A.2. Yocto Linux Kernel Architecture and Branching Strategies

As mentioned earlier, a key goal of the Yocto Project is to present the developer with a kernel that has a clear and continuous history that is visible to the user. The architecture and mechanisms, in particular the branching strategies, used achieve that goal in a manner similar to upstream Linux kernel development in kernel.org.

You can think of a Yocto Linux kernel as consisting of a baseline Linux kernel with added features logically structured on top of the baseline. The features are tagged and organized by way of a branching strategy implemented by the Yocto Project team using the Source Code Manager (SCM) Git.

Notes

  • Git is the obvious SCM for meeting the Yocto Linux kernel organizational and structural goals described in this section. Not only is Git the SCM for Linux kernel development in kernel.org but, Git continues to grow in popularity and supports many different work flows, front-ends and management techniques.

  • You can find documentation on Git at http://git-scm.com/documentation. You can also get an introduction to Git as it applies to the Yocto Project in the "Git" section in the Yocto Project Reference Manual. The latter reference provides an overview of Git and presents a minimal set of Git commands that allows you to be functional using Git. You can use as much, or as little, of what Git has to offer to accomplish what you need for your project. You do not have to be a "Git Expert" in order to use it with the Yocto Project.

Using Git's tagging and branching features, the Yocto Project team creates kernel branches at points where functionality is no longer shared and thus, needs to be isolated. For example, board-specific incompatibilities would require different functionality and would require a branch to separate the features. Likewise, for specific kernel features, the same branching strategy is used.

This "tree-like" architecture results in a structure that has features organized to be specific for particular functionality, single kernel types, or a subset of kernel types. Thus, the user has the ability to see the added features and the commits that make up those features. In addition to being able to see added features, the user can also view the history of what made up the baseline Linux kernel.

Another consequence of this strategy results in not having to store the same feature twice internally in the tree. Rather, the kernel team stores the unique differences required to apply the feature onto the kernel type in question.

Note

The Yocto Project team strives to place features in the tree such that features can be shared by all boards and kernel types where possible. However, during development cycles or when large features are merged, the team cannot always follow this practice. In those cases, the team uses isolated branches to merge features.

BSP-specific code additions are handled in a similar manner to kernel-specific additions. Some BSPs only make sense given certain kernel types. So, for these types, the team creates branches off the end of that kernel type for all of the BSPs that are supported on that kernel type. From the perspective of the tools that create the BSP branch, the BSP is really no different than a feature. Consequently, the same branching strategy applies to BSPs as it does to kernel features. So again, rather than store the BSP twice, the team only stores the unique differences for the BSP across the supported multiple kernels.

While this strategy can result in a tree with a significant number of branches, it is important to realize that from the developer's point of view, there is a linear path that travels from the baseline kernel.org, through a select group of features and ends with their BSP-specific commits. In other words, the divisions of the kernel are transparent and are not relevant to the developer on a day-to-day basis. From the developer's perspective, this path is the "master" branch in Git terms. The developer does not need to be aware of the existence of any other branches at all. Of course, value exists in the having these branches in the tree, should a person decide to explore them. For example, a comparison between two BSPs at either the commit level or at the line-by-line code diff level is now a trivial operation.

The following illustration shows the conceptual Yocto Linux kernel.

In the illustration, the "Kernel.org Branch Point" marks the specific spot (or Linux kernel release) from which the Yocto Linux kernel is created. From this point forward in the tree, features and differences are organized and tagged.

The "Yocto Project Baseline Kernel" contains functionality that is common to every kernel type and BSP that is organized further along in the tree. Placing these common features in the tree this way means features do not have to be duplicated along individual branches of the tree structure.

From the "Yocto Project Baseline Kernel", branch points represent specific functionality for individual Board Support Packages (BSPs) as well as real-time kernels. The illustration represents this through three BSP-specific branches and a real-time kernel branch. Each branch represents some unique functionality for the BSP or for a real-time Yocto Linux kernel.

In this example structure, the "Real-time (rt) Kernel" branch has common features for all real-time Yocto Linux kernels and contains more branches for individual BSP-specific real-time kernels. The illustration shows three branches as an example. Each branch points the way to specific, unique features for a respective real-time kernel as they apply to a given BSP.

The resulting tree structure presents a clear path of markers (or branches) to the developer that, for all practical purposes, is the Yocto Linux kernel needed for any given set of requirements.

Note

Keep in mind the figure does not take into account all the supported Yocto Linux kernels, but rather shows a single generic kernel just for conceptual purposes. Also keep in mind that this structure represents the Yocto Project Source Repositories that are either pulled from during the build or established on the host development system prior to the build by either cloning a particular kernel's Git repository or by downloading and unpacking a tarball.

Working with the kernel as a structured tree follows recognized community best practices. In particular, the kernel as shipped with the product, should be considered an "upstream source" and viewed as a series of historical and documented modifications (commits). These modifications represent the development and stabilization done by the Yocto Project kernel development team.

Because commits only change at significant release points in the product life cycle, developers can work on a branch created from the last relevant commit in the shipped Yocto Project Linux kernel. As mentioned previously, the structure is transparent to the developer because the kernel tree is left in this state after cloning and building the kernel.