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<chapter id='kernel-concepts'>

<title>Yocto Project Kernel Concepts</title>

<section id='concepts-org'>
    <title>Introduction</title>
    <para>
        This chapter provides conceptual information about the kernel:
        <itemizedlist>
            <listitem><para>Kernel Goals</para></listitem>
            <listitem><para>Kernel Development and Maintenance Overview</para></listitem>
            <listitem><para>Kernel Architecture</para></listitem>
            <listitem><para>Kernel Tools</para></listitem>
        </itemizedlist>
    </para>
</section>

    <section id='kernel-goals'>
        <title>Kernel Goals</title>
        <para>
            The complexity of embedded kernel design has increased dramatically.
            Whether it is managing multiple implementations of a particular feature or tuning and
            optimizing board specific features, both flexibility and maintainability are key concerns.
            The Linux kernels available through the Yocto Project are presented with the embedded
            developer's needs in mind and have evolved to assist in these key concerns.
            For example, prior methods such as applying hundreds of patches to an extracted
            tarball have been replaced with proven techniques that allow easy inspection,
            bisection and analysis of changes.
            Application of these techniques also creates a platform for performing integration and
            collaboration with the thousands of upstream development projects.
        </para>
        <para>
            With all these considerations in mind, the Yocto Project's kernel and development team
            strives to attain these goals:
        <itemizedlist>
            <listitem><para>Allow the end user to leverage community best practices to seamlessly
            manage the development, build and debug cycles.</para></listitem>
            <listitem><para>Create a platform for performing integration and collaboration with the
            thousands of upstream development projects that exist.</para></listitem>
            <listitem><para>Provide mechanisms that support many different work flows, front-ends and
            management techniques.</para></listitem>
            <listitem><para>Deliver the most up-to-date kernel possible while still ensuring that
            the baseline kernel is the most stable official release.</para></listitem>
            <listitem><para>Include major technological features as part of the Yocto Project's
            upward revision strategy.</para></listitem>
            <listitem><para>Present a kernel Git repository that, similar to the upstream
            <filename>kernel.org</filename> tree,
            has a clear and continuous history.</para></listitem>
            <listitem><para>Deliver a key set of supported kernel types, where each type is tailored
            to meet a specific use (e.g. networking, consumer, devices, and so forth).</para></listitem>
            <listitem><para>Employ a Git branching strategy that, from a developer's point of view,
            results in a linear path from the baseline <filename>kernel.org</filename>,
            through a select group of features and
            ends with their BSP-specific commits.</para></listitem>
        </itemizedlist>
        </para>
    </section>

    <section id='kernel-big-picture'>
        <title>Yocto Project Kernel Development and Maintenance Overview</title>
        <para>
            Kernels available through the Yocto Project, like other kernels, are based off the Linux
            kernel releases from <ulink url='http://www.kernel.org'></ulink>.
            At the beginning of a major development cycle, the Yocto Project team
            chooses its kernel based on factors such as release timing, the anticipated release
            timing of final upstream <filename>kernel.org</filename> versions, and Yocto Project
            feature requirements.
            Typically, the kernel chosen is in the
            final stages of development by the community.
            In other words, the kernel is in the release
            candidate or "rc" phase and not yet a final release.
            But, by being in the final stages of external development, the team knows that the
            <filename>kernel.org</filename> final release will clearly be within the early stages of
            the Yocto Project development window.
        </para>
        <para>
            This balance allows the team to deliver the most up-to-date kernel
            possible, while still ensuring that the team has a stable official release for
            the baseline Linux kernel version.
        </para>
        <para>
            The ultimate source for kernels available through the Yocto Project are released kernels
            from <filename>kernel.org</filename>.
            In addition to a foundational kernel from <filename>kernel.org</filename>, the
            kernels available contain a mix of important new mainline
            developments, non-mainline developments (when there is no alternative),
            Board Support Package (BSP) developments,
            and custom features.
            These additions result in a commercially released Yocto Project Linux kernel that caters
            to specific embedded designer needs for targeted hardware.
        </para>
        <para>
            Once a kernel is officially released, the Yocto Project team goes into
            their next development cycle, or upward revision (uprev) cycle, while still
            continuing maintenance on the released kernel.
            It is important to note that the most sustainable and stable way
            to include feature development upstream is through a kernel uprev process.
            Back-porting hundreds of individual fixes and minor features from various
            kernel versions is not sustainable and can easily compromise quality.
        </para>
        <para>
            During the uprev cycle, the Yocto Project team uses an ongoing analysis of
            kernel development, BSP support, and release timing to select the best
            possible <filename>kernel.org</filename> version.
            The team continually monitors community kernel
            development to look for significant features of interest.
            The team does consider back-porting large features if they have a significant advantage.
            User or community demand can also trigger a back-port or creation of new
            functionality in the Yocto Project baseline kernel during the uprev cycle.
        </para>
        <para>
            Generally speaking, every new kernel both adds features and introduces new bugs.
            These consequences are the basic properties of upstream kernel development and are
            managed by the Yocto Project team's kernel strategy.
            It is the Yocto Project team's policy to not back-port minor features to the released kernel.
            They only consider back-porting significant technological jumps - and, that is done
            after a complete gap analysis.
            The reason for this policy is that back-porting any small to medium sized change
            from an evolving kernel can easily create mismatches, incompatibilities and very
            subtle errors.
        </para>
        <para>
            These policies result in both a stable and a cutting
            edge kernel that mixes forward ports of existing features and significant and critical
            new functionality.
            Forward porting functionality in the kernels available through the Yocto Project kernel
            can be thought of as a "micro uprev."
            The many “micro uprevs” produce a kernel version with a mix of
            important new mainline, non-mainline, BSP developments and feature integrations.
            This kernel gives insight into new features and allows focused
            amounts of testing to be done on the kernel, which prevents
            surprises when selecting the next major uprev.
            The quality of these cutting edge kernels is evolving and the kernels are used in leading edge
            feature and BSP development.
        </para>
    </section>

    <section id='kernel-architecture'>
        <title>Kernel Architecture</title>
        <para>
            This section describes the architecture of the kernels available through the
            Yocto Project and provides information
            on the mechanisms used to achieve that architecture.
        </para>

        <section id='architecture-overview'>
            <title>Overview</title>
            <para>
                As mentioned earlier, a key goal of the Yocto Project is to present the
                developer with
                a kernel that has a clear and continuous history that is visible to the user.
                The architecture and mechanisms used achieve that goal in a manner similar to the
                upstream <filename>kernel.org</filename>.
            </para>
            <para>
                You can think of a Yocto Project kernel as consisting of a baseline Linux kernel with
                added features logically structured on top of the baseline.
                The features are tagged and organized by way of a branching strategy implemented by the
                source code manager (SCM) Git.
                For information on Git as applied to the Yocto Project, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>" section in the
                Yocto Project Development Manual.
            </para>
            <para>
                The result is that the user has the ability to see the added features and
                the commits that make up those features.
                In addition to being able to see added features, the user can also view the history of what
                made up the baseline kernel.
            </para>
            <para>
                The following illustration shows the conceptual Yocto Project kernel.
            </para>
            <para>
                <imagedata fileref="figures/kernel-architecture-overview.png" width="6in" depth="7in" align="center" scale="100" />
            </para>
            <para>
                In the illustration, the "Kernel.org Branch Point"
                marks the specific spot (or release) from
                which the Yocto Project kernel is created.
                From this point "up" in the tree, features and differences are organized and tagged.
            </para>
            <para>
                The "Yocto Project Baseline Kernel" contains functionality that is common to every kernel
                type and BSP that is organized further up the tree.
                Placing these common features in the
                tree this way means features don't have to be duplicated along individual branches of the
                structure.
            </para>
            <para>
                From the Yocto Project Baseline Kernel, branch points represent specific functionality
                for individual BSPs as well as real-time kernels.
                The illustration represents this through three BSP-specific branches and a real-time
                kernel branch.
                Each branch represents some unique functionality for the BSP or a real-time kernel.
            </para>
            <para>
                In this example structure, the real-time kernel branch has common features for all
                real-time kernels and contains
                more branches for individual BSP-specific real-time kernels.
                The illustration shows three branches as an example.
                Each branch points the way to specific, unique features for a respective real-time
                kernel as they apply to a given BSP.
            </para>
            <para>
                The resulting tree structure presents a clear path of markers (or branches) to the
                developer that, for all practical purposes, is the kernel needed for any given set
                of requirements.
            </para>
        </section>

        <section id='branching-and-workflow'>
            <title>Branching Strategy and Workflow</title>
            <para>
                The Yocto Project team creates kernel branches at points where functionality is
                no longer shared and thus, needs to be isolated.
                For example, board-specific incompatibilities would require different functionality
                and would require a branch to separate the features.
                Likewise, for specific kernel features, the same branching strategy is used.
            </para>
            <para>
                This branching strategy results in a tree that has features organized to be specific
                for particular functionality, single kernel types, or a subset of kernel types.
                This strategy also results in not having to store the same feature twice
                internally in the tree.
                Rather, the kernel team stores the unique differences required to apply the
                feature onto the kernel type in question.
                <note>
                    The Yocto Project team strives to place features in the tree such that they can be
                    shared by all boards and kernel types where possible.
                    However, during development cycles or when large features are merged,
                    the team cannot always follow this practice.
                    In those cases, the team uses isolated branches to merge features.
                </note>
            </para>
            <para>
                BSP-specific code additions are handled in a similar manner to kernel-specific additions.
                Some BSPs only make sense given certain kernel types.
                So, for these types, the team creates branches off the end of that kernel type for all
                of the BSPs that are supported on that kernel type.
                From the perspective of the tools that create the BSP branch, the BSP is really no
                different than a feature.
                Consequently, the same branching strategy applies to BSPs as it does to features.
                So again, rather than store the BSP twice, the team only stores the unique
                differences for the BSP across the supported multiple kernels.
            </para>
            <para>
                While this strategy can result in a tree with a significant number of branches, it is
                important to realize that from the developer's point of view, there is a linear
                path that travels from the baseline <filename>kernel.org</filename>, through a select
                group of features and ends with their BSP-specific commits.
                In other words, the divisions of the kernel are transparent and are not relevant
                to the developer on a day-to-day basis.
                From the developer's perspective, this path is the "master" branch.
                The developer does not need to be aware of the existence of any other branches at all.
                Of course, there is value in the existence of these branches
                in the tree, should a person decide to explore them.
                For example, a comparison between two BSPs at either the commit level or at the line-by-line
                code <filename>diff</filename> level is now a trivial operation.
            </para>
            <para>
                Working with the kernel as a structured tree follows recognized community best practices.
                In particular, the kernel as shipped with the product, should be
                considered an "upstream source" and viewed as a series of
                historical and documented modifications (commits).
                These modifications represent the development and stabilization done
                by the Yocto Project kernel development team.
            </para>
            <para>
                Because commits only change at significant release points in the product life cycle,
                developers can work on a branch created
                from the last relevant commit in the shipped Yocto Project kernel.
                As mentioned previously, the structure is transparent to the developer
                because the kernel tree is left in this state after cloning and building the kernel.
            </para>
        </section>

        <section id='source-code-manager-git'>
            <title>Source Code Manager - Git</title>
            <para>
                The Source Code Manager (SCM) is Git.
                This SCM is the obvious mechanism for meeting the previously mentioned goals.
                Not only is it the SCM for <filename>kernel.org</filename> but,
                Git continues to grow in popularity and supports many different work flows,
                front-ends and management techniques.
            </para>
            <para>
                You can find documentation on Git at <ulink url='http://git-scm.com/documentation'></ulink>.
                You can also get an introduction to Git as it applies to the Yocto Project in the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>"
                section in the Yocto Project Development Manual.
                These referenced sections overview Git and describe a minimal set of
                commands that allows you to be functional using Git.
                <note>
                    You can use as much, or as little, of what Git has to offer to accomplish what
                    you need for your project.
                    You do not have to be a "Git Master" in order to use it with the Yocto Project.
                </note>
            </para>
        </section>
    </section>

    <section id='kernel-configuration'>
        <title>Kernel Configuration</title>
        <para>
            Kernel configuration, along with kernel features, defines how a kernel
            image is built for the Yocto Project.
            Through configuration settings, you can customize a Yocto Project kernel to be
            specific to particular hardware.
            For example, you can specify sound support or networking support.
            This section describes basic concepts behind Kernel configuration within the
            Yocto Project and references you to other areas for specific configuration
            applications.
        </para>

        <para>
            Conceptually, configuration of a Yocto Project kernel occurs similarly to that needed for any
            Linux kernel.
            The build process for a Yocto Project kernel uses a <filename>.config</filename> file, which
            is created through the Linux Kernel Configuration (LKC) tool.
            You can directly set various configurations in the
            <filename>.config</filename> file by using the <filename>menuconfig</filename>
            tool as built by BitBake.
            You can also define configurations in the file by using configuration fragments.
            <note>
                It is not recommended that you edit the <filename>.config</filename> file directly.
            </note>
            Here are some brief descriptions of the ways you can affect the
            <filename>.config</filename> file:
            <itemizedlist>
                <listitem><para><emphasis>The <filename>menuconfig</filename> Tool:</emphasis>
                    One of many front-ends that allows you to define kernel configurations.
                    Some others are <filename>make config</filename>,
                    <filename>make nconfig</filename>, and <filename>make gconfig</filename>.
                    In the Yocto Project environment, you must use BitBake to build the
                    <filename>menuconfig</filename> tool before you can use it to define
                    configurations:
                    <literallayout class='monospaced'>
     $ bitbake linux-yocto -c menuconfig
                    </literallayout>
                    After the tool is built, you can interact with it normally.
                    You can see how <filename>menuconfig</filename> is used to change a simple
                    kernel configuration in the
                    "<ulink url='&YOCTO_DOCS_DEV_URL;#configuring-the-kernel'>Configuring the Kernel</ulink>"
                    section of the Yocto Project Development Manual.
                    For general information on <filename>menuconfig</filename>, see
                    <ulink url='http://en.wikipedia.org/wiki/Menuconfig'></ulink>.
                    </para></listitem>
                <listitem><para><emphasis>Configuration Fragments:</emphasis> A file with a
                    list of kernel options just as they would appear syntactically in the
                    <filename>.config</filename> file.
                    Configuration fragments are typically logical groupings and are assembled
                    by the OpenEmbedded build system to produce input used by the LKC
                    that ultimately generates the <filename>.config</filename> file.</para>
                    <para>The
                    <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'>KERNEL_FEATURES</ulink></filename>
                    variable can be used to list configuration fragments.
                    For further discussion on applying configuration fragments, see the
                    "<ulink url='&YOCTO_DOCS_BSP_URL;#bsp-filelayout-kernel'>Linux Kernel Configuration</ulink>"
                    section in the Yocto Project Board Support Package (BSP) Guide.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='kernel-tools'>
        <title>Kernel Tools</title>
        <para>
            Since most standard workflows involve moving forward with an existing tree by
            continuing to add and alter the underlying baseline, the tools that manage
            the Yocto Project's kernel construction are largely hidden from the developer to
            present a simplified view of the kernel for ease of use.
        </para>
        <para>
            Fundamentally, the kernel tools that manage and construct the
            Yocto Project kernel accomplish the following:
            <itemizedlist>
                <listitem><para>Group patches into named, reusable features.</para></listitem>
                <listitem><para>Allow top-down control of included features.</para></listitem>
                <listitem><para>Bind kernel configurations to kernel patches and features.</para></listitem>
                <listitem><para>Present a seamless Git repository that blends Yocto Project value
                    with the <filename>kernel.org</filename> history and development.</para></listitem>
            </itemizedlist>
        </para>
    </section>
</chapter>
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