Poky is the reference Yocto Project distribution. It contains some of basic components (called the build system) of OpenEmbedded and a set of metadata for creating embedded distributions for a number of targets. It is platform independent and performs cross-compiling using the BitBake tool (a task scheduler), OpenEmbedded-Core, and a default set of metadata, as shown in the following figure. It provides the mechanism to build and combine thousands of distributed open source projects.

The Poky build system is poised to become the reference in the industrial world as evinces by industry leaders such as Wind River, Intel, Montavista, and Mentor Graphics.

BitBake, the build engine, is a task scheduler (like GNU Make) which parses several scripts (shell and Python, for example).

Once the environment is built, BitBake will execute the task that has been requested. If no task is provided, BitBake will run the default task, called build.

To run a task, BitBake will first look for an environment variable called do_ <task name>, which will contain the task code to execute (in Python or a shell). So, to compile a Yocto recipe, use the code contained in the do_compile variable.

In short, from the information contained in the recipes (or metadata), it downloads the sources of projects from the Internet, a local directory, or a version-control system (such as Git), and then builds in the order determined by the dependency graph generated dynamically. Finally, it installs binaries, generates the corresponding package, and builds the final image, which can be installed on the target (Raspberry Pi for us).

The following picture shows how BitBake works:

The OpenEmbedded-Core metadata collection (meta in the following diagram) provides the engine of the Poky build tool. It is designed to provide the core features (several recipes). It provides support for six different processor architectures (ARM, x86, x86-64, PowerPC, MIPS, and MIPS64), supporting only QEMU-emulated machines.

The organization of OpenEmbedded-Core is depicted here:

This layer includes different recipes, which describe how to fetch, configure, compile and package applications and images.

Metadata, which is composed of a mix of Python and shell script text files (.conf, .bb, .bbclass, and .inc), provides a tremendously flexible system. Metadata refers to the build instructions themselves as well as the data used to control what things get built and to affect how they are built. The metadata also includes commands and data used to indicate which versions of software are used and where they are obtained from. Poky uses this to extend OpenEmbedded-Core and includes two different layers, which are another metadata subset. Here are their details:

We will discover metadata in depth through Chapter 4, Understanding the BitBake tool.