# Working with Advanced Metadata (`yocto-kernel-cache`)

## Overview

In addition to supporting configuration fragments and patches, the Yocto Project kernel tools also support rich 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 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 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 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.

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 `KMACHINE` variable. This variable is typically set to the same value as the `MACHINE` variable, which is used by 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 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 `KBRANCH` variable must be set to indicate the branch.

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:

         KERNEL_FEATURES
         LINUX_KERNEL_TYPE

`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" 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 `SRC_URI`.

You can use the `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" section.

## Kernel Metadata Syntax

The kernel Metadata consists of three primary types of files: `scc` `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. 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 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. 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 `FILESEXTRAPATHS` if you are creating Metadata in recipe-space, or the top level of `yocto-kernel-cache` if you are creating Metadata outside of the recipe-space.

### 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" section.

Within the `smp.scc` file, the `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.

The description file can include multiple `kconf` statements, one per fragment.

As described in the "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" and "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" 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" section. The `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 `require` 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"

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.

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.

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 Yocto Project Board Support Package (BSP) Developer's Guide.

#### 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 `KMACHINE`, `KTYPE`, and `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 `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" 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" 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

#### 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`):

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: `KMACHINE`, `KTYPE`, and `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" 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 `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 `${` `THISDIR` `}/${` `PN` `}`. See the "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 `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 `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 `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" section.

You can find information on how to create patches and BSP descriptions in the "Patches" and "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.

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 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 `KMACHINE`, `KTYPE`, `KARCH`, and `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.