Linux development at every level. The Cross Linux From Scratch project provides a simple guide to hand-crafting a system, the various build frameworks and slimline versions of desktop distributions provide a sliding scale of automation for the creation of a distribution, and the complete stacks provided by the likes of Android and MeeGo provide an integrated foundation for further development.

Of the range of embedded Linux tools, the projects generating the most press are the likes of Android and MeeGo. This is partially due to the large corporate sponsors behind these projects, but it may also be because they are starting to change the face of embedded Linux by commoditizing the software stack for subsets of embedded devices. In so doing, Android and MeeGo may suggest a possible future development path for embedded Linux as a whole: for many applications, an off-the-shelf stack could deliver an enticingly short time-to-market along with the added incentive of high community involvement in the wider project. Whether or not this model can support enough diversity to make it appropriate across the embedded ecosystem is not clear, but the potential benefits to manufacturers and end users may make it an attractive way to deliver Linux in a range of other embedded markets.