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+<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
+[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >
+<!--SPDX-License-Identifier: CC-BY-2.0-UK-->
+
+<chapter id='kernel-dev-advanced'>
+<title>Working with Advanced Metadata (<filename>yocto-kernel-cache</filename>)</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_REF_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 is
+        to help you manage the complexity of the configuration and sources
+        used to support multiple BSPs and Linux kernel types.
+    </para>
+
+    <para>
+        Kernel Metadata exists in many places.
+        One area in the Yocto Project
+        <ulink url='&YOCTO_DOCS_OM_URL;#source-repositories'>Source Repositories</ulink>
+        is the <filename>yocto-kernel-cache</filename> Git repository.
+        You can find this repository grouped under the "Yocto Linux Kernel"
+        heading in the
+        <ulink url='&YOCTO_GIT_URL;'>Yocto Project Source Repositories</ulink>.
+    </para>
+
+    <para>
+        Kernel development tools ("kern-tools") exist also in the Yocto
+        Project Source Repositories under the "Yocto Linux Kernel" heading
+        in the <filename>yocto-kernel-tools</filename> Git repository.
+        The recipe that builds these tools is
+        <filename>meta/recipes-kernel/kern-tools/kern-tools-native_git.bb</filename>
+        in the
+        <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>
+        (e.g. <filename>poky</filename>).
+    </para>
+</section>
+
+<section id='using-kernel-metadata-in-a-recipe'>
+    <title>Using Kernel Metadata in a Recipe</title>
+
+    <para>
+        As mentioned in the introduction, the Yocto Project contains kernel
+        Metadata, which is located in the
+        <filename>yocto-kernel-cache</filename> Git repository.
+        This Metadata defines Board Support Packages (BSPs) that
+        correspond to definitions in linux-yocto recipes for corresponding BSPs.
+        A BSP consists of an aggregation of kernel policy and enabled
+        hardware-specific features.
+        The BSP can be influenced from within the linux-yocto recipe.
+        <note>
+            A Linux kernel recipe that contains kernel Metadata (e.g.
+            inherits from the <filename>linux-yocto.inc</filename> 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
+        <filename>MACHINE</filename> variable, which is used by
+        <ulink url='&YOCTO_DOCS_REF_URL;#bitbake-term'>BitBake</ulink>.
+        However, in some cases, the variable might instead refer to the
+        underlying platform of the <filename>MACHINE</filename>.
+    </para>
+
+    <para>
+        Multiple BSPs can reuse the same <filename>KMACHINE</filename>
+        name if they are built using the same BSP description.
+        Multiple Corei7-based BSPs could share the same "intel-corei7-64"
+        value for <filename>KMACHINE</filename>.
+        It is important to realize that <filename>KMACHINE</filename> is
+        just for kernel mapping, while <filename>MACHINE</filename>
+        is the machine type within a BSP Layer.
+        Even with this distinction, however, these two variables can hold
+        the same value.
+        See the <link linkend='bsp-descriptions'>BSP Descriptions</link>
+        section for more information.
+    </para>
+
+    <para>
+        Every linux-yocto style recipe must also indicate the Linux kernel
+        source repository branch used to build the Linux kernel.
+        The <ulink url='&YOCTO_DOCS_REF_URL;#var-KBRANCH'><filename>KBRANCH</filename></ulink>
+        variable must be set to indicate the branch.
+        <note>
+            You can use the <filename>KBRANCH</filename> value to define an
+            alternate branch typically with a machine override as shown here
+            from the <filename>meta-yocto-bsp</filename> layer:
+            <literallayout class='monospaced'>
+     KBRANCH_edgerouter = "standard/edgerouter"
+            </literallayout>
+        </note>
+    </para>
+
+    <para>
+        The linux-yocto style recipes can optionally define the following
+        variables:
+        <literallayout class='monospaced'>
+     KERNEL_FEATURES
+     LINUX_KERNEL_TYPE
+        </literallayout>
+    </para>
+
+    <para>
+        <ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_KERNEL_TYPE'><filename>LINUX_KERNEL_TYPE</filename></ulink>
+        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 <filename>KMACHINE</filename>,
+        <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.
+    </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
+        <ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'><filename>KERNEL_FEATURES</filename></ulink>
+        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/netfilter.scc",
+        specify:
+        <literallayout class='monospaced'>
+     KERNEL_FEATURES += "features/netfilter/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>yocto-kernel-cache</filename> repository.
+        Each branch of this repository contains "features" and "cfg"
+        subdirectories at the top-level.
+        For more information, see the
+        "<link linkend='kernel-metadata-syntax'>Kernel Metadata Syntax</link>"
+        section.
+    </para>
+</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'>
+     <replaceable>base</replaceable>/
+        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
+        <replaceable>base</replaceable>, 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 the top level of
+        <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/yocto-kernel-cache/tree/'><filename>yocto-kernel-cache</filename></ulink>
+        if you are creating
+        <link linkend='metadata-outside-the-recipe-space'>Metadata outside of the recipe-space</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 a <filename>.scc</filename> file
+            that describes the fragment.
+        </para>
+
+        <para>
+            As an example, consider the Symmetric Multi-Processing (SMP)
+            fragment used with the <filename>linux-yocto-4.12</filename>
+            kernel as defined outside of the recipe space (i.e.
+            <filename>yocto-kernel-cache</filename>).
+            This Metadata consists of two files: <filename>smp.scc</filename>
+            and <filename>smp.cfg</filename>.
+            You can find these files in the <filename>cfg</filename> directory
+            of the <filename>yocto-4.12</filename> branch in the
+            <filename>yocto-kernel-cache</filename> Git repository:
+            <literallayout class='monospaced'>
+     cfg/smp.scc:
+        define KFEATURE_DESCRIPTION "Enable SMP for 32 bit builds"
+        define KFEATURE_COMPATIBILITY all
+
+        kconf hardware smp.cfg
+
+     cfg/smp.cfg:
+        CONFIG_SMP=y
+        CONFIG_SCHED_SMT=y
+        # Increase default NR_CPUS from 8 to 64 so that platform with
+        # more than 8 processors can be all activated at boot time
+        CONFIG_NR_CPUS=64
+        # The following is needed when setting NR_CPUS to something
+        # greater than 8 on x86 architectures, it should be automatically
+        # disregarded by Kconfig when using a different arch
+        CONFIG_X86_BIGSMP=y
+            </literallayout>
+            You can find general information on configuration fragment files in
+            the
+            "<link linkend='creating-config-fragments'>Creating Configuration Fragments</link>"
+            section.
+        </para>
+
+        <para>
+            Within the <filename>smp.scc</filename> file, the
+            <ulink url='&YOCTO_DOCS_REF_URL;#var-KFEATURE_DESCRIPTION'><filename>KFEATURE_DESCRIPTION</filename></ulink>
+            statement provides a short description of the fragment.
+            Higher level kernel tools use this description.
+        </para>
+
+        <para>
+            Also within the <filename>smp.scc</filename> file, the
+            <filename>kconf</filename> command includes 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='validating-configuration'>Validating Configuration</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 (i.e.
+            <filename>.patch</filename> files).
+        </para>
+
+        <para>
+            A typical patch includes a description file and the patch itself.
+            As an example, consider the build patches used with the
+            <filename>linux-yocto-4.12</filename> kernel as defined outside of
+            the recipe space (i.e. <filename>yocto-kernel-cache</filename>).
+            This Metadata consists of several files:
+            <filename>build.scc</filename> and a set of
+            <filename>*.patch</filename> files.
+            You can find these files in the <filename>patches/build</filename>
+            directory of the <filename>yocto-4.12</filename> branch in the
+            <filename>yocto-kernel-cache</filename> Git repository.
+        </para>
+
+        <para>
+            The following listings show the <filename>build.scc</filename>
+            file and part of the
+            <filename>modpost-mask-trivial-warnings.patch</filename> file:
+            <literallayout class='monospaced'>
+     patches/build/build.scc:
+        patch arm-serialize-build-targets.patch
+        patch powerpc-serialize-image-targets.patch
+        patch kbuild-exclude-meta-directory-from-distclean-processi.patch
+
+        # applied by kgit
+        # patch kbuild-add-meta-files-to-the-ignore-li.patch
+
+        patch modpost-mask-trivial-warnings.patch
+        patch menuconfig-check-lxdiaglog.sh-Allow-specification-of.patch
+
+     patches/build/modpost-mask-trivial-warnings.patch:
+        From bd48931bc142bdd104668f3a062a1f22600aae61 Mon Sep 17 00:00:00 2001
+        From: Paul Gortmaker &lt;paul.gortmaker@windriver.com&gt;
+        Date: Sun, 25 Jan 2009 17:58:09 -0500
+        Subject: [PATCH] modpost: mask trivial warnings
+
+        Newer HOSTCC will complain about various stdio fcns because
+                          .
+                          .
+                          .
+ 	        char *dump_write = NULL, *files_source = NULL;
+ 	        int opt;
+        --
+        2.10.1
+
+        generated by cgit v0.10.2 at 2017-09-28 15:23:23 (GMT)
+            </literallayout>
+            The description file can include multiple patch statements where
+            each statement handles a single patch.
+            In the example <filename>build.scc</filename> file, five patch
+            statements exist for the five patches in the directory.
+        </para>
+
+        <para>
+            You can create a typical <filename>.patch</filename> file using
+            <filename>diff -Nurp</filename> or
+            <filename>git format-patch</filename> commands.
+            For information on how to create patches, see the
+            "<link linkend='using-devtool-to-patch-the-kernel'>Using <filename>devtool</filename> to Patch the Kernel</link>"
+            and
+            "<link linkend='using-traditional-kernel-development-to-patch-the-kernel'>Using Traditional Kernel Development to Patch the Kernel</link>"
+            sections.
+        </para>
+    </section>
+
+    <section id='features'>
+        <title>Features</title>
+
+        <para>
+            Features are complex kernel Metadata types that consist
+            of configuration fragments, patches, and possibly other feature
+            description files.
+            As an example, consider the following generic listing:
+            <literallayout class='monospaced'>
+     features/<replaceable>myfeature</replaceable>.scc
+        define KFEATURE_DESCRIPTION "Enable <replaceable>myfeature</replaceable>"
+
+        patch 0001-<replaceable>myfeature</replaceable>-core.patch
+        patch 0002-<replaceable>myfeature</replaceable>-interface.patch
+
+        include cfg/<replaceable>myfeature</replaceable>_dependency.scc
+        kconf non-hardware <replaceable>myfeature</replaceable>.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 with
+            the <filename>include</filename> command.
+        </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 kernel of a
+            specific type (e.g. a real-time kernel).
+            Syntactically, kernel types are no different than features
+            as described in the "<link linkend='features'>Features</link>"
+            section.
+            The
+            <ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_KERNEL_TYPE'><filename>LINUX_KERNEL_TYPE</filename></ulink>
+            variable in the kernel recipe selects the kernel type.
+            For example, in the <filename>linux-yocto_4.12.bb</filename>
+            kernel recipe found in
+            <filename>poky/meta/recipes-kernel/linux</filename>, a
+            <ulink url='&YOCTO_DOCS_BB_URL;#require-inclusion'><filename>require</filename></ulink>
+            directive includes the
+            <filename>poky/meta/recipes-kernel/linux/linux-yocto.inc</filename>
+            file, which has the following statement that defines the default
+            kernel type:
+            <literallayout class='monospaced'>
+     LINUX_KERNEL_TYPE ??= "standard"
+            </literallayout>
+        </para>
+
+        <para>
+            Another example would be the real-time kernel (i.e.
+            <filename>linux-yocto-rt_4.12.bb</filename>).
+            This kernel recipe directly sets the kernel type as follows:
+            <literallayout class='monospaced'>
+     LINUX_KERNEL_TYPE = "preempt-rt"
+            </literallayout>
+            <note>
+                You can find kernel recipes in the
+                <filename>meta/recipes-kernel/linux</filename> directory of the
+                <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>
+                (e.g. <filename>poky/meta/recipes-kernel/linux/linux-yocto_4.12.bb</filename>).
+                See the "<link linkend='using-kernel-metadata-in-a-recipe'>Using Kernel Metadata in a Recipe</link>"
+                section for more information.
+            </note>
+        </para>
+
+        <para>
+            Three kernel types ("standard", "tiny", and "preempt-rt") are
+            supported for Linux Yocto kernels:
+            <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>
+            For any given kernel type, the Metadata is defined by the
+            <filename>.scc</filename> (e.g. <filename>standard.scc</filename>).
+            Here is a partial listing for the <filename>standard.scc</filename>
+            file, which is found in the <filename>ktypes/standard</filename>
+            directory of the <filename>yocto-kernel-cache</filename> Git
+            repository:
+            <literallayout class='monospaced'>
+     # Include this kernel type fragment to get the standard features and
+     # configuration values.
+
+     # Note: if only the features are desired, but not the configuration
+     #       then this should be included as:
+     #             include ktypes/standard/standard.scc nocfg
+     #       if no chained configuration is desired, include it as:
+     #             include ktypes/standard/standard.scc nocfg inherit
+
+
+
+     include ktypes/base/base.scc
+     branch standard
+
+     kconf non-hardware standard.cfg
+
+     include features/kgdb/kgdb.scc
+                .
+                .
+                .
+
+     include cfg/net/ip6_nf.scc
+     include cfg/net/bridge.scc
+
+     include cfg/systemd.scc
+
+     include features/rfkill/rfkill.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 (i.e. <filename>*.scc</filename> files)
+            combine kernel types with hardware-specific features.
+            The hardware-specific Metadata is typically defined
+            independently in the BSP layer, and then aggregated with each
+            supported kernel type.
+            <note>
+                For BSPs supported by the Yocto Project, the BSP description
+                files are located in the <filename>bsp</filename> directory
+                of the
+                <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/yocto-kernel-cache/tree/bsp'><filename>yocto-kernel-cache</filename></ulink>
+                repository organized under the "Yocto Linux Kernel" heading
+                in the
+                <ulink url='http://git.yoctoproject.org/cgit/cgit.cgi'>Yocto Project Source Repositories</ulink>.
+            </note>
+        </para>
+
+        <para>
+            This section overviews the BSP description structure, the
+            aggregation concepts, and presents a detailed example using
+            a BSP supported by the Yocto Project (i.e. BeagleBone Board).
+            For complete information on BSP layer file hierarchy, see the
+            <ulink url='&YOCTO_DOCS_BSP_URL;'>Yocto Project Board Support Package (BSP) Developer's Guide</ulink>.
+        </para>
+
+        <section id='bsp-description-file-overview'>
+            <title>Overview</title>
+
+            <para>
+                For simplicity, consider the following root BSP layer
+                description files for the BeagleBone board.
+                These files employ both a structure and naming convention
+                for consistency.
+                The naming convention for the file is as follows:
+                <literallayout class='monospaced'>
+     <replaceable>bsp_root_name</replaceable>-<replaceable>kernel_type</replaceable>.scc
+                </literallayout>
+                Here are some example root layer BSP filenames for the
+                BeagleBone Board BSP, which is supported by the Yocto Project:
+                <literallayout class='monospaced'>
+     beaglebone-standard.scc
+     beaglebone-preempt-rt.scc
+                </literallayout>
+                Each file uses the root name (i.e "beaglebone") BSP name
+                followed by the kernel type.
+            </para>
+
+            <para>
+                Examine the <filename>beaglebone-standard.scc</filename>
+                file:
+                <literallayout class='monospaced'>
+     define KMACHINE beaglebone
+     define KTYPE standard
+     define KARCH arm
+
+     include ktypes/standard/standard.scc
+     branch beaglebone
+
+     include beaglebone.scc
+
+     # default policy for standard kernels
+     include features/latencytop/latencytop.scc
+     include features/profiling/profiling.scc
+                </literallayout>
+                Every top-level BSP description file 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 example supports the "beaglebone" machine for the
+                "standard" kernel and the "arm" 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 the kernel type defined in your kernel
+                Metadata as it is here, you only need to ensure that the
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_KERNEL_TYPE'><filename>LINUX_KERNEL_TYPE</filename></ulink>
+                variable in the kernel recipe and the
+                <filename>KTYPE</filename> variable in the BSP description
+                file match.
+            </para>
+
+            <para>
+                To separate your kernel policy from your hardware configuration,
+                you include a kernel type (<filename>ktype</filename>), such as
+                "standard".
+                In the previous example, this is done using the following:
+                <literallayout class='monospaced'>
+     include ktypes/standard/standard.scc
+                </literallayout>
+                This file aggregates all the configuration fragments, patches,
+                and features that make up your standard kernel policy.
+                See the "<link linkend='kernel-types'>Kernel Types</link>"
+                section for more information.
+            </para>
+
+            <para>
+                To aggregate common configurations and features specific to the
+                kernel for <replaceable>mybsp</replaceable>, use the following:
+                <literallayout class='monospaced'>
+     include <replaceable>mybsp</replaceable>.scc
+                </literallayout>
+                You can see that in the BeagleBone example with the following:
+                <literallayout class='monospaced'>
+     include beaglebone.scc
+                </literallayout>
+                For information on how to break a complete
+                <filename>.config</filename> file into the various
+                configuration fragments, see the
+                "<link linkend='creating-config-fragments'>Creating Configuration Fragments</link>"
+                section.
+            </para>
+
+            <para>
+                Finally, if you have any configurations specific to the
+                hardware that are not in a <filename>*.scc</filename> file,
+                you can include them as follows:
+                <literallayout class='monospaced'>
+     kconf hardware <replaceable>mybsp</replaceable>-<replaceable>extra</replaceable>.cfg
+                </literallayout>
+                The BeagleBone example does not include these types of
+                configurations.
+                However, the Malta 32-bit board does ("mti-malta32").
+                Here is the <filename>mti-malta32-le-standard.scc</filename>
+                file:
+                <literallayout class='monospaced'>
+     define KMACHINE mti-malta32-le
+     define KMACHINE qemumipsel
+     define KTYPE standard
+     define KARCH mips
+
+     include ktypes/standard/standard.scc
+     branch mti-malta32
+
+     include mti-malta32.scc
+     kconf hardware mti-malta32-le.cfg
+                </literallayout>
+            </para>
+        </section>
+
+        <section id='bsp-description-file-example-minnow'>
+            <title>Example</title>
+
+            <para>
+                Many real-world examples are more complex.
+                Like any other <filename>.scc</filename> file, BSP
+                descriptions can aggregate features.
+                Consider the Minnow BSP definition given the
+                <filename>linux-yocto-4.4</filename> branch of the
+                <filename>yocto-kernel-cache</filename> (i.e.
+                <filename>yocto-kernel-cache/bsp/minnow/minnow.scc</filename>):
+                <note>
+                    Although the Minnow Board BSP is unused, the Metadata
+                    remains and is being used here just as an example.
+                </note>
+                <literallayout class='monospaced'>
+         include cfg/x86.scc
+         include features/eg20t/eg20t.scc
+         include cfg/dmaengine.scc
+         include features/power/intel.scc
+         include cfg/efi.scc
+         include features/usb/ehci-hcd.scc
+         include features/usb/ohci-hcd.scc
+         include features/usb/usb-gadgets.scc
+         include features/usb/touchscreen-composite.scc
+         include cfg/timer/hpet.scc
+         include features/leds/leds.scc
+         include features/spi/spidev.scc
+         include features/i2c/i2cdev.scc
+         include features/mei/mei-txe.scc
+
+         # Earlyprintk and port debug requires 8250
+         kconf hardware cfg/8250.cfg
+
+         kconf hardware minnow.cfg
+         kconf hardware minnow-dev.cfg
+                </literallayout>
+            </para>
+
+            <para>
+                The <filename>minnow.scc</filename> description file includes
+                a hardware configuration fragment
+                (<filename>minnow.cfg</filename>) specific to the Minnow
+                BSP as well as several more general configuration
+                fragments and features enabling hardware found on the
+                machine.
+                This <filename>minnow.scc</filename> description file is then
+                included in each of the three
+                "minnow" description files for the supported kernel types
+                (i.e. "standard", "preempt-rt", and "tiny").
+                Consider the "minnow" description for the "standard" kernel
+                type (i.e. <filename>minnow-standard.scc</filename>:
+                <literallayout class='monospaced'>
+         define KMACHINE minnow
+         define KTYPE standard
+         define KARCH i386
+
+         include ktypes/standard
+
+         include minnow.scc
+
+         # Extra minnow configs above the minimal defined in minnow.scc
+         include cfg/efi-ext.scc
+         include features/media/media-all.scc
+         include features/sound/snd_hda_intel.scc
+
+         # The following should really be in standard.scc
+         # USB live-image support
+         include cfg/usb-mass-storage.scc
+         include cfg/boot-live.scc
+
+         # Basic profiling
+         include features/latencytop/latencytop.scc
+         include features/profiling/profiling.scc
+
+         # Requested drivers that don't have an existing scc
+         kconf hardware minnow-drivers-extra.cfg
+                </literallayout>
+                The <filename>include</filename> command midway through the file
+                includes the <filename>minnow.scc</filename> description that
+                defines all enabled hardware for the BSP that is common to
+                all kernel types.
+                Using this command significantly reduces duplication.
+            </para>
+
+            <para>
+                Now consider the "minnow" description for the "tiny" kernel
+                type (i.e. <filename>minnow-tiny.scc</filename>):
+                <literallayout class='monospaced'>
+        define KMACHINE minnow
+        define KTYPE tiny
+        define KARCH i386
+
+        include ktypes/tiny
+
+        include minnow.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 "minnow"
+                description file.
+            </para>
+
+            <para>
+                Notice again the three critical variables:
+                <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>.
+                Of these variables, only <filename>KTYPE</filename>
+                has changed to specify the "tiny" kernel type.
+            </para>
+        </section>
+    </section>
+</section>
+
+<section id='kernel-metadata-location'>
+    <title>Kernel Metadata Location</title>
+
+    <para>
+        Kernel Metadata always exists outside of the kernel tree either
+        defined in a kernel recipe (recipe-space) or outside of the recipe.
+        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 kernel Metadata kept outside the
+        recipe-space.
+        Working with Metadata in this area 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-<replaceable>my_bsp_layer</replaceable>/
+     `-- 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>
+
+        <para>
+            If the BSP description is in recipe space, you cannot simply list
+            the <filename>*.scc</filename> in the <filename>SRC_URI</filename>
+            statement.
+            You need to use the following form from your kernel append file:
+            <literallayout class='monospaced'>
+     SRC_URI_append_<replaceable>myplatform</replaceable> = " \
+        file://<replaceable>myplatform</replaceable>;type=kmeta;destsuffix=<replaceable>myplatform</replaceable> \
+        "
+            </literallayout>
+        </para>
+    </section>
+
+    <section id='metadata-outside-the-recipe-space'>
+        <title>Metadata Outside the Recipe-Space</title>
+
+        <para>
+            When stored outside of the recipe-space, the kernel Metadata
+            files reside in a separate repository.
+            The OpenEmbedded build system adds the Metadata to the build as
+            a "type=kmeta" repository through the
+            <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
+            variable.
+            As an example, consider the following <filename>SRC_URI</filename>
+            statement from the <filename>linux-yocto_4.12.bb</filename>
+            kernel recipe:
+            <literallayout class='monospaced'>
+     SRC_URI = "git://git.yoctoproject.org/linux-yocto-4.12.git;name=machine;branch=${KBRANCH}; \
+                git://git.yoctoproject.org/yocto-kernel-cache;type=kmeta;name=meta;branch=yocto-4.12;destsuffix=${KMETA}"
+            </literallayout>
+            <filename>${KMETA}</filename>, in this context, is simply used to
+            name the directory into which the Git fetcher places the Metadata.
+            This behavior is no different than any multi-repository
+            <filename>SRC_URI</filename> statement used in a recipe (e.g.
+            see the previous section).
+        </para>
+
+        <para>
+            You can keep kernel Metadata in a "kernel-cache", which is a
+            directory containing configuration fragments.
+            As with any Metadata kept outside the recipe-space, you simply
+            need to use the <filename>SRC_URI</filename> statement with the
+            "type=kmeta" attribute.
+            Doing so makes the kernel Metadata available during the
+            configuration phase.
+        </para>
+
+        <para>
+            If you modify the Metadata, you must not forget to update the
+            <filename>SRCREV</filename> 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='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
+            Yocto Linux Kernel Git repositories use:
+            <literallayout class='monospaced'>
+     <replaceable>common</replaceable>/<replaceable>kernel_type</replaceable>/<replaceable>machine</replaceable>
+            </literallayout>
+        </para>
+
+        <para>
+            If you had two kernel types, "standard" and "small" for
+            instance, three machines, and <replaceable>common</replaceable>
+            as <filename>mydir</filename>, the branches in your
+            Git repository might look like this:
+            <literallayout class='monospaced'>
+     mydir/base
+     mydir/standard/base
+     mydir/standard/machine_a
+     mydir/standard/machine_b
+     mydir/standard/machine_c
+     mydir/small/base
+     mydir/small/machine_a
+            </literallayout>
+        </para>
+
+        <para>
+            This organization can help clarify the branch relationships.
+            In this case, <filename>mydir/standard/machine_a</filename>
+            includes everything in <filename>mydir/base</filename> and
+            <filename>mydir/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>mydir/standard</filename> and
+                <filename>mydir/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
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-KMACHINE'><filename>KMACHINE</filename></ulink>,
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-KTYPE'><filename>KTYPE</filename></ulink>,
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-KARCH'><filename>KARCH</filename></ulink>,
+                and
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-KFEATURE_DESCRIPTION'><filename>KFEATURE_DESCRIPTION</filename></ulink>.
+                </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>
+<!--
+vim: expandtab tw=80 ts=4
+-->