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+.. SPDX-License-Identifier: CC-BY-2.0-UK
+
+*******************************************************
+Working with Advanced Metadata (``yocto-kernel-cache``)
+*******************************************************
+
+.. _kernel-dev-advanced-overview:
+
+Overview
+========
+
+In addition to supporting configuration fragments and patches, the Yocto
+Project kernel tools also support rich
+:term:`Metadata` 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.
+
+Kernel Metadata exists in many places. One area in the
+:ref:`overview-manual/overview-manual-development-environment:yocto project source repositories`
+is the ``yocto-kernel-cache`` Git repository. You can find this repository
+grouped under the "Yocto Linux Kernel" heading in the
+:yocto_git:`Yocto Project Source Repositories <>`.
+
+Kernel development tools ("kern-tools") exist also in the Yocto Project
+Source Repositories under the "Yocto Linux Kernel" heading in the
+``yocto-kernel-tools`` Git repository. The recipe that builds these
+tools is ``meta/recipes-kernel/kern-tools/kern-tools-native_git.bb`` in
+the :term:`Source Directory` (e.g.
+``poky``).
+
+Using Kernel Metadata in a Recipe
+=================================
+
+As mentioned in the introduction, the Yocto Project contains kernel
+Metadata, which is located in the ``yocto-kernel-cache`` 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
+   linux-yocto.inc
+   file) is said to be a "linux-yocto style" recipe.
+
+Every linux-yocto style recipe must define the
+:term:`KMACHINE` variable. This
+variable is typically set to the same value as the ``MACHINE`` variable,
+which is used by :term:`BitBake`.
+However, in some cases, the variable might instead refer to the
+underlying platform of the ``MACHINE``.
+
+Multiple BSPs can reuse the same ``KMACHINE`` name if they are built
+using the same BSP description. Multiple Corei7-based BSPs could share
+the same "intel-corei7-64" value for ``KMACHINE``. It is important to
+realize that ``KMACHINE`` is just for kernel mapping, while ``MACHINE``
+is the machine type within a BSP Layer. Even with this distinction,
+however, these two variables can hold the same value. See the `BSP
+Descriptions <#bsp-descriptions>`__ section for more information.
+
+Every linux-yocto style recipe must also indicate the Linux kernel
+source repository branch used to build the Linux kernel. The
+:term:`KBRANCH` variable must be set
+to indicate the branch.
+
+.. note::
+
+   You can use the
+   KBRANCH
+   value to define an alternate branch typically with a machine override
+   as shown here from the
+   meta-yocto-bsp
+   layer:
+   ::
+
+           KBRANCH_edgerouter = "standard/edgerouter"
+
+
+The linux-yocto style recipes can optionally define the following
+variables:
+
+  - :term:`KERNEL_FEATURES`
+
+  - :term:`LINUX_KERNEL_TYPE`
+
+:term:`LINUX_KERNEL_TYPE`
+defines the kernel type to be used in assembling the configuration. If
+you do not specify a ``LINUX_KERNEL_TYPE``, it defaults to "standard".
+Together with ``KMACHINE``, ``LINUX_KERNEL_TYPE`` 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 "`Kernel Types <#kernel-types>`__"
+section for more information on kernel types.
+
+During the build, the kern-tools search for the BSP description file
+that most closely matches the ``KMACHINE`` and ``LINUX_KERNEL_TYPE``
+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.
+
+The tools first search for the ``KMACHINE`` and then for the
+``LINUX_KERNEL_TYPE``. If the tools cannot find a partial match, they
+will use the sources from the ``KBRANCH`` and any configuration
+specified in the :term:`SRC_URI`.
+
+You can use the
+:term:`KERNEL_FEATURES`
+variable to include features (configuration fragments, patches, or both)
+that are not already included by the ``KMACHINE`` and
+``LINUX_KERNEL_TYPE`` variable combination. For example, to include a
+feature specified as "features/netfilter/netfilter.scc", specify:
+::
+
+   KERNEL_FEATURES += "features/netfilter/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 kernel Metadata itself. The examples here are taken from the
+``yocto-kernel-cache`` repository. Each branch of this repository
+contains "features" and "cfg" subdirectories at the top-level. For more
+information, see the "`Kernel Metadata
+Syntax <#kernel-metadata-syntax>`__" section.
+
+Kernel Metadata Syntax
+======================
+
+The kernel Metadata consists of three primary types of files: ``scc``
+[1]_ 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 kernel Metadata 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 kernel Metadata:
+
+-  Features
+
+-  Board Support Packages (BSPs)
+
+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. `Kernel types <#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 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:
+::
+
+   base/
+      bsp/
+      cfg/
+      features/
+      ktypes/
+      patches/
+
+The ``bsp`` directory contains the `BSP
+descriptions <#bsp-descriptions>`__. The remaining directories all
+contain "features". Separating ``bsp`` from the rest of the structure
+aids conceptualizing intended usage.
+
+Use these guidelines to help place your ``scc`` description files within
+the structure:
+
+-  If your file contains only configuration fragments, place the file in
+   the ``cfg`` directory.
+
+-  If your file contains only source-code fixes, place the file in the
+   ``patches`` directory.
+
+-  If your file encapsulates a major feature, often combining sources
+   and configurations, place the file in ``features`` directory.
+
+-  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 ``ktypes`` directory.
+
+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 ``cfg``, ``features``, ``patches``, and ``ktypes``,
+contain "features" as far as the kernel tools are concerned.
+
+Paths used in kernel Metadata files are relative to base, which is
+either
+:term:`FILESEXTRAPATHS` if
+you are creating Metadata in `recipe-space <#recipe-space-metadata>`__,
+or the top level of
+:yocto_git:`yocto-kernel-cache </cgit/cgit.cgi/yocto-kernel-cache/tree/>`
+if you are creating `Metadata outside of the
+recipe-space <#metadata-outside-the-recipe-space>`__.
+
+.. [1]
+   ``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 ``scc`` files to be description files.
+
+Configuration
+-------------
+
+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 (``.cfg``) and a ``.scc``
+file that describes the fragment.
+
+As an example, consider the Symmetric Multi-Processing (SMP) fragment
+used with the ``linux-yocto-4.12`` kernel as defined outside of the
+recipe space (i.e. ``yocto-kernel-cache``). This Metadata consists of
+two files: ``smp.scc`` and ``smp.cfg``. You can find these files in the
+``cfg`` directory of the ``yocto-4.12`` branch in the
+``yocto-kernel-cache`` Git repository:
+::
+
+   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
+
+You can find general information on configuration
+fragment files in the "`Creating Configuration
+Fragments <#creating-config-fragments>`__" section.
+
+Within the ``smp.scc`` file, the
+:term:`KFEATURE_DESCRIPTION`
+statement provides a short description of the fragment. Higher level
+kernel tools use this description.
+
+Also within the ``smp.scc`` file, the ``kconf`` command includes 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 "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
+   kconf
+   statements, one per fragment.
+
+As described in the "`Validating
+Configuration <#validating-configuration>`__" section, you can use the
+following BitBake command to audit your configuration:
+::
+
+   $ bitbake linux-yocto -c kernel_configcheck -f
+
+Patches
+-------
+
+Patch descriptions are very similar to configuration fragment
+descriptions, which are described in the previous section. However,
+instead of a ``.cfg`` file, these descriptions work with source patches
+(i.e. ``.patch`` files).
+
+A typical patch includes a description file and the patch itself. As an
+example, consider the build patches used with the ``linux-yocto-4.12``
+kernel as defined outside of the recipe space (i.e.
+``yocto-kernel-cache``). This Metadata consists of several files:
+``build.scc`` and a set of ``*.patch`` files. You can find these files
+in the ``patches/build`` directory of the ``yocto-4.12`` branch in the
+``yocto-kernel-cache`` Git repository.
+
+The following listings show the ``build.scc`` file and part of the
+``modpost-mask-trivial-warnings.patch`` file:
+::
+
+   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 <paul.gortmaker@windriver.com>
+      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)
+
+The description file can
+include multiple patch statements where each statement handles a single
+patch. In the example ``build.scc`` file, five patch statements exist
+for the five patches in the directory.
+
+You can create a typical ``.patch`` file using ``diff -Nurp`` or
+``git format-patch`` commands. For information on how to create patches,
+see the "`Using ``devtool`` to Patch the
+Kernel <#using-devtool-to-patch-the-kernel>`__" and "`Using Traditional
+Kernel Development to Patch the
+Kernel <#using-traditional-kernel-development-to-patch-the-kernel>`__"
+sections.
+
+Features
+--------
+
+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:
+::
+
+   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
+
+This example shows how the ``patch`` and ``kconf`` commands are used as well
+as how an additional feature description file is included with the
+``include`` command.
+
+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 ``KERNEL_FEATURES`` variable of the
+Linux kernel recipe. See the "`Using Kernel Metadata in a
+Recipe <#using-kernel-metadata-in-a-recipe>`__" section earlier in the
+manual.
+
+Kernel Types
+------------
+
+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 "`Features <#features>`__" section. The
+:term:`LINUX_KERNEL_TYPE`
+variable in the kernel recipe selects the kernel type. For example, in
+the ``linux-yocto_4.12.bb`` kernel recipe found in
+``poky/meta/recipes-kernel/linux``, a
+:ref:`require <bitbake:require-inclusion>` directive
+includes the ``poky/meta/recipes-kernel/linux/linux-yocto.inc`` file,
+which has the following statement that defines the default kernel type:
+::
+
+   LINUX_KERNEL_TYPE ??= "standard"
+
+Another example would be the real-time kernel (i.e.
+``linux-yocto-rt_4.12.bb``). This kernel recipe directly sets the kernel
+type as follows:
+::
+
+   LINUX_KERNEL_TYPE = "preempt-rt"
+
+.. note::
+
+   You can find kernel recipes in the
+   meta/recipes-kernel/linux
+   directory of the
+   Source Directory
+   (e.g.
+   poky/meta/recipes-kernel/linux/linux-yocto_4.12.bb
+   ). See the "
+   Using Kernel Metadata in a Recipe
+   " section for more information.
+
+Three kernel types ("standard", "tiny", and "preempt-rt") are supported
+for Linux Yocto kernels:
+
+-  "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.
+
+-  "preempt-rt": Applies the ``PREEMPT_RT`` patches and the
+   configuration options required to build a real-time Linux kernel.
+   This kernel type inherits from the "standard" kernel type.
+
+-  "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.
+
+For any given kernel type, the Metadata is defined by the ``.scc`` (e.g.
+``standard.scc``). Here is a partial listing for the ``standard.scc``
+file, which is found in the ``ktypes/standard`` directory of the
+``yocto-kernel-cache`` Git repository:
+::
+
+   # 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
+
+As with any ``.scc`` file, a kernel type definition can aggregate other
+``.scc`` files with ``include`` commands. These definitions can also
+directly pull in configuration fragments and patches with the ``kconf``
+and ``patch`` commands, respectively.
+
+.. note::
+
+   It is not strictly necessary to create a kernel type
+   .scc
+   file. The Board Support Package (BSP) file can implicitly define the
+   kernel type using a
+   define
+   KTYPE
+   myktype
+   line. See the "
+   BSP Descriptions
+   " section for more information.
+
+BSP Descriptions
+----------------
+
+BSP descriptions (i.e. ``*.scc`` 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
+   bsp
+   directory of the
+   yocto-kernel-cache
+   repository organized under the "Yocto Linux Kernel" heading in the
+   Yocto Project Source Repositories
+   .
+
+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 :doc:`../bsp-guide/bsp-guide`.
+
+.. _bsp-description-file-overview:
+
+Description Overview
+~~~~~~~~~~~~~~~~~~~~
+
+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:
+::
+
+   bsp_root_name-kernel_type.scc
+
+Here are some example root layer
+BSP filenames for the BeagleBone Board BSP, which is supported by the
+Yocto Project:
+::
+
+   beaglebone-standard.scc
+   beaglebone-preempt-rt.scc
+
+Each file uses the root name (i.e "beaglebone") BSP name followed by the
+kernel type.
+
+Examine the ``beaglebone-standard.scc`` file:
+::
+
+   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
+
+Every top-level BSP description file
+should define the :term:`KMACHINE`,
+:term:`KTYPE`, and
+:term:`KARCH` 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.
+
+Be aware that a hard link between the ``KTYPE`` 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
+:term:`LINUX_KERNEL_TYPE`
+variable in the kernel recipe and the ``KTYPE`` variable in the BSP
+description file match.
+
+To separate your kernel policy from your hardware configuration, you
+include a kernel type (``ktype``), such as "standard". In the previous
+example, this is done using the following:
+::
+
+   include ktypes/standard/standard.scc
+
+This file aggregates all the configuration
+fragments, patches, and features that make up your standard kernel
+policy. See the "`Kernel Types <#kernel-types>`__" section for more
+information.
+
+To aggregate common configurations and features specific to the kernel
+for mybsp, use the following:
+::
+
+   include mybsp.scc
+
+You can see that in the BeagleBone example with the following:
+::
+
+   include beaglebone.scc
+
+For information on how to break a complete ``.config`` file into the various
+configuration fragments, see the "`Creating Configuration
+Fragments <#creating-config-fragments>`__" section.
+
+Finally, if you have any configurations specific to the hardware that
+are not in a ``*.scc`` file, you can include them as follows:
+::
+
+   kconf hardware mybsp-extra.cfg
+
+The BeagleBone example does not include these
+types of configurations. However, the Malta 32-bit board does
+("mti-malta32"). Here is the ``mti-malta32-le-standard.scc`` file:
+::
+
+   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
+
+.. _bsp-description-file-example-minnow:
+
+Example
+~~~~~~~
+
+Many real-world examples are more complex. Like any other ``.scc`` file,
+BSP descriptions can aggregate features. Consider the Minnow BSP
+definition given the ``linux-yocto-4.4`` branch of the
+``yocto-kernel-cache`` (i.e.
+``yocto-kernel-cache/bsp/minnow/minnow.scc``):
+
+.. note::
+
+   Although the Minnow Board BSP is unused, the Metadata remains and is
+   being used here just as an example.
+
+::
+
+   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
+
+The ``minnow.scc`` description file includes a hardware configuration
+fragment (``minnow.cfg``) specific to the Minnow BSP as well as several
+more general configuration fragments and features enabling hardware
+found on the machine. This ``minnow.scc`` 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.
+``minnow-standard.scc``:
+::
+
+   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
+
+The ``include`` command midway through the file includes the ``minnow.scc`` description
+that defines all enabled hardware for the BSP that is common to all
+kernel types. Using this command significantly reduces duplication.
+
+Now consider the "minnow" description for the "tiny" kernel type (i.e.
+``minnow-tiny.scc``):
+::
+
+   define KMACHINE minnow
+   define KTYPE tiny
+   define KARCH i386
+
+   include ktypes/tiny
+
+   include minnow.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" 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.
+
+Notice again the three critical variables:
+:term:`KMACHINE`,
+:term:`KTYPE`, and
+:term:`KARCH`. Of these variables, only
+``KTYPE`` has changed to specify the "tiny" kernel type.
+
+Kernel Metadata Location
+========================
+
+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.
+
+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 "`Modifying an Existing
+Recipe <#modifying-an-existing-recipe>`__" section.
+
+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.
+
+Recipe-Space Metadata
+---------------------
+
+When stored in recipe-space, the kernel Metadata files reside in a
+directory hierarchy below
+:term:`FILESEXTRAPATHS`. For
+a linux-yocto recipe or for a Linux kernel recipe derived by copying and
+modifying
+``oe-core/meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb`` to
+a recipe in your layer, ``FILESEXTRAPATHS`` is typically set to
+``${``\ :term:`THISDIR`\ ``}/${``\ :term:`PN`\ ``}``.
+See the "`Modifying an Existing
+Recipe <#modifying-an-existing-recipe>`__" section for more information.
+
+Here is an example that shows a trivial tree of kernel Metadata stored
+in recipe-space within a BSP layer:
+::
+
+   meta-my_bsp_layer/
+   `-- recipes-kernel
+       `-- linux
+           `-- linux-yocto
+               |-- bsp-standard.scc
+               |-- bsp.cfg
+               `-- standard.cfg
+
+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 ``.scc`` files on the
+:term:`SRC_URI`. BitBake parses them
+and fetches any files referenced in the ``.scc`` files by the
+``include``, ``patch``, or ``kconf`` commands. Because of this, it is
+necessary to bump the recipe :term:`PR`
+value when changing the content of files not explicitly listed in the
+``SRC_URI``.
+
+If the BSP description is in recipe space, you cannot simply list the
+``*.scc`` in the ``SRC_URI`` statement. You need to use the following
+form from your kernel append file:
+::
+
+   SRC_URI_append_myplatform = " \
+       file://myplatform;type=kmeta;destsuffix=myplatform \
+       "
+
+Metadata Outside the Recipe-Space
+---------------------------------
+
+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
+:term:`SRC_URI` variable. As an
+example, consider the following ``SRC_URI`` statement from the
+``linux-yocto_4.12.bb`` kernel recipe:
+::
+
+   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}"
+
+
+``${KMETA}``, 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 ``SRC_URI`` statement used in a recipe (e.g.
+see the previous section).
+
+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 ``SRC_URI`` statement with
+the "type=kmeta" attribute. Doing so makes the kernel Metadata available
+during the configuration phase.
+
+If you modify the Metadata, you must not forget to update the ``SRCREV``
+statements in the kernel's recipe. In particular, you need to update the
+``SRCREV_meta`` variable to match the commit in the ``KMETA`` branch you
+wish to use. Changing the data in these branches and not updating the
+``SRCREV`` statements to match will cause the build to fetch an older
+commit.
+
+Organizing Your Source
+======================
+
+Many recipes based on the ``linux-yocto-custom.bb`` 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.
+
+Repository organization strategies exist that maximize source reuse,
+remove redundancy, and logically order your changes. This section
+presents strategies for the following cases:
+
+-  Encapsulating patches in a feature description and only including the
+   patches in the BSP descriptions of the applicable boards.
+
+-  Creating a machine branch in your kernel source repository and
+   applying the patches on that branch only.
+
+-  Creating a feature branch in your kernel source repository and
+   merging that branch into your BSP when needed.
+
+The approach you take is entirely up to you and depends on what works
+best for your development model.
+
+Encapsulating Patches
+---------------------
+
+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 "`BSP Descriptions <#bsp-descriptions>`__" section.
+
+You can find information on how to create patches and BSP descriptions
+in the "`Patches <#patches>`__" and "`BSP
+Descriptions <#bsp-descriptions>`__" sections.
+
+Machine Branches
+----------------
+
+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.
+
+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 ``KBRANCH`` to use for the board as follows:
+::
+
+   KBRANCH = "mynewbranch"
+
+Another method is to use the ``branch`` command in the BSP
+description:
+
+   mybsp.scc:
+      define KMACHINE mybsp
+      define KTYPE standard
+      define KARCH i386
+      include standard.scc
+
+      branch mynewbranch
+
+      include mybsp-hw.scc
+
+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:
+::
+
+   common/kernel_type/machine
+
+If you had two kernel types, "standard" and "small" for instance, three
+machines, and common as ``mydir``, the branches in your Git repository
+might look like this:
+:
+
+   mydir/base
+   mydir/standard/base
+   mydir/standard/machine_a
+   mydir/standard/machine_b
+   mydir/standard/machine_c
+   mydir/small/base
+   mydir/small/machine_a
+
+This organization can help clarify the branch relationships. In this
+case, ``mydir/standard/machine_a`` includes everything in ``mydir/base``
+and ``mydir/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: Git will not allow you to use
+   mydir/standard
+   and
+   mydir/standard/machine_a
+   because it would have to create a file and a directory named
+   "standard".
+
+Feature Branches
+----------------
+
+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 ``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-hw.scc
+
+.. _scc-reference:
+
+SCC Description File Reference
+==============================
+
+This section provides a brief reference for the commands you can use
+within an SCC description file (``.scc``):
+
+-  ``branch [ref]``: Creates a new branch relative to the current branch
+   (typically ``${KTYPE}``) using the currently checked-out branch, or
+   "ref" if specified.
+
+-  ``define``: Defines variables, such as
+   :term:`KMACHINE`,
+   :term:`KTYPE`,
+   :term:`KARCH`, and
+   :term:`KFEATURE_DESCRIPTION`.
+
+-  ``include SCC_FILE``: Includes an SCC file in the current file. The
+   file is parsed as if you had inserted it inline.
+
+-  ``kconf [hardware|non-hardware] CFG_FILE``: Queues a configuration
+   fragment for merging into the final Linux ``.config`` file.
+
+-  ``git merge GIT_BRANCH``: Merges the feature branch into the current
+   branch.
+
+-  ``patch PATCH_FILE``: Applies the patch to the current Git branch.
+
+