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>

    <para>
        In particular, the kernel tools allow you to specify only what you
        must, and nothing more.
        Where a complete Linux kernel <filename>.config</filename> includes
        all the automatically selected <filename>CONFIG</filename> options,
        the configuration fragments only need to contain the highest level
        visible <filename>CONFIG</filename> options as presented by the Linux
        kernel <filename>menuconfig</filename> system.
        This reduces your maintenance effort and allows you
        to further separate your configuration in ways that make sense for
        your project.
        A common split is policy and hardware.
        For example, all your kernels might support
        the <filename>proc</filename> and <filename>sys</filename> filesystems,
        but only specific boards will require sound, USB, or specific drivers.
        Specifying these individually allows you to aggregate them
        together as needed, but maintain them in only one place.
        Similar logic applies to source changes.
    </para>

    <para>
        Original Text:
        <literallayout class='monospaced'>
In addition to configuration fragments and patches, the Yocto Project kernel
tools support rich metadata which you can use to define complex policies and
BSP support. The purpose of the metadata and the tools to manage it, known as
the kern-tools (kern-tools-native_git.bb), is to assist in managing the
complexity of the configuration and sources in support of multiple BSPs and
Linux kernel types.

In particular, the kernel tools allow you to specify only what you must, and
nothing more.  Where a complete Linux kernel .config includes all the
automatically selected CONFIG options, the configuration fragments only need to
contain the highest level visible CONFIG options as presented by the Linux
kernel menuconfig system. This reduces your maintenance effort and allows you
to further separate your configuration in ways that make sense for your project.
A common split is policy and hardware. For example, all your kernels may support
the proc and sys filesystems, but only specific boards will require sound, usb,
or specific drivers. Specifying these individually allows you to aggregate them
together as needed, but maintain them in only one place. Similar logic applies
to source changes.
        </literallayout>
    </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 enablement.
        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 BitBake (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'>
     KBRANCH
     KERNEL_FEATURES
     KBRANCH_DEFAULT
     LINUX_KERNEL_TYPE
        </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 <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 inforation 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 <filename>SRC_URI</filename>.
    </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>

    <para>
        Original Text.
        <literallayout class='monospaced'>
The metadata provided with any linux-yocto style Linux kernel sources must
define a BSP that corresponds to the definition laid out in the recipe. A BSP
consists of an aggregation of kernel policy and hardware specific feature
enablement. This can be influenced from within the recipe.

Every linux-yocto style recipe must define the following variables:

	KMACHINE

KMACHINE is typically set to the same value as used within the recipe-space BSP
definition, such as "routerstationpro" or "fri2". However, multiple BSPs can
reuse the same KMACHINE name if they are built using the same BSP description
(see 3.3.5). The meta-intel "fri2" and "fri2-noemgd" are good examples of such
a situation where each specifies KMACHINE as "fri2".

They may optionally define the following variables:
	KBRANCH
	KERNEL_FEATURES
	KBRANCH_DEFAULT
	LINUX_KERNEL_TYPE

KBRANCH_DEFAULT defines the default source branch within the Linux kernel source
repository to be used to build the Linux kernel. It is used as the default value
for KBRANCH which may define an alternate branch, typically with a machine
override, such as:

KBRANCH_fri2 = "standard/fri2"

Unless you specify otherwise, KBRANCH_DEFAULT is initialized to "master".

LINUX_KERNEL_TYPE defines the kernel type to be used in assembling the
configuration and defaults to "standard" if you do not specify otherwise.
Together with KMACHINE, this defines the search arguments used by the Yocto
Project Linux kernel tools to find the appropriate description within the
metadata with which to build out the sources and configuration. The linux-yocto
recipes define "standard", "tiny", and "preempt-rt" kernel types. See 3.3.4 for
more inforation on kernel types.

During the build, the kern-tools will search for the BSP description file that
most closely matches the KMACHINE and LINUX_KERNEL_TYPE passed in from the
recipe. It will use the first BSP description it finds matching both variables.
Failing that it will issue a warning such as the following:

	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

	In this example KMACHINE was set to "fri2-broken" and LINUX_KERNEL_TYPE
	was set to "broken".

It will then search first for the KMACHINE and then
for the LINUX_KERNEL_TYPE. If it cannot find a partial match, it will use the
sources from the KBRANCH and any configuration specified in the SRC_URI.

KERNEL_FEATURES can be used to include features (configuration fragments,
patches, or both) that are not already included by the KMACHINE and
LINUX_KERNEL_TYPE combination. To include a feature specified as
"features/netfilter.scc" for example, specify:

KERNEL_FEATURES += "features/netfilter.scc"

To include a feature called "cfg/sound.scc" just for the qemux86 machine,
specify:

KERNEL_FEATURES_append_qemux86 = "cfg/sound.scc"

The value of the entries in KERNEL_FEATURES are dependent on their location
within the metadata itself. The examples here are taken from the
linux-yocto-3.4 repository where "features" and "cfg" are subdirectories of the
metadata directory. For details, see 3.3.

	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 above is accurate for 1.3 and later
	      releases of the Yocto Project.
        </literallayout>
    </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>

    <para>
        Original Text:
        <literallayout class='monospaced'>
This meta-data can be defined along with the Linux kernel recipe (recipe-space)
as partially described in section 2.2 as well as within the Linux kernel sources
themselves (in-tree).

Where you choose to store the meta-data depends on what you want to do and how
you intend to work. If you are unfamiliar with the Linux kernel and only wish
to apply a config and possibly a couple of patches provided to you by others,
you may find the recipe-space mechanism to be easier to work with. This is also
a good approach if you are working with Linux kernel sources you do not control
or if you just don't want to maintain a Linux kernel git repository on your own.

If you are doing active kernel development and are already maintaining a Linux
kernel git repository of your own, you may find it more convenient to work with
the meta-data in the same repository as the Linux kernel sources. This can make
iterative development of the Linux kernel more efficient outside of the bitbake
environment.

Regardless of where the meta-data is stored, the syntax as
described in the following sections applies equally.
        </literallayout>
    </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>${THISDIR}/${</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>

        <para>
            Original text:
            <literallayout class='monospaced'>
When stored in recipe-space, the meta-data files reside in a directory hierarchy
below FILESEXTRAPATHS, which is typically set to ${THISDIR}/${PN} for a
linux-yocto or linux-yocto-custom derived Linux kernel recipe. See 2.2.

By way of example, a trivial tree of meta-data stored in recipe-space within a
BSP layer might look like the following:

meta/
`-- recipes-kernel
    `-- linux
        `-- linux-yocto
            |-- bsp-standard.scc
            |-- bsp.cfg
            `-- standard.cfg

When the meta-data is stored in recipe-space, you must take steps to ensure
bitbake has the necessary information to decide which files to fetch and when
they need to be fetched again.

It is only necessary to specify the .scc files on the SRC_URI; bitbake will
parse them and fetch any files referenced in the .scc files by the include,
patch, or kconf commands. Because of this, it is necessary to bump the recipe PR
value when changing the content of files not explicitly listed in the SRC_URI.
            </literallayout>
        </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>
            Original text:
            <literallayout class='monospaced'>
When stored in-tree, the meta-data files reside in the "meta" directory of the
Linux kernel sources. They may be present in the same branch as the sources,
such as "master", or in their own orphan branch, typically named "meta". An
orphan branch in git is a branch with unique history and content to the other
branches in the repository. This is useful to track meta-data changes
independently from the sources of the Linux kernel, while still keeping them
together in the same repository. For the purposes of this document, we will
discuss all in-tree meta-data as residing below the "meta/cfg/kernel-cache"
directory.

By way of example, a trivial tree of meta-data stored in a custom Linux kernel
git repository might look like the following:

meta/
`-- cfg
    `-- kernel-cache
        |-- bsp-standard.scc
        |-- bsp.cfg
        `-- standard.cfg

To use a specific branch for the meta-data, specify the branch in the KMETA
variable in your Linux kernel recipe, for example:

	KMETA = "meta"

To use the same branch as the sources, set KMETA to the empty string:

	KMETA = ""

If you are working with your own sources and want to create an orphan meta
branch, you can do so using the following commands from within your Linux kernel
git repository:

	$ git checkout --orphan meta
	$ git rm -rf .
	$ git commit --allow-empty -m "Create orphan meta branch"
            </literallayout>
        </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
                <filename>cfg</filename>.</para></listitem>
            <listitem><para>If your file contains
                only source-code fixes, place the file in
                <filename>patches</filename>.</para></listitem>
            <listitem><para>If your file encapsulates
                a major feature, often combining sources and configurations,
                place the file in <filename>features</filename>.
                </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>.
                </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>

    <para>
        Original text:
        <literallayout class='monospaced'>
The Yocto Project Linux kernel tools meta-data consists of three primary types
of files: scc* description files, configuration fragments, and patches. The scc
files define variables and include or otherwise reference any of the three file
types. The description files are used to aggregate all types of meta-data into
what ultimately describes the sources and the configuration required to build a
Linux kernel tailored to a specific machine.

The scc description files are used to define two fundamental types of meta-data:
	o Features
	o BSPs

Features aggregate sources in the form of patches and configuration in the form
of configuration fragments into a modular reusable unit. Features are used to
implement conceptually separate meta-data descriptions like pure configuration
fragments, simple patches, complex features, and kernel types (ktypes). Kernel
types define general kernel features and policy to be reused in the BSPs.

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

While the meta-data 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 meta-data. The following
meta-data file hierarchy is recommended:

	&lt;base&gt;/
		bsp/
		cfg/
		features/
		ktypes/
		patches/

The bsp directory should contain the BSP descriptions, described in detail in
3.3.5. The remaining directories all contain "features"; the separation is meant
to aid in conceptualizing their intended usage.  A simple guide to determine
where your scc description file should go is as follows. If it contains only
configuration fragments, it belongs in cfg. If it contains only source-code
fixes, it belongs in patches. If it encapsulates a major feature, often
combining sources and configurations, it belongs in features. If it aggregates
non-hardware configuration and patches in order to define a base kernel policy
or major kernel type to be reused across multiple BSPs, it belongs in ktypes.
The line between these can easily become blurred, especially as out-of-tree
features are slowly merged upstream over time. Also remember that this is purely
logical organization and has no impact on the functionality of the meta-data as
all of cfg, features, patches, and ktypes, contain "features" as far as the
Yocto Project Linux kernel tools are concerned.

Paths used in meta-data files are relative to &lt;base&gt; which is either
FILESEXTRAPATHS if you are creating meta-data in recipe-space (see 3.2.1), or
meta/cfg/kernel-cache/ if you are creating meta-data in-tree (see 3.2.2).

* scc 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 it to be a description file.
        </literallayout>
    </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>
            <filename>KFEATURE_DESCRIPTION</filename> 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>

        <para>
        Original text:
            <literallayout class='monospaced'>
The simplest unit of meta-data is the configuration-only feature. It consists of
one or more Linux kernel configuration parameters in a .cfg file (as described
in section XYZ) and an scc file describing the fragment. The SMP fragment
included in the linux-yocto-3.4 git repository consists of the following two
files:

cfg/smp.scc:
	define KFEATURE_DESCRIPTION "Enable SMP"
	kconf hardware smp.cfg

cfg/smp.cfg:
	CONFIG_SMP=y
	CONFIG_SCHED_SMT=y

See 2.3.1 for details on creating configuration fragments.

KFEATURE_DESCRIPTION provides a short description of the fragment, the
primary use is for higher level tooling, such as the Yocto Project BSP Tools
(TODO:Citation).

The "kconf" command is used to include the actual configuration fragment in an
scc file, and the "hardware" keyword identifies the fragment as being hardware
enabling, as opposed to general policy (which would use the keyword
"non-hardware"). The distinction is made for the benefit of the configuration
validation tools which will warn you if a hardware fragment overrides a policy
set by a non-hardware fragment.

As described in 2.3.1, the following bitbake command can be used to audit your
configuration:

	$ bitbake linux-yocto -c kernel_configcheck -f

The description file can include multiple kconf statements, one per fragment.
            </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>

        <para>
            Original text:
            <literallayout class='monospaced'>
Patches are described in a very similar way to configuration fragments (see
3.3.1). Instead of a .cfg file, they work with source patches. A typical patch
includes a description file and the patch itself:

patches/mypatch.scc:
	patch mypatch.patch

patches/mypatch.patch:
	&lt;typical patch created with 'diff -Nurp' or 'git format-patch'&gt;

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

        <para>
        Original text:
            <literallayout class='monospaced'>
Features are a combination of configuration fragments and patches, or, more
accurately, configuration fragments and patches are simple forms of a feature, a
more complex meta-data type. In addition to the kconf and patch commands,
features often aggregate description files with the include command.

A hypothetical example of a feature description file might look like the
following:

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

Features are typically less granular than configuration fragments and are more
likely than configurations fragments and patches to be the types of things you
will want to specify in the KERNEL_FEATURES variable of the Linux kernel recipe
(see 3.1).
            </literallayout>
        </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 KTYPE myktype</filename>
            line.
            See the "<link linkend='bsp-descriptions'>BSP Descriptions</link>"
            section for more information.
        </note>

        <para>
            Original text:
            <literallayout class='monospaced'>
Kernel types, or ktypes, are used to aggregate all non-hardware configuration
fragments together with any patches you want to use for all Linux kernel builds
of the specified ktype. In short, ktypes are where you define a high-level
kernel policy. Syntactically, however, they are no different than features (see
3.3.3).  preempt-rt, and tiny. The ktype is selected by the LINUX_KERNEL_TYPE
variable in the recipe (see 3.1).

By way of example, the linux-yocto-3.4 tree defines three ktypes: standard,
tiny, and preempt-rt. The standard kernel type includes the generic Linux kernel
policy of the Yocto Project linux-yocto kernel recipes. This includes things
like which filesystems, which networking options, which core kernel features,
and which debugging and tracing optoins are supported. The preempt-rt kernel
type applies the PREEMPT_RT patches and the configuration options required to
build a real-time Linux kernel. It inherits from standard. The tiny kernel type
is independent from the standard configuration and defines a bare minimum
configuration meant to serve as a base for very small Linux kernels. Tiny does
not currently include any source changes, but it may in the future.

The standard ktype is defined by standard.scc:
	# 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

As with any scc file, a ktype definition can aggregate other scc files with the
include command, or directly pull in configuration fragments and patches with
the kconf and patch commands, respectively.

Note: It is not strictly necessary to create a ktype scc file. The BSP file can
      define the ktype implicitly with a "define KTYPE myktype" line. See 3.3.5.
            </literallayout>
        </para>
    </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
            <filename>KMACHINE</filename>, <filename>KTYPE</filename>,
            and <filename>KARCH</filename> 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
            <filename>LINUX_KERNEL_TYPE</filename> 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 enablement 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>

        <para>
            Original text:
            <literallayout class='monospaced'>
BSP descriptions combine kernel types (see 3.3.4) with hardware-specific
features (see 3.3.3). The hardware specific portion is typically defined
independently, and then aggregated with each supported kernel type. Consider a
simple example:

mybsp.scc:
	define KMACHINE mybsp
	define KTYPE standard
	define KARCH i386

	kconf mybsp.cfg

Every BSP description should include the definition of the KMACHINE, KTYPE, and
KARCH variables. These variables allow the build-system to identify this
description as meeting the criteria set by the recipe being built. This
particular description can be said to support the "mybsp" machine for the
"standard" kernel type and the "i386" architecture. Note that there is no hard
link between the KTYPE and a ktype description file. If you do not have kernel
types defined in your meta-data, you only need to ensure that the recipe
LINUX_KERNEL_TYPE and the KTYPE here match.

NOTE: future versions of the tooling make the specification of KTYPE in the BSP
      optional.

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" (see 3.3.4) and
including that description in the BSP description. You might also have multiple
hardware configurations that you aggregate into a single hardware description
file which you could include here, rather than referencing a single .cfg file.
Consider the following:

mybsp.scc:
	define KMACHINE mybsp
	define KTYPE standard
	define KARCH i386

	include standard.scc
	include mybsp.scc

In the above example standard.scc aggregates all the configuration fragments,
patches, and features that make up your standard kernel policy whereas mybsp.scc
aggregates all those necessary to support the hardware available on the mybsp
machine. For information on how to break a complete .config into the various
fragments, see 2.3.1.

Many real-world examples are more complex. Like any other scc file, BSP
descriptions can aggregate features. Consider the Fish River Island II (fri2)
BSP definitions from the linux-yocto-3.4 repository:

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

The fri2.scc description file includes a hardware configuration fragment
(fri2.cfg) specific to the fri2 BSP as well as several more general
configuration fragments and features enabling hardware found on the fri2. This
description is then included in each of the three machine-ktype descriptions
(standard, preempt-rt, and tiny). Consider the fri2 standard description:

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

Note the "include fri2.scc" line about midway through the file. By defining all
hardware enablement common to the BSP for all kernel types, duplication is
significantly reduced.

This description introduces a few more variables and commands worthy of further
discussion.  Note the "branch" command which is used to create a
machine-specific branch into which source changes can be applied. With this
branch set up, the "git merge" command uses the git SCM to merge in a feature
branch "emgd-1.14".  This could also be handled with the patch command, but for
commonly used features such as this, feature branches can be a convenient
mechanism (see 3.5).

Next consider the fri2 tiny description:

fri2-tiny.scc:
	define KMACHINE fri2
	define KTYPE tiny
	define KARCH i386

	include ktypes/tiny/tiny.scc
	branch fri2

	include fri2.scc

As you might expect, the tiny description includes quite a bit less. In fact,
it includes only the minimal policy defined by the tiny ktype and the
hardware-specific configuration required for boot and the most basic
functionality of the system as defined in the base fri2 description file. Note
again the three critical variables: KMACHINE, KTYPE, and KARCH. Of these, only
the KTYPE has changed, now set to "tiny".
            </literallayout>
        </para>
    </section>
</section>

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

    <para>
        The "<link linkend='using-kernel-metadata-in-a-recipe'>Using Kernel Metadata in a Recipe</link>"
        section introduced the <filename>KBRANCH</filename> variable, which
        defines the source branch to use from the Linux kernel Git repository
        you are using.
        Many linux-yocto-custom derived recipes will be using Linux kernel
        sources with only a single branch: "master".
        However, when you are working with multiple boards and architectures,
        you are likely to run into the situation where a series of patches
        are needed for one board to boot.
        Sometimes, these patches are works-in-progress or fundamentally wrong,
        yet still necessary for specific boards.
        In these situations, you most likely do not want to include these
        patches in every kernel you build.
        You have a couple of options.
    </para>

    <para>
        First, you could encapsulate these patches in a feature description
        and only include them in the BSP description for the board(s) that
        require them.
        For more information, see the
        "<link linkend='patches'>Patches</link>" and
        "<link linkend='bsp-descriptions'>BSP Descriptions</link>" sections.
    </para>

    <para>
        Alternatively, you can create a branch in your Linux kernel sources
        and apply the patches there.
        You can then specify this new branch as the
        <filename>KBRANCH</filename> to use for this board.
        You can do this in the recipe with the
        <filename>KBRANCH</filename> variable:
        <literallayout class='monospaced'>
     KBRANCH = "mynewbranch"
        </literallayout>
        or in the BSP description using the "branch" command:
        <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>
        The approach you take, feature or branch, is entirely up to you
        and depends on what works best for your development model.
        If you are actively working on board support, you may find that
        working within a branch is more practical than trying to continually
        reintegrate your patches into a feature.
        On the other hand, if you are simply reusing some patches from an
        external tree and are not working on them, you may find the
        encapsulated feature to be appropriate as it does not require the
        additional complexity of branching in your Linux kernel sources.
    </para>

    <para>
        If you are supporting multiple boards and architectures and 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;ktype&gt;/&lt;machine&gt;
        </literallayout>
    </para>

    <para>
        If you had two ktypes, standard and small for instance, and three
        machines, your Git tree 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 relationship of the branches to
        each other.
        In this case, "common/standard/machine_a" would include everything in
        "common/base" and "common/standard/base".
        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: it will not allow you to use
            "common/standard" and "common/standard/machine_a" because it
            would have to create a file and a directory named "standard".
        </note>
    </para>

    <para>
        Original text:
        <literallayout class='monospaced'>
Section 3.1 introduced the KBRANCH variable which defines the source branch to
use from the Linux kernel git repository you are using. Many linux-yocto-custom
derived recipes will be using Linux kernel sources with only a single branch:
"master". However, when you are working with multiple boards and architectures,
you are likely to run into the situation where a series of patches are needed
for one board to boot. Sometimes these patches are works in progress or
fundamentally wrong, yet still necessary for specific boards. In these
situations, you most likely do not want to include these patches in every kernel
you build. You have a couple of options.

First, you could encapsulate these patches in a feature description and only
include them in the BSP description for the board(s) that require them (see
3.3.2 and 3.3.5).

Alternatively, you can create a branch in your Linux kernel sources and apply
the patches there. You can then specify this new branch as the KBRANCH to use
for this board. You can do this in the recipe with the KBRANCH variable:

	KBRANCH = "mynewbranch"

or in the BSP description using the "branch" command:

mybsp.scc:
	define KMACHINE mybsp
	define KTYPE standard
	define KARCH i386
	include standard.scc

	branch mynewbranch

	include mybsp.scc

The decision of which approach to take, feature or branch, is entirely up to you
and depends on what works best for your development model. If you are actively
working on board support, you may find that working within a branch is more
practical than trying to continually reintegrate your patches into a feature. On
the other hand, if you are simply reusing some patches from an external tree and
are not working on them, you may find the encapsulated feature to be appropriate
as it does not require the additional complexity of branching in your Linux
kernel sources.

If you are supporting multiple boards and architectures and find yourself with
numerous branches, you might consider using a hierarchical branching system
similar to what the linux-yocto Linux kernel repositories use:

	&lt;common&gt;/&lt;ktype&gt;/&lt;machine&gt;

If you had two ktypes, standard and small for instance, and three machines, your
git tree might look like this:

	common/base
	common/standard/base
	common/standard/machine_a
	common/standard/machine_b
	common/standard/machine_c
	common/small/base
	common/small/machine_a

This organization can help clarify the relationship of the branches to
each other. In this case, "common/standard/machine_a" would include everything in
"common/base" and "common/standard/base". 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: it will not allow you to use
      "common/standard" and "common/standard/machine_a" because it would have to
      create a file and a directory named "standard".
        </literallayout>
    </para>
</section>

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

    <para>
        During active development a new feature, it can be more efficient
        to work with that feature as a branch, rather than as a set of
        patches which 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>

    <para>
        Original text:
        <literallayout class='monospaced'>
During active development a new feature, it can be more efficient to work with
that feature as a branch, rather than as a set of patches which have to be
regularly updated. The Yocto Project Linux kernel tools provide for this with
the "git merge" command.

To merge a feature branch into a BSP, insert the "git merge" command after any
branch commands:

mybsp.scc:
	define KMACHINE mybsp
	define KTYPE standard
	define KARCH i386
	include standard.scc

	branch mynewbranch
	git merge myfeature

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

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

    <para>
        This section provides short descriptions for the commands you can
        use from an <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>
                <para><emphasis>TODO:</emphasis> Bruce, we need to clarify
                the "relative to the current branch" bit.</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 <filename>scc</filename> file in the current file.
                It will be parsed as if inserted 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>

    <para>
        Original text:
        <literallayout class='monospaced'>
* branch [ref]

  Create a new branch relative to the current branch (typically ${KTYPE}) using
  the currently checked-out branch, or "ref" if specified.

  TODO: Bruce, we need to clarify the "relative to the current branch" bit.

* define

  Define variables, such as KMACHINE, KTYPE, KARCH, and KFEATURE_DESCRIPTION.

* include SCC_FILE

  Include an scc file in the current file. It will be parsed as if inserted
  inline.

* kconf [hardware|non-hardware] CFG_FILE

  Queue a configuration fragment for merging into the final Linux .config file.

* merge (or "git merge") GIT_BRANCH

  Merge the feature branch into the current branch.

* patch PATCH_FILE

  Apply the patch to the current git branch.
        </literallayout>
    </para>
</section>

</chapter>
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