# Yocto Project Kernel Concepts

## Introduction

This chapter provides conceptual information about the Yocto Project kernel:

- Kernel Goals

- Yocto Project Kernel Development and Maintenance Overview

- Kernel Architecture

- Kernel Tools

## Kernel Goals

The complexity of embedded kernel design has increased dramatically. Whether it is managing multiple implementations of a particular feature or tuning and optimizing board specific features, flexibility and maintainability are key concerns. The Yocto Project Linux kernel is presented with the embedded developer's needs in mind and has evolved to assist in these key concerns. For example, prior methods such as applying hundreds of patches to an extracted tarball have been replaced with proven techniques that allow easy inspection, bisection and analysis of changes. Application of these techniques also creates a platform for performing integration and collaboration with the thousands of upstream development projects.

With all these considerations in mind, the Yocto Project kernel and development team strives to attain these goals:

- Allow the end user to leverage community best practices to seamlessly manage the development, build and debug cycles.

- Create a platform for performing integration and collaboration with the thousands of upstream development projects that exist.

- Provide mechanisms that support many different work flows, front-ends and management techniques.

- Deliver the most up-to-date kernel possible while still ensuring that the baseline kernel is the most stable official release.

- Include major technological features as part of Yocto Project's up-rev strategy.

- Present a git tree, that just like the upstream kernel.org tree, has a clear and continuous history.

- Deliver a key set of supported kernel types, where each type is tailored to a specific use case (i.g. networking, consumer, devices, and so forth).

- Employ a git branching strategy that from a customer's point of view results in a linear path from the baseline kernel.org, through a select group of features and ends with their BSP-specific commits.

## Yocto Project Kernel Development and Maintenance Overview

Yocto Project kernel, like other kernels, is based off the Linux kernel release from . At the beginning of our major development cycle, we choose our Yocto Project kernel based on factors like release timing, the anticipated release timing of "final" (i.e. non "rc") upstream kernel.org versions, and Yocto Project feature requirements. Typically this will be a kernel that is in the final stages of development by the community (i.e. still in the release candidate or "rc" phase) and not yet a final release. But by being in the final stages of external development, we know that the kernel.org final release will clearly land within the early stages of the Yocto Project development window.

This balance allows us to deliver the most up-to-date kernel as possible, while still ensuring that we have a stable official release as our baseline kernel version.

The ultimate source for the Yocto Project kernel is a released kernel from kernel.org. In addition to a foundational kernel from kernel.org the released Yocto Project kernel contains a mix of important new mainline developments, non-mainline developments (when there is no alternative), Board Support Package (BSP) developments, and custom features. These additions result in a commercially released Yocto Project kernel that caters to specific embedded designer needs for targeted hardware.

Once a Yocto Project kernel is officially released the Yocto Project team goes into their next development cycle, or "uprev" cycle while continuing maintenance on the released kernel. It is important to note that the most sustainable and stable way to include feature development upstream is through a kernel uprev process. Back-porting of hundreds of individual fixes and minor features from various kernel versions is not sustainable and can easily compromise quality. During the uprev cycle, the Yocto Project team uses an ongoing analysis of kernel development, BSP support, and release timing to select the best possible kernel.org version. The team continually monitors community kernel development to look for significant features of interest. The team does consider back-porting large features if they have a significant advantage. User or community demand can also trigger a back-port or creation of new functionality in the Yocto Project baseline kernel during the uprev cycle.

Generally speaking, every new kernel both adds features and introduces new bugs. These consequences are the basic properties of upstream kernel development and are managed by the Yocto Project team's kernel strategy. It is the Yocto Project team's policy to not back-port minor features to the released kernel. They only consider back-porting significant technological jumps - and, that is done after a complete gap analysis. The reason for this policy is that simply back-porting any small to medium sized change from an evolving kernel can easily create mismatches, incompatibilities and very subtle errors.

These policies result in both a stable and a cutting edge kernel that mixes forward ports of existing features and significant and critical new functionality. Forward porting functionality in the Yocto Project kernel can be thought of as a "micro uprev." The many “micro uprevs” produce a kernel version with a mix of important new mainline, non-mainline, BSP developments and feature integrations. This kernel gives insight into new features and allows focused amounts of testing to be done on the kernel, which prevents surprises when selecting the next major uprev. The quality of these cutting edge kernels is evolving and the kernels are used in leading edge feature and BSP development.

## Kernel Architecture

This section describes the architecture of the Yocto Project kernel and provides information on the mechanisms used to achieve that architecture.

### Overview

As mentioned earlier, a key goal of 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 used achieve that goal in a manner similar to the upstream kernel.org.

You can think of the Yocto Project kernel as consisting of a baseline 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 source code manager (SCM) git. The result is that 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 kernel as well.

The following illustration shows the conceptual Yocto Project kernel.

In the illustration, the "kernel.org Branch Point" marks the specific spot (or release) from which the Yocto Project kernel is created. From this point "up" 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 up the tree. Placing these common features in the tree this way means features don't have to be duplicated along individual branches of the structure.

From the Yocto Project Baseline Kernel branch points represent specific functionality for individual 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 a real-time kernel.

In this example structure, the real-time kernel branch has common features for all real-time 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 user that for all practical purposes is the kernel needed for any given set of requirements.

### Branching Strategy and Workflow

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 branching strategy results in a tree that has features organized to be specific for particular functionality, single kernel types, or a subset of kernel types. This strategy results in not having to store the same feature twice internally in the tree. Rather we store the unique differences required to apply the feature onto the kernel type in question.

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

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, we create 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 features. So again, rather than store the BSP twice, only the unique differences for the BSP across the supported multiple kernels are uniquely stored.

While this strategy can result in a tree with a significant number of branches, it is important to realize that from the user'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 user's perspective, this is the "master" branch. They do not need not be aware of the existence of any other branches at all. Of course there is value in the existence of 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.

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 kernel. As mentioned previously, the structure is transparent to the user because the kernel tree is left in this state after cloning and building the kernel.

### Source Code Manager - git

The Source Code Manager (SCM) is git and it is the obvious mechanism for meeting the previously mentioned goals. Not only is it the SCM for kernel.org but git continues to grow in popularity and supports many different work flows, front-ends and management techniques.

It should be noted that you can use as much, or as little, of what git has to offer as is appropriate to your project.

## Kernel Tools

Since most standard workflows involve moving forward with an existing tree by continuing to add and alter the underlying baseline, the tools that manage Yocto Project's kernel construction are largely hidden from the developer to present a simplified view of the kernel for ease of use.

The fundamental properties of the tools that manage and construct the kernel are:

- the ability to group patches into named, reusable features

- to allow top down control of included features

- the binding of kernel configuration to kernel patches/features

- the presentation of a seamless git repository that blends Yocto Project value with the kernel.org history and development