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+
+
+
+
+
+ Yocto Project Kernel Architecture and Use Manual
+
+
+
+ Introduction
+
+ Yocto Project presents the kernel as a fully patched, history-clean git
+ repository.
+ The git tree represents the selected features, board support,
+ and configurations extensively tested by Yocto Project.
+ The Yocto Project kernel allows the end user to leverage community
+ best practices to seamlessly manage the development, build and debug cycles.
+
+
+ This manual describes the Yocto Project kernel by providing information
+ on its history, organization, benefits, and use.
+ The manual consists of two sections:
+
+ Concepts - Describes concepts behind the kernel.
+ You will understand how the kernel is organized and why it is organized in
+ the way it is. You will understand the benefits of the kernel's organization
+ and the mechanisms used to work with the kernel and how to apply it in your
+ design process.
+ Using the Kernel - Describes best practices and "how-to" information
+ that lets you put the kernel to practical use. Some examples are "How to Build a
+ Project Specific Tree", "How to Examine Changes in a Branch", and "Saving Kernel
+ Modifications."
+
+
+
+ For more information on the kernel, see the following links:
+
+
+
+
+
+
+ You can find more information on Yocto Project by visiting the website at
+ .
+
+
+
+
+
+ Concepts
+
+ This section provides conceptual information about the Yocto Project kernel:
+
+ Kernel Goals
+ Yocto Project Kernel Development and Maintenance Overview
+ Kernel Architecture
+ Kernel Tools
+
+
+
+ Kernel Goals
+
+ The complexity of embedded kernel design has increased dramatically.
+ Whether it is managing multiple implementations of a particular feature or tuning and
+ optimizing board specific features, flexibility and maintainability are key concerns.
+ The Yocto Project Linux kernel is presented with the embedded
+ developer's needs in mind and has evolved to assist in these key concerns.
+ For example, prior methods such as applying hundreds of patches to an extracted
+ tarball have been replaced with proven techniques that allow easy inspection,
+ bisection and analysis of changes.
+ Application of these techniques also creates a platform for performing integration and
+ collaboration with the thousands of upstream development projects.
+
+
+ With all these considerations in mind, the Yocto Project kernel and development team
+ strives to attain these goals:
+
+ Allow the end user to leverage community best practices to seamlessly
+ manage the development, build and debug cycles.
+ Create a platform for performing integration and collaboration with the
+ thousands of upstream development projects that exist.
+ Provide mechanisms that support many different work flows, front-ends and
+ management techniques.
+ Deliver the most up-to-date kernel possible while still ensuring that
+ the baseline kernel is the the most stable official release.
+ Include major technological features as part of Yocto Project's up-rev
+ strategy.
+ Present a git tree, that just like the upstream kernel.org tree, has a
+ clear and continuous history.
+ Deliver a key set of supported kernel types, where each type is tailored
+ to a specific use case (i.g. networking, consumer, devices, and so forth).
+ Employ a git branching strategy that from a customer's point of view
+ results in a linear path from the baseline kernel.org, through a select group of features and
+ ends with their BSP-specific commits.
+
+
+
+
+
+ Yocto Project Kernel Development and Maintenance Overview
+
+ Yocto Project kernel, like other kernels, is based off the Linux kernel release
+ from .
+ At the beginning of our major development cycle, we choose our Yocto Project kernel
+ based on factors like release timing, the anticipated release timing of "final" (i.e. non "rc")
+ upstream kernel.org versions, and Yocto Project feature requirements.
+ Typically this will be a kernel that is in the
+ final stages of development by the community (i.e. still in the release
+ candidate or "rc" phase) and not yet a final release.
+ But by being in the final stages of external development, we know that the
+ kernel.org final release will clearly land within the early stages of
+ the Yocto Project development window.
+
+
+ This balance allows us to deliver the most up-to-date kernel
+ as possible, while still ensuring that we have a stable official release as
+ our baseline kernel version.
+
+
+ The following figure represents the overall place the Yocto Project kernel fills.
+
+
+
+
+
+ In the figure the ultimate source for the Yocto Project kernel is a released kernel
+ from kernel.org.
+ In addition to a foundational kernel from kernel.org the commercially released
+ Yocto Project kernel contains a mix of important new mainline
+ developments, non-mainline developments, Board Support Package (BSP) developments,
+ and custom features.
+ These additions result in a commercially released Yocto Project kernel that caters
+ to specific embedded designer needs for targeted hardware.
+
+
+ Once a Yocto Project kernel is officially released the Yocto Project team goes into
+ their next development cycle, or "uprev" cycle.
+ It is important to note that the most sustainable and stable way
+ to include feature development upstream is through a kernel uprev process.
+ Back-porting of hundreds of individual fixes and minor features from various
+ kernel versions is not sustainable and can easily compromise quality.
+ During the uprev cycle, the Yocto Project team uses an ongoing analysis of
+ kernel development, BSP support, and release timing to select the best
+ possible kernel.org version.
+ The team continually monitors community kernel
+ development to look for significant features of interest.
+ The illustration depicts this by showing the team looking back to kernel.org for new features,
+ BSP features, and significant bug fixes.
+ The team does consider back-porting large features if they have a significant advantage.
+ User or community demand can also trigger a back-port or creation of new
+ functionality in the Yocto Project baseline kernel during the uprev cycle.
+
+
+ Generally speaking, every new kernel both adds features and introduces new bugs.
+ These consequences are the basic properties of upstream kernel development and are
+ managed by the Yocto Project team's kernel strategy.
+ It is the Yocto Project team's policy to not back-port minor features to the released kernel.
+ They only consider back-porting significant technological jumps - and, that is done
+ after a complete gap analysis.
+ The reason for this policy is that simply back-porting any small to medium sized change
+ from an evolving kernel can easily create mismatches, incompatibilities and very
+ subtle errors.
+
+
+ These policies result in both a stable and a cutting
+ edge kernel that mixes forward ports of existing features and significant and critical
+ new functionality.
+ Forward porting functionality in the Yocto Project kernel can be thought of as a
+ "micro uprev."
+ The many “micro uprevs” produce a kernel version with a mix of
+ important new mainline, non-mainline, BSP developments and feature integrations.
+ This kernel gives insight into new features and allows focused
+ amounts of testing to be done on the kernel, which prevents
+ surprises when selecting the next major uprev.
+ The quality of these cutting edge kernels is evolving and the kernels are used in very special
+ cases for BSP and feature development.
+
+
+
+
+ Kernel Architecture
+
+ This section describes the architecture of the Yocto Project kernel and provides information
+ on the mechanisms used to achieve that architecture.
+
+
+
+ Overview
+
+ As mentioned earlier, a key goal of Yocto Project is to present the developer with
+ a kernel that has a clear and continuous history that is visible to the user.
+ The architecture and mechanisms used achieve that goal in a manner similar to the
+ upstream kernel.org.
+
+
+
+ You can think of the Yocto Project kernel as consisting of a baseline kernel with
+ added features logically structured on top of the baseline.
+ The features are tagged and organized by way of a branching strategy implemented by the
+ source code manager (SCM) git.
+ The result is that the user has the ability to see the added features and
+ the commits that make up those features.
+ In addition to being able to see added features, the user can also view the history of what
+ made up the baseline kernel as well.
+
+
+ The following illustration shows the conceptual Yocto Project kernel.
+
+
+
+
+
+ In the illustration, the "kernel.org Branch Point" marks the specific spot (or release) from
+ which the Yocto Project kernel is created. From this point "up" in the tree features and
+ differences are organized and tagged.
+
+
+ The "Yocto Project Baseline Kernel" contains functionality that is common to every kernel
+ type and BSP that is organized further up the tree. Placing these common features in the
+ tree this way means features don't have to be duplicated along individual branches of the
+ structure.
+
+
+ From the Yocto Project Baseline Kernel branch points represent specific functionality
+ for individual BSPs as well as real-time kernels.
+ The illustration represents this through three BSP-specific branches and a real-time
+ kernel branch.
+ Each branch represents some unique functionality for the BSP or a real-time kernel.
+
+
+ The real-time kernel branch has common features for all real-time kernels and contains
+ more branches for individual BSP-specific real-time kernels.
+ The illustration shows three branches as an example.
+ Each branch points the way to specific, unique features for a respective real-time
+ kernel as they apply to a given BSP.
+
+
+ The resulting tree structure presents a clear path of markers (or branches) to the user
+ that for all practical purposes is the kernel needed for any given set of requirements.
+
+
+
+
+ Branching Strategy and Workflow
+
+ The Yocto Project team creates kernel branches at points where functionality is
+ no longer shared and thus, needs to be isolated.
+ For example, board-specific incompatibilities would require different functionality
+ and would require a branch to separate the features.
+ Likewise, for specific kernel features the same branching strategy is used.
+ This branching strategy results in a tree that has features organized to be specific
+ for particular functionality, single kernel types, or a subset of kernel types.
+ This strategy results in not having to store the same feature twice internally in the
+ tree.
+ Rather we store the unique differences required to apply the feature onto the kernel type
+ in question.
+
+
+ BSP-specific code additions are handled in a similar manner to kernel-specific additions.
+ Some BSPs only make sense given certain kernel types.
+ So, for these types, we create branches off the end of that kernel type for all
+ of the BSPs that are supported on that kernel type.
+ From the perspective of the tools that create the BSP branch, the BSP is really no
+ different than a feature.
+ Consequently, the same branching strategy applies to BSPs as it does to features.
+ So again, rather than store the BSP twice, only the unique differences for the BSP across
+ the supported multiple kernels are uniquely stored.
+
+
+ While this strategy results in a tree with a significant number of branches, it is
+ important to realize that from the customer's point of view, there is a linear
+ path that travels from the baseline kernel.org, through a select group of features and
+ ends with their BSP-specific commits.
+ In other words, the divisions of the kernel are transparent and are not relevant
+ to the developer on a day-to-day basis.
+ From the customer's perspective, this is the "master" branch.
+ They do not need not be aware of the existence of any other branches at all.
+ Of course there is value in the existence of these branches
+ in the tree, should a person decide to explore them.
+ For example, a comparison between two BSPs at either the commit level or at the line-by-line
+ code diff level is now a trivial operation.
+
+
+ Working with the kernel as a structured tree follows recognized community best practices.
+ In particular, the kernel as shipped with the product should be
+ considered an 'upstream source' and viewed as a series of
+ historical and documented modifications (commits).
+ These modifications represent the development and stabilization done
+ by the Yocto Project kernel development team.
+
+
+ Because commits only change at significant release points in the product life cycle,
+ developers can work on a branch created
+ from the last relevant commit in the shipped Yocto Project kernel.
+ As mentioned previously, the structure is transparent to the user
+ because the kernel tree is left in this state after cloning and building the kernel.
+
+
+
+
+ Source Code Manager - git
+
+ The Source Code Manager (SCM) is git and it is the obvious mechanism for meeting the
+ previously mentioned goals.
+ Not only is it the SCM for kernel.org but git continues to grow in popularity and
+ supports many different work flows, front-ends and management techniques.
+
+
+ It should be noted that you can use as much, or as little, of what git has to offer
+ as is appropriate to your project.
+
+
+
+
+
+
+
+
+
+
+
+
+
+ How to get things accomplished with the kernel
+
+ This section describes how to accomplish tasks involving the kernel's tree structure.
+ The information covers the following:
+
+ Tree construction
+ Build strategies
+ Series & Configuration Compiler
+ kgit
+ Workflow examples
+ Source Code Manager (SCM)
+ Board Support Package (BSP) template migration
+ BSP creation
+ Patching
+ Updating BSP patches and configuration
+ guilt
+ scc file example
+ "dirty" string
+ Transition kernel layer
+
+
+
+
+ Tree Construction
+
+The Yocto Project kernel repository, as shipped with the product, is created by
+compiling and executing the set of feature descriptions for every BSP/feature
+in the product. Those feature descriptions list all necessary patches,
+configuration, branching, tagging and feature divisions found in the kernel.
+
+
+The files used to describe all the valid features and BSPs in the Yocto Project
+kernel can be found in any clone of the kernel git tree. The directory
+wrs/cfg/kernel-cache/ is a snapshot of all the kernel configuration and
+feature descriptions (.scc) that were used to build the kernel repository.
+It should however be noted, that browsing the snapshot of feature
+descriptions and patches is not an effective way to determine what is in a
+particular kernel branch. Using git directly to get insight into the changes
+in a branch is more efficient and a more flexible way to inspect changes to
+the kernel. Examples of using git to inspect kernel commits are in the
+following sections.
+
+
+As a reminder, it is envisioned that a ground up reconstruction of the
+complete kernel tree is an action only taken by Yocto Project staff during an
+active development cycle. When an end user creates a project, it takes
+advantage of this complete tree in order to efficiently place a git tree
+within their project.
+
+
+The general flow of the project specific kernel tree construction is as follows:
+
+ a top level kernel feature is passed to the kernel build subsystem,
+ normally this is a BSP for a particular kernel type.
+
+ the file that describes the top level feature is located by searching
+ system directories:
+
+
+ the kernel-cache under linux/wrs/cfg/kernel-cache
+ kernel-*-cache directories in layers
+ configured and default templates
+
+
+ In a typical build a feature description of the format:
+ <bsp name>-<kernel type>.scc is the target of the search.
+
+
+ once located, the feature description is compiled into a simple script
+ of actions, or an existing equivalent script which was part of the
+ shipped kernel is located.
+
+ extra features are appended to the top level feature description. Extra
+ features can come from the command line, the configure script or
+ templates.
+
+ each extra feature is located, compiled and appended to the script from
+ step #3
+
+ the script is executed, and a meta-series is produced. The meta-series
+ is a description of all the branches, tags, patches and configuration that
+ need to be applied to the base git repository to completely create the
+ "bsp_name-kernel_type".
+
+ the base repository (normally kernel.org) is cloned, and the actions
+ listed in the meta-series are applied to the tree.
+
+ the git repository is left with the desired branch checked out and any
+ required branching, patching and tagging has been performed.
+
+
+
+
+The tree is now ready for configuration and compilation. Those two topics will
+be covered below.
+
+
+The end user generated meta-series adds to the kernel as shipped with
+ the Yocto Project release. Any add-ons and configuration data are applied
+ to the end of an existing branch. The full repository generation that
+ is found in the linux-2.6-windriver.git is the combination of all
+ supported boards and configurations.
+
+
+
+This technique is flexible and allows the seamless blending of an immutable
+history with additional deployment specific patches. Any additions to the
+kernel become an integrated part of the branches.
+
+
+It is key that feature descriptions indicate if any branches are
+ required, since the build system cannot automatically decide where a
+ BSP should branch or if that branch point needs a name with
+ significance. There is a single restriction enforced by the compilation
+ phase:
+
+ A BSP must create a branch of the format <bsp name>-<kernel type>.
+
+ This means that all merged/support BSPs must indicate where to start
+ its branch from, with the right name, in its .scc files. The scc
+ section describes the available branching commands in more detail.
+
+
+
+
+A summary of end user tree construction activities follow:
+
+ compile and link a full top-down kernel description from feature descriptions
+ execute the complete description to generate a meta-series
+ interpret the meta-series to create a customized git repository for the
+ board
+ migrate configuration fragments and configure the kernel
+ checkout the BSP branch and build
+
+
+
+
+
+ Build Strategy
+
+There are some prerequisites that must be met before starting the compilation
+phase of the kernel build system:
+
+
+ There must be a kernel git repository indicated in the SRC_URI.
+ There must be a branch <bsp name>-<kernel type>.
+
+
+
+These are typically met by running tree construction/patching phase of the
+build system, but can be achieved by other means. Examples of alternate work
+flows such as bootstrapping a BSP are provided below.
+
+
+Before building a kernel it is configured by processing all of the
+configuration "fragments" specified by the scc feature descriptions. As the
+features are compiled, associated kernel configuration fragments are noted
+and recorded in the meta-series in their compilation order. The
+fragments are migrated, pre-processed and passed to the Linux Kernel
+Configuration subsystem (lkc) as raw input in the form of a .config file.
+The lkc uses its own internal dependency constraints to do the final
+processing of that information and generates the final .config that will
+be used during compilation.
+
+
+Kernel compilation is started, using the board's architecture and other
+relevant values from the board template, and a kernel image is produced.
+
+
+The other thing that you will first see once you configure a kernel is that
+it will generate a build tree that is separate from your git source tree.
+This build dir will be called "linux-<BSPname>-<kerntype>-build" where
+kerntype is one of standard, cg``
+e, etc. This functionality is done by making
+use of the existing support that is within the kernel.org tree by default.
+
+
+What this means, is that all the generated files (that includes the final
+".config" itself, all ".o" and ".a" etc) are now in this directory. Since
+the git source tree can contain any number of BSPs, all on their own branch,
+you now can easily switch between builds of BSPs as well, since each one also
+has their own separate build directory.
+
+
+
+
+ Series & Configuration Compiler (SCC)
+
+In early versions of the product, kernel patches were simply listed in a flat
+file called "patches.list", and then quilt was added as a tool to help
+traverse this list, which in quilt terms was called a "series" file.
+
+
+Before the 2.0 release, it was already apparent that a static series file was
+too inflexible, and that the series file had to become more dynamic and rely
+on certain state (like kernel type) in order to determine whether a patch was
+to be used or not. The 2.0 release already made use of some stateful
+construction of series files, but since the delivery mechanism was unchanged
+(tar + patches + series files), most people were not aware of anything really
+different. The 3.0 release continues with this stateful construction of
+series files, but since the delivery mechanism is changed (git + branches) it
+now is more apparent to people.
+
+
+As was previously mentioned, scc is a "series and configuration
+compiler". Its role is to combine feature descriptions into a format that can
+be used to generate a meta-series. A meta series contains all the required
+information to construct a complete set of branches that are required to
+build a desired board and feature set. The meta series is interpreted by the
+kgit tools to create a git repository that could be built.
+
+
+To illustrate how scc works, a feature description must first be understood.
+A feature description is simply a small bash shell script that is executed by
+scc in a controlled environment. Each feature description describes a set of
+operations that add patches, modify existing patches or configure the
+kernel. It is key that feature descriptions can include other features, and
+hence allow the division of patches and configuration into named, reusable
+containers.
+
+
+Each feature description can use any of the following valid scc commands:
+
+ shell constructs: bash conditionals and other utilities can be used in a feature
+ description. During compilation, the working directory is the feature
+ description itself, so any command that is "raw shell" and not from the
+ list of supported commands, can not directly modify a git repository.
+
+ patch <relative path>/<patch name>: outputs a patch to be included in a feature's patch set. Only the name of
+ the patch is supplied, the path is calculated from the currently set
+ patch directory, which is normally the feature directory itself.
+
+ patch_trigger >condition< >action< <tgt>: indicate that a trigger should be set to perform an action on a
+ patch.
+
+The conditions can be:
+
+
+ arch:<comma separated arch list or "all">
+ plat:<comma separated platform list or "all">
+
+The action can be:
+
+ exclude: This is used in exceptional situations where a patch
+ cannot be applied for certain reasons (arch or platform).
+ When the trigger is satisfied the patch will be removed from
+ the patch list.
+ include: This is used to include a patch only for a specific trigger.
+ Like exclude, this should only be used when necessary.
+ It takes 1 argument, the patch to include.
+
+
+ include <feature name> [after <feature>]: includes a feature for processing. The feature is "expanded" at the
+ position of the include directive. This means that any patches,
+ configuration or sub-includes of the feature will appear in the final
+ series before the commands that follow the include.
+
+ include searches the include directories for a matching feature name,
+ include directories are passed to scc by the caller using -I <path> and
+ is transparent to the feature script. This means that <feature name> must
+ be relative to one of the search paths. For example, if
+ /opt/kernel-cache/feat/sched.scc is to be included and scc is invoked
+ with -I /opt/kernel-cache, then a feature would issue "include
+ feat/sched.scc" to include the feature.
+
+
+ The optional "after" directive allows a feature to modify the existing
+ order of includes and insert a feature after the named feature is
+ processed. Note: the "include foo after bar" must be issued before "bar"
+ is processed, so is normally only used by a new top level feature to
+ modify the order of features in something it is including.
+
+ exclude <feature name>: Indicates that a particular feature should *not* be included even if an
+ 'include' directive is found. The exclude must be issued before the
+ include is processed, so is normally only used by a new top level feature
+ to modify the order of features in something it is including.
+
+ git <command>: Issues any git command during tree construction. Note: this command is
+ not validated/sanitized so care must be taken to not damage the
+ tree. This can be used to script branching, tagging, pulls or other git
+ operations.
+
+ dir <directory>: changes the working directory for "patch" directives. This can be used to
+ shorten a long sequence of patches by not requiring a common relative
+ directory to be issued each time.
+
+ kconf <type> <fragment name>: associates a kernel config frag with the feature.
+ <type> can be
+ "hardware" or "non-hardware" and is used by the kernel configuration
+ subsystem to audit configuration. <fragment name> is the name of a file
+ in the current feature directory that contains a series of kernel
+ configuration options. There is no restriction on the chosen fragment
+ name, although a suffix of ".cfg" is recommended. Multiple fragment
+ specifications are supported.
+
+ branch <branch name>: creates a branch in the tree. All subsequent patch commands will be
+ applied to the new branch and changes isolated from the rest of the
+ repository.
+
+ scc_leaf <base feature> <branch name>: Performs a combination feature include and branch. This is mainly a
+ convenience directive, but has significance to some build system bindings
+ as a sentinel to indicate that this intends to create a branch that is
+ valid for kernel compilation.
+
+ tag <tag name>: Tags the tree. The tag will be applied in processing order, so will
+ be after already applied patches and precede patches yet to be applied.
+
+ define <var> <value>: Creates a variable with a particular value that can be used in subsequent
+ feature descriptions.
+
+
+
+
+
+
+
+
+ Workflow Examples
+
+
+As previously noted, the Yocto Project kernel has built in git/guilt
+integration, but these utilities are not the only way to work with the kernel
+repository. Yocto Project has not made changes to git, or other tools that
+invalidate alternate workflows. Additionally, the way the kernel repository
+is constructed uses only core git functionality allowing any number of tools
+or front ends to use the resulting tree.
+
+This section contains several workflow examples.
+
+
+
+ Change Inspection: Kernel Changes/Commits
+
+A common question when working with a BSP/kernel is: "What changes have been applied to this tree?"
+
+
+In previous Yocto Project releases, there were a collection of directories that
+contained patches to the kernel, those patches could be inspected, grep'd or
+otherwise used to get a general feeling for changes. This sort of patch
+inspection is not an efficient way to determine what has been done to the
+kernel, since there are many optional patches that are selected based on the
+kernel type and feature description, not to mention patches that are actually
+in directories that are not being searched.
+
+
+A more effective way to determine what has changed in the kernel is to use
+git and inspect / search the kernel tree. This is a full view of not only the
+source code modifications, but the reasoning behind the changes.
+
+
+ What Changed in a BSP?
+
+These examples could continue for some time, since the Yocto Project git
+repository doesn't break existing git functionality and there are nearly
+endless permutations of those commands. Also note that unless a commit range
+is given (<kernel type>..<bsp>-<kernel type>), kernel.org history is blended
+with Yocto Project changes
+
+
+ # full description of the changes
+ > git whatchanged <kernel type>..<bsp>-<kernel type>
+ > eg: git whatchanged standard..common_pc-standard
+
+ # summary of the changes
+ > git log ‐‐pretty=oneline ‐‐abbrev-commit <kernel type>..<bsp>-<kernel type>
+
+ # source code changes (one combined diff)
+ > git diff <kernel type>..<bsp>-<kernel type>
+ > git show <kernel type>..<bsp>-<kernel type>
+
+ # dump individual patches per commit
+ > git format-patch -o <dir> <kernel type>..<bsp>-<kernel type>
+
+ # determine the change history of a particular file
+ > git whatchanged <path to file>
+
+ # determine the commits which touch each line in a file
+ > git blame <path to file>
+
+
+
+
+ Show a Particular Feature or Branch Change
+
+Significant features or branches are tagged in the Yocto Project tree to divide
+changes. Remember to first determine (or add) the tag of interest. Note:
+there will be many tags, since each BSP branch is tagged, kernel.org tags and
+feature tags are all present.
+
+
+ # show the changes tagged by a feature
+ > git show <tag>
+ > eg: git show yaffs2
+
+ # determine which branches contain a feature
+ > git branch ‐‐contains <tag>
+
+ # show the changes in a kernel type
+ > git whatchanged wrs_base..<kernel type>
+ > eg: git whatchanged wrs_base..standard
+
+
+Many other comparisons can be done to isolate BSP changes, such as comparing
+to kernel.org tags (v2.6.27.18, etc), per subsystem comparisons (git
+whatchanged mm) or many other types of checks.
+
+
+
+
+
+ Development: Saving Kernel Modifications
+
+Another common operation is to build a Yocto Project supplied BSP, make some
+changes, rebuild and test. Those local changes often need to be exported,
+shared or otherwise maintained.
+
+
+Since the Yocto Project kernel source tree is backed by git, this activity is
+greatly simplified and is much easier than in previous releases. git tracks
+file modifications, additions and deletions, which allows the developer to
+modify the code and later realize that the changes should be saved, and
+easily determine what was changed. It also provides many tools to commit,
+undo and export those modifications.
+
+
+There are many ways to perform this action, and the technique employed
+depends on the destination for the patches, which could be any of:
+
+ bulk storage
+ internal sharing either through patches or using git
+ external submission
+ export for integration into another SCM
+
+
+
+The destination of the patches also incluences the method of gathering them
+due to issues such as:
+
+ bisectability
+ commit headers
+ division of subsystems for separate submission / review
+
+
+
+
+ Bulk Export
+
+If patches are simply being stored outside of the kernel source repository,
+either permanently or temporarily, then there are several methods that can be
+used.
+
+
+Note the "bulk" in this discussion, these techniques are not appropriate for
+full integration of upstream submission, since they do not properly divide
+changes or provide an avenue for per-change commit messages. This example
+assumes that changes have not been committed incrementally during development
+and simply must be gathered and exported.
+
+ # bulk export of ALL modifications without separation or division
+ # of the changes
+
+ > git add .
+ > git commit -s -a -m >commit message<
+ or
+ > git commit -s -a # and interact with $EDITOR
+
+
+
+These operations have captured all the local changes in the project source
+tree in a single git commit, and that commit is also stored in the project's
+source tree.
+
+
+Once exported, those changes can then be restored manually, via a template or
+through integration with the default_kernel. Those topics are covered in
+future sections.
+
+
+
+
+ Incremental/Planned Sharing
+
+Note: unlike the previous "bulk" section, the following examples assume that
+changes have been incrementally committed to the tree during development and
+now are being exported.
+
+
+During development the following commands will be of interest, but for full
+git documentation refer to the git man pages or an online resource such as
+http://github.com
+
+ # edit a file
+ > vi >path</file
+ # stage the change
+ > git add >path</file
+ # commit the change
+ > git commit -s
+ # remove a file
+ > git rm >path</file
+ # commit the change
+ > git commit -s
+
+ ... etc.
+
+
+
+Distributed development with git is possible by having a universally agreed
+upon unique commit identifier (set by the creator of the commit) mapping to a
+specific changeset with a specific parent. This ID is created for you when
+you create a commit, and will be re-created when you amend/alter or re-apply
+a commit. As an individual in isolation, this is of no interest, but if you
+intend to share your tree with normal git push/pull operations for
+distributed development, you should consider the ramifications of changing a
+commit that you've already shared with others.
+
+
+Assuming that the changes have *not* been pushed upstream, or pulled into
+another repository, both the commit content and commit messages associated
+with development can be update via:
+
+ > git add >path</file
+ > git commit ‐‐amend
+ > git rebase or git rebase -i
+
+
+
+Again, assuming that the changes have *not* been pushed upstream, and that
+there are no pending works in progress (use "git status" to check) then
+commits can be reverted (undone) via:
+
+ # remove the commit, update working tree and remove all
+ # traces of the change
+ > git reset ‐‐hard HEAD^
+ # remove the commit, but leave the files changed and staged for re-commit
+ > git reset ‐‐soft HEAD^
+ # remove the commit, leave file change, but not staged for commit
+ > git reset ‐‐mixed HEAD^
+
+
+
+Branches can be created, changes cherry-picked or any number of git
+operations performed until the commits are in good order for pushing upstream
+or pull requests. After a push or pull, commits are normally considered
+'permanent' and should not be modified, only incrementally changed in new
+commits. This is standard "git" workflow and Yocto Project recommends the
+kernel.org best practices.
+
+It is recommend to tag or branch before adding changes to a Yocto Project
+ BSP (or creating a new one), since the branch or tag provides a
+ reference point to facilitate locating and exporting local changes.
+
+
+
+ Export Internally Via Patches
+
+Committed changes can be extracted from a working directory by exporting them
+as patches. Those patches can be used for upstream submission, placed in a
+Yocto Project template for automatic kernel patching or many other common uses.
+
+
+ # >first commit> can be a tag if one was created before development
+ # began. It can also be the parent branch if a branch was created
+ # before development began.
+
+ > git format-patch -o <dir> <first commit>..<last commit>
+
+
+
+
+ In other words:
+
+ # identify commits of interest.
+
+ # if the tree was tagged before development
+ > git format-patch -o <save dir> <tag>
+
+ # if no tags are available
+ > git format-patch -o <save dir> HEAD^ # last commit
+ > git format-patch -o <save dir> HEAD^^ # last 2 commits
+ > git whatchanged # identify last commit
+ > git format-patch -o <save dir> <commit id>
+ > git format-patch -o <save dir> <rev-list>
+
+
+
+
+The result is a directory with sequentially numbered patches, that when
+applied to a repository using "git am", will reproduce the original commit
+and all related information (author, date, commit log, etc) will be
+preserved. Note that new commit IDs will be generated upon reapplication,
+reflecting that the commit is now applied to an underlying commit with a
+different ID.
+
+
+See the "template patching" example for how to use the patches to
+automatically apply to a new kernel build.
+
+
+
+
+ Export Internally Via git
+
+Committed changes can also be exported from a working directory by pushing
+(or by making a pull request) the changes into a master repository. Those
+same change can then be pulled into a new kernel build at a later time using this command form:
+
+ git push ssh://<master server>/<path to repo> <local branch>:<remote branch>
+
+For example:
+
+ > push ssh://openlinux.windriver.com/pub/git/kernel-2.6.27 common_pc-standard:common_pc-standard
+
+A pull request entails using "git request-pull" to compose an email to the
+maintainer requesting that a branch be pulled into the master repository, see
+http://github.com/guides/pull-requests for an example.
+
+
+Other commands such as 'git stash' or branching can also be used to save
+changes, but are not covered in this document.
+
+
+See the section "importing from another SCM" for how a git push to the
+default_kernel, can be used to automatically update the builds of all users
+of a central git repository.
+
+
+
+
+
+ Export for External (Upstream) Submission
+
+If patches are to be sent for external submission, they can be done via a
+pull request if the patch series is large or the maintainer prefers to pull
+changes. But commonly, patches are sent as email series for easy review and
+integration.
+
+
+Before sending patches for review ensure that you understand the
+standard of the community in question and follow their best practices. For
+example, kernel patches should follow standards such as:
+
+
+
+ Documentation/SubmittingPatches (in any linux kernel source tree)
+
+
+
+The messages used to commit changes are a large part of these standards, so
+ensure that the headers for each commit have the required information. If the
+initial commits were not properly documented or don't meet those standards
+rebasing via git rebase -i offer an opportunity to manipulate the commits and
+get them into the required format. Other techniques such as branching and
+cherry picking commits are also viable options.
+
+
+Once complete, patches are sent via email to the maintainer(s) or lists that
+review and integrate changes. "git send-email" is commonly used to ensure
+that patches are properly formatted for easy application and avoid mailer
+induced patch damage.
+
+
+An example of dumping patches for external submission follows:
+
+ # dump the last 4 commits
+ > git format-patch ‐‐thread -n -o ~/rr/ HEAD^^^^
+ > git send-email ‐‐compose ‐‐subject '[RFC 0/N] <patch series summary>' \
+ ‐‐to foo@yoctoproject.org ‐‐to bar@yoctoproject.org \
+ ‐‐cc list@yoctoproject.org ~/rr
+ # the editor is invoked for the 0/N patch, and when complete the entire
+ # series is sent via email for review
+
+
+
+
+
+ Export for Import into Other SCM
+
+Using any one of the previously discussed techniques, commits can be exported
+as patches for import into another SCM. Note however, that if those patches
+are manually applied to a secondary tree and then that secondary tree is
+checked into the SCM, then it often results in lost information (like commit
+logs) and so it is not recommended.
+
+
+Many SCMs can directly import git commits, or can translate git patches to
+not lose information. Those facilities are SCM dependent and should be used
+whenever possible.
+
+
+
+
+
+ SCM: Working with the Yocto Project Kernel in Another SCM
+
+This is not the same as the exporting of patches to another SCM, but instead
+is concerned with kernel development that is done completely in another
+environment, but built with the Yocto Project build system. In this scenario two
+things must happen:
+
+ The delivered Yocto Project kernel must be exported into the second
+ SCM.
+ Development must be exported from that secondary SCM into a
+ format that can be used by the Yocto Project build system.
+
+
+
+ Exporting Delivered Kernel to SCM
+
+Depending on the SCM it may be possible to export the entire Yocto Project
+kernel git repository, branches and all, into a new environment. This is the
+preferred method, since it has the most flexibility and potential to maintain
+the meta data associated with each commit.
+
+
+When a direct import mechanism is not available, it is still possible to
+export a branch (or series of branches) and check them into a new
+repository.
+
+
+The following commands illustrate some of the steps that could be used to
+import the common_pc-standard kernel into a secondary SCM
+
+ > git checkout common_pc-standard
+ > cd .. ; echo linux/.git > .cvsignore
+ > cvs import -m "initial import" linux MY_COMPANY start
+
+The CVS repo could now be relocated and used in a centralized manner.
+
+
+The following commands illustrate how two BSPs could be condensed and merged
+into a second SCM:
+
+ > git checkout common_pc-standard
+ > git merge cav_ebt5800-standard
+ # resolve any conflicts and commit them
+ > cd .. ; echo linux/.git > .cvsignore
+ > cvs import -m "initial import" linux MY_COMPANY start
+
+
+
+
+
+ Importing Changes for Build
+
+Once development has reached a suitable point in the second development
+environment, changes can either be exported as patches or imported into git
+directly (if a conversion/import mechanism is available for the SCM).
+
+If changes are exported as patches, they can be placed in a template and
+automatically applied to the kernel during patching. See the template patch
+example for details.
+
+
+If changes are imported directly into git, they must be propagated to the
+wrll-linux-2.6.27/git/default_kernel bare clone of each individual build
+to be present when the kernel is checked out.
+
+The following example illustrates one variant of this workflow:
+
+ # on master git repository
+ > cd linux-2.6.27
+ > git tag -d common_pc-standard-mark
+ > git pull ssh://<foo>@<bar>/pub/git/kernel-2.6.27 common_pc-standard:common_pc-standard
+ > git tag common_pc-standard-mark
+
+ # on each build machine (or NFS share, etc)
+ > cd wrll-linux-2.6.27/git/default_kernel
+ > git fetch ssh://<foo>@<master server>/pub/git/kernel-2.6.27
+
+ # in the build, perform a from-scratch build of Linux and the new changes
+ # will be checked out and built.
+ > make linux
+
+
+
+
+
+
+ BSP: Template Migration from 2.0
+
+The move to a git-backed kernel build system in 3.0 introduced a small new
+requirement for any BSP that is not integrated into the GA release of the
+product: branching information.
+
+
+As was previously mentioned in the background sections, branching information
+is always required, since the kernel build system cannot make intelligent
+branching decisions and must rely on the developer. This branching
+information is provided via a .scc file.
+
+
+A BSP template in 2.0 contained build system information (config.sh, etc) and
+kernel patching information in the 'linux' subdirectory. The same holds true
+in 3.0, with only minor changes in the kernel patching directory.
+The ".smudge" files are now ".scc" files and now contain a full description
+ of the kernel branching, patching and configuration for the BSP. Where in
+ 2.0, they only contained kernel patching information.
+
+
+The following illustrates the migration of a simple 2.0 BSP template to the
+new 3.0 kernel build system.
+
+
+Note: all operations are from the root of a customer layer.
+
+
+ templates/
+ `‐‐ board
+ `‐‐ my_board
+ |‐‐ config.sh
+ |‐‐ include
+ `‐‐ linux
+ `‐‐ 2.6.x
+ |‐‐ knl-base.cfg
+ |‐‐ bsp.patch
+ `‐‐ my_bsp.smudge
+
+ > mv templates/board/my_board/linux/2.6.x/* templates/board/my_board/linux
+ > rm -rf templates/board/my_board/linux/2.6.x/
+ > mv templates/board/my_board/linux/my_bsp.smudge \
+ templates/board/my_board/linux/my_bsp-standard.scc
+ > echo "kconf hardware knl-base.cfg" >> \
+ templates/board/my_board/linux/my_bsp-standard.scc
+ > vi templates/board/my_board/linux/my_bsp-standard.scc
+ # add the following at the top of the file
+ scc_leaf ktypes/standard my_bsp-standard
+
+ templates/
+ `‐‐ board
+ `‐‐ my_board
+ |‐‐ config.sh
+ |‐‐ include
+ `‐‐ linux
+ |‐‐ knl-base.cfg
+ |‐‐ bsp.patch
+ `‐‐ my_bsp-standard.scc
+
+
+That's it. Configure and build.
+
+There is a naming convention for the .scc file, which allows the build
+ system to locate suitable feature descriptions for a board:
+
+
+ <bsp name>-<kernel type>.scc
+
+
+ if this naming convention isn't followed your feature description will
+ not be located and a build error thrown.
+
+
+
+
+ BSP: Creating a New BSP
+
+Although it is obvious that the structure of a new BSP uses the migrated
+directory structure from the previous example,the first question is whether
+or not the BSP is started from scratch.
+
+
+If Yocto Project has a similar BSP, it is often easier to clone and update,
+rather than start from scratch. If the mainline kernel has support, it is
+easier to branch from the -standard kernel and begin development (and not be
+concerned with undoing existing changes). This section covers both options.
+
+
+In almost every scenario, the LDAT build system bindings must be completed
+before either cloning or starting a new BSP from scratch. This is simply
+because the board template files are required to configure a project/build
+and create the necessary environment to begin working directly with the
+kernel. If it is desired to start immediately with kernel development and
+then add LDAT bindings, see the "bootstrapping a BSP" section.
+
+
+ Creating the BSP from Scratch
+
+To create the BSP from scratch you need to do the following:
+
+ Create a board template for the new BSP in a layer.
+ Configure a build with the board.
+ Configure a kernel.
+
+
+
+Following is an example showing all three steps. You start by creating a board template for the new BSP in a layer.
+
+ templates/
+ `‐‐ board
+ `‐‐ my_bsp
+ |‐‐ include
+ |‐‐ config.sh
+ `‐‐ linux
+ |‐‐ my_bsp.cfg
+ `‐‐ my_bsp-standard.scc
+
+ > cat config.sh
+ TARGET_BOARD="my_bsp"
+ TARGET_LINUX_LINKS="bzImage"
+ TARGET_SUPPORTED_KERNEL="standard"
+ TARGET_SUPPORTED_ROOTFS="glibc_std"
+ BANNER="This BSP is *NOT* supported"
+ TARGET_PROCFAM="pentium4"
+ TARGET_PLATFORMS="GPP"
+
+ > cat include
+ cpu/x86_32_i686
+ karch/i386
+
+ > cat linux/my_bsp-standard.scc
+ scc_leaf ktypes/standard/standard.scc my_bsp-standard
+
+ > cat linux/my_bsp.cfg
+ CONFIG_X86=y
+ CONFIG_SMP=y
+ CONFIG_VT=y
+ # etc, etc, etc
+
+
+
+Something like the following can now be added to a board build, and
+a project can be started:
+
+ ‐‐enable-board=my_bsp \
+ ‐‐with-layer=custom_bsp
+
+
+
+Now you can configure a kernel:
+
+ > make -C build linux.config
+
+
+
+You now have a kernel tree, which is branched and has no patches, ready for
+development.
+
+
+
+
+ Cloning an Existing BSP
+
+Cloning an existing BSP from the shipped product is similar to the "from
+scratch" option and there are two distinct ways to achieve this goal:
+
+ Create a board template for the new BSP in a layer.
+ Clone the .scc and board config.
+
+
+
+The first method is similar to the from scratch BSP where you create a board template for the new
+BSP. Although in this case, copying an existing board template from
+wrll-wrlinux/templates/board would be appropriate, since we are cloning an
+existing BSP. Edit the config.sh, include and other board options for the new
+BSP.
+
+
+The second method is to clone the .scc and board config.
+To do this, in the newly created board template, create a linux subdirectory and export
+the .scc and configuration from the source BSP in the published Yocto Project
+kernel. During construction, all of the configuration and patches were
+captured, so it is simply a matter of extracting them.
+
+
+Extraction can be accomplished using four different techniques:
+
+ Config and patches from the bare default_kernel.
+ Clone default_kernel and checkout wrs_base.
+ Clone default_kernel and checkout BSP branch.
+ Branch from the Yocto Project BSP.
+
+
+
+Technique 1: config and patches from the bare default_kernel
+
+ > cd layers/wrll-linux-2.6.27/git/default_kernel
+ > git show checkpoint_end | filterdiff -i '*common_pc*' | patch -s -p2 -d /tmp
+
+ # This will create two directories: cfg and patches.
+
+ > cd /tmp/cfg/kernel-cache/bsp/common_pc/
+
+ # This directory contains all the patches and .scc files used to construct
+ # the BSP in the shipped tree. Copy the patches to the new BSP template,
+ # and add them to the .scc file created above. See "template patching" if
+ # more details are required.
+
+
+
+Technique 2: clone default_kernel and checkout wrs_base
+
+ > git clone layers/wrll-linux-2.6.27/git/default_kernel windriver-2.6.27
+ > cd windriver-2.6.27
+ > git checkout wrs_base
+ > cd wrs/cfg/kernel-cache/bsp/common_pc
+
+# again, this directory has all the patches and .scc files used to construct
+# the BSP
+
+
+
+Technique 3: clone default_kernel and checkout BSP branch
+
+ > git clone layers/wrll-linux-2.6.27/git/default_kernel windriver-2.6.27
+ > cd windriver-2.6.27
+ > git checkout common_pc-standard
+ > git whatchanged
+ # browse patches and determine which ones are of interest, say there are
+ # 3 patches of interest
+ > git format-patch -o <path to BSP template>/linux HEAD^^^
+ # update the .scc file to add the patches, see "template patches" if
+ # more details are required
+
+
+
+Technique #4: branch from the Yocto Project BSP
+This is potentially the most "different" technique, but is actually
+ the easiest to support and leverages the infrastructure. rtcore BSPs
+ are created in a similar manner to this.
+
+
+
+In this technique the .scc file in the board template is slightly different
+ and indicates that the BSP should branch after the base Yocto Project BSP
+ of the correct kernel type, so to start a new BSP that inherits the
+ kernel patches of the common_pc-standard, the following would be done:
+
+ > cat linux/my_bsp-standard.scc
+ scc_leaf bsp/common_pc/common_pc-standard.scc my_bsp-standard
+
+
+
+ And only kernel configuration (not patches) need be contained in the
+ board template.
+
+
+ This has the advantage of automatically picking up updates to the BSP
+ and not duplicating any patches for a similar board.
+
+
+
+
+ BSP: Bootstrapping
+
+The previous examples created the board templates and configured a build
+before beginning work on a new BSP. It is also possible for advanced users to
+simply treat the Yocto Project git repository as an upstream source and begin
+BSP development directly on the repository. This is the closest match to how
+the kernel community at large would operate.
+
+
+Two techniques exist to accomplish this:
+
+
+Technique 1: upstream workflow
+
+ > git clone layers/wrll-linux-2.6.27/git/default_kernel windriver-2.6.27
+ > cd windriver-2.6.27
+ > git checkout -b my_bsp-standard common_pc-standard
+
+ # edit files, import patches, generally do BSP development
+
+ # at this point we can create the BSP template, and export the kernel
+ # changes using one of the techniques discussed in that section. For
+ # example, It is possible to push these changes, directly into the
+ # default_kernel and never directly manipulate or export patch files
+
+
+
+Technique 2: Yocto Project kernel build workflow
+
+
+ Create the BSP branch from the appropriate kernel type
+
+ > cd linux
+ # the naming convention for auto-build is <bsp>-<kernel type>
+ > git checkout -b my_bsp-standard standard
+
+
+
+Make changes, import patches, etc.
+
+ > ../../host-cross/bin/guilt init
+ # 'wrs/patches/my_bsp-standard' has now been created to
+ # manage the branches patches
+
+ # option 1: edit files, guilt import
+ > ../../host-cross/bin/guilt new extra-version.patch
+ > vi Makefile
+ > ../../host-cross/bin/guilt refresh
+ # add a header
+ > ../../host-cross/bin/guilt header -e
+ # describe the patch using best practices, like the example below:
+
+ ‐‐‐>‐‐‐>‐‐‐> cut here
+ From: Bruce Ashfield <bruce.ashfield@windriver.com>
+
+ Adds an extra version to the kernel
+
+ Modify the main EXTRAVERSION to show our bsp name
+
+ Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
+ ‐‐‐>‐‐‐>‐‐‐> cut here
+
+ # option 2: import patches
+ > git am <patch>
+ or
+ > git apply <patch>
+ > git add <files>
+ > git commit -s
+
+ # configure the board, save relevant options
+ > make ARCH=<arch> menuconfig
+
+ # save the cfg changes for reconfiguration
+ > mkdir wrs/cfg/<cache>/my_bsp
+ > vi wrs/cfg/<cache>/my_bsp/my_bsp.cfg
+
+ # classify the patches
+ > ../../host-cross/bin/kgit classify create <kernel-foo-cache>/my_bsp/my_bsp
+ # test build
+ > cd ..
+ > make linux TARGET_BOARD=my_bsp kprofile=my_bsp use_current_branch=1
+
+
+
+ Assuming the patches have been exported to the correct location, Future
+ builds will now find the board, apply the patches to the base tree and make
+ the relevant branches and structures and the special build options are no
+ longer required.
+
+
+
+
+
+ Patching
+
+The most common way to apply patches to the kernel is via a template.
+However, for more advanced applications (such as the sharing of patches between
+multiple sub-features) it is possible to patch the kernel-cache.
+This section covers both scenarios.
+
+
+ Patching: Template
+
+kernel
+templates follow the same rules as any LDAT template. A directory should be
+created in a recognized template location, with a 'linux' subdirectory. The
+'linux' directory triggers LDAT to pass the dir as a potential patch location
+to the kernel build system. Any .scc files found in that directory, will be
+automatically appended to the end of the BSP branch (for the configured
+board).
+
+
+This behavior is essentially the same since previous product
+releases. The only exception is the use of ".scc", which allows kernel
+configuration AND patches to be applied in a template.
+
+
+If creating a full template is not required, a .scc file can be placed at
+the top of the build, along with configuration and patches. The build
+system will pickup the .scc and add it onto the patch list automatically
+
+
+As an example, consider a simple template to update a BP:
+
+ > cat templates/feature/extra_version/linux/extra_version.scc
+ patch 0001-extraversion-add-Wind-River-identifier.patch
+
+
+
+To illustrate how the previous template patch was created, the following
+steps were performed:
+
+ > cd <board build>/build/linux
+ > vi Makefile
+ # modify EXTRAVERSION to have a unique string
+ > git commit -s -m "extraversion: add Yocto Project identifier" Makefile
+ > git format-patch -o <path to layer>/templates/feature/extra_version/linux/
+ > echo "patch 0001-extraversion-add-Wind-River-identifier.patch" > \
+ <path to layer>/templates/feature/extra_version/linux/extra_version.scc
+
+
+
+This next example creates a template with a linux subdirectory, just as we
+ always have for previous releases.
+
+ > mkdir templates/features/my_feature/linux
+
+
+
+ In that directory place your feature description, your
+ patch and configuration (if required).
+
+ > ls templates/features/my_feature/linux
+
+ version.patch
+ my_feature.scc
+ my_feature.cfg
+
+
+
+ The .scc file describes the patches, configuration and
+ where in the patch order the feature should be inserted.
+
+ patch version.patch
+ kconf non-hardware my_feature.cfg
+
+
+
+ Configure your build with the new template
+
+ ‐‐with-template=features/my_feature
+
+
+
+Build the kernel
+
+ > make linux
+
+
+
+
+
+ Patching: Kernel Cache
+
+As previously mentioned, this example is included for completeness, and is for more advanced
+applications (such as the sharing of patches between multiple sub-features).
+Most patching should be done via templates, since that interface is
+guaranteed not to change and the kernel-cache interface carries no such
+guarantee.
+
+
+At the top of a layer, create a kernel cache. The build system will recognize
+any directory of the name 'kernel-*-cache' as a kernel cache.
+
+ > cd <my layer>
+ >mkdir kernel-temp-cache
+
+
+
+Make a directory with the BSP
+
+ > mkdir kernel-temp-cache
+ > mkdir kernel-temp-cache/my_feat
+
+
+
+Create the feature files as they were in technique #1
+
+ > echo "patch my_patch.path" > kernel-temp-cache/my_feat/my_feature.scc
+
+
+
+Configure the build with the feature added to the kernel type
+
+ ‐‐with-kernel=standard+my_feat/my_feature.scc
+
+
+
+Build the kernel
+
+ > make linux
+
+
+
+
+
+
+ BSP: Updating Patches and Configuration
+
+As was described in the "template patching" example, it is simple
+to add patches to a BSP via a template, but often, it is desirable
+to experiment and test patches before committing them to a template.
+You can do this by modifying the BSP source.
+
+
+Start as follows:
+
+ > cd linux
+ > git checkout <bspname>-<kernel name>
+
+ > git am <patch>
+
+
+
+Or you can do this:
+
+ > kgit-import -t patch <patch>
+
+ > cd ..
+ > make linux
+
+
+
+For details on conflict resolution and patch application, see the
+git manual, or other suitable online references.
+
+ > git am <mbox>
+ # conflict
+ > git apply ‐‐reject .git/rebase-apply/0001
+ # resolve conflict
+ > git am ‐‐resolved (or git am ‐‐skip, git am ‐‐abort)
+ # continue until complete
+
+
+
+Here is another example:
+
+ # merge the patches
+ # 1) single patch
+ > git am <mbox>
+ > git apply <patch<
+ > kgit import -t patch <patch>
+
+ # 2) multiple patches
+ > git am <mbox>
+ > kgit import -t dir <dir>
+
+ # if kgit -t dir is used, a patch resolution cycle such
+ # as this can be used:
+
+ > kgit import -t dir <dir>
+ # locate rejects and resolve
+ # options:
+ > wiggle ‐‐replace <path to file> <path to reject>
+ > guilt refresh
+ or
+ > # manual resolution
+ > git add <files>
+ > git commit -s
+ or
+ > git apply ‐‐reject .git/rebase-apply/0001
+ > git add <files>
+ > git am ‐‐resolved
+ or
+ > # merge tool of choice
+
+ # continue series:
+
+ > kgit import -t dir <dir>
+ or
+ > git am ‐‐continue
+
+
+
+Once all the patches have been tested and are satisfactory, they
+should be exported via the techniques described in "saving kernel
+modifications."
+
+
+Once the kernel has been patched and configured for a BSP, it's
+configuration commonly needs to be modified. This can be done by
+running [menu|x]config on the kernel tree, or working with
+configuration fragments.
+
+
+Using menuconfig, the operation is as follows:
+
+ > make linux.menuconfig
+ > make linux.rebuild
+
+
+
+Once complete, the changes are in linux-<bsp>-<kernel type>-build/.config.
+To permanently save these changes, compare the .config before and after the
+menuconfig, and place those changes in a configuration fragment in the
+template of your choice.
+
+
+Using configuration fragments, the operation is as follows (using the
+si_is8620 as an example BSP):
+
+ > vi linux/wrs/cfg/kernel-cache/bsp/si_is8620/si_is8620.cfg
+ > make linux.reconfig
+ > make linux.rebuild
+
+
+
+The modified configuration fragment can simply be copied out of the
+linux/wrs/.. directory and placed in the appropriate template for future
+application.
+
+
+
+
+
+
+
+
+ "-dirty" String
+
+If kernel images are being built with -dirty on the end of the version
+string, this simply means that there are modification in the source
+directory that haven't been committed.
+
+ > git status
+
+
+
+The above git command will indicate modified, removed or added files. Those changes should
+be committed to the tree (even if they will never be saved, or exported
+for future use) and the kernel rebuilt.
+
+
+To brute force pickup and commit all such pending changes enter the following:
+
+ > git add .
+ > git commit -s -a -m "getting rid of -dirty"
+
+
+
+And then rebuild the kernel
+
+
+
+
+ Kernel: Transition Kernel Layer
+
+In order to temporarily use a different base kernel in Yocto Project
+Linux 3.0 you need to do the following:
+
+ Create a custom kernel layer.
+ Create a git repository of the transition kernel.
+
+
+
+Once those requirements are met multiple boards and kernels can
+be built. The cost of setup is only paid once and then additional
+BSPs and options can be added.
+
+
+This creates a transition kernel layer to evaluate functionality
+of some other kernel with the goal of easing transition to an
+integrated and validated Yocto Project kernel.
+
+
+The next few sections describe the process:
+
+
+ Creating a Custom Kernel Layer
+
+The custom kernel layer must have the following minimum
+elements:
+
+ An include of the shipped Yocto Project kernel layer.
+ A kernel-cache with an override of the standard kernel type.
+
+
+
+This allows the inheritance of the kernel build infrastructure,
+while overriding the list of patches that should be applied to
+the base kernel.
+
+
+The kernel layer can optionally include an override to the base
+Yocto Project Linux BSP to inhibit the application of BSP specific
+patches. If a custom BSP is being used, this is not required.
+
+
+
+
+ git Repo of the Transition Kernel
+
+The kernel build system requires a base kernel repository to
+seed the build process. This repository must be found in the
+same layer as the build infrastructure (i.e wrll-linux-2.6.27)
+in the 'git' subdir, with the name 'default_kernel'
+
+Since Yocto Project Linux ships with a default_kernel
+(the validated Yocto Project kernel) in the wrll-linux-2.6.27
+kernel layer, that must be removed and replaced with the
+transition kernel.
+
+If the Yocto Project install cannot be directly modified
+with the new default kernel, then the path to the transition
+kernel layer's 'git' subdir must be passed to the build
+process via:
+
+linux_GIT_BASE=<absolute path to layer>/git
+
+
+
+If the transition kernel has not been delivered via git,
+then a git repo should be created, and bare cloned into
+place. Creating this repository is as simple as:
+
+ > tar zxvf temp_kernel.tgz
+ > cd temp_kernel
+ > git init
+ > git add .
+ > git commit -a -m "Transition kernel baseline"
+
+ 'temp_kernel' can now be cloned into place via:
+
+ > cd <path to git base>/git
+ > git clone ‐‐bare <path to temp_kernel/temp_kernel default_kernel
+
+
+
+
+
+ Building the Kernel
+
+Once these prerequisites have been met, the kernel can be
+built with:
+
+ > make linux
+
+
+
+The new base kernel will be cloned into place and have any patches
+indicated in the transition kernel's cache (or templates) applied.
+The kernel build will detect the non-Yocto Project base repo and
+use the HEAD of the tree for the build.
+
+
+
+
+ Example
+
+This example creates a kernel layer to build the latest
+kernel.org tree as the 'common_pc' BSP.
+
+ > cd <path to layers>
+ > mkdir wrll-linux-my_version
+ > cd wrll-linux-my_version
+ > echo "wrll-linux-2.6.27" > include
+ > mkdir -p kernel-cache/ktypes/standard
+ > mkdir -p kernel-cache/bsp/common_pc
+ > echo "v2.6.29" > kernel-cache/kver
+ > echo "branch common_pc-standard" > kernel-cache/bsp/common_pc/common_pc.scc
+ > echo "kconf hardware common_pc.cfg" >> kernel-cache/bsp/common_pc/common_pc.scc
+ > echo "CONFIG_FOO=y" > kernel-cache/bsp/common_pc/common_pc.cfg
+ > mkdir git
+ > cd git
+ > git clone ‐‐bare git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git default_kernel
+
+
+
+Configure a build to use the new layer. This means that:
+
+ ‐‐enable-kernel-version=my_version
+
+
+
+Should be used to override the shipped default.
+
+
+To build the kernel:
+
+ > cd build
+ > make linux_GIT_BASE=<layer path>/wrll-linux-my_version/git linux
+
+
+
+If this is to build without some user intervention (passing of the
+GIT_BASE), you must do the clone into the wrll-linux-2.6.27/git directory.
+
+Unless you define valid "hardware.kcf" and "non-hardware.kcf" some
+non fatal warnings will be seen. They can be fixed by populating these
+files in the kernel-cache with valid hardware and non hardware config
+options.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+