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-
-<chapter id='kernel-concepts'>
-
-<title>Yocto Project Kernel Concepts</title>
-
-<section id='concepts-org'>
- <title>Introduction</title>
- <para>
- This chapter provides conceptual information about the kernel:
- <itemizedlist>
- <listitem><para>Kernel Goals</para></listitem>
- <listitem><para>Kernel Development and Maintenance Overview</para></listitem>
- <listitem><para>Kernel Architecture</para></listitem>
- <listitem><para>Kernel Tools</para></listitem>
- </itemizedlist>
- </para>
-</section>
-
- <section id='kernel-goals'>
- <title>Kernel Goals</title>
- <para>
- 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, both flexibility and maintainability are key concerns.
- The Linux kernels available through the Yocto Project are presented with the embedded
- developer's needs in mind and have 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.
- </para>
- <para>
- With all these considerations in mind, the Yocto Project's kernel and development team
- strives to attain these goals:
- <itemizedlist>
- <listitem><para>Allow the end user to leverage community best practices to seamlessly
- manage the development, build and debug cycles.</para></listitem>
- <listitem><para>Create a platform for performing integration and collaboration with the
- thousands of upstream development projects that exist.</para></listitem>
- <listitem><para>Provide mechanisms that support many different work flows, front-ends and
- management techniques.</para></listitem>
- <listitem><para>Deliver the most up-to-date kernel possible while still ensuring that
- the baseline kernel is the most stable official release.</para></listitem>
- <listitem><para>Include major technological features as part of the Yocto Project's
- upward revision strategy.</para></listitem>
- <listitem><para>Present a kernel Git repository that, similar to the upstream
- <filename>kernel.org</filename> tree,
- has a clear and continuous history.</para></listitem>
- <listitem><para>Deliver a key set of supported kernel types, where each type is tailored
- to meet a specific use (e.g. networking, consumer, devices, and so forth).</para></listitem>
- <listitem><para>Employ a Git branching strategy that, from a developer's point of view,
- results in a linear path from the baseline <filename>kernel.org</filename>,
- through a select group of features and
- ends with their BSP-specific commits.</para></listitem>
- </itemizedlist>
- </para>
- </section>
-
- <section id='kernel-big-picture'>
- <title>Yocto Project Kernel Development and Maintenance Overview</title>
- <para>
- Kernels available through the Yocto Project, like other kernels, are based off the Linux
- kernel releases from <ulink url='http://www.kernel.org'></ulink>.
- At the beginning of a major development cycle, the Yocto Project team
- chooses its kernel based on factors such as release timing, the anticipated release
- timing of final upstream <filename>kernel.org</filename> versions, and Yocto Project
- feature requirements.
- Typically, the kernel chosen is in the
- final stages of development by the community.
- In other words, the kernel is in the release
- candidate or "rc" phase and not yet a final release.
- But, by being in the final stages of external development, the team knows that the
- <filename>kernel.org</filename> final release will clearly be within the early stages of
- the Yocto Project development window.
- </para>
- <para>
- This balance allows the team to deliver the most up-to-date kernel
- possible, while still ensuring that the team has a stable official release for
- the baseline Linux kernel version.
- </para>
- <para>
- The ultimate source for kernels available through the Yocto Project are released kernels
- from <filename>kernel.org</filename>.
- In addition to a foundational kernel from <filename>kernel.org</filename>, the
- kernels available contain a mix of important new mainline
- developments, non-mainline developments (when there is no alternative),
- Board Support Package (BSP) developments,
- and custom features.
- These additions result in a commercially released Yocto Project Linux kernel that caters
- to specific embedded designer needs for targeted hardware.
- </para>
- <para>
- Once a kernel is officially released, the Yocto Project team goes into
- their next development cycle, or upward revision (uprev) cycle, while still
- continuing maintenance on the released kernel.
- 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 hundreds of individual fixes and minor features from various
- kernel versions is not sustainable and can easily compromise quality.
- </para>
- <para>
- 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 <filename>kernel.org</filename> version.
- The team continually monitors community kernel
- development to look for significant features of interest.
- 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.
- </para>
- <para>
- 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 back-porting any small to medium sized change
- from an evolving kernel can easily create mismatches, incompatibilities and very
- subtle errors.
- </para>
- <para>
- 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 kernels available through 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 leading edge
- feature and BSP development.
- </para>
- </section>
-
- <section id='kernel-architecture'>
- <title>Kernel Architecture</title>
- <para>
- This section describes the architecture of the kernels available through the
- Yocto Project and provides information
- on the mechanisms used to achieve that architecture.
- </para>
-
- <section id='architecture-overview'>
- <title>Overview</title>
- <para>
- As mentioned earlier, a key goal of the 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 <filename>kernel.org</filename>.
- </para>
- <para>
- You can think of a Yocto Project kernel as consisting of a baseline Linux 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.
- For information on Git as applied to the Yocto Project, see the
- "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>" section in the
- Yocto Project Development Manual.
- </para>
- <para>
- 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.
- </para>
- <para>
- The following illustration shows the conceptual Yocto Project kernel.
- </para>
- <para>
- <imagedata fileref="figures/kernel-architecture-overview.png" width="6in" depth="7in" align="center" scale="100" />
- </para>
- <para>
- 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.
- </para>
- <para>
- 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.
- </para>
- <para>
- 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.
- </para>
- <para>
- In this example structure, 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.
- </para>
- <para>
- The resulting tree structure presents a clear path of markers (or branches) to the
- developer that, for all practical purposes, is the kernel needed for any given set
- of requirements.
- </para>
- </section>
-
- <section id='branching-and-workflow'>
- <title>Branching Strategy and Workflow</title>
- <para>
- 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.
- </para>
- <para>
- 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 also results in not having to store the same feature twice
- internally in the tree.
- Rather, the kernel team stores the unique differences required to apply the
- feature onto the kernel type in question.
- <note>
- The Yocto Project team strives to place features in the tree such that they can be
- shared by all boards and kernel types where possible.
- However, during development cycles or when large features are merged,
- the team cannot always follow this practice.
- In those cases, the team uses isolated branches to merge features.
- </note>
- </para>
- <para>
- 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, the team creates 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, the team only stores the unique
- differences for the BSP across the supported multiple kernels.
- </para>
- <para>
- While this strategy can result in a tree with a significant number of branches, it is
- important to realize that from the developer's point of view, there is a linear
- path that travels from the baseline <filename>kernel.org</filename>, 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 developer's perspective, this path is the "master" branch.
- The developer does not need to 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 <filename>diff</filename> level is now a trivial operation.
- </para>
- <para>
- 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.
- </para>
- <para>
- 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 developer
- because the kernel tree is left in this state after cloning and building the kernel.
- </para>
- </section>
-
- <section id='source-code-manager-git'>
- <title>Source Code Manager - Git</title>
- <para>
- The Source Code Manager (SCM) is Git.
- This SCM is the obvious mechanism for meeting the previously mentioned goals.
- Not only is it the SCM for <filename>kernel.org</filename> but,
- Git continues to grow in popularity and supports many different work flows,
- front-ends and management techniques.
- </para>
- <para>
- You can find documentation on Git at <ulink url='http://git-scm.com/documentation'></ulink>.
- You can also get an introduction to Git as it applies to the Yocto Project in the
- "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>"
- section in the Yocto Project Development Manual.
- These referenced sections overview Git and describe a minimal set of
- commands that allows you to be functional using Git.
- <note>
- You can use as much, or as little, of what Git has to offer to accomplish what
- you need for your project.
- You do not have to be a "Git Master" in order to use it with the Yocto Project.
- </note>
- </para>
- </section>
- </section>
-
- <section id='kernel-configuration'>
- <title>Kernel Configuration</title>
- <para>
- Kernel configuration, along with kernel features, defines how a kernel
- image is built for the Yocto Project.
- Through configuration settings, you can customize a Yocto Project kernel to be
- specific to particular hardware.
- For example, you can specify sound support or networking support.
- This section describes basic concepts behind Kernel configuration within the
- Yocto Project and references you to other areas for specific configuration
- applications.
- </para>
-
- <para>
- Conceptually, configuration of a Yocto Project kernel occurs similarly to that needed for any
- Linux kernel.
- The build process for a Yocto Project kernel uses a <filename>.config</filename> file, which
- is created through the Linux Kernel Configuration (LKC) tool.
- You can directly set various configurations in the
- <filename>.config</filename> file by using the <filename>menuconfig</filename>
- tool as built by BitBake.
- You can also define configurations in the file by using configuration fragments.
- <note>
- It is not recommended that you edit the <filename>.config</filename> file directly.
- </note>
- Here are some brief descriptions of the ways you can affect the
- <filename>.config</filename> file:
- <itemizedlist>
- <listitem><para><emphasis>The <filename>menuconfig</filename> Tool:</emphasis>
- One of many front-ends that allows you to define kernel configurations.
- Some others are <filename>make config</filename>,
- <filename>make nconfig</filename>, and <filename>make gconfig</filename>.
- In the Yocto Project environment, you must use BitBake to build the
- <filename>menuconfig</filename> tool before you can use it to define
- configurations:
- <literallayout class='monospaced'>
- $ bitbake linux-yocto -c menuconfig
- </literallayout>
- After the tool is built, you can interact with it normally.
- You can see how <filename>menuconfig</filename> is used to change a simple
- kernel configuration in the
- "<ulink url='&YOCTO_DOCS_DEV_URL;#configuring-the-kernel'>Configuring the Kernel</ulink>"
- section of the Yocto Project Development Manual.
- For general information on <filename>menuconfig</filename>, see
- <ulink url='http://en.wikipedia.org/wiki/Menuconfig'></ulink>.
- </para></listitem>
- <listitem><para><emphasis>Configuration Fragments:</emphasis> A file with a
- list of kernel options just as they would appear syntactically in the
- <filename>.config</filename> file.
- Configuration fragments are typically logical groupings and are assembled
- by the OpenEmbedded build system to produce input used by the LKC
- that ultimately generates the <filename>.config</filename> file.</para>
- <para>The
- <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'>KERNEL_FEATURES</ulink></filename>
- variable can be used to list configuration fragments.
- For further discussion on applying configuration fragments, see the
- "<ulink url='&YOCTO_DOCS_BSP_URL;#bsp-filelayout-kernel'>Linux Kernel Configuration</ulink>"
- section in the Yocto Project Board Support Package (BSP) Guide.
- </para></listitem>
- </itemizedlist>
- </para>
- </section>
-
- <section id='kernel-tools'>
- <title>Kernel Tools</title>
- <para>
- Since most standard workflows involve moving forward with an existing tree by
- continuing to add and alter the underlying baseline, the tools that manage
- the Yocto Project's kernel construction are largely hidden from the developer to
- present a simplified view of the kernel for ease of use.
- </para>
- <para>
- Fundamentally, the kernel tools that manage and construct the
- Yocto Project kernel accomplish the following:
- <itemizedlist>
- <listitem><para>Group patches into named, reusable features.</para></listitem>
- <listitem><para>Allow top-down control of included features.</para></listitem>
- <listitem><para>Bind kernel configurations to kernel patches and features.</para></listitem>
- <listitem><para>Present a seamless Git repository that blends Yocto Project value
- with the <filename>kernel.org</filename> history and development.</para></listitem>
- </itemizedlist>
- </para>
- </section>
-</chapter>
-<!--
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-
-<chapter id='kernel-how-to'>
-
-<title>Working with the Yocto Project Kernel</title>
-
-
-<section id='actions-org'>
- <title>Introduction</title>
- <para>
- This chapter describes how to accomplish tasks involving a kernel's tree structure.
- The information is designed to help the developer that wants to modify the Yocto
- Project kernel and contribute changes upstream to the Yocto Project.
- The information covers the following:
- <itemizedlist>
- <listitem><para>Tree construction</para></listitem>
- <listitem><para>Build strategies</para></listitem>
- <listitem><para>Workflow examples</para></listitem>
- </itemizedlist>
- </para>
-</section>
-
- <section id='tree-construction'>
- <title>Tree Construction</title>
- <para>
- This section describes construction of the Yocto Project kernel source repositories
- as accomplished by the Yocto Project team to create kernel repositories.
- These kernel repositories are found under the heading "Yocto Linux Kernel" at
- <ulink url='&YOCTO_GIT_URL;/cgit.cgi'>&YOCTO_GIT_URL;/cgit.cgi</ulink>
- and can be shipped as part of a Yocto Project release.
- The team creates these repositories 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 a kernel.
- Thus, the Yocto Project kernel repository (or tree) is built.
- </para>
- <para>
- The existence of this tree allows you to access and clone a particular
- Yocto Project kernel repository and use it to build images based on their configurations
- and features.
- </para>
- <para>
- You can find the files used to describe all the valid features and BSPs
- in the Yocto Project kernel in any clone of the Yocto Project kernel source repository
- Git tree.
- For example, the following command clones the Yocto Project baseline kernel that
- branched off of <filename>linux.org</filename> version 3.4:
- <literallayout class='monospaced'>
- $ git clone git://git.yoctoproject.org/linux-yocto-3.4
- </literallayout>
- For another example of how to set up a local Git repository of the Yocto Project
- kernel files, see the
- "<ulink url='&YOCTO_DOCS_DEV_URL;#local-kernel-files'>Yocto Project Kernel</ulink>" bulleted
- item in the Yocto Project Development Manual.
- </para>
- <para>
- Once you have cloned the kernel Git repository on your local machine, you can
- switch to the <filename>meta</filename> branch within the repository.
- Here is an example that assumes the local Git repository for the kernel is in
- a top-level directory named <filename>linux-yocto-3.4</filename>:
- <literallayout class='monospaced'>
- $ cd ~/linux-yocto-3.4
- $ git checkout -b meta origin/meta
- </literallayout>
- Once you have checked out and switched to the <filename>meta</filename> branch,
- you can see a snapshot of all the kernel configuration and feature descriptions that are
- used to build that particular kernel repository.
- These descriptions are in the form of <filename>.scc</filename> files.
- </para>
- <para>
- You should realize, however, that browsing your local kernel repository
- for feature descriptions and patches is not an effective way to determine what is in a
- particular kernel branch.
- Instead, you should use Git directly to discover the changes in a branch.
- Using Git is an efficient and flexible way to inspect changes to the kernel.
- For examples showing how to use Git to inspect kernel commits, see the following sections
- in this chapter.
- <note>
- Ground up reconstruction of the complete kernel tree is an action only taken by the
- Yocto Project team during an active development cycle.
- When you create a clone of the kernel Git repository, you are simply making it
- efficiently available for building and development.
- </note>
- </para>
- <para>
- The following steps describe what happens when the Yocto Project Team constructs
- the Yocto Project kernel source Git repository (or tree) found at
- <ulink url='&YOCTO_GIT_URL;/cgit.cgi'></ulink> given the
- introduction of a new top-level kernel feature or BSP.
- These are the actions that effectively create the tree
- that includes the new feature, patch or BSP:
- <orderedlist>
- <listitem><para>A top-level kernel feature is passed to the kernel build subsystem.
- Normally, this feature is a BSP for a particular kernel type.</para></listitem>
- <listitem><para>The file that describes the top-level feature is located by searching
- these system directories:
- <itemizedlist>
- <listitem><para>The in-tree kernel-cache directories, which are located
- in <filename>meta/cfg/kernel-cache</filename></para></listitem>
- <listitem><para>Areas pointed to by <filename>SRC_URI</filename> statements
- found in recipes</para></listitem>
- </itemizedlist>
- For a typical build, the target of the search is a
- feature description in an <filename>.scc</filename> file
- whose name follows this format:
- <literallayout class='monospaced'>
- <bsp_name>-<kernel_type>.scc
- </literallayout>
- </para></listitem>
- <listitem><para>Once located, the feature description is either compiled into a simple script
- of actions, or into an existing equivalent script that is already part of the
- shipped kernel.</para></listitem>
- <listitem><para>Extra features are appended to the top-level feature description.
- These features can come from the
- <ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'><filename>KERNEL_FEATURES</filename></ulink>
- variable in recipes.</para></listitem>
- <listitem><para>Each extra feature is located, compiled and appended to the script
- as described in step three.</para></listitem>
- <listitem><para>The script is executed to produce a series of <filename>meta-*</filename>
- directories.
- These directories are descriptions of all the branches, tags, patches and configurations that
- need to be applied to the base Git repository to completely create the
- source (build) branch for the new BSP or feature.</para></listitem>
- <listitem><para>The base repository is cloned, and the actions
- listed in the <filename>meta-*</filename> directories are applied to the
- tree.</para></listitem>
- <listitem><para>The Git repository is left with the desired branch checked out and any
- required branching, patching and tagging has been performed.</para></listitem>
- </orderedlist>
- </para>
- <para>
- The kernel tree is now ready for developer consumption to be locally cloned,
- configured, and built into a Yocto Project kernel specific to some target hardware.
- <note><para>The generated <filename>meta-*</filename> directories add 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
- official Yocto Project kernel repositories at
- <ulink url='&YOCTO_GIT_URL;/cgit.cgi'>http://git.yoctoproject.org/cgit.cgi</ulink>
- is the combination of all supported boards and configurations.</para>
- <para>The technique the Yocto Project team uses is flexible and allows for seamless
- blending of an immutable history with additional patches specific to a
- deployment.
- Any additions to the kernel become an integrated part of the branches.</para>
- </note>
- </para>
- </section>
-
- <section id='build-strategy'>
- <title>Build Strategy</title>
- <para>
- Once a local Git repository of the Yocto Project kernel exists on a development system,
- you can consider the compilation phase of kernel development - building a kernel image.
- Some prerequisites exist that are validated by the build process before compilation
- starts:
- </para>
-
- <itemizedlist>
- <listitem><para>The
- <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink> points
- to the kernel Git repository.</para></listitem>
- <listitem><para>A BSP build branch exists.
- This branch has the following form:
- <literallayout class='monospaced'>
- <kernel_type>/<bsp_name>
- </literallayout></para></listitem>
- </itemizedlist>
-
- <para>
- The OpenEmbedded build system makes sure these conditions exist before attempting compilation.
- Other means, however, do exist, such as as bootstrapping a BSP, see
- the "<link linkend='workflow-examples'>Workflow Examples</link>".
- </para>
-
- <para>
- Before building a kernel, the build process verifies the tree
- and configures the kernel by processing all of the
- configuration "fragments" specified by feature descriptions in the <filename>.scc</filename>
- files.
- As the features are compiled, associated kernel configuration fragments are noted
- and recorded in the <filename>meta-*</filename> series of directories in their compilation order.
- The fragments are migrated, pre-processed and passed to the Linux Kernel
- Configuration subsystem (<filename>lkc</filename>) as raw input in the form
- of a <filename>.config</filename> file.
- The <filename>lkc</filename> uses its own internal dependency constraints to do the final
- processing of that information and generates the final <filename>.config</filename> file
- that is used during compilation.
- </para>
-
- <para>
- Using the board's architecture and other relevant values from the board's template,
- kernel compilation is started and a kernel image is produced.
- </para>
-
- <para>
- The other thing that you notice once you configure a kernel is that
- the build process generates a build tree that is separate from your kernel's local Git
- source repository tree.
- This build tree has a name that uses the following form, where
- <filename>${MACHINE}</filename> is the metadata name of the machine (BSP) and "kernel_type" is one
- of the Yocto Project supported kernel types (e.g. "standard"):
- <literallayout class='monospaced'>
- linux-${MACHINE}-<kernel_type>-build
- </literallayout>
- </para>
-
- <para>
- The existing support in the <filename>kernel.org</filename> tree achieves this
- default functionality.
- </para>
-
- <para>
- This behavior means that all the generated files for a particular machine or BSP are now in
- the build tree directory.
- The files include the final <filename>.config</filename> file, all the <filename>.o</filename>
- files, the <filename>.a</filename> files, and so forth.
- Since each machine or BSP has its own separate build directory in its own separate branch
- of the Git repository, you can easily switch between different builds.
- </para>
- </section>
-
- <section id='workflow-examples'>
- <title>Workflow Examples</title>
-
- <para>
- As previously noted, the Yocto Project kernel has built-in Git integration.
- However, these utilities are not the only way to work with the kernel repository.
- The Yocto Project has not made changes to Git or to other tools that
- would invalidate alternate workflows.
- Additionally, the way the kernel repository is constructed results in using
- only core Git functionality, thus allowing any number of tools or front ends to use the
- resulting tree.
- </para>
-
- <para>
- This section contains several workflow examples.
- Many of the examples use Git commands.
- You can find Git documentation at
- <ulink url='http://git-scm.com/documentation'></ulink>.
- You can find a simple overview of using Git with the Yocto Project in the
- "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>"
- section of the Yocto Project Development Manual.
- </para>
-
- <section id='change-inspection-kernel-changes-commits'>
- <title>Change Inspection: Changes/Commits</title>
-
- <para>
- A common question when working with a kernel is:
- "What changes have been applied to this tree?"
- </para>
-
- <para>
- In projects that have a collection of directories that
- contain patches to the kernel, it is possible to inspect or "grep" the contents
- of the directories to get a general feel for the changes.
- This sort of patch inspection is not an efficient way to determine what has been
- done to the kernel.
- The reason it is inefficient is because there are many optional patches that are
- selected based on the kernel type and the feature description.
- Additionally, patches could exist in directories that are not included in the search.
- </para>
-
- <para>
- A more efficient way to determine what has changed in the branch is to use
- Git and inspect or search the kernel tree.
- This method gives you a full view of not only the source code modifications,
- but also provides the reasons for the changes.
- </para>
-
- <section id='what-changed-in-a-kernel'>
- <title>What Changed in a Kernel?</title>
-
- <para>
- Following are a few examples that show how to use Git commands to examine changes.
- Because Git repositories in the Yocto Project do not break existing Git
- functionality, and because there exists many permutations of these types of
- Git commands, many methods exist by which you can discover changes.
- <note>
- In the following examples, unless you provide a commit range,
- <filename>kernel.org</filename> history is blended with Yocto Project
- kernel changes.
- You can form ranges by using branch names from the kernel tree as the
- upper and lower commit markers with the Git commands.
- You can see the branch names through the web interface to the
- Yocto Project source repositories at
- <ulink url='http://git.yoctoproject.org/cgit.cgi'></ulink>.
- For example, the branch names for the <filename>linux-yocto-3.4</filename>
- kernel repository can be seen at
- <ulink url='http://git.yoctoproject.org/cgit.cgi/linux-yocto-3.4/refs/heads'></ulink>.
- </note>
- To see a full range of the changes, use the
- <filename>git whatchanged</filename> command and specify a commit range
- for the branch (<filename><commit>..<commit></filename>).
- </para>
-
- <para>
- Here is an example that looks at what has changed in the
- <filename>emenlow</filename> branch of the
- <filename>linux-yocto-3.4</filename> kernel.
- The lower commit range is the commit associated with the
- <filename>standard/base</filename> branch, while
- the upper commit range is the commit associated with the
- <filename>standard/emenlow</filename> branch.
- <literallayout class='monospaced'>
- $ git whatchanged origin/standard/base..origin/standard/emenlow
- </literallayout>
- </para>
-
- <para>
- To see a summary of changes use the <filename>git log</filename> command.
- Here is an example using the same branches:
- <literallayout class='monospaced'>
- $ git log --oneline origin/standard/base..origin/standard/emenlow
- </literallayout>
- The <filename>git log</filename> output might be more useful than
- the <filename>git whatchanged</filename> as you get
- a short, one-line summary of each change and not the entire commit.
- </para>
-
- <para>
- If you want to see code differences associated with all the changes, use
- the <filename>git diff</filename> command.
- Here is an example:
- <literallayout class='monospaced'>
- $ git diff origin/standard/base..origin/standard/emenlow
- </literallayout>
- </para>
-
- <para>
- You can see the commit log messages and the text differences using the
- <filename>git show</filename> command:
- Here is an example:
- <literallayout class='monospaced'>
- $ git show origin/standard/base..origin/standard/emenlow
- </literallayout>
- </para>
-
- <para>
- You can create individual patches for each change by using the
- <filename>git format-patch</filename> command.
- Here is an example that that creates patch files for each commit and
- places them in your <filename>Documents</filename> directory:
- <literallayout class='monospaced'>
- $ git format-patch -o $HOME/Documents origin/standard/base..origin/standard/emenlow
- </literallayout>
- </para>
- </section>
-
- <section id='show-a-particular-feature-or-branch-change'>
- <title>Show a Particular Feature or Branch Change</title>
-
- <para>
- Developers use tags in the Yocto Project kernel tree to divide changes for significant
- features or branches.
- Once you know a particular tag, you can use Git commands
- to show changes associated with the tag and find the branches that contain
- the feature.
- <note>
- Because BSP branch, <filename>kernel.org</filename>, and feature tags are all
- present, there could be many tags.
- </note>
- The <filename>git show <tag></filename> command shows changes that are tagged by
- a feature.
- Here is an example that shows changes tagged by the <filename>systemtap</filename>
- feature:
- <literallayout class='monospaced'>
- $ git show systemtap
- </literallayout>
- You can use the <filename>git branch --contains <tag></filename> command
- to show the branches that contain a particular feature.
- This command shows the branches that contain the <filename>systemtap</filename>
- feature:
- <literallayout class='monospaced'>
- $ git branch --contains systemtap
- </literallayout>
- </para>
-
- <para>
- You can use many other comparisons to isolate BSP and kernel changes.
- For example, you can compare against <filename>kernel.org</filename> tags
- such as the <filename>v3.4</filename> tag.
- </para>
- </section>
- </section>
-
- <section id='development-saving-kernel-modifications'>
- <title>Development: Saving Kernel Modifications</title>
-
- <para>
- Another common operation is to build a BSP supplied by the Yocto Project, make some
- changes, rebuild, and then test.
- Those local changes often need to be exported, shared or otherwise maintained.
- </para>
-
- <para>
- Since the Yocto Project kernel source tree is backed by Git, this activity is
- much easier as compared to with previous releases.
- Because Git tracks file modifications, additions and deletions, it is easy
- to modify the code and later realize that you need to save the changes.
- It is also easy to determine what has changed.
- This method also provides many tools to commit, undo and export those modifications.
- </para>
-
- <para>
- This section and its sub-sections, describe general application of Git's
- <filename>push</filename> and <filename>pull</filename> commands, which are used to
- get your changes upstream or source your code from an upstream repository.
- The Yocto Project provides scripts that help you work in a collaborative development
- environment.
- For information on these scripts, see the
- "<ulink url='&YOCTO_DOCS_DEV_URL;#pushing-a-change-upstream'>Using Scripts to Push a Change
- Upstream and Request a Pull</ulink>" and
- "<ulink url='&YOCTO_DOCS_DEV_URL;#submitting-a-patch'>Using Email to Submit a Patch</ulink>"
- sections in the Yocto Project Development Manual.
- </para>
-
- <para>
- There are many ways to save kernel modifications.
- The technique employed
- depends on the destination for the patches:
-
- <itemizedlist>
- <listitem><para>Bulk storage</para></listitem>
- <listitem><para>Internal sharing either through patches or by using Git</para></listitem>
- <listitem><para>External submissions</para></listitem>
- <listitem><para>Exporting for integration into another Source Code
- Manager (SCM)</para></listitem>
- </itemizedlist>
- </para>
-
- <para>
- Because of the following list of issues, the destination of the patches also influences
- the method for gathering them:
-
- <itemizedlist>
- <listitem><para>Bisectability</para></listitem>
- <listitem><para>Commit headers</para></listitem>
- <listitem><para>Division of subsystems for separate submission or review</para></listitem>
- </itemizedlist>
- </para>
-
- <section id='bulk-export'>
- <title>Bulk Export</title>
-
- <para>
- This section describes how you can "bulk" export changes that have not
- been separated or divided.
- This situation works well when you are simply storing patches outside of the kernel
- source repository, either permanently or temporarily, and you are not committing
- incremental changes during development.
- <note>
- This technique is not appropriate for full integration of upstream submission
- because changes are not properly divided and do not provide an avenue for per-change
- commit messages.
- Therefore, this example assumes that changes have not been committed incrementally
- during development and that you simply must gather and export them.
- </note>
- <literallayout class='monospaced'>
- # bulk export of ALL modifications without separation or division
- # of the changes
-
- $ git add .
- $ git commit -s -a -m <msg>
- or
- $ git commit -s -a # and interact with $EDITOR
- </literallayout>
- </para>
-
- <para>
- The previous operations capture 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.
- </para>
-
- <para>
- Once the changes are exported, you can restore them manually using a template
- or through integration with the <filename>default_kernel</filename>.
- </para>
-
- </section>
-
- <section id='incremental-planned-sharing'>
- <title>Incremental/Planned Sharing</title>
-
- <para>
- This section describes how to save modifications when you are making incremental
- commits or practicing planned sharing.
- The examples in this section assume that you have incrementally committed
- changes to the tree during development and now need to export them.
- The sections that follow
- describe how you can export your changes internally through either patches or by
- using Git commands.
- </para>
-
- <para>
- During development, the following commands are of interest.
- For full Git documentation, refer to the Git documentation at
- <ulink url='http://github.com'></ulink>.
-
- <literallayout class='monospaced'>
- # 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.
- </literallayout>
- </para>
-
- <para>
- Distributed development with Git is possible when you use a universally
- agreed-upon unique commit identifier (set by the creator of the commit) that maps to a
- specific change set with a specific parent.
- This identifier is created for you when
- you create a commit, and is re-created when you amend, alter or re-apply
- a commit.
- As an individual in isolation, this is of no interest.
- However, if you
- intend to share your tree with normal Git <filename>push</filename> and
- <filename>pull</filename> operations for
- distributed development, you should consider the ramifications of changing a
- commit that you have already shared with others.
- </para>
-
- <para>
- Assuming that the changes have not been pushed upstream, or pulled into
- another repository, you can update both the commit content and commit messages
- associated with development by using the following commands:
-
- <literallayout class='monospaced'>
- $ Git add <path>/file
- $ Git commit --amend
- $ Git rebase or Git rebase -i
- </literallayout>
- </para>
-
- <para>
- Again, assuming that the changes have not been pushed upstream, and that
- no pending works-in-progress exist (use <filename>git status</filename> to check), then
- you can revert (undo) commits by using the following commands:
-
- <literallayout class='monospaced'>
- # 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^
- </literallayout>
- </para>
-
- <para>
- You can create branches, "cherry-pick" changes, or perform any number of Git
- operations until the commits are in good order for pushing upstream
- or for pull requests.
- After a <filename>push</filename> or <filename>pull</filename> command,
- commits are normally considered
- "permanent" and you should not modify them.
- If the commits need to be changed, you can incrementally do so with new commits.
- These practices follow standard Git workflow and the <filename>kernel.org</filename> best
- practices, which is recommended.
- <note>
- It is recommended to tag or branch before adding changes to a Yocto Project
- BSP or before creating a new one.
- The reason for this recommendation is because the branch or tag provides a
- reference point to facilitate locating and exporting local changes.
- </note>
- </para>
-
- <section id='export-internally-via-patches'>
- <title>Exporting Changes Internally by Using Patches</title>
-
- <para>
- This section describes how you can extract committed changes from a working directory
- by exporting them as patches.
- Once the changes have been extracted, you can use the patches for upstream submission,
- place them in a Yocto Project template for automatic kernel patching,
- or apply them in many other common uses.
- </para>
-
- <para>
- This example shows how to create a directory with sequentially numbered patches.
- Once the directory is created, you can apply it to a repository using the
- <filename>git am</filename> command to reproduce the original commit and all
- the related information such as author, date, commit log, and so forth.
- <note>
- The new commit identifiers (ID) will be generated upon re-application.
- This action reflects that the commit is now applied to an underlying commit
- with a different ID.
- </note>
- <literallayout class='monospaced'>
- # <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>
- </literallayout>
- </para>
-
- <para>
- In other words:
- <literallayout class='monospaced'>
- # 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>
- </literallayout>
- </para>
- </section>
-
- <section id='export-internally-via-git'>
- <title>Exporting Changes Internally by Using Git</title>
-
- <para>
- This section describes how you can export changes from a working directory
- by pushing the changes into a master repository or by making a pull request.
- Once you have pushed the changes to the master repository, you can then
- pull those same changes into a new kernel build at a later time.
- </para>
-
- <para>
- Use this command form to push the changes:
- <literallayout class='monospaced'>
- $ git push ssh://<master_server>/<path_to_repo>
- <local_branch>:<remote_branch>
- </literallayout>
- </para>
-
- <para>
- For example, the following command pushes the changes from your local branch
- <filename>yocto/standard/common-pc/base</filename> to the remote branch with the same name
- in the master repository <filename>//git.mycompany.com/pub/git/kernel-3.4</filename>.
- <literallayout class='monospaced'>
- $ git push ssh://git.mycompany.com/pub/git/kernel-3.4 \
- yocto/standard/common-pc/base:yocto/standard/common-pc/base
- </literallayout>
- </para>
-
- <para>
- A pull request entails using the <filename>git request-pull</filename> command to compose
- an email to the
- maintainer requesting that a branch be pulled into the master repository, see
- <ulink url='http://github.com/guides/pull-requests'></ulink> for an example.
- <note>
- Other commands such as <filename>git stash</filename> or branching can also be used to save
- changes, but are not covered in this document.
- </note>
- </para>
- </section>
- </section>
-
- <section id='export-for-external-upstream-submission'>
- <title>Exporting Changes for External (Upstream) Submission</title>
-
- <para>
- This section describes how to export changes for external upstream submission.
- If the patch series is large or the maintainer prefers to pull
- changes, you can submit these changes by using a pull request.
- However, it is common to send patches as an email series.
- This method allows easy review and integration of the changes.
- <note>
- Before sending patches for review be sure you understand the
- community standards for submitting and documenting changes and follow their best practices.
- For example, kernel patches should follow standards such as:
- <itemizedlist>
- <listitem><para>
- <ulink url='http://linux.yyz.us/patch-format.html'></ulink></para></listitem>
- <listitem><para>Documentation/SubmittingPatches (in any linux
- kernel source tree)</para></listitem>
- </itemizedlist>
- </note>
- </para>
-
- <para>
- The messages used to commit changes are a large part of these standards.
- Consequently, be sure that the headers for each commit have the required information.
- For information on how to follow the Yocto Project commit message standards, see the
- "<ulink url='&YOCTO_DOCS_DEV_URL;#how-to-submit-a-change'>How to Submit a
- Change</ulink>" section in the Yocto Project Development Manual.
- </para>
-
- <para>
- If the initial commits were not properly documented or do not meet those standards,
- you can re-base by using the <filename>git rebase -i</filename> command to
- manipulate the commits and
- get them into the required format.
- Other techniques such as branching and cherry-picking commits are also viable options.
- </para>
-
- <para>
- Once you complete the commits, you can generate the email that sends the patches
- to the maintainer(s) or lists that review and integrate changes.
- The command <filename>git send-email</filename> is commonly used to ensure
- that patches are properly
- formatted for easy application and avoid mailer-induced patch damage.
- </para>
-
- <para>
- The following is an example of dumping patches for external submission:
- <literallayout class='monospaced'>
- # 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
- </literallayout>
- </para>
- </section>
-
- <section id='export-for-import-into-other-scm'>
- <title>Exporting Changes for Import into Another SCM</title>
-
- <para>
- When you want to export changes for import into another
- Source Code Manager (SCM), you can use any of the previously discussed
- techniques.
- However, if the patches are manually applied to a secondary tree and then
- that tree is checked into the SCM, you can lose change information such as
- commit logs.
- This process is not recommended.
- </para>
-
- <para>
- Many SCMs can directly import Git commits, or can translate Git patches so that
- information is not lost.
- Those facilities are SCM-dependent and you should use them whenever possible.
- </para>
- </section>
- </section>
-
- <section id='scm-working-with-the-yocto-project-kernel-in-another-scm'>
- <title>Working with the Yocto Project Kernel in Another SCM</title>
-
- <para>
- This section describes kernel development in an SCM other than Git,
- which is not the same as exporting changes to another SCM described earlier.
- For this scenario, you use the OpenEmbedded build system to
- develop the kernel in a different SCM.
- The following must be true for you to accomplish this:
- <itemizedlist>
- <listitem><para>The delivered Yocto Project kernel must be exported into the second
- SCM.</para></listitem>
- <listitem><para>Development must be exported from that secondary SCM into a
- format that can be used by the OpenEmbedded build system.</para></listitem>
- </itemizedlist>
- </para>
-
- <section id='exporting-delivered-kernel-to-scm'>
- <title>Exporting the Delivered Kernel to the SCM</title>
-
- <para>
- Depending on the SCM, it might be possible to export the entire Yocto Project
- kernel Git repository, branches and all, into a new environment.
- This method is preferred because it has the most flexibility and potential to maintain
- the meta data associated with each commit.
- </para>
-
- <para>
- 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.
- </para>
-
- <para>
- The following commands illustrate some of the steps you could use to
- import the <filename>yocto/standard/common-pc/base</filename>
- kernel into a secondary SCM:
- <literallayout class='monospaced'>
- $ git checkout yocto/standard/common-pc/base
- $ cd .. ; echo linux/.git > .cvsignore
- $ cvs import -m "initial import" linux MY_COMPANY start
- </literallayout>
- </para>
-
- <para>
- You could now relocate the CVS repository and use it in a centralized manner.
- </para>
-
- <para>
- The following commands illustrate how you can condense and merge two BSPs into a
- second SCM:
- <literallayout class='monospaced'>
- $ git checkout yocto/standard/common-pc/base
- $ git merge yocto/standard/common-pc-64/base
- # resolve any conflicts and commit them
- $ cd .. ; echo linux/.git > .cvsignore
- $ cvs import -m "initial import" linux MY_COMPANY start
- </literallayout>
- </para>
- </section>
-
- <section id='importing-changes-for-build'>
- <title>Importing Changes for the Build</title>
-
- <para>
- Once development has reached a suitable point in the second development
- environment, you need to export the changes as patches.
- To export them, place the changes in a recipe and
- automatically apply them to the kernel during patching.
- </para>
- </section>
- </section>
-
- <section id='bsp-creating'>
- <title>Creating a BSP Based on an Existing Similar BSP</title>
-
- <para>
- This section overviews the process of creating a BSP based on an
- existing similar BSP.
- The information is introductory in nature and does not provide step-by-step examples.
- For detailed information on how to create a new BSP, see
- the "<ulink url='&YOCTO_DOCS_BSP_URL;#creating-a-new-bsp-layer-using-the-yocto-bsp-script'>Creating a New BSP Layer Using the yocto-bsp Script</ulink>" section in the
- Yocto Project Board Support Package (BSP) Developer's Guide, or see the
- <ulink url='&YOCTO_WIKI_URL;/wiki/Transcript:_creating_one_generic_Atom_BSP_from_another'>Transcript:_creating_one_generic_Atom_BSP_from_another</ulink>
- wiki page.
- </para>
-
- <para>
- The basic steps you need to follow are:
- <orderedlist>
- <listitem><para><emphasis>Make sure you have set up a local Source Directory:</emphasis>
- You must create a local
- <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>
- by either creating a Git repository (recommended) or
- extracting a Yocto Project release tarball.</para></listitem>
- <listitem><para><emphasis>Choose an existing BSP available with the Yocto Project:</emphasis>
- Try to map your board features as closely to the features of a BSP that is
- already supported and exists in the Yocto Project.
- Starting with something as close as possible to your board makes developing
- your BSP easier.
- You can find all the BSPs that are supported and ship with the Yocto Project
- on the Yocto Project's Download page at
- <ulink url='&YOCTO_HOME_URL;/download'></ulink>.</para></listitem>
- <listitem><para><emphasis>Be sure you have the Base BSP:</emphasis>
- You need to either have a local Git repository of the base BSP set up or
- have downloaded and extracted the files from a release BSP tarball.
- Either method gives you access to the BSP source files.</para></listitem>
- <listitem><para><emphasis>Make a copy of the existing BSP, thus isolating your new
- BSP work:</emphasis>
- Copying the existing BSP file structure gives you a new area in which to work.</para></listitem>
- <listitem><para><emphasis>Make configuration and recipe changes to your new BSP:</emphasis>
- Configuration changes involve the files in the BSP's <filename>conf</filename>
- directory.
- Changes include creating a machine-specific configuration file and editing the
- <filename>layer.conf</filename> file.
- The configuration changes identify the kernel you will be using.
- Recipe changes include removing, modifying, or adding new recipe files that
- instruct the build process on what features to include in the image.</para></listitem>
- <listitem><para><emphasis>Prepare for the build:</emphasis>
- Before you actually initiate the build, you need to set up the build environment
- by sourcing the environment initialization script.
- After setting up the environment, you need to make some build configuration
- changes to the <filename>local.conf</filename> and <filename>bblayers.conf</filename>
- files.</para></listitem>
- <listitem><para><emphasis>Build the image:</emphasis>
- The OpenEmbedded build system uses BitBake to create the image.
- You need to decide on the type of image you are going to build (e.g. minimal, base,
- core, sato, and so forth) and then start the build using the <filename>bitbake</filename>
- command.</para></listitem>
- </orderedlist>
- </para>
- </section>
-
- <section id='tip-dirty-string'>
- <title>"-dirty" String</title>
-
- <para>
- If kernel images are being built with "-dirty" on the end of the version
- string, this simply means that modifications in the source
- directory have not been committed.
- <literallayout class='monospaced'>
- $ git status
- </literallayout>
- </para>
-
- <para>
- You can use the above Git command to report modified, removed, or added files.
- You should commit those changes to the tree regardless of whether they will be saved,
- exported, or used.
- Once you commit the changes you need to rebuild the kernel.
- </para>
-
- <para>
- To brute force pickup and commit all such pending changes, enter the following:
- <literallayout class='monospaced'>
- $ git add .
- $ git commit -s -a -m "getting rid of -dirty"
- </literallayout>
- </para>
-
- <para>
- Next, rebuild the kernel.
- </para>
- </section>
- </section>
-</chapter>
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projects / thirdparty / openembedded / openembedded-core-contrib.git / commitdiff
