3 Working with Advanced Metadata (yocto-kernel-cache)

3.1 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”) are also available 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).

3.2 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 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.

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:

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 they find that matches 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.

3.3 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 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 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.

3.3.1 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.

Note

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

3.3.2 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, there are five patch statements 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.

3.3.3 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.

3.3.4 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"

Note

You can find kernel recipes in the meta/recipes-kernel/linux directory of the Yocto Project Source Repositories (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.

3.3.5 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 Yocto Project Board Support Package Developer’s Guide.

3.3.5.1 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 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 there is no hard link between the KTYPE variable and a kernel type description file. 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

3.3.5.2 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: KMACHINE, KTYPE, and KARCH. Of these variables, only KTYPE has changed to specify the “tiny” kernel type.

3.4 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.

3.4.1 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 \
    "

3.4.2 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.

3.5 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.

Here are repository organization strategies maximizing source reuse, removing redundancy, and logically ordering 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.

3.5.1 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.

3.5.2 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”.

3.5.3 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

3.6 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.