%poky; ] > Working with Advanced Metadata
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, known as the kern-tools (kern-tools-native_git.bb), is to help you manage the complexity of the configuration and sources used to support multiple BSPs and Linux kernel types.
Using Kernel Metadata in a Recipe The kernel sources in the Yocto Project contain kernel Metadata, which is located in the meta branches of the kernel source Git repositories. This Metadata defines Board Support Packages (BSPs) that correspond to definitions in linux-yocto recipes for the same BSPs. A BSP consists of an aggregation of kernel policy and hardware-specific feature enablements. The BSP can be influenced from within the linux-yocto recipe. Linux kernel source that contains kernel Metadata is said to be "linux-yocto style" kernel source. A Linux kernel recipe that inherits from the linux-yocto.inc include 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 (e.g. "edgerouter" or "fri2"). Multiple BSPs can reuse the same KMACHINE name if they are built using the same BSP description. The "fri2" and "fri2-noemgd" BSP combination in the meta-intel layer is a good example of two BSPs using the same KMACHINE value (i.e. "fri2"). See the BSP Descriptions section for more information. The linux-yocto style recipes can optionally define the following variables: KBRANCH KERNEL_FEATURES KBRANCH_DEFAULT LINUX_KERNEL_TYPE KBRANCH_DEFAULT defines the Linux kernel source repository's default branch to use to build the Linux kernel. The value is used as the default for KBRANCH, which can define an alternate branch typically with a machine override as follows: KBRANCH_fri2 = "standard/fri2" Unless you specify otherwise, KBRANCH_DEFAULT initializes to "master". 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 such as the following: WARNING: Can't find any BSP hardware or required configuration fragments. WARNING: Looked at meta/cfg/broken/fri2-broken/hdw_frags.txt and meta/cfg/broken/fri2-broken/required_frags.txt in directory: meta/cfg/broken/fri2-broken In this example, KMACHINE was set to "fri2-broken" and LINUX_KERNEL_TYPE was set to "broken". 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.scc", specify: KERNEL_FEATURES += "features/netfilter.scc" To include a feature called "cfg/sound.scc" just for the qemux86 machine, specify: KERNEL_FEATURES_append_qemux86 = "cfg/sound.scc" The value of the entries in KERNEL_FEATURES are dependent on their location within the kernel Metadata itself. The examples here are taken from the linux-yocto-3.4 repository where "features" and "cfg" are subdirectories within the meta/cfg/kernel-cache directory. For more information, see the "Kernel Metadata Syntax" section. The processing of the these variables has evolved some between the 0.9 and 1.3 releases of the Yocto Project and associated kern-tools sources. The descriptions in this section are accurate for 1.3 and later releases of the Yocto Project.
Kernel Metadata Location Kernel Metadata can be defined in either the kernel recipe (recipe-space) or in the kernel tree (in-tree). Where you choose to define the Metadata depends on what you want to do and how you intend to work. Regardless of where you define the kernel Metadata, the syntax used applies equally. 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 the kernel Metadata in the same repository as the Linux kernel sources. This method can make iterative development of the Linux kernel more efficient outside of the BitBake environment.
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.
In-Tree Metadata When stored in-tree, the kernel Metadata files reside in the meta directory of the Linux kernel sources. The meta directory can be present in the same repository branch as the sources, such as "master", or meta can be its own orphan branch. An orphan branch in Git is a branch with unique history and content to the other branches in the repository. Orphan branches are useful to track Metadata changes independently from the sources of the Linux kernel, while still keeping them together in the same repository. For the purposes of this document, we will discuss all in-tree Metadata as residing below the meta/cfg/kernel-cache directory. Following is an example that shows how a trivial tree of Metadata is stored in a custom Linux kernel Git repository: meta/ `-- cfg `-- kernel-cache |-- bsp-standard.scc |-- bsp.cfg `-- standard.cfg To use a branch different from where the sources reside, specify the branch in the KMETA variable in your Linux kernel recipe. Here is an example: KMETA = "meta" To use the same branch as the sources, set KMETA to an empty string: KMETA = "" If you are working with your own sources and want to create an orphan meta branch, use these commands from within your Linux kernel Git repository: $ git checkout --orphan meta $ git rm -rf . $ git commit --allow-empty -m "Create orphan meta branch" If you modify the Metadata in the linux-yocto meta branch, 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.
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 meta/cfg/kernel-cache/ if you are creating Metadata in-tree.
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 an .scc file that describes the fragment. The Symmetric Multi-Processing (SMP) fragment included in the linux-yocto-3.4 Git repository consists of the following two files: cfg/smp.scc: define KFEATURE_DESCRIPTION "Enable SMP" kconf hardware smp.cfg cfg/smp.cfg: CONFIG_SMP=y CONFIG_SCHED_SMT=y You can find information on configuration fragment files in the "Creating Configuration Fragments" section of the Yocto Project Development Manual and in the "Generating Configuration Files" section earlier in this manual. KFEATURE_DESCRIPTION provides a short description of the fragment. Higher level kernel tools use this description. The kconf command is used to include the actual configuration fragment in an .scc file, and the "hardware" keyword identifies the fragment as being hardware enabling, as opposed to general policy, which would use the "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 "Generating Configuration Files" 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. A typical patch includes a description file and the patch itself: patches/mypatch.scc: patch mypatch.patch patches/mypatch.patch: typical-patch You can create the typical .patch file using diff -Nurp or git format-patch. The description file can include multiple patch statements, one per patch.
Features Features are complex kernel Metadata types that consist of configuration fragments (kconf), patches (patch), and possibly other feature description files (include). Here is an example that shows a feature description file: 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. 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 kernels of a specific type. 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. See the "Using Kernel Metadata in a Recipe" section for more information. As an example, the linux-yocto-3.4 tree defines three kernel types: "standard", "tiny", and "preempt-rt": "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. The "standard" kernel type is defined by standard.scc: # Include this kernel type fragment to get the standard features and # configuration values. # Include all standard features include standard-nocfg.scc kconf non-hardware standard.cfg # individual cfg block section include cfg/fs/devtmpfs.scc include cfg/fs/debugfs.scc include cfg/fs/btrfs.scc include cfg/fs/ext2.scc include cfg/fs/ext3.scc include cfg/fs/ext4.scc include cfg/net/ipv6.scc include cfg/net/ip_nf.scc include cfg/net/ip6_nf.scc include cfg/net/bridge.scc As with any .scc file, a 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 combine kernel types with hardware-specific features. The hardware-specific portion is typically defined independently, and then aggregated with each supported kernel type. Consider this simple BSP description that supports the "mybsp" machine: mybsp.scc: define KMACHINE mybsp define KTYPE standard define KARCH i386 kconf mybsp.cfg Every BSP description 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 simple example supports the "mybsp" machine for the "standard" kernel and the "i386" 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 kernel types defined in your kernel Metadata, you only need to ensure that the kernel recipe's LINUX_KERNEL_TYPE variable and the KTYPE variable in the BSP description file match. Future versions of the tooling make the specification of KTYPE in the BSP optional. If you did want to separate your kernel policy from your hardware configuration, you could do so by specifying a kernel type, such as "standard" and including that description file in the BSP description file. See the "Kernel Types" section for more information. You might also have multiple hardware configurations that you aggregate into a single hardware description file that you could include in the BSP description file, rather than referencing a single .cfg file. Consider the following: mybsp.scc: define KMACHINE mybsp define KTYPE standard define KARCH i386 include standard.scc include mybsp-hw.scc In the above example, standard.scc aggregates all the configuration fragments, patches, and features that make up your standard kernel policy whereas mybsp-hw.scc aggregates all those necessary to support the hardware available on the "mybsp" machine. For information on how to break a complete .config file into the various configuration fragments, see the "Generating Configuration Files" section. Many real-world examples are more complex. Like any other .scc file, BSP descriptions can aggregate features. Consider the Fish River Island 2 (fri2) BSP definition from the linux-yocto-3.4 Git repository: fri2.scc: kconf hardware fri2.cfg include cfg/x86.scc include features/eg20t/eg20t.scc include cfg/dmaengine.scc include features/ericsson-3g/f5521gw.scc include features/power/intel.scc include cfg/efi.scc include features/usb/ehci-hcd.scc include features/usb/ohci-hcd.scc include features/iwlwifi/iwlwifi.scc The fri2.scc description file includes a hardware configuration fragment (fri2.cfg) specific to the Fish River Island 2 BSP as well as several more general configuration fragments and features enabling hardware found on the machine. This description file is then included in each of the three "fri2" description files for the supported kernel types (i.e. "standard", "preempt-rt", and "tiny"). Consider the "fri2" description for the "standard" kernel type: fri2-standard.scc: define KMACHINE fri2 define KTYPE standard define KARCH i386 include ktypes/standard/standard.scc branch fri2 git merge emgd-1.14 include fri2.scc # Extra fri2 configs above the minimal defined in fri2.scc include cfg/efi-ext.scc include features/drm-emgd/drm-emgd.scc include cfg/vesafb.scc # default policy for standard kernels include cfg/usb-mass-storage.scc The include command midway through the file includes the fri2.scc description that defines all hardware enablements for the BSP that is common to all kernel types. Using this command significantly reduces duplication. This "fri2" standard description introduces a few more variables and commands that are worth further discussion. Notice the branch fri2 command, which creates a machine-specific branch into which source changes are applied. With this branch set up, the git merge command uses Git to merge in a feature branch named "emgd-1.14". You could also handle this with the patch command. However, for commonly used features such as this, feature branches are a convenient mechanism. See the "Feature Branches" section for more information. Now consider the "fri2" description for the "tiny" kernel type: fri2-tiny.scc: define KMACHINE fri2 define KTYPE tiny define KARCH i386 include ktypes/tiny/tiny.scc branch fri2 include fri2.scc As you might expect, the "tiny" description includes quite a bit less. In fact, it includes only the minimal policy defined by the "tiny" kernel type and the hardware-specific configuration required for booting the machine along with the most basic functionality of the system as defined in the base "fri2" description file. Notice again the three critical variables: KMACHINE, KTYPE, and KARCH. Of these variables, only the KTYPE has changed. It is now set to "tiny".
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 linux-yocto Linux kernel 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.