# Introduction

## Overview

Regardless of how you intend to make use of the Yocto Project, chances are you will work with the Linux kernel. This manual provides background information on the Yocto Linux kernel Metadata, describes common tasks you can perform using the kernel tools, and shows you how to use the kernel Metadata needed to work with the kernel inside the Yocto Project.

Each Yocto Project release has a set of linux-yocto recipes, whose Git repositories you can view in the Yocto Source Repositories under the "Yocto Linux Kernel" heading. New recipes for the release track the latest upstream developments and introduce newly supported platforms. Previous recipes in the release are refreshed and supported for at least one additional release. As they align, these previous releases are updated to include the latest from the Long Term Support Initiative (LTSI) project. Also included is a linux-yocto development recipe (`linux-yocto-dev.bb`) should you want to work with the very latest in upstream Linux kernel development and kernel Metadata development.

The Yocto Project also provides a powerful set of kernel tools for managing Linux kernel sources and configuration data. You can use these tools to make a single configuration change, apply multiple patches, or work with your own kernel sources.

In particular, the kernel tools allow you to generate configuration fragments that specify only what you must, and nothing more. Configuration fragments only need to contain the highest level visible `CONFIG` options as presented by the Linux kernel `menuconfig` system. Contrast this against a complete Linux kernel `.config`, which includes all the automatically selected `CONFIG` options. This efficiency reduces your maintenance effort and allows you to further separate your configuration in ways that make sense for your project. A common split separates policy and hardware. For example, all your kernels might support the `proc` and `sys` filesystems, but only specific boards require sound, USB, or specific drivers. Specifying these configurations individually allows you to aggregate them together as needed, but maintains them in only one place. Similar logic applies to separating source changes.

If you do not maintain your own kernel sources and need to make only minimal changes to the sources, the released recipes provide a vetted base upon which to layer your changes. Doing so allows you to benefit from the continual kernel integration and testing performed during development of the Yocto Project.

If, instead, you have a very specific Linux kernel source tree and are unable to align with one of the official linux-yocto recipes, an alternative exists by which you can use the Yocto Project Linux kernel tools with your own kernel sources.

## Other Resources

The sections that follow provide instructions for completing specific Linux kernel development tasks. These instructions assume you are comfortable working with BitBake recipes and basic open-source development tools. Understanding these concepts will facilitate the process of working with the kernel recipes. If you find you need some additional background, please be sure to review and understand the following documentation:

- Yocto Project Quick Start

- The "Modifying Temporary Source Code" section in the Yocto Project Development Manual

- The "Understanding and Creating Layers" section in the Yocto Project Development Manual

- The "Modifying the Kernel" section in the Yocto Project Development Manual.

Finally, while this document focuses on the manual creation of recipes, patches, and configuration files, the Yocto Project Board Support Package (BSP) tools are available to automate this process with existing content and work well to create the initial framework and boilerplate code. For details on these tools, see the "Using the Yocto Project's BSP Tools" section in the Yocto Project Board Support Package (BSP) Developer's Guide.