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<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_QS_URL;'>Yocto Project Quick Start</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>

    <para>
        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
        "<ulink url='&YOCTO_DOCS_BSP_URL;#using-the-yocto-projects-bsp-tools'>Using the Yocto Project's BSP Tools</ulink>"
        section in the Yocto Project Board Support Package (BSP) Developer's
        Guide.
    </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_QS_URL;#yp-resources'>Setting Up to Use the Yocto Project</ulink>"
                section in the Yocto Project Quick Start 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 the
                <ulink url='&YOCTO_DOCS_BSP_URL;#managing-kernel-patches-and-config-items-with-yocto-kernel'><filename>yocto-kernel</filename></ulink>
                script as well as <filename>menuconfig</filename>.
                The <filename>yocto-kernel</filename> script lets
                you interactively set up kernel configurations.
                </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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