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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >
<chapter id='kernel-dev-intro'>
<title>Introduction</title>
<section id='kernel-dev-overview'>
<title>Overview</title>
<para>
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
<ulink url='&YOCTO_DOCS_REF_URL;#metadata'>Metadata</ulink>,
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.
</para>
<para>
Each Yocto Project release has a set of Yocto Linux kernel recipes,
whose Git repositories you can view in the Yocto
<ulink url='&YOCTO_GIT_URL;'>Source Repositories</ulink> under
the "Yocto Linux Kernel" heading.
New recipes for the release track the latest Linux kernel
upstream developments from
<ulink url='http://www.kernel.org'></ulink> 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
"<link linkend='kernel-big-picture'>Yocto Project Kernel Development and Maintenance</link>"
section.
</para>
<para>
Also included is a Yocto Linux kernel development recipe
(<filename>linux-yocto-dev.bb</filename>) should you want to work
with the very latest in upstream Yocto Linux kernel development and
kernel Metadata development.
<note>
For more on Yocto Linux kernels, see the
"<link linkend='kernel-big-picture'>Yocto Project Kernel Development and Maintenance</link>
section.
</note>
</para>
<para>
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.
</para>
<para>
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 <filename>CONFIG</filename> options as presented by the
Yocto Linux kernel <filename>menuconfig</filename> system.
Contrast this against a complete Yocto Linux kernel
<filename>.config</filename> file, which includes all the automatically
selected <filename>CONFIG</filename> 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
<filename>proc</filename> and <filename>sys</filename> 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.
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
The remainder of this manual provides instructions for completing
specific Linux kernel development tasks.
These instructions assume you are comfortable working with
<ulink url='http://openembedded.org/wiki/Bitbake'>BitBake</ulink>
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:
<itemizedlist>
<listitem><para>
<ulink url='&YOCTO_DOCS_BRIEF_URL;'>Yocto Project Quick Build</ulink>
document.
</para></listitem>
<listitem><para>
<ulink url='&YOCTO_DOCS_OM_URL;'>Yocto Project Overview and Concepts Manual</ulink>.
</para></listitem>
<listitem><para>
<ulink url='&YOCTO_DOCS_SDK_URL;#using-devtool-in-your-sdk-workflow'><filename>devtool</filename> workflow</ulink>
as described in the Yocto Project Application Development and
the Extensible Software Development Kit (eSDK) manual.
</para></listitem>
<listitem><para>
The
"<ulink url='&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers'>Understanding and Creating Layers</ulink>"
section in the Yocto Project Development Tasks Manual.
</para></listitem>
<listitem><para>
The
"<link linkend='kernel-modification-workflow'>Kernel Modification Workflow</link>"
section.
</para></listitem>
</itemizedlist>
</para>
</section>
<section id='kernel-modification-workflow'>
<title>Kernel Modification Workflow</title>
<para>
Kernel modification involves changing the Yocto Project kernel,
which could involve changing configuration options as well as adding
new kernel recipes.
Configuration changes can be added in the form of configuration
fragments, while recipe modification comes through the kernel's
<filename>recipes-kernel</filename> area in a kernel layer you create.
</para>
<para>
This section presents a high-level overview of the Yocto Project
kernel modification workflow.
The illustration and accompanying list provide general information
and references for further information.
<imagedata fileref="figures/kernel-dev-flow.png"
width="9in" depth="5in" align="center" scalefit="1" />
</para>
<para>
<orderedlist>
<listitem><para>
<emphasis>Set up Your Host Development System to Support
Development Using the Yocto Project</emphasis>:
See the
"<ulink url='&YOCTO_DOCS_DEV_URL;#dev-manual-start'>Setting Up the Development Host to Use the Yocto Project</ulink>"
section in the Yocto Project Development Tasks Manual for
options on how to get a build host ready to use the Yocto
Project.
</para></listitem>
<listitem><para>
<emphasis>Set Up Your Host Development System for Kernel Development:</emphasis>
It is recommended that you use <filename>devtool</filename>
and an extensible SDK for kernel development.
Alternatively, you can use traditional kernel development
methods with the Yocto Project.
Either way, there are steps you need to take to get the
development environment ready.</para>
<para>Using <filename>devtool</filename> and the eSDK requires
that you have a clean build of the image and that you are
set up with the appropriate eSDK.
For more information, see the
"<link linkend='getting-ready-to-develop-using-devtool'>Getting Ready to Develop Using <filename>devtool</filename></link>"
section.</para>
<para>Using traditional kernel development requires that you
have the kernel source available in an isolated local Git
repository.
For more information, see the
"<link linkend='getting-ready-for-traditional-kernel-development'>Getting Ready for Traditional Kernel Development</link>"
section.
</para></listitem>
<listitem><para>
<emphasis>Make Changes to the Kernel Source Code if
applicable:</emphasis>
Modifying the kernel does not always mean directly
changing source files.
However, if you have to do this, you make the changes to the
files in the eSDK's Build Directory if you are using
<filename>devtool</filename>.
For more information, see the
"<link linkend='using-devtool-to-patch-the-kernel'>Using <filename>devtool</filename> to Patch the Kernel</link>"
section.</para>
<para>If you are using traditional kernel development, you
edit the source files in the kernel's local Git repository.
For more information, see the
"<link linkend='using-traditional-kernel-development-to-patch-the-kernel'>Using Traditional Kernel Development to Patch the Kernel</link>"
section.
</para></listitem>
<listitem><para>
<emphasis>Make Kernel Configuration Changes if
Applicable:</emphasis>
If your situation calls for changing the kernel's
configuration, you can use
<link linkend='using-menuconfig'><filename>menuconfig</filename></link>,
which allows you to interactively develop and test the
configuration changes you are making to the kernel.
Saving changes you make with <filename>menuconfig</filename>
updates the kernel's <filename>.config</filename> file.
<note><title>Warning</title>
Try to resist the temptation to directly edit an
existing <filename>.config</filename> file, which is
found in the Build Directory among the source code
used for the build.
Doing so, can produce unexpected results when the
OpenEmbedded build system regenerates the configuration
file.
</note>
Once you are satisfied with the configuration
changes made using <filename>menuconfig</filename>
and you have saved them, you can directly compare the
resulting <filename>.config</filename> file against an
existing original and gather those changes into a
<link linkend='creating-config-fragments'>configuration fragment file</link>
to be referenced from within the kernel's
<filename>.bbappend</filename> file.</para>
<para>Additionally, if you are working in a BSP layer
and need to modify the BSP's kernel's configuration,
you can use <filename>menuconfig</filename>.
</para></listitem>
<listitem><para>
<emphasis>Rebuild the Kernel Image With Your Changes:</emphasis>
Rebuilding the kernel image applies your changes.
Depending on your target hardware, you can verify your changes
on actual hardware or perhaps QEMU.
</para></listitem>
</orderedlist>
The remainder of this developer's guide covers common tasks typically
used during kernel development, advanced Metadata usage, and Yocto Linux
kernel maintenance concepts.
</para>
</section>
</chapter>
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