Regardless of how you intend to make use of the Yocto Project, chances are you will work with the Linux kernel. This manual describes how to set up your build host to support kernel development, introduces the kernel development process, provides background information on the Yocto Linux kernel Metadata, describes common tasks you can perform using the kernel tools, shows you how to use the kernel Metadata needed to work with the kernel inside the Yocto Project, and provides insight into how the Yocto Project team develops and maintains Yocto Linux kernel Git repositories and Metadata.
Each Yocto Project release has a set of Yocto Linux kernel 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 Linux kernel upstream developments from http://www.kernel.org and introduce newly-supported platforms. Previous recipes in the release are refreshed and supported for at least one additional Yocto Project release. As they align, these previous releases are updated to include the latest from the Long Term Support Initiative (LTSI) project. You can learn more about Yocto Linux kernels and LTSI in the "Yocto Project Kernel Development and Maintenance" section.
Also included is a Yocto Linux kernel development recipe
(linux-yocto-dev.bb
) should you want to work
with the very latest in upstream Yocto Linux kernel development and
kernel Metadata development.
The Yocto Project also provides a powerful set of kernel tools for managing Yocto 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
Yocto Linux kernel menuconfig
system.
Contrast this against a complete Yocto Linux kernel
.config
file, 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 Yocto Linux kernel recipes, an alternative exists by which you can use the Yocto Project Linux kernel tools with your own kernel sources.
The remainder of this manual provides 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 Build document.
devtool
workflow
as described in the Yocto Project Application Development and
the Extensible Software Development Kit (eSDK) manual.
The "Understanding and Creating Layers" section in the Yocto Project Development Tasks Manual.
The "Kernel Modification Workflow" section.