path: root/documentation/kernel-dev/kernel-dev-advanced.xml

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<chapter id='kernel-dev-advanced'>
<title>Working with Advanced Metadata</title>

<section id='kernel-dev-advanced-overview'>
    <title>Overview</title>

    <para>
        In addition to supporting configuration fragments and patches, the
        Yocto Project kernel tools also support rich
        <ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink> that you can
        use to define complex policies and Board Support Package (BSP) support.
        The purpose of the Metadata and the tools that manage it, known as
        the kern-tools (<filename>kern-tools-native_git.bb</filename>), is
        to help you manage the complexity of the configuration and sources
        used to support multiple BSPs and Linux kernel types.
    </para>
</section>

<section id='using-kernel-metadata-in-a-recipe'>
    <title>Using Kernel Metadata in a Recipe</title>

    <para>
        The kernel sources in the Yocto Project contain kernel Metadata, which is
        located in the <filename>meta</filename> branches of the kernel source
        Git repositories.
        This Metadata defines Board Support Packages (BSPs) that
        correspond to definitions in linux-yocto recipes for the same BSPs.
        A BSP consists of an aggregation of kernel policy and hardware-specific
        feature enablements.
        The BSP can be influenced from within the linux-yocto recipe.
        <note>
            Linux kernel source that contains kernel Metadata is said to be
            "linux-yocto style" kernel source.
            A Linux kernel recipe that inherits from the
            <filename>linux-yocto.inc</filename> include file is said to be a
            "linux-yocto style" recipe.
        </note>
    </para>

    <para>
        Every linux-yocto style recipe must define the
        <ulink url='&YOCTO_DOCS_REF_URL;#var-KMACHINE'><filename>KMACHINE</filename></ulink>
        variable.
        This variable is typically set to the same value as the
        <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>
        variable, which is used by
        <ulink url='&YOCTO_DOCS_DEV_URL;#bitbake-term'>BitBake</ulink>
        (e.g. "routerstationpro" or "fri2").
        Multiple BSPs can reuse the same <filename>KMACHINE</filename>
        name if they are built using the same BSP description.
        The "fri2" and "fri2-noemgd" BSP combination
        in the <filename>meta-intel</filename>
        layer is a good example of two BSPs using the same
        <filename>KMACHINE</filename> value (i.e. "fri2").
        See the <link linkend='bsp-descriptions'>BSP Descriptions</link> section
        for more information.
    </para>

    <para>
        The linux-yocto style recipes can optionally define the following
        variables:
        <literallayout class='monospaced'>
     <ulink url='&YOCTO_DOCS_REF_URL;#var-KBRANCH'>KBRANCH</ulink>
     <ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'>KERNEL_FEATURES</ulink>
     <ulink url='&YOCTO_DOCS_REF_URL;#var-KBRANCH_DEFAULT'>KBRANCH_DEFAULT</ulink>
     <ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_KERNEL_TYPE'>LINUX_KERNEL_TYPE</ulink>
        </literallayout>
        <filename>KBRANCH_DEFAULT</filename> defines the Linux kernel source
        repository's default branch to use to build the Linux kernel.
        The value is used as the default for <filename>KBRANCH</filename>, which
        can define an alternate branch typically with a machine override as
        follows:
        <literallayout class='monospaced'>
     KBRANCH_fri2 = "standard/fri2"
        </literallayout>
        Unless you specify otherwise, <filename>KBRANCH_DEFAULT</filename>
        initializes to "master".
    </para>

    <para>
        <filename>LINUX_KERNEL_TYPE</filename> defines the kernel type to be
        used in assembling the configuration.
        If you do not specify a <filename>LINUX_KERNEL_TYPE</filename>,
        it defaults to "standard".
        Together with
        <ulink url='&YOCTO_DOCS_REF_URL;#var-KMACHINE'><filename>KMACHINE</filename></ulink>,
        <filename>LINUX_KERNEL_TYPE</filename> defines the search
        arguments used by the kernel tools to find the
        appropriate description within the kernel Metadata with which to
        build out the sources and configuration.
        The linux-yocto recipes define "standard", "tiny", and "preempt-rt"
        kernel types.
        See the <link linkend='kernel-types'>Kernel Types</link> section
        for more information on kernel types.
    </para>

    <para>
        During the build, the kern-tools search for the BSP description
        file that most closely matches the <filename>KMACHINE</filename>
        and <filename>LINUX_KERNEL_TYPE</filename> variables passed in from the
        recipe.
        The tools use the first BSP description it finds that match
        both variables.
        If the tools cannot find a match, they issue a warning such as
        the following:
        <literallayout class='monospaced'>
     WARNING: Can't find any BSP hardware or required configuration fragments.
     WARNING: Looked at meta/cfg/broken/fri2-broken/hdw_frags.txt and
              meta/cfg/broken/fri2-broken/required_frags.txt in directory:
              meta/cfg/broken/fri2-broken
        </literallayout>
        In this example, <filename>KMACHINE</filename> was set to "fri2-broken"
        and <filename>LINUX_KERNEL_TYPE</filename> was set to "broken".
    </para>

    <para>
        The tools first search for the <filename>KMACHINE</filename> and
        then for the <filename>LINUX_KERNEL_TYPE</filename>.
        If the tools cannot find a partial match, they will use the
        sources from the <filename>KBRANCH</filename> and any configuration
        specified in the
        <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>.
    </para>

    <para>
        You can use the <filename>KERNEL_FEATURES</filename> variable
        to include features (configuration fragments, patches, or both) that
        are not already included by the <filename>KMACHINE</filename> and
        <filename>LINUX_KERNEL_TYPE</filename> variable combination.
        For example, to include a feature specified as "features/netfilter.scc",
        specify:
        <literallayout class='monospaced'>
     KERNEL_FEATURES += "features/netfilter.scc"
        </literallayout>
        To include a feature called "cfg/sound.scc" just for the
        <filename>qemux86</filename> machine, specify:
        <literallayout class='monospaced'>
     KERNEL_FEATURES_append_qemux86 = "cfg/sound.scc"
        </literallayout>
        The value of the entries in <filename>KERNEL_FEATURES</filename>
        are dependent on their location within the kernel Metadata itself.
        The examples here are taken from the
        <filename>linux-yocto-3.4</filename> repository where "features"
        and "cfg" are subdirectories within the
        <filename>meta/cfg/kernel-cache</filename> directory.
        For more information, see the
        "<link linkend='kernel-metadata-syntax'>Kernel Metadata Syntax</link>" section.
        <note>
            The processing of the these variables has evolved some between the
	        0.9 and 1.3 releases of the Yocto Project and associated
	        kern-tools sources.
            The descriptions in this section are accurate for 1.3 and later
	        releases of the Yocto Project.
        </note>
    </para>
</section>

<section id='kernel-metadata-location'>
    <title>Kernel Metadata Location</title>

    <para>
        Kernel Metadata can be defined in either the kernel recipe
        (recipe-space) or in the kernel tree (in-tree).
        Where you choose to define the Metadata depends on what you want
        to do and how you intend to work.
        Regardless of where you define the kernel Metadata, the syntax used
        applies equally.
    </para>

    <para>
        If you are unfamiliar with the Linux kernel and only wish
        to apply a configuration and possibly a couple of patches provided to
        you by others, the recipe-space method is recommended.
        This method is also a good approach if you are working with Linux kernel
        sources you do not control or if you just do not want to maintain a
        Linux kernel Git repository on your own.
        For partial information on how you can define kernel Metadata in
        the recipe-space, see the
        "<link linkend='modifying-an-existing-recipe'>Modifying an Existing Recipe</link>"
        section.
    </para>

    <para>
        Conversely, if you are actively developing a kernel and are already
        maintaining a Linux kernel Git repository of your own, you might find
        it more convenient to work with the kernel Metadata in the same
        repository as the Linux kernel sources.
        This method can make iterative development of the Linux kernel
        more efficient outside of the BitBake environment.
    </para>

    <section id='recipe-space-metadata'>
        <title>Recipe-Space Metadata</title>

        <para>
            When stored in recipe-space, the kernel Metadata files reside in a
            directory hierarchy below
            <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>.
            For a linux-yocto recipe or for a Linux kernel recipe derived
            by copying and modifying
            <filename>oe-core/meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb</filename>
            to a recipe in your layer, <filename>FILESEXTRAPATHS</filename>
            is typically set to
            <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-THISDIR'><filename>THISDIR</filename></ulink><filename>}/${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink><filename>}</filename>.
            See the "<link linkend='modifying-an-existing-recipe'>Modifying an Existing Recipe</link>"
            section for more information.
        </para>

        <para>
            Here is an example that shows a trivial tree of kernel Metadata
            stored in recipe-space within a BSP layer:
            <literallayout class='monospaced'>
     meta-my_bsp_layer/
     `-- recipes-kernel
         `-- linux
             `-- linux-yocto
                 |-- bsp-standard.scc
                 |-- bsp.cfg
                 `-- standard.cfg
            </literallayout>
        </para>

        <para>
            When the Metadata is stored in recipe-space, you must take
            steps to ensure BitBake has the necessary information to decide
            what files to fetch and when they need to be fetched again.
            It is only necessary to specify the <filename>.scc</filename>
            files on the
            <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>.
            BitBake parses them and fetches any files referenced in the
            <filename>.scc</filename> files by the <filename>include</filename>,
            <filename>patch</filename>, or <filename>kconf</filename> commands.
            Because of this, it is necessary to bump the recipe
            <ulink url='&YOCTO_DOCS_REF_URL;#var-PR'><filename>PR</filename></ulink>
            value when changing the content of files not explicitly listed
            in the <filename>SRC_URI</filename>.
        </para>
    </section>

    <section id='in-tree-metadata'>
        <title>In-Tree Metadata</title>

        <para>
            When stored in-tree, the kernel Metadata files reside in the
            <filename>meta</filename> directory of the Linux kernel sources.
            The <filename>meta</filename> directory can be present in the
            same repository branch as the sources,
            such as "master", or <filename>meta</filename> can be its own
            orphan branch.
            <note>
                An orphan branch in Git is a branch with unique history and
                content to the other branches in the repository.
                Orphan branches are useful to track Metadata changes
                independently from the sources of the Linux kernel, while
                still keeping them together in the same repository.
            </note>
            For the purposes of this document, we will discuss all
            in-tree Metadata as residing below the
            <filename>meta/cfg/kernel-cache</filename> directory.
        </para>

        <para>
            Following is an example that shows how a trivial tree of Metadata
            is stored in a custom Linux kernel Git repository:
            <literallayout class='monospaced'>
     meta/
     `-- cfg
         `-- kernel-cache
             |-- bsp-standard.scc
             |-- bsp.cfg
             `-- standard.cfg
            </literallayout>
        </para>

        <para>
            To use a branch different from where the sources reside,
            specify the branch in the <filename>KMETA</filename> variable
            in your Linux kernel recipe.
            Here is an example:
            <literallayout class='monospaced'>
     KMETA = "meta"
            </literallayout>
            To use the same branch as the sources, set
            <filename>KMETA</filename> to an empty string:
            <literallayout class='monospaced'>
     KMETA = ""
            </literallayout>
            If you are working with your own sources and want to create an
            orphan <filename>meta</filename> branch, use these commands
            from within your Linux kernel Git repository:
            <literallayout class='monospaced'>
     $ git checkout --orphan meta
     $ git rm -rf .
     $ git commit --allow-empty -m "Create orphan meta branch"
            </literallayout>
        </para>

        <para>
            If you modify the Metadata in the linux-yocto
            <filename>meta</filename> branch, you must not forget to update
            the
            <ulink url='&YOCTO_DOCS_REF_URL;#var-SRCREV'><filename>SRCREV</filename></ulink>
            statements in the kernel's recipe.
            In particular, you need to update the
            <filename>SRCREV_meta</filename> variable to match the commit in
            the <filename>KMETA</filename> branch you wish to use.
            Changing the data in these branches and not updating the
            <filename>SRCREV</filename> statements to match will cause the
            build to fetch an older commit.
        </para>
    </section>
</section>

<section id='kernel-metadata-syntax'>
    <title>Kernel Metadata Syntax</title>

    <para>
        The kernel Metadata consists of three primary types of files:
        <filename>scc</filename>
        <footnote>
            <para>
                <filename>scc</filename> stands for Series Configuration
                Control, but the naming has less significance in the
                current implementation of the tooling than it had in the
                past.
                Consider <filename>scc</filename> files to be description files.
            </para>
        </footnote>
        description files, configuration fragments, and patches.
        The <filename>scc</filename> files define variables and include or
        otherwise reference any of the three file types.
        The description files are used to aggregate all types of kernel
        Metadata into
        what ultimately describes the sources and the configuration required
        to build a Linux kernel tailored to a specific machine.
    </para>

    <para>
        The <filename>scc</filename> description files are used to define two
        fundamental types of kernel Metadata:
        <itemizedlist>
            <listitem><para>Features</para></listitem>
            <listitem><para>Board Support Packages (BSPs)</para></listitem>
        </itemizedlist>
    </para>

    <para>
        Features aggregate sources in the form of patches and configuration
        fragments into a modular reusable unit.
        You can use features to implement conceptually separate kernel
        Metadata descriptions such as pure configuration fragments,
        simple patches, complex features, and kernel types.
        <link linkend='kernel-types'>Kernel types</link> define general
        kernel features and policy to be reused in the BSPs.
    </para>

    <para>
        BSPs define hardware-specific features and aggregate them with kernel
        types to form the final description of what will be assembled and built.
    </para>

    <para>
        While the kernel Metadata syntax does not enforce any logical
        separation of configuration fragments, patches, features or kernel
        types, best practices dictate a logical separation of these types
        of Metadata.
        The following Metadata file hierarchy is recommended:
        <literallayout class='monospaced'>
     &lt;base&gt;/
        bsp/
        cfg/
        features/
        ktypes/
        patches/
        </literallayout>
    </para>

    <para>
        The <filename>bsp</filename> directory contains the
        <link linkend='bsp-descriptions'>BSP descriptions</link>.
        The remaining directories all contain "features".
        Separating <filename>bsp</filename> from the rest of the structure
        aids conceptualizing intended usage.
    </para>

    <para>
        Use these guidelines to help place your <filename>scc</filename>
        description files within the structure:
        <itemizedlist>
            <listitem><para>If your file contains
                only configuration fragments, place the file in the
                <filename>cfg</filename> directory.</para></listitem>
            <listitem><para>If your file contains
                only source-code fixes, place the file in the
                <filename>patches</filename> directory.</para></listitem>
            <listitem><para>If your file encapsulates
                a major feature, often combining sources and configurations,
                place the file in <filename>features</filename> directory.
                </para></listitem>
            <listitem><para>If your file aggregates
                non-hardware configuration and patches in order to define a
                base kernel policy or major kernel type to be reused across
                multiple BSPs, place the file in <filename>ktypes</filename>
                directory.
                </para></listitem>
        </itemizedlist>
    </para>

    <para>
        These distinctions can easily become blurred - especially as
        out-of-tree features slowly merge upstream over time.
        Also, remember that how the description files are placed is
        a purely logical organization and has no impact on the functionality
        of the kernel Metadata.
        There is no impact because all of <filename>cfg</filename>,
        <filename>features</filename>, <filename>patches</filename>, and
        <filename>ktypes</filename>, contain "features" as far as the kernel
        tools are concerned.
    </para>

    <para>
        Paths used in kernel Metadata files are relative to
        <filename>&lt;base&gt;</filename>, which is either
        <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>
        if you are creating Metadata in
        <link linkend='recipe-space-metadata'>recipe-space</link>,
        or <filename>meta/cfg/kernel-cache/</filename> if you are creating
        Metadata <link linkend='in-tree-metadata'>in-tree</link>.
    </para>

    <section id='configuration'>
        <title>Configuration</title>

        <para>
            The simplest unit of kernel Metadata is the configuration-only
            feature.
            This feature consists of one or more Linux kernel configuration
            parameters in a configuration fragment file
            (<filename>.cfg</filename>) and an <filename>.scc</filename> file
            that describes the fragment.
        </para>

        <para>
            The Symmetric Multi-Processing (SMP) fragment included in the
            <filename>linux-yocto-3.4</filename> Git repository
            consists of the following two files:
            <literallayout class='monospaced'>
     cfg/smp.scc:
        define KFEATURE_DESCRIPTION "Enable SMP"
        kconf hardware smp.cfg

     cfg/smp.cfg:
        CONFIG_SMP=y
        CONFIG_SCHED_SMT=y
            </literallayout>
            You can find information on configuration fragment files in the
            "<ulink url='&YOCTO_DOCS_DEV_URL;#creating-config-fragments'>Creating Configuration Fragments</ulink>"
            section of the Yocto Project Development Manual and in
            the "<link linkend='generating-configuration-files'>Generating Configuration Files</link>"
            section earlier in this manual.
        </para>

        <para>
            <ulink url='&YOCTO_DOCS_REF_URL;#var-KFEATURE_DESCRIPTION'><filename>KFEATURE_DESCRIPTION</filename></ulink>
            provides a short description of the fragment.
            Higher level kernel tools use this description.
        </para>

        <para>
            The <filename>kconf</filename> command is used to include the
            actual configuration fragment in an <filename>.scc</filename>
            file, and the "hardware" keyword identifies the fragment as
            being hardware enabling, as opposed to general policy,
            which would use the "non-hardware" keyword.
            The distinction is made for the benefit of the configuration
            validation tools, which warn you if a hardware fragment
            overrides a policy set by a non-hardware fragment.
            <note>
                The description file can include multiple
                <filename>kconf</filename> statements, one per fragment.
            </note>
        </para>

        <para>
            As described in the
            "<link linkend='generating-configuration-files'>Generating Configuration Files</link>"
            section, you can use the following BitBake command to audit your
            configuration:
            <literallayout class='monospaced'>
     $ bitbake linux-yocto -c kernel_configcheck -f
            </literallayout>
        </para>
    </section>

    <section id='patches'>
        <title>Patches</title>

        <para>
            Patch descriptions are very similar to configuration fragment
            descriptions, which are described in the previous section.
            However, instead of a <filename>.cfg</filename> file, these
            descriptions work with source patches.
        </para>

        <para>
            A typical patch includes a description file and the patch itself:
            <literallayout class='monospaced'>
     patches/mypatch.scc:
        patch mypatch.patch

     patches/mypatch.patch:
        &lt;typical-patch&gt;
            </literallayout>
            You can create the typical <filename>.patch</filename>
            file using <filename>diff -Nurp</filename> or
            <filename>git format-patch</filename>.
        </para>

        <para>
            The description file can include multiple patch statements,
            one per patch.
        </para>
    </section>

    <section id='features'>
        <title>Features</title>

        <para>
            Features are complex kernel Metadata types that consist
            of configuration fragments (<filename>kconf</filename>), patches
            (<filename>patch</filename>), and possibly other feature
            description files (<filename>include</filename>).
        </para>

        <para>
            Here is an example that shows a feature description file:
            <literallayout class='monospaced'>
     features/myfeature.scc
        define KFEATURE_DESCRIPTION "Enable myfeature"

        patch 0001-myfeature-core.patch
        patch 0002-myfeature-interface.patch

        include cfg/myfeature_dependency.scc
        kconf non-hardware myfeature.cfg
            </literallayout>
            This example shows how the <filename>patch</filename> and
            <filename>kconf</filename> commands are used as well as
            how an additional feature description file is included.
        </para>

        <para>
            Typically, features are less granular than configuration
            fragments and are more likely than configuration fragments
            and patches to be the types of things you want to specify
            in the <filename>KERNEL_FEATURES</filename> variable of the
            Linux kernel recipe.
            See the "<link linkend='using-kernel-metadata-in-a-recipe'>Using Kernel Metadata in a Recipe</link>"
            section earlier in the manual.
        </para>
    </section>

    <section id='kernel-types'>
        <title>Kernel Types</title>

        <para>
            A kernel type defines a high-level kernel policy by
            aggregating non-hardware configuration fragments with
            patches you want to use when building a Linux kernels of a
            specific type.
            Syntactically, kernel types are no different than features
            as described in the "<link linkend='features'>Features</link>"
            section.
            The <filename>LINUX_KERNEL_TYPE</filename> variable in the kernel
            recipe selects the kernel type.
            See the "<link linkend='using-kernel-metadata-in-a-recipe'>Using Kernel Metadata in a Recipe</link>"
            section for more information.
        </para>

        <para>
            As an example, the <filename>linux-yocto-3.4</filename>
            tree defines three kernel types: "standard",
            "tiny", and "preempt-rt":
            <itemizedlist>
                <listitem><para>"standard":
                    Includes the generic Linux kernel policy of the Yocto
                    Project linux-yocto kernel recipes.
                    This policy includes, among other things, which file
                    systems, networking options, core kernel features, and
                    debugging and tracing options are supported.
                    </para></listitem>
                <listitem><para>"preempt-rt":
                    Applies the <filename>PREEMPT_RT</filename>
                    patches and the configuration options required to
                    build a real-time Linux kernel.
                    This kernel type inherits from the "standard" kernel type.
                    </para></listitem>
                <listitem><para>"tiny":
                    Defines a bare minimum configuration meant to serve as a
                    base for very small Linux kernels.
                    The "tiny" kernel type is independent from the "standard"
                    configuration.
                    Although the "tiny" kernel type does not currently include
                    any source changes, it might in the future.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            The "standard" kernel type is defined by
            <filename>standard.scc</filename>:
            <literallayout class='monospaced'>
     # Include this kernel type fragment to get the standard features and
     # configuration values.

     # Include all standard features
     include standard-nocfg.scc

     kconf non-hardware standard.cfg

     # individual cfg block section
     include cfg/fs/devtmpfs.scc
     include cfg/fs/debugfs.scc
     include cfg/fs/btrfs.scc
     include cfg/fs/ext2.scc
     include cfg/fs/ext3.scc
     include cfg/fs/ext4.scc

     include cfg/net/ipv6.scc
     include cfg/net/ip_nf.scc
     include cfg/net/ip6_nf.scc
     include cfg/net/bridge.scc
            </literallayout>
        </para>

        <para>
            As with any <filename>.scc</filename> file, a
            kernel type definition can aggregate other
            <filename>.scc</filename> files with
            <filename>include</filename> commands.
            These definitions can also directly pull in
            configuration fragments and patches with the
            <filename>kconf</filename> and <filename>patch</filename>
            commands, respectively.
        </para>

        <note>
            It is not strictly necessary to create a kernel type
            <filename>.scc</filename> file.
            The Board Support Package (BSP) file can implicitly define
            the kernel type using a <filename>define
            <ulink url='&YOCTO_DOCS_REF_URL;#var-KTYPE'>KTYPE</ulink> myktype</filename>
            line.
            See the "<link linkend='bsp-descriptions'>BSP Descriptions</link>"
            section for more information.
        </note>
    </section>

    <section id='bsp-descriptions'>
        <title>BSP Descriptions</title>

        <para>
            BSP descriptions combine kernel types with hardware-specific
            features.
            The hardware-specific portion is typically defined
            independently, and then aggregated with each supported kernel
            type.
            Consider this simple BSP description that supports the "mybsp"
            machine:
            <literallayout class='monospaced'>
     mybsp.scc:
        define KMACHINE mybsp
        define KTYPE standard
        define KARCH i386

        kconf mybsp.cfg
            </literallayout>
            Every BSP description should define the
            <ulink url='&YOCTO_DOCS_REF_URL;#var-KMACHINE'><filename>KMACHINE</filename></ulink>,
            <ulink url='&YOCTO_DOCS_REF_URL;#var-KTYPE'><filename>KTYPE</filename></ulink>,
            and <ulink url='&YOCTO_DOCS_REF_URL;#var-KARCH'><filename>KARCH</filename></ulink>
            variables.
            These variables allow the OpenEmbedded build system to identify
            the description as meeting the criteria set by the recipe being
            built.
            This simple example supports the "mybsp" machine for the "standard"
            kernel and the "i386" architecture.
        </para>

        <para>
            Be aware that a hard link between the
            <filename>KTYPE</filename> variable and a kernel type
            description file does not exist.
            Thus, if you do not have kernel types defined in your kernel
            Metadata, you only need to ensure that the kernel recipe's
            <ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_KERNEL_TYPE'><filename>LINUX_KERNEL_TYPE</filename></ulink>
            variable and the <filename>KTYPE</filename> variable in the
            BSP description file match.
            <note>
                Future versions of the tooling make the specification of
                <filename>KTYPE</filename> in the BSP optional.
            </note>
        </para>

        <para>
            If you did want to separate your kernel policy from your
            hardware configuration, you could do so by specifying a kernel
            type, such as "standard" and including that description file
            in the BSP description file.
            See the "<link linkend='kernel-types'>Kernel Types</link>" section
            for more information.
        </para>

        <para>
            You might also have multiple hardware configurations that you
            aggregate into a single hardware description file that you
            could include in the BSP description file, rather than referencing
            a single <filename>.cfg</filename> file.
            Consider the following:
            <literallayout class='monospaced'>
     mybsp.scc:
        define KMACHINE mybsp
        define KTYPE standard
        define KARCH i386

        include standard.scc
        include mybsp-hw.scc
            </literallayout>
        </para>

        <para>
            In the above example, <filename>standard.scc</filename>
            aggregates all the configuration fragments, patches, and
            features that make up your standard kernel policy whereas
            <filename>mybsp-hw.scc</filename> aggregates all those necessary
            to support the hardware available on the "mybsp" machine.
            For information on how to break a complete
            <filename>.config</filename> file into the various
            configuration fragments, see the
            "<link linkend='generating-configuration-files'>Generating Configuration Files</link>"
            section.
        </para>

        <para>
            Many real-world examples are more complex.
            Like any other <filename>.scc</filename> file, BSP
            descriptions can aggregate features.
            Consider the Fish River Island 2 (fri2)
            BSP definition from the <filename>linux-yocto-3.4</filename>
            Git repository:
            <literallayout class='monospaced'>
     fri2.scc:
        kconf hardware fri2.cfg

        include cfg/x86.scc
        include features/eg20t/eg20t.scc
        include cfg/dmaengine.scc
        include features/ericsson-3g/f5521gw.scc
        include features/power/intel.scc
        include cfg/efi.scc
        include features/usb/ehci-hcd.scc
        include features/usb/ohci-hcd.scc
        include features/iwlwifi/iwlwifi.scc
            </literallayout>
        </para>

        <para>
            The <filename>fri2.scc</filename> description file includes
            a hardware configuration fragment
            (<filename>fri2.cfg</filename>) specific to the Fish River
            Island 2 BSP as well as several more general configuration
            fragments and features enabling hardware found on the
            machine.
            This description file is then included in each of the three
            "fri2" description files for the supported kernel types
            (i.e. "standard", "preempt-rt", and "tiny").
            Consider the "fri2" description for the "standard" kernel
            type:
            <literallayout class='monospaced'>
     fri2-standard.scc:
        define KMACHINE fri2
        define KTYPE standard
        define KARCH i386

        include ktypes/standard/standard.scc
        branch fri2

        git merge emgd-1.14

        include fri2.scc

        # Extra fri2 configs above the minimal defined in fri2.scc
        include cfg/efi-ext.scc
        include features/drm-emgd/drm-emgd.scc
        include cfg/vesafb.scc

        # default policy for standard kernels
        include cfg/usb-mass-storage.scc
            </literallayout>
            The <filename>include</filename> command midway through the file
            includes the <filename>fri2.scc</filename> description that
            defines all hardware enablements for the BSP that is common to all
            kernel types.
            Using this command significantly reduces duplication.
        </para>

        <para>
            This "fri2" standard description introduces a few more variables
            and commands that are worth further discussion.
            Notice the <filename>branch fri2</filename> command, which creates
            a machine-specific branch into which source changes are applied.
            With this branch set up, the <filename>git merge</filename> command
            uses Git to merge in a feature branch named "emgd-1.14".
            You could also handle this with the <filename>patch</filename>
            command.
            However, for commonly used features such as this, feature branches
            are a convenient mechanism.
            See the "<link linkend='feature-branches'>Feature Branches</link>"
            section for more information.
        </para>

        <para>
            Now consider the "fri2" description for the "tiny" kernel type:
            <literallayout class='monospaced'>
     fri2-tiny.scc:
        define KMACHINE fri2
        define KTYPE tiny
        define KARCH i386

        include ktypes/tiny/tiny.scc
        branch fri2

        include fri2.scc
            </literallayout>
            As you might expect, the "tiny" description includes quite a
            bit less.
            In fact, it includes only the minimal policy defined by the
            "tiny" kernel type and the hardware-specific configuration required
            for booting the machine along with the most basic functionality of
            the system as defined in the base "fri2" description file.
        </para>

        <para>
            Notice again the three critical variables:
            <filename>KMACHINE</filename>, <filename>KTYPE</filename>,
            and <filename>KARCH</filename>.
            Of these variables, only the <filename>KTYPE</filename> has changed.
            It is now set to "tiny".
        </para>
    </section>
</section>

<section id='organizing-your-source'>
    <title>Organizing Your Source</title>

    <para>
        Many recipes based on the <filename>linux-yocto-custom.bb</filename>
        recipe use Linux kernel sources that have only a single
        branch - "master".
        This type of repository structure is fine for linear development
        supporting a single machine and architecture.
        However, if you work with multiple boards and architectures,
        a kernel source repository with multiple branches is more
        efficient.
        For example, suppose you need a series of patches for one board to boot.
        Sometimes, these patches are works-in-progress or fundamentally wrong,
        yet they are still necessary for specific boards.
        In these situations, you most likely do not want to include these
        patches in every kernel you build (i.e. have the patches as part of
        the lone "master" branch).
        It is situations like these that give rise to multiple branches used
        within a Linux kernel sources Git repository.
    </para>

    <para>
        Repository organization strategies exist that maximize source reuse,
        remove redundancy, and logically order your changes.
        This section presents strategies for the following cases:
        <itemizedlist>
            <listitem><para>Encapsulating patches in a feature description
                and only including the patches in the BSP descriptions of
                the applicable boards.</para></listitem>
            <listitem><para>Creating a machine branch in your
                kernel source repository and applying the patches on that
                branch only.</para></listitem>
            <listitem><para>Creating a feature branch in your
                kernel source repository and merging that branch into your
                BSP when needed.</para></listitem>
        </itemizedlist>
    </para>

    <para>
        The approach you take is entirely up to you
        and depends on what works best for your development model.
    </para>

    <section id='encapsulating-patches'>
        <title>Encapsulating Patches</title>

        <para>
            if you are reusing patches from an external tree and are not
            working on the patches, you might find the encapsulated feature
            to be appropriate.
            Given this scenario, you do not need to create any branches in the
            source repository.
            Rather, you just take the static patches you need and encapsulate
            them within a feature description.
            Once you have the feature description, you simply include that into
            the BSP description as described in the
            "<link linkend='bsp-descriptions'>BSP Descriptions</link>"
            section.
        </para>

        <para>
            You can find information on how to create patches and BSP
            descriptions in the "<link linkend='patches'>Patches</link>" and
            "<link linkend='bsp-descriptions'>BSP Descriptions</link>"
            sections.
        </para>
    </section>

    <section id='machine-branches'>
        <title>Machine Branches</title>

        <para>
            When you have multiple machines and architectures to support,
            or you are actively working on board support, it is more
            efficient to create branches in the repository based on
            individual machines.
            Having machine branches allows common source to remain in the
            "master" branch with any features specific to a machine stored
            in the appropriate machine branch.
            This organization method frees you from continually reintegrating
            your patches into a feature.
        </para>

        <para>
            Once you have a new branch, you can set up your kernel Metadata
            to use the branch a couple different ways.
            In the recipe, you can specify the new branch as the
            <filename>KBRANCH</filename> to use for the board as
            follows:
            <literallayout class='monospaced'>
     KBRANCH = "mynewbranch"
            </literallayout>
            Another method is to use the <filename>branch</filename> command
            in the BSP description:
            <literallayout class='monospaced'>
     mybsp.scc:
        define KMACHINE mybsp
        define KTYPE standard
        define KARCH i386
        include standard.scc

        branch mynewbranch

        include mybsp-hw.scc
            </literallayout>
        </para>

        <para>
            If you find
            yourself with numerous branches, you might consider using a
            hierarchical branching system similar to what the linux-yocto Linux
            kernel repositories use:
            <literallayout class='monospaced'>
     &lt;common&gt;/&lt;kernel_type&gt;/&lt;machine&gt;
            </literallayout>
        </para>

        <para>
            If you had two kernel types, "standard" and "small" for
            instance, and three machines, the branches in your
            Git repository might look like this:
            <literallayout class='monospaced'>
     common/base
     common/standard/base
     common/standard/machine_a
     common/standard/machine_b
     common/standard/machine_c
     common/small/base
     common/small/machine_a
            </literallayout>
        </para>

        <para>
            This organization can help clarify the branch relationships.
            In this case, <filename>common/standard/machine_a</filename>
            includes everything in <filename>common/base</filename> and
            <filename>common/standard/base</filename>.
            The "standard" and "small" branches add sources specific to those
            kernel types that for whatever reason are not appropriate for the
            other branches.
            <note>The "base" branches are an artifact of the way Git manages
                its data internally on the filesystem: Git will not allow you
                to use <filename>common/standard</filename> and
                <filename>common/standard/machine_a</filename> because it
                would have to create a file and a directory named "standard".
            </note>
        </para>
    </section>

    <section id='feature-branches'>
        <title>Feature Branches</title>

        <para>
            When you are actively developing new features, it can be more
            efficient to work with that feature as a branch, rather than
            as a set of patches that have to be regularly updated.
            The Yocto Project Linux kernel tools provide for this with
            the <filename>git merge</filename> command.
        </para>

        <para>
            To merge a feature branch into a BSP, insert the
            <filename>git merge</filename> command after any
            <filename>branch</filename> commands:
            <literallayout class='monospaced'>
     mybsp.scc:
        define KMACHINE mybsp
        define KTYPE standard
        define KARCH i386
        include standard.scc

        branch mynewbranch
        git merge myfeature

        include mybsp-hw.scc
            </literallayout>
        </para>
    </section>
</section>

<section id='scc-reference'>
    <title>SCC Description File Reference</title>

    <para>
        This section provides a brief reference for the commands you can use
        within an SCC description file (<filename>.scc</filename>):
        <itemizedlist>
            <listitem><para><filename>branch [ref]</filename>:
                Creates a new branch relative to the current branch
                (typically <filename>${KTYPE}</filename>) using
                the currently checked-out branch, or "ref" if specified.
                </para></listitem>
            <listitem><para><filename>define</filename>:
                Defines variables, such as <filename>KMACHINE</filename>,
                <filename>KTYPE</filename>, <filename>KARCH</filename>,
                and <filename>KFEATURE_DESCRIPTION</filename>.</para></listitem>
            <listitem><para><filename>include SCC_FILE</filename>:
                Includes an SCC file in the current file.
                The file is parsed as if you had inserted it inline.
                </para></listitem>
            <listitem><para><filename>kconf [hardware|non-hardware] CFG_FILE</filename>:
                Queues a configuration fragment for merging into the final
                Linux <filename>.config</filename> file.</para></listitem>
            <listitem><para><filename>git merge GIT_BRANCH</filename>:
                Merges the feature branch into the current branch.
                </para></listitem>
            <listitem><para><filename>patch PATCH_FILE</filename>:
                Applies the patch to the current Git branch.</para></listitem>
        </itemizedlist>
    </para>
</section>

</chapter>
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