269 lines
13 KiB
XML
269 lines
13 KiB
XML
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
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[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >
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<chapter id='kernel-dev-intro'>
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<title>Introduction</title>
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<section id='kernel-dev-overview'>
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<title>Overview</title>
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<para>
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Regardless of how you intend to make use of the Yocto Project,
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chances are you will work with the Linux kernel.
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This manual describes how to set up your build host to support
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kernel development, introduces the kernel development process,
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provides background information on the Yocto Linux kernel
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<ulink url='&YOCTO_DOCS_REF_URL;#metadata'>Metadata</ulink>,
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describes common tasks you can perform using the kernel tools,
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shows you how to use the kernel Metadata needed to work with
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the kernel inside the Yocto Project, and provides insight into how
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the Yocto Project team develops and maintains Yocto Linux kernel
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Git repositories and Metadata.
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</para>
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<para>
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Each Yocto Project release has a set of Yocto Linux kernel recipes,
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whose Git repositories you can view in the Yocto
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<ulink url='&YOCTO_GIT_URL;'>Source Repositories</ulink> under
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the "Yocto Linux Kernel" heading.
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New recipes for the release track the latest Linux kernel
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upstream developments from
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<ulink url='http://www.kernel.org'></ulink> and introduce
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newly-supported platforms.
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Previous recipes in the release are refreshed and supported for at
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least one additional Yocto Project release.
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As they align, these previous releases are updated to include the
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latest from the Long Term Support Initiative (LTSI) project.
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You can learn more about Yocto Linux kernels and LTSI in the
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"<link linkend='kernel-big-picture'>Yocto Project Kernel Development and Maintenance</link>"
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section.
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</para>
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<para>
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Also included is a Yocto Linux kernel development recipe
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(<filename>linux-yocto-dev.bb</filename>) should you want to work
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with the very latest in upstream Yocto Linux kernel development and
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kernel Metadata development.
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<note>
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For more on Yocto Linux kernels, see the
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"<link linkend='kernel-big-picture'>Yocto Project Kernel Development and Maintenance</link>
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section.
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</note>
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</para>
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<para>
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The Yocto Project also provides a powerful set of kernel
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tools for managing Yocto Linux kernel sources and configuration data.
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You can use these tools to make a single configuration change,
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apply multiple patches, or work with your own kernel sources.
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</para>
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<para>
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In particular, the kernel tools allow you to generate configuration
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fragments that specify only what you must, and nothing more.
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Configuration fragments only need to contain the highest level
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visible <filename>CONFIG</filename> options as presented by the
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Yocto Linux kernel <filename>menuconfig</filename> system.
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Contrast this against a complete Yocto Linux kernel
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<filename>.config</filename> file, which includes all the automatically
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selected <filename>CONFIG</filename> options.
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This efficiency reduces your maintenance effort and allows you
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to further separate your configuration in ways that make sense for
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your project.
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A common split separates policy and hardware.
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For example, all your kernels might support the
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<filename>proc</filename> and <filename>sys</filename> filesystems,
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but only specific boards require sound, USB, or specific drivers.
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Specifying these configurations individually allows you to aggregate
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them together as needed, but maintains them in only one place.
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Similar logic applies to separating source changes.
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</para>
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<para>
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If you do not maintain your own kernel sources and need to make
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only minimal changes to the sources, the released recipes provide a
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vetted base upon which to layer your changes.
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Doing so allows you to benefit from the continual kernel
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integration and testing performed during development of the
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Yocto Project.
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</para>
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<para>
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If, instead, you have a very specific Linux kernel source tree
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and are unable to align with one of the official Yocto Linux kernel
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recipes, an alternative exists by which you can use the Yocto
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Project Linux kernel tools with your own kernel sources.
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</para>
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<para>
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The remainder of this manual provides instructions for completing
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specific Linux kernel development tasks.
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These instructions assume you are comfortable working with
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<ulink url='http://openembedded.org/wiki/Bitbake'>BitBake</ulink>
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recipes and basic open-source development tools.
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Understanding these concepts will facilitate the process of working
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with the kernel recipes.
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If you find you need some additional background, please be sure to
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review and understand the following documentation:
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<itemizedlist>
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<listitem><para>
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<ulink url='&YOCTO_DOCS_QS_URL;'>Yocto Project Quick Start</ulink>
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</para></listitem>
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<listitem><para>
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<ulink url='&YOCTO_DOCS_SDK_URL;#using-devtool-in-your-sdk-workflow'><filename>devtool</filename> workflow</ulink>
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as described in the Yocto Project Application Development and
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the Extensible Software Development Kit (eSDK) manual.
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</para></listitem>
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<listitem><para>
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The
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"<ulink url='&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers'>Understanding and Creating Layers</ulink>"
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section in the Yocto Project Development Tasks Manual.
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</para></listitem>
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<listitem><para>
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The
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"<link linkend='kernel-modification-workflow'>Kernel Modification Workflow</link>"
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section.
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</para></listitem>
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</itemizedlist>
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</para>
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<para>
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Finally, while this document focuses on the manual creation of
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recipes, patches, and configuration files, the Yocto Project
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Board Support Package (BSP) tools are available to automate
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this process with existing content and work well to create the
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initial framework and boilerplate code.
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For details on these tools, see the
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"<ulink url='&YOCTO_DOCS_BSP_URL;#using-the-yocto-projects-bsp-tools'>Using the Yocto Project's BSP Tools</ulink>"
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section in the Yocto Project Board Support Package (BSP) Developer's
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Guide.
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</para>
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</section>
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<section id='kernel-modification-workflow'>
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<title>Kernel Modification Workflow</title>
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<para>
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Kernel modification involves changing the Yocto Project kernel,
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which could involve changing configuration options as well as adding
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new kernel recipes.
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Configuration changes can be added in the form of configuration
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fragments, while recipe modification comes through the kernel's
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<filename>recipes-kernel</filename> area in a kernel layer you create.
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</para>
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<para>
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This section presents a high-level overview of the Yocto Project
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kernel modification workflow.
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The illustration and accompanying list provide general information
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and references for further information.
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<imagedata fileref="figures/kernel-dev-flow.png"
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width="9in" depth="5in" align="center" scalefit="1" />
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</para>
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<para>
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<orderedlist>
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<listitem><para>
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<emphasis>Set Up Your Host Development System to Support
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Development Using the Yocto Project:</emphasis>
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See the
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"<ulink url='&YOCTO_DOCS_QS_URL;#yp-resources'>Setting Up to Use the Yocto Project</ulink>"
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section in the Yocto Project Quick Start for options on how
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to get a build host ready to use the Yocto Project.
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</para></listitem>
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<listitem><para>
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<emphasis>Set Up Your Host Development System for Kernel Development:</emphasis>
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It is recommended that you use <filename>devtool</filename>
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and an extensible SDK for kernel development.
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Alternatively, you can use traditional kernel development
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methods with the Yocto Project.
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Either way, there are steps you need to take to get the
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development environment ready.</para>
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<para>Using <filename>devtool</filename> and the eSDK requires
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that you have a clean build of the image and that you are
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set up with the appropriate eSDK.
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For more information, see the
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"<link linkend='getting-ready-to-develop-using-devtool'>Getting Ready to Develop Using <filename>devtool</filename></link>"
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section.</para>
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<para>Using traditional kernel development requires that you
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have the kernel source available in an isolated local Git
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repository.
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For more information, see the
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"<link linkend='getting-ready-for-traditional-kernel-development'>Getting Ready for Traditional Kernel Development</link>"
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section.
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</para></listitem>
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<listitem><para>
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<emphasis>Make Changes to the Kernel Source Code if
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applicable:</emphasis>
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Modifying the kernel does not always mean directly
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changing source files.
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However, if you have to do this, you make the changes to the
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files in the eSDK's Build Directory if you are using
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<filename>devtool</filename>.
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For more information, see the
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"<link linkend='using-devtool-to-patch-the-kernel'>Using <filename>devtool</filename> to Patch the Kernel</link>"
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section.</para>
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<para>If you are using traditional kernel development, you
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edit the source files in the kernel's local Git repository.
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For more information, see the
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"<link linkend='using-traditional-kernel-development-to-patch-the-kernel'>Using Traditional Kernel Development to Patch the Kernel</link>"
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section.
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</para></listitem>
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<listitem><para>
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<emphasis>Make Kernel Configuration Changes if
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Applicable:</emphasis>
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If your situation calls for changing the kernel's
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configuration, you can use
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<link linkend='using-menuconfig'><filename>menuconfig</filename></link>,
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which allows you to interactively develop and test the
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configuration changes you are making to the kernel.
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Saving changes you make with <filename>menuconfig</filename>
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updates the kernel's <filename>.config</filename> file.
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<note><title>Warning</title>
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Try to resist the temptation to directly edit an
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existing <filename>.config</filename> file, which is
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found in the Build Directory among the source code
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used for the build.
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Doing so, can produce unexpected results when the
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OpenEmbedded build system regenerates the configuration
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file.
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</note>
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Once you are satisfied with the configuration
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changes made using <filename>menuconfig</filename>
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and you have saved them, you can directly compare the
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resulting <filename>.config</filename> file against an
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existing original and gather those changes into a
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<link linkend='creating-config-fragments'>configuration fragment file</link>
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to be referenced from within the kernel's
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<filename>.bbappend</filename> file.</para>
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<para>Additionally, if you are working in a BSP layer
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and need to modify the BSP's kernel's configuration,
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you can use the
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<ulink url='&YOCTO_DOCS_BSP_URL;#managing-kernel-patches-and-config-items-with-yocto-kernel'><filename>yocto-kernel</filename></ulink>
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script as well as <filename>menuconfig</filename>.
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The <filename>yocto-kernel</filename> script lets
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you interactively set up kernel configurations.
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</para></listitem>
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<listitem><para>
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<emphasis>Rebuild the Kernel Image With Your Changes:</emphasis>
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Rebuilding the kernel image applies your changes.
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Depending on your target hardware, you can verify your changes
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on actual hardware or perhaps QEMU.
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</para></listitem>
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</orderedlist>
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The remainder of this developer's guide covers common tasks typically
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used during kernel development, advanced Metadata usage, and Yocto Linux
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kernel maintenance concepts.
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</para>
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</section>
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</chapter>
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<!--
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vim: expandtab tw=80 ts=4
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-->
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