poky/documentation/kernel-manual/yocto-project-kernel-manual.xml

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<article id='intro'>
   <imagedata fileref="figures/yocto-project-transp.png" width="6in" depth="1in" align="right" scale="25" />

<section id='fake-title'>
    <title>Yocto Project Kernel Architecture and Use Manual</title>
</section>

<section id='introduction'>
    <title>Introduction</title>
    <para>
        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.
    </para>
    <para>
        This manual describes the Yocto Project kernel by providing information
        on its history, organization, benefits, and use.
        The manual consists of two sections:
        <itemizedlist>
            <listitem><para>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.</para></listitem>
            <listitem><para>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."</para></listitem>
        </itemizedlist>
    </para>
    <para>
        For more information on the kernel, see the following links:
        <itemizedlist>
            <listitem><para><ulink url='http://ldn.linuxfoundation.org/book/1-a-guide-kernel-development-process'></ulink></para></listitem>
            <listitem><para><ulink url='http://userweb.kernel.org/~akpm/stuff/tpp.txt'></ulink></para></listitem>
            <listitem><para><ulink url='http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob_plain;f=Documentation/HOWTO;hb=HEAD'></ulink></para></listitem> 
        </itemizedlist>
        <para> 
        You can find more information on Yocto Project by visiting the website at
        <ulink url='http://www.yoctoproject.org'></ulink>.
        </para>
    </para>
</section>

<section id='concepts'>
    <title>Concepts</title>
    <para>
        This section provides conceptual information about the Yocto Project kernel:
        <itemizedlist>
            <listitem><para>Kernel Goals</para></listitem>
            <listitem><para>Yocto Project Kernel Development and Maintenance Overview</para></listitem>
            <listitem><para>Kernel Architecture</para></listitem>
            <listitem><para>Kernel Tools</para></listitem>
        </itemizedlist>
    </para>
    <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, 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.
        </para>
        <para>
            With all these considerations in mind, the Yocto Project 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 the most stable official release.</para></listitem>
            <listitem><para>Include major technological features as part of Yocto Project's up-rev 
            strategy.</para></listitem>
            <listitem><para>Present a git tree, that just like the upstream kernel.org 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 a specific use case (i.g. networking, consumer, devices, and so forth).</para></listitem>
            <listitem><para>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.</para></listitem>
        </itemizedlist>
        </para>
    </section>

    <section id='kernel-big-picture'>
        <title>Yocto Project Kernel Development and Maintenance Overview</title>
        <para>
            Yocto Project kernel, like other kernels, is based off the Linux kernel release
            from <ulink url='http://www.kernel.org'></ulink>.  
            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.
        </para>
        <para>
            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.
        </para>
        <para>
            The following figure represents the overall place the Yocto Project kernel fills.
        </para>
        <para>
        <imagedata fileref="figures/kernel-big-picture.png" width="6in" depth="4in" align="center" scale="100" />
        </para>
        <para>
            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. 
        </para>
        <para>
            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. 
        </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 simply 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 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.
        </para>
    </section>

    <section id='kernel-architecture'>
        <title>Kernel Architecture</title>
        <para>
            This section describes the architecture of the Yocto Project kernel 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 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.
                
            </para>
            <para>
                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.
            </para>
            <para>
                The following illustration shows the conceptual Yocto Project kernel.
            </para>
            <para>
                <imagedata fileref="figures/kernel-architecture-overview.png" width="6in" depth="4in" 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>
                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 user
                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.
                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.
            </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, 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.
            </para>
            <para>
                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.
            </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 user
                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 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.
            </para>
            <note><para> 
                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.
            </para></note>
        </section>
    </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
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>
The fundamental properties of the tools that manage and construct the
kernel are:
<itemizedlist>
    <listitem><para>the ability to group patches into named, reusable features</para></listitem>
    <listitem><para>to allow top down control of included features</para></listitem>
    <listitem><para>the binding of kernel configuration to kernel patches/features</para></listitem>
    <listitem><para>the presentation of a seamless git repository that blends Yocto Project value with the kernel.org history and development</para></listitem>
</itemizedlist>
</para>
<para>
The tools that construct a kernel tree will be discussed later in this 
document. The following tools form the foundation of the Yocto Project 
kernel toolkit:
<itemizedlist>
    <listitem><para>git  : distributed revision control system created by Linus Torvalds</para></listitem>
    <listitem><para>guilt: quilt on top of git</para></listitem>
    <listitem><para>*cfg : kernel configuration management and classification</para></listitem>
    <listitem><para>kgit*: Yocto Project kernel tree creation and management tools</para></listitem>
    <listitem><para>scc  : series &amp; configuration compiler</para></listitem>
</itemizedlist>
</para>
    </section> 
</section>




<!-- <section id='concepts2'>
    <title>Kernel Concepts</title>
    <itemizedlist>
        <listitem><para>What tools and commands are used with the kernel.</para></listitem>
        <listitem><para>Source Control Manager (SCM).</para></listitem>
        <listitem><para>What are some workflows that you can apply using the kernel.</para></listitem>
    </itemizedlist>
</section> -->

<section id='actions'>
    <title>How to get things accomplished with the kernel</title>
    <para>
        This section describes how to accomplish tasks involving the kernel's tree structure. 
        The information covers the following:
        <itemizedlist> 
            <listitem><para>Tree construction</para></listitem>
            <listitem><para>Build strategies</para></listitem>
            <listitem><para>Series &amp; Configuration Compiler</para></listitem>
            <listitem><para>kgit</para></listitem>
            <listitem><para>Workflow examples</para></listitem>
            <listitem><para>Source Code Manager (SCM)</para></listitem>
            <listitem><para>Board Support Package (BSP) template migration</para></listitem>
            <listitem><para>BSP creation</para></listitem>
            <listitem><para>Patching</para></listitem>
            <listitem><para>Updating BSP patches and configuration</para></listitem>
            <listitem><para>guilt</para></listitem>
            <listitem><para>scc file example</para></listitem>
            <listitem><para>"dirty" string</para></listitem>
            <listitem><para>Transition kernel layer</para></listitem>
        </itemizedlist>
    </para>

    <section id='tree-construction'>
        <title>Tree Construction</title>
        <para>
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.
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
The general flow of the project specific kernel tree construction is as follows:
<orderedlist>
    <listitem><para>a top level kernel feature is passed to the kernel build subsystem,
     normally this is a BSP for a particular kernel type.</para></listitem>

    <listitem><para>the file that describes the top level feature is located by searching
     system directories:</para>
      
       <itemizedlist>
            <listitem><para>the kernel-cache under linux/wrs/cfg/kernel-cache</para></listitem>
            <listitem><para>kernel-*-cache directories in layers</para></listitem>
            <listitem><para>configured and default templates</para></listitem>
        </itemizedlist>

     <para>In a typical build a feature description of the format:
          &lt;bsp name&gt;-&lt;kernel type&gt;.scc is the target of the search.
     </para></listitem>

     <listitem><para>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.</para></listitem>

     <listitem><para>extra features are appended to the top level feature description. Extra
     features can come from the command line, the configure script or 
     templates.</para></listitem>

     <listitem><para>each extra feature is located, compiled and appended to the script from
     step #3</para></listitem>

     <listitem><para>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".</para></listitem>

     <listitem><para>the base repository (normally kernel.org) is cloned, and the actions
     listed in the meta-series 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 tree is now ready for configuration and compilation. Those two topics will
be covered below.
</para>

<note><para>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.
</para></note>

<para>
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.
</para>

<note><para>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:
    </para>
    <para>A BSP must create a branch of the format &lt;bsp name&gt;-&lt;kernel type&gt;.</para>

    <para>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.
    </para>
</note>

<para>
A summary of end user tree construction activities follow:
<itemizedlist>
    <listitem><para>compile and link a full top-down kernel description from feature descriptions</para></listitem>
    <listitem><para>execute the complete description to generate a meta-series</para></listitem>
    <listitem><para>interpret the meta-series to create a customized git repository for the
    board</para></listitem>
    <listitem><para>migrate configuration fragments and configure the kernel</para></listitem>
    <listitem><para>checkout the BSP branch and build</para></listitem>
</itemizedlist>
</para>
    </section>

    <section id='build-strategy'>
        <title>Build Strategy</title>
<para>
There are some prerequisites that must be met before starting the compilation
phase of the kernel build system:
</para>
<itemizedlist>
    <listitem><para>There must be a kernel git repository indicated in the SRC_URI.</para></listitem>
    <listitem><para>There must be a branch &lt;bsp name&gt;-&lt;kernel type&gt;.</para></listitem>
</itemizedlist>

<para>
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.
</para>
<para>
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.
</para>
<para>
Kernel compilation is started, using the board's architecture and other
relevant values from the board template, and a kernel image is produced.
</para>
<para>
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-&lt;BSPname&gt;-&lt;kerntype&gt;-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.
</para>
<para>
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.
</para>
    </section>

    <section id='scc'>
        <title>Series &amp; Configuration Compiler (SCC)</title>
<para>
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.
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
Each feature description can use any of the following valid scc commands:
<itemizedlist>
    <listitem><para>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.</para></listitem>

    <listitem><para>patch &lt;relative path&gt;/&lt;patch name&gt;: 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.</para></listitem>

    <listitem><para>patch_trigger &gt;condition&lt; &gt;action&lt; &lt;tgt&gt;: indicate that a trigger should be set to perform an action on a 
    patch.</para>

<para>The conditions can be:

         <itemizedlist>
             <listitem><para>arch:&lt;comma separated arch list or "all"&gt;</para></listitem>
             <listitem><para>plat:&lt;comma separated platform list or "all"&gt;</para></listitem>
         </itemizedlist></para>
<para>The action can be:
         <itemizedlist>
             <listitem><para>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.</para></listitem>
             <listitem><para>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.</para></listitem>
         </itemizedlist></para></listitem>

         <listitem><para>include &lt;feature name&gt; [after &lt;feature&gt;]: 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.</para>
    <para>
    include searches the include directories for a matching feature name,
    include directories are passed to scc by the caller using -I &lt;path&gt; and
    is transparent to the feature script. This means that &lt;feature name&gt; 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.
</para>
<para>
    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.</para></listitem>

  <listitem><para>exclude &lt;feature name&gt;: 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.</para></listitem>

  <listitem><para>git &lt;command&gt;: 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.</para></listitem>

  <listitem><para>dir &lt;directory&gt;: 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.</para></listitem>

  <listitem><para>kconf &lt;type&gt; &lt;fragment name&gt;: associates a kernel config frag with the feature. 
     &lt;type&gt; can be
    "hardware" or "non-hardware" and is used by the kernel configuration
    subsystem to audit configuration. &lt;fragment name&gt; 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.</para></listitem>

  <listitem><para>branch &lt;branch name&gt;: 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.</para></listitem>

  <listitem><para>scc_leaf &lt;base feature&gt; &lt;branch name&gt;: 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.</para></listitem>
   
  <listitem><para>tag &lt;tag name&gt;: 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.</para></listitem>

  <listitem><para>define &lt;var&gt; &lt;value&gt;: Creates a variable with a particular value that can be used in subsequent
    feature descriptions.</para></listitem>
</itemizedlist>

</para>
    </section>

    <section id='kgit-tools'>
        <title>kgit Tools</title>
<para>
The kgit tools are responsible for constructing and maintaining the Wind
River kernel repository. These activities include importing, exporting, and
applying patches as well as sanity checking and branch management.  From the
developers perspective, the kgit tools are hidden and rarely require
interactive use. But one tool in particular that warrants further description
is "kgit-meta".
</para>
<para>
kgit-meta is the actual application of feature description(s) to a kernel repo.
In other words, it is responsible for interpreting the meta series generated
from a scc compiled script. As a result, kgit-meta is coupled to the set of
commands permitted in a .scc feature description (listed in the scc section).
kgit-meta understands both the meta series format and how to use git and
guilt to modify a base git repository.  It processes a meta-series line by
line, branching, tagging, patching and tracking changes that are made to the
base git repository.
</para>
<para>
Once kgit-meta has processed a meta-series, it leaves the repository with the
last branch checked out, and creates the necessary guilt infrastructure to
inspect the tree, or add to it via using guilt. As was previously mentioned,
guilt is not required, but is provided as a convenience. Other utilities such
as quilt, stgit, git or others can also be used to manipulate the git
repository.
</para>
    </section>

    <section id='workflow-examples'>
        <title>Workflow Examples</title>

        <para>
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.</para>
<para>
This section contains several workflow examples.
</para>

        <section id='change-inspection-kernel-changes-commits'>
            <title>Change Inspection: Kernel Changes/Commits</title>
<para>
A common question when working with a BSP/kernel is: "What changes have been applied to this tree?"
</para>
<para>
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.
</para>
<para>
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.
</para>
            <section id='what-changed-in-a-bsp'>
                <title>What Changed in a BSP?</title>
<para>
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 (&lt;kernel type&gt;..&lt;bsp&gt;-&lt;kernel type&gt;), kernel.org history is blended
with Yocto Project changes
</para>
<literallayout class='monospaced'>
     # full description of the changes
     &gt; git whatchanged &lt;kernel type&gt;..&lt;bsp&gt;-&lt;kernel type&gt;
        &gt; eg: git whatchanged standard..common_pc-standard

     # summary of the changes
     &gt; git log &dash;&dash;pretty=oneline &dash;&dash;abbrev-commit &lt;kernel type&gt;..&lt;bsp&gt;-&lt;kernel type&gt;

     # source code changes (one combined diff)
     &gt; git diff &lt;kernel type&gt;..&lt;bsp&gt;-&lt;kernel type&gt;
     &gt; git show &lt;kernel type&gt;..&lt;bsp&gt;-&lt;kernel type&gt;

     # dump individual patches per commit
     &gt; git format-patch -o &lt;dir&gt; &lt;kernel type&gt;..&lt;bsp&gt;-&lt;kernel type&gt;

     # determine the change history of a particular file
     &gt; git whatchanged &lt;path to file&gt;
   
     # determine the commits which touch each line in a file
     &gt; git blame &lt;path to file&gt;
</literallayout>
            </section>
 
            <section id='show-a-particular-feature-or-branch-change'>
                <title>Show a Particular Feature or Branch Change</title>
<para>
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.
</para>
<literallayout class='monospaced'>
     # show the changes tagged by a feature
     &gt; git show &lt;tag&gt; 
        &gt; eg: git show yaffs2

     # determine which branches contain a feature
     &gt; git branch &dash;&dash;contains &lt;tag&gt;

     # show the changes in a kernel type
     &gt; git whatchanged wrs_base..&lt;kernel type&gt;
        &gt; eg: git whatchanged wrs_base..standard
</literallayout>
<para>
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.
</para>
            </section>
        </section>

        <section id='development-saving-kernel-modifications'>
            <title>Development: Saving Kernel Modifications</title>
<para>
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.
</para>
<para>
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.
</para>
<para>
There are many ways to perform this action, and the technique employed
depends on the destination for the patches, which could be any of:
<itemizedlist>
    <listitem><para>bulk storage</para></listitem>
    <listitem><para>internal sharing either through patches or using git</para></listitem>
    <listitem><para>external submission</para></listitem>
    <listitem><para>export for integration into another SCM</para></listitem>
</itemizedlist>
</para>
<para>
The destination of the patches also incluences the method of gathering them
due to issues such as:
<itemizedlist>
    <listitem><para>bisectability</para></listitem>
    <listitem><para>commit headers</para></listitem>
    <listitem><para>division of subsystems for separate submission / review</para></listitem>
</itemizedlist>
</para>

            <section id='bulk-export'>
                <title>Bulk Export</title>
<para>
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.
</para>
<para>
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.
<literallayout class='monospaced'>
     # bulk export of ALL modifications without separation or division
     # of the changes 

     &gt; git add .
     &gt; git commit -s -a -m &gt;commit message&lt; 
        or
     &gt; git commit -s -a # and interact with $EDITOR
</literallayout>
</para>
<para>
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.
</para>
<para>
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.
</para>
            </section>

            <section id='incremental-planned-sharing'>
                <title>Incremental/Planned Sharing</title>
<para>
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.
</para>
<para>
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
<literallayout class='monospaced'>
     # edit a file
     &gt; vi &gt;path&lt;/file
     # stage the change
     &gt; git add &gt;path&lt;/file
     # commit the change
     &gt; git commit -s
     # remove a file
     &gt; git rm &gt;path&lt;/file
     # commit the change
     &gt; git commit -s

     ... etc.
</literallayout>
</para>
<para>
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.
</para>
<para>
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:
<literallayout class='monospaced'>
     &gt; git add &gt;path&lt;/file
     &gt; git commit &dash;&dash;amend
     &gt; git rebase or git rebase -i
</literallayout>
</para>
<para>   
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:
<literallayout class='monospaced'>
     # remove the commit, update working tree and remove all
     # traces of the change
     &gt; git reset &dash;&dash;hard HEAD^
     # remove the commit, but leave the files changed and staged for re-commit
     &gt; git reset &dash;&dash;soft HEAD^
     # remove the commit, leave file change, but not staged for commit
     &gt; git reset &dash;&dash;mixed HEAD^
</literallayout>
</para>
<para>
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.
</para>
<note><para>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.
</para></note>
          
                <section id='export-internally-via-patches'>
                    <title>Export Internally Via Patches</title>
<para>
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.

<literallayout class='monospaced'>
     # &gt;first commit&gt; 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.

     &gt; git format-patch -o &lt;dir&gt; &lt;first commit&gt;..&lt;last commit&gt;
</literallayout>
</para>

<para>
  In other words:
<literallayout class='monospaced'>
     # identify commits of interest.

     # if the tree was tagged before development
     &gt; git format-patch -o &lt;save dir&gt; &lt;tag&gt;

     # if no tags are available
     &gt; git format-patch -o &lt;save dir&gt; HEAD^  # last commit
     &gt; git format-patch -o &lt;save dir&gt; HEAD^^ # last 2 commits
     &gt; git whatchanged # identify last commit
     &gt; git format-patch -o &lt;save dir&gt; &lt;commit id&gt;
     &gt; git format-patch -o &lt;save dir&gt; &lt;rev-list&gt;
</literallayout>
</para> 

<para>
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.
</para>
<para>
See the "template patching" example for how to use the patches to
automatically apply to a new kernel build.
</para>
                </section>

                <section id='export-internally-via-git'>
                    <title>Export Internally Via git</title>
<para>
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:
<literallayout class='monospaced'>
     git push ssh://&lt;master server&gt;/&lt;path to repo&gt; &lt;local branch&gt;:&lt;remote branch&gt;
</literallayout>
For example:
<literallayout class='monospaced'>
     &gt; push ssh://openlinux.windriver.com/pub/git/kernel-2.6.27 common_pc-standard:common_pc-standard
</literallayout>
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.
</para>
<para>
Other commands such as 'git stash' or branching can also be used to save
changes, but are not covered in this document.
</para>
<para>
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.
</para>
                </section>
            </section>

            <section id='export-for-external-upstream-submission'>
                <title>Export for External (Upstream) Submission</title>
<para>
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.
</para>
<note><para>
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:
<itemizedlist>
    <listitem><para><ulink url='http://userweb.kernel.org/~akpm/stuff/tpp.txt'></ulink></para></listitem>
    <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>
</para></note>
<para>
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.
</para>
<para>
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.
</para>
<para>
An example of dumping patches for external submission follows:
<literallayout class='monospaced'>
     # dump the last 4 commits 
     &gt; git format-patch &dash;&dash;thread -n -o ~/rr/ HEAD^^^^
     &gt; git send-email &dash;&dash;compose &dash;&dash;subject '[RFC 0/N] &lt;patch series summary&gt;' \
      &dash;&dash;to foo@yoctoproject.org &dash;&dash;to bar@yoctoproject.org \
      &dash;&dash;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>Export for Import into Other SCM</title>
<para>
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.
</para>
<para>
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.
</para>
            </section>
        </section>

        <section id='scm-working-with-the-yocto-project-kernel-in-another-scm'>
            <title>SCM: Working with the Yocto Project Kernel in Another SCM</title>
<para>
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:
<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 Yocto Project build system.</para></listitem>
</itemizedlist>
</para>
            <section id='exporting-delivered-kernel-to-scm'>
                <title>Exporting Delivered Kernel to SCM</title>
<para>
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.
</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 that could be used to
import the common_pc-standard kernel into a secondary SCM
<literallayout class='monospaced'>
     &gt; git checkout common_pc-standard
     &gt; cd .. ; echo linux/.git &gt; .cvsignore
     &gt; cvs import -m "initial import" linux MY_COMPANY start
</literallayout>
The CVS repo could now be relocated and used in a centralized manner. 
</para>
<para>
The following commands illustrate how two BSPs could be condensed and merged
into a second SCM:
<literallayout class='monospaced'>
     &gt; git checkout common_pc-standard
     &gt; git merge cav_ebt5800-standard
     # resolve any conflicts and commit them
     &gt; cd .. ; echo linux/.git &gt; .cvsignore
     &gt; cvs import -m "initial import" linux MY_COMPANY start
</literallayout>
</para>
            </section>

            <section id='importing-changes-for-build'>
                <title>Importing Changes for Build</title>
<para>
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).
</para>
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.
<para>
</para>
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.
<para>
The following example illustrates one variant of this workflow:
<literallayout class='monospaced'>
     # on master git repository
     &gt; cd linux-2.6.27
     &gt; git tag -d common_pc-standard-mark
     &gt; git pull ssh://&lt;foo&gt;@&lt;bar&gt;/pub/git/kernel-2.6.27 common_pc-standard:common_pc-standard
     &gt; git tag common_pc-standard-mark

     # on each build machine (or NFS share, etc)
     &gt; cd wrll-linux-2.6.27/git/default_kernel
     &gt; git fetch ssh://&lt;foo&gt;@&lt;master server&gt;/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.
     &gt; make linux
</literallayout>
</para>
            </section>
        </section>

        <section id='bsp-template-migration-from-2'>
            <title>BSP: Template Migration from 2.0</title>
<para>
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.
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
The following illustrates the migration of a simple 2.0 BSP template to the
new 3.0 kernel build system.
</para>
<note><para>
Note: all operations are from the root of a customer layer.
</para></note>
<literallayout class='monospaced'>
     templates/
     `&dash;&dash; board
         `&dash;&dash; my_board
             |&dash;&dash; config.sh
             |&dash;&dash; include
             `&dash;&dash; linux
                 `&dash;&dash; 2.6.x
                     |&dash;&dash; knl-base.cfg
                     |&dash;&dash; bsp.patch
                     `&dash;&dash; my_bsp.smudge

     &gt; mv templates/board/my_board/linux/2.6.x/* templates/board/my_board/linux
     &gt; rm -rf templates/board/my_board/linux/2.6.x/
     &gt; mv templates/board/my_board/linux/my_bsp.smudge \
     templates/board/my_board/linux/my_bsp-standard.scc
     &gt; echo "kconf hardware knl-base.cfg" &gt;&gt; \
     templates/board/my_board/linux/my_bsp-standard.scc
     &gt; 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/
     `&dash;&dash; board
         `&dash;&dash; my_board
             |&dash;&dash; config.sh
             |&dash;&dash; include
             `&dash;&dash; linux
                  |&dash;&dash; knl-base.cfg
                  |&dash;&dash; bsp.patch
                  `&dash;&dash; my_bsp-standard.scc
</literallayout>
<para>
That's it. Configure and build.
</para>
<note><para>There is a naming convention for the .scc file, which allows the build
      system to locate suitable feature descriptions for a board:
</para></note>
<literallayout class='monospaced'>
     &lt;bsp name&gt;-&lt;kernel type&gt;.scc
</literallayout>
<para>    
      if this naming convention isn't followed your feature description will
      not be located and a build error thrown.
</para>
        </section>

        <section id='bsp-creating-a-new-bsp'>
            <title>BSP: Creating a New BSP</title>
<para>
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. 
</para>
<para>
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.
</para>
<para>
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.
</para>
            <section id='creating-from-scratch'>
                <title>Creating the BSP from Scratch</title>
<para>
To create the BSP from scratch you need to do the following:
<orderedlist>
    <listitem><para>Create a board template for the new BSP in a layer.</para></listitem>
    <listitem><para>Configure a build with the board.</para></listitem>
    <listitem><para>Configure a kernel.</para></listitem>
</orderedlist>
</para>
<para>
Following is an example showing all three steps.  You start by creating a board template for the new BSP in a layer.
<literallayout class='monospaced'>
     templates/
     `&dash;&dash; board
         `&dash;&dash; my_bsp
             |&dash;&dash; include
             |&dash;&dash; config.sh
             `&dash;&dash; linux
                  |&dash;&dash; my_bsp.cfg
                  `&dash;&dash; my_bsp-standard.scc           

     &gt; 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"

     &gt; cat include 
     cpu/x86_32_i686
     karch/i386

     &gt; cat linux/my_bsp-standard.scc
     scc_leaf ktypes/standard/standard.scc my_bsp-standard

     &gt; cat linux/my_bsp.cfg
     CONFIG_X86=y
     CONFIG_SMP=y                                                                                                    
     CONFIG_VT=y
     # etc, etc, etc
</literallayout>
</para>
<para>
Something like the following can now be added to a board build, and
a project can be started:
<literallayout class='monospaced'>
     &dash;&dash;enable-board=my_bsp \
     &dash;&dash;with-layer=custom_bsp
</literallayout>
</para>
<para>
Now you can configure a kernel:
<literallayout class='monospaced'>
     &gt; make -C build linux.config
</literallayout>
</para>
<para>
You now have a kernel tree, which is branched and has no patches, ready for
development.
</para>
            </section> 

            <section id='cloning-an-existing-bsp'>
                 <title>Cloning an Existing BSP</title>
<para>
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:
<itemizedlist>
     <listitem><para>Create a board template for the new BSP in a layer.</para></listitem>
     <listitem><para>Clone the .scc and board config.</para></listitem>
</itemizedlist>
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
Extraction can be accomplished using four different techniques:
<itemizedlist>
    <listitem><para>Config and patches from the bare default_kernel.</para></listitem>
    <listitem><para>Clone default_kernel and checkout wrs_base.</para></listitem>
    <listitem><para>Clone default_kernel and checkout BSP branch.</para></listitem>
    <listitem><para>Branch from the Yocto Project BSP.</para></listitem>
</itemizedlist>
</para>
<para>
Technique 1: config and patches from the bare default_kernel
<literallayout class='monospaced'>
     &gt; cd layers/wrll-linux-2.6.27/git/default_kernel
     &gt; git show checkpoint_end | filterdiff -i '*common_pc*' | patch -s -p2 -d /tmp
  
     # This will create two directories: cfg and patches. 

     &gt; 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.
</literallayout>
</para>
<para>
Technique 2: clone default_kernel and checkout wrs_base
<literallayout class='monospaced'>
     &gt; git clone layers/wrll-linux-2.6.27/git/default_kernel windriver-2.6.27
     &gt; cd windriver-2.6.27
     &gt; git checkout wrs_base
     &gt; cd wrs/cfg/kernel-cache/bsp/common_pc

# again, this directory has all the patches and .scc files used to construct
# the BSP
</literallayout>
</para>
<para>
Technique 3: clone default_kernel and checkout BSP branch  
<literallayout class='monospaced'>
     &gt; git clone layers/wrll-linux-2.6.27/git/default_kernel windriver-2.6.27
     &gt; cd windriver-2.6.27
     &gt; git checkout common_pc-standard
     &gt; git whatchanged
     # browse patches and determine which ones are of interest, say there are
     # 3 patches of interest
     &gt; git format-patch -o &lt;path to BSP template&gt;/linux HEAD^^^
     # update the .scc file to add the patches, see "template patches" if
     # more details are required
</literallayout>
</para>
<para>
Technique #4: branch from the Yocto Project BSP
<note><para>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.
</para></note>
</para>
<para>
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:
<literallayout class='monospaced'>
     &gt; cat linux/my_bsp-standard.scc
     scc_leaf bsp/common_pc/common_pc-standard.scc my_bsp-standard
</literallayout>
</para>
<para>
  And only kernel configuration (not patches) need be contained in the 
  board template. 
</para>
<para>
  This has the advantage of automatically picking up updates to the BSP
  and not duplicating any patches for a similar board.
</para>
            </section>     

            <section id='bsp-bootstrapping'>
                <title>BSP: Bootstrapping</title>
<para>
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.
</para>
<para>
Two techniques exist to accomplish this:
</para>
<para>
Technique 1: upstream workflow
<literallayout class='monospaced'>
     &gt; git clone layers/wrll-linux-2.6.27/git/default_kernel windriver-2.6.27
     &gt; cd windriver-2.6.27
     &gt; 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
</literallayout>
</para>
<para>
Technique 2: Yocto Project kernel build workflow
</para>
<para>
  Create the BSP branch from the appropriate kernel type
<literallayout class='monospaced'>
     &gt; cd linux
     # the naming convention for auto-build is &lt;bsp&gt;-&lt;kernel type&gt;
     &gt; git checkout -b my_bsp-standard standard
</literallayout>
</para>
<para>
Make changes, import patches, etc.
<literallayout class='monospaced'>
     &gt; ../../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
       &gt; ../../host-cross/bin/guilt new extra-version.patch
       &gt; vi Makefile
       &gt; ../../host-cross/bin/guilt refresh
       # add a header
       &gt; ../../host-cross/bin/guilt header -e
       # describe the patch using best practices, like the example below:

     &dash;&dash;&dash;&gt;&dash;&dash;&dash;&gt;&dash;&dash;&dash;&gt; cut here
     From: Bruce Ashfield &lt;bruce.ashfield@windriver.com&gt;

     Adds an extra version to the kernel

     Modify the main EXTRAVERSION to show our bsp name

     Signed-off-by: Bruce Ashfield &lt;bruce.ashfield@windriver.com&gt;
     &dash;&dash;&dash;&gt;&dash;&dash;&dash;&gt;&dash;&dash;&dash;&gt; cut here

     # option 2: import patches
       &gt; git am &lt;patch&gt;
          or
       &gt; git apply &lt;patch&gt;
       &gt; git add &lt;files&gt;
       &gt; git commit -s

     # configure the board, save relevant options
       &gt; make ARCH=&lt;arch&gt; menuconfig

     # save the cfg changes for reconfiguration
       &gt; mkdir wrs/cfg/&lt;cache&gt;/my_bsp
       &gt; vi wrs/cfg/&lt;cache&gt;/my_bsp/my_bsp.cfg

     # classify the patches
       &gt; ../../host-cross/bin/kgit classify create &lt;kernel-foo-cache&gt;/my_bsp/my_bsp
     # test build
       &gt; cd ..
       &gt; make linux TARGET_BOARD=my_bsp kprofile=my_bsp use_current_branch=1
</literallayout>
</para>
<para>
 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.
</para>
            </section>  
        </section>

        <section id='patching'>
             <title>Patching</title>
<para>
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.
</para>
            <section id='patching-template'>
                <title>Patching: Template</title>
<para>
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).
</para>
<para>
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.
</para>
<note><para>
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
</para></note>
<para>
As an example, consider a simple template to update a BP:
<literallayout class='monospaced'>
     &gt; cat templates/feature/extra_version/linux/extra_version.scc
     patch 0001-extraversion-add-Wind-River-identifier.patch
</literallayout>
</para>
<para>
To illustrate how the previous template patch was created, the following
steps were performed:
<literallayout class='monospaced'>
     &gt; cd &lt;board build&gt;/build/linux
     &gt; vi Makefile
     # modify EXTRAVERSION to have a unique string
     &gt; git commit -s -m "extraversion: add Yocto Project identifier" Makefile
     &gt; git format-patch -o &lt;path to layer&gt;/templates/feature/extra_version/linux/
     &gt; echo "patch 0001-extraversion-add-Wind-River-identifier.patch" &gt; \
              &lt;path to layer&gt;/templates/feature/extra_version/linux/extra_version.scc
</literallayout>
</para>
<para>
This next example creates a template with a linux subdirectory, just as we
    always have for previous releases.
<literallayout class='monospaced'>
     &gt; mkdir templates/features/my_feature/linux
</literallayout>
</para>
<para>
    In that directory place your feature description, your
    patch and configuration (if required).
<literallayout class='monospaced'>
     &gt; ls templates/features/my_feature/linux

          version.patch
          my_feature.scc
          my_feature.cfg
</literallayout>
</para>
<para>
    The .scc file describes the patches, configuration and
    where in the patch order the feature should be inserted.
<literallayout class='monospaced'>
       patch version.patch
       kconf non-hardware my_feature.cfg
</literallayout>
</para>
<para>
    Configure your build with the new template
<literallayout class='monospaced'>
     &dash;&dash;with-template=features/my_feature
</literallayout>
</para>
<para>
Build the kernel
<literallayout class='monospaced'>
     &gt; make linux
</literallayout>
</para>
            </section>

            <section id='patching-kernel-cache'>
                 <title>Patching: Kernel Cache</title>
<para>
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.
</para>
<para>
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.
<literallayout class='monospaced'>
     &gt; cd &lt;my layer&gt;
     &gt;mkdir kernel-temp-cache
</literallayout>
</para>
<para>
Make a directory with the BSP
<literallayout class='monospaced'>
     &gt; mkdir kernel-temp-cache
     &gt; mkdir kernel-temp-cache/my_feat
</literallayout>
</para>
<para>
Create the feature files as they were in technique #1
<literallayout class='monospaced'>
     &gt; echo "patch my_patch.path" &gt; kernel-temp-cache/my_feat/my_feature.scc
</literallayout>
</para>
<para>
Configure the build with the feature added to the kernel type
<literallayout class='monospaced'>
     &dash;&dash;with-kernel=standard+my_feat/my_feature.scc
</literallayout>
</para>
<para>
Build the kernel
<literallayout class='monospaced'>
     &gt; make linux
</literallayout>
</para>
            </section>
        </section>
 
        <section id='bsp-updating-patches-and-configuration'>
            <title>BSP: Updating Patches and Configuration</title>
<para>
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. 
</para>
<para>
Start as follows:
<literallayout class='monospaced'>
     &gt; cd linux
     &gt; git checkout &lt;bspname&gt;-&lt;kernel name&gt;

     &gt; git am &lt;patch&gt;
</literallayout>
</para>
<para>
Or you can do this:
<literallayout class='monospaced'>
     &gt; kgit-import -t patch &lt;patch&gt;

     &gt; cd ..
     &gt; make linux
</literallayout>
</para>
<para>
For details on conflict resolution and patch application, see the
git manual, or other suitable online references.
<literallayout class='monospaced'>
     &gt; git am &lt;mbox&gt;
     # conflict
     &gt; git apply &dash;&dash;reject .git/rebase-apply/0001
     # resolve conflict
     &gt; git am &dash;&dash;resolved (or git am &dash;&dash;skip, git am &dash;&dash;abort)
     # continue until complete
</literallayout>
</para>
<para>
Here is another example:
<literallayout class='monospaced'>
     # merge the patches
     # 1) single patch
     &gt; git am &lt;mbox&gt;
     &gt; git apply &lt;patch&lt;
     &gt; kgit import -t patch &lt;patch&gt;

     # 2) multiple patches
     &gt; git am &lt;mbox&gt;
     &gt; kgit import -t dir &lt;dir&gt;

     # if kgit -t dir is used, a patch resolution cycle such
     # as this can be used:

     &gt; kgit import -t dir &lt;dir&gt;
     # locate rejects and resolve
     # options:
       &gt; wiggle &dash;&dash;replace &lt;path to file&gt; &lt;path to reject&gt;
       &gt; guilt refresh
           or
       &gt; # manual resolution
       &gt; git add &lt;files&gt;
       &gt; git commit -s
           or
       &gt; git apply &dash;&dash;reject .git/rebase-apply/0001
       &gt; git add &lt;files&gt;
       &gt; git am &dash;&dash;resolved
           or
       &gt; # merge tool of choice

      # continue series:

       &gt; kgit import -t dir &lt;dir&gt;
           or
       &gt; git am &dash;&dash;continue
</literallayout>
</para>
<para>
Once all the patches have been tested and are satisfactory, they
should be exported via the techniques described in "saving kernel
modifications."
</para>
<para>
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.
</para>
<para>
Using menuconfig, the operation is as follows:
<literallayout class='monospaced'>
     &gt; make linux.menuconfig
     &gt; make linux.rebuild
</literallayout>
</para>
<para>
Once complete, the changes are in linux-&lt;bsp&gt;-&lt;kernel type&gt;-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.
</para>
<para>
Using configuration fragments, the operation is as follows (using the
si_is8620 as an example BSP):
<literallayout class='monospaced'>
     &gt; vi linux/wrs/cfg/kernel-cache/bsp/si_is8620/si_is8620.cfg
     &gt; make linux.reconfig
     &gt; make linux.rebuild
</literallayout>
</para>
<para>
The modified configuration fragment can simply be copied out of the
linux/wrs/.. directory and placed in the appropriate template for future
application.
</para>
        </section>
   
        <section id='tools-guilt'>
            <title>Tools: guilt</title>
<para>
Yocto Project has guilt integrated as a kernel tool; therefore users that are
familiar with quilt may wish to use this tool to pop, push and refresh 
their patches. Note: guilt should only be used for local operations, once
a set of changes has been pushed or pulled, they should no longer be popped
or refresh by guilt, since popping, refreshing and re-pushing patches 
changes their commit IDs and creating non-fast forward branches.
</para>
<para>
The following example illustrates how to add patches a Yocto Project 
BSP branch via guilt:
<literallayout class='monospaced'>
     &gt; cd build/linux
     &gt; git checkout common_pc-standard
     &gt; guilt new extra.patch
     # edit files, make changes, etc
     &gt; guilt refresh
     &gt; guilt top
   extra.patch
  
     # export that patch to an external location
     &gt; kgit export -p top /tmp
</literallayout>
</para>
<para>
Other guilt operations of interest are:
<literallayout class='monospaced'>
  > guilt push, guilt push -a
  > guilt pop
  > guilt applied, guilt unapplied
  > guilt top
  > guilt refresh
  > guilt header -e
  > guilt next
</literallayout>
</para>
<note><para>
Guilt only uses git commands and git plumbing to perform its operations, 
anything that guilt does can also be done using git directly. It is provided
as a convenience utility, but is not required and the developer can use whatever
tools or workflow they wish.
</para></note>
<para>
The following builds from the above instructions to show how guilt can be
used to assist in getting your BSP kernel patches ready. You should follow
the above instructions up to and including 'make linux.config'. In this
example I will create a new commit (patch) from scratch and import another
fictitious patch from some external public git tree (ie, a commit with full
message, signoff etc.). Please ensure you have host-cross/bin in your path.
<literallayout class='monospaced'>
     %> cd linux
     %> guilt-init
     %> guilt-new -m fill_me_in_please first_one.patch
     %> touch somefile.txt
     %> guilt-add somefile.txt
     %> guilt-header -e
     %> guilt-refresh
     %> guilt-import path_to_some_patch/patch_filename
     %> guilt-push
</literallayout>
</para>
<para>
Here are a few notes about the above:
<itemizedlist>
     <listitem><para>guilt-header -e &dash;&dash; this will open editing of the patch header in
      EDITOR.  As with a git commit the first line is the short log and
      should be just that short and concise message about the commit. Follow
      the short log with lines of text that will be the long description but
      note Do not put a blank line after the short log. As usual you will
      want to follow this with a blank line and then a signoff line.</para></listitem>

    <listitem><para>The last line in the example above has 2 dots on the end. If you
      don't add the 2 periods on the end guilt will think you are sending
      just one patch. The wrong one!</para></listitem>

    <listitem><para>The advantage to using guilt over not using guilt is that if you have a
      review comment in the first patch (first_one.patch in the case of this
      example) it is very easy to use guilt to pop the other patches off
      allowing you to make the necessary changes without having to use more
      inventive git type strategies.</para></listitem>
</itemizedlist>
</para>
        </section>

        <section id='tools-scc-file-example'>
            <title>Tools: scc File Example</title>
<para>
This section provides some scc file examples:  leaf node, 'normal' mode, and transforms.
</para>
            <section id='leaf-node'>
                <title>Leaf Node</title>
<para>
The following example is a BSP branch with no child branches - a leaf on the tree.
<literallayout class='monospaced'>
     # these are optional, but allow standalone tree construction
     define WRS_BOARD  &lt;name&gt;
     define WRS_KERNEL &lt;kern type&gt;
     define WRS_ARCH   &lt;arch&gt;

     scc_leaf ktypes/standard common_pc-standard
     #              ^                 ^
     #              +&dash;&dash; parent        + branch name

     include common_pc.scc
     #     ^
     #     +&dash;&dash;&dash; include another feature
</literallayout>
</para>
            </section>

            <section id='normal-mode'>
                <title>'Normal' Mode</title>
<para>
Here is an example of 'normal' mode:
<literallayout class='monospaced'>
     #                 +&dash;&dash;&dash;&dash; name of file to read
     #                 v
     kconf hardware common_pc.cfg
     # ^      ^
     # |      +&dash;&dash; 'type: hardware or non-hardware
     # |
     # +&dash;&dash;&dash; kernel config

     # patches
     patch 0002-atl2-add-atl2-driver.patch
     patch 0003-net-remove-LLTX-in-atl2-driver.patch
     patch 0004-net-add-net-poll-support-for-atl2-driver.patch
</literallayout>
</para>

            </section>

            <section id='transforms'>
                <title>Transforms</title>
<para>
This section shows an example of transforms:
<literallayout class='monospaced'>
     # either of the next two options will trigger an 'auto'
     # branch from existing ones, since they change the commit
     # order and hence must construct their own branch

     # this changes the order of future includes, if the
     # passed feature is detected, the first feature is
     # included AFTER it
     include features/rt/rt.scc after features/kgdb/kgdb
     # this also changes the order of existing branches
     # this prevents the named feature from ever being
     # included
     exclude features/dynamic_ftrace/dynamic_ftrace.scc

     # inherit the standard kernel
     include ktypes/standard/standard


     # LTT supplies this, so we don't want the sub-chunk from RT.
     patch_trigger arch:all exclude ftrace-upstream-tracepoints.patch
     # ...but we still want the one unique tracepoint it added.
     patch tracepoint-add-for-sched_resched_task.patch

     # these will change the named patches in the series into
     # &lt;patch name&gt;.patch.&lt;feature name&gt;
     # where the substituted patch is in this directory
     patch_trigger arch:all ctx_mod dynamic_printk.patch
     patch_trigger arch:all ctx_mod 0001-Implement-futex-macros-for-ARM.patch
     # unconditionally exclude a patch
     patch_trigger arch:all exclude ftrace-fix-ARM-crash.patch
</literallayout>
</para>
            </section>
        </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 there are modification in the source
directory that haven't been committed.
<literallayout class='monospaced'>
     &gt; git status
</literallayout>
</para>
<para>
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.
</para>
<para>
To brute force pickup and commit all such pending changes enter the following:
<literallayout class='monospaced'>
     &gt; git add .
     &gt; git commit -s -a -m "getting rid of -dirty"
</literallayout>
</para>
<para>
And then rebuild the kernel
</para>
        </section>

        <section id='kernel-transition-kernel-layer'>
            <title>Kernel: Transition Kernel Layer</title>
<para>
In order to temporarily use a different base kernel in Yocto Project
Linux 3.0 you need to do the following:
<orderedlist>
    <listitem><para>Create a custom kernel layer.</para></listitem>
    <listitem><para>Create a git repository of the transition kernel.</para></listitem>
</orderedlist>
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
The next few sections describe the process:
</para>
            <section id='creating-a-custom-kernel-layer'>
                <title>Creating a Custom Kernel Layer</title>
<para>
The custom kernel layer must have the following minimum
elements:
<itemizedlist>
    <listitem><para>An include of the shipped Yocto Project kernel layer.</para></listitem>
    <listitem><para>A kernel-cache with an override of the standard kernel type.</para></listitem>
</itemizedlist>
</para>
<para>
This allows the inheritance of the kernel build infrastructure,
while overriding the list of patches that should be applied to
the base kernel.
</para>
<para>
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.
</para>
            </section>

            <section id='git-repo-of-the-transition-kernel'>
                <title>git Repo of the Transition Kernel</title>
<para>
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'
</para>
<para>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.
</para>
<para>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:
<programlisting>
linux_GIT_BASE=&lt;absolute path to layer&gt;/git
</programlisting>
</para>
<para>
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:
<literallayout class='monospaced'>
     &gt; tar zxvf temp_kernel.tgz
     &gt; cd temp_kernel
     &gt; git init
     &gt; git add .
     &gt; git commit -a -m "Transition kernel baseline"

     'temp_kernel' can now be cloned into place via:

     &gt; cd &lt;path to git base&gt;/git
     &gt; git clone &dash;&dash;bare &lt;path to temp_kernel/temp_kernel default_kernel
</literallayout>
</para>
            </section>

            <section id='building-the-kernel'>
                <title>Building the Kernel</title>
<para>
Once these prerequisites have been met, the kernel can be
built with:
<literallayout class='monospaced'>
     &gt; make linux
</literallayout>
</para>
<para>
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.
</para>
            </section>

            <section id='example'>
                <title>Example</title>
<para>
This example creates a kernel layer to build the latest
kernel.org tree as the 'common_pc' BSP.
<literallayout class='monospaced'>
     &gt; cd &lt;path to layers&gt;
     &gt; mkdir wrll-linux-my_version
     &gt; cd wrll-linux-my_version
     &gt; echo "wrll-linux-2.6.27" &gt; include
     &gt; mkdir -p kernel-cache/ktypes/standard
     &gt; mkdir -p kernel-cache/bsp/common_pc
     &gt; echo "v2.6.29" &gt; kernel-cache/kver
     &gt; echo "branch common_pc-standard" &gt; kernel-cache/bsp/common_pc/common_pc.scc
     &gt; echo "kconf hardware common_pc.cfg" &gt;&gt; kernel-cache/bsp/common_pc/common_pc.scc
     &gt; echo "CONFIG_FOO=y" &gt; kernel-cache/bsp/common_pc/common_pc.cfg
     &gt; mkdir git
     &gt; cd git
     &gt; git clone &dash;&dash;bare git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git default_kernel
</literallayout>
</para>
<para>
Configure a build to use the new layer. This means that:
<literallayout class='monospaced'>
     &dash;&dash;enable-kernel-version=my_version
</literallayout>
</para>
<para>
Should be used to override the shipped default.
</para>
<para>
To build the kernel:
<literallayout class='monospaced'>
     &gt; cd build
     &gt; make linux_GIT_BASE=&lt;layer path&gt;/wrll-linux-my_version/git linux
</literallayout>
</para>
<para>
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.
</para>
<note><para>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.
</para></note>
            </section>
        </section>
    </section>





<!--    <itemizedlist>
        <listitem><para>Introduction to this section.</para></listitem>
        <listitem><para>Constructing a project-specific kernel tree.</para></listitem>
        <listitem><para>Building the kernel.</para></listitem>
        <listitem><para>Seeing what has changed.</para></listitem>
        <listitem><para>Seeing what has changed in a particular branch.</para></listitem>
        <listitem><para>Modifying the kernel.</para></listitem>
        <listitem><para>Saving modifications.</para></listitem>
        <listitem><para>Storing patches outside of the kernel source repository (bulk export).</para></listitem>
        <listitem><para>Working with incremental changes.</para></listitem>
        <listitem><para>Extracting commited changes from a working directory (exporting internally through
            patches.</para></listitem>
        <listitem><para>Pushing commited changes.</para></listitem>
        <listitem><para>Exporting for external (upstream) submission.</para></listitem>
        <listitem><para>Exporting for import into another Source Control Manager (SCM).</para></listitem>
        <listitem><para>Working with the Yocto Project kernel in another SCM.</para>
            <itemizedlist>
                <listitem><para>Exporting the delivered kernel to an SCM.</para></listitem>
                <listitem><para>Importing changed for the build.</para></listitem>
            </itemizedlist></listitem>
        <listitem><para>Migrating templates from version 2.0.</para></listitem>
        <listitem><para>Creating a new Board Support Package (BSP).</para>
            <itemizedlist>
                <listitem><para>Creating from scratch.</para></listitem>
                <listitem><para>Cloning.</para></listitem>
            </itemizedlist></listitem>
        <listitem><para>BSP bootstrapping.</para></listitem>
        <listitem><para>Applying patches to the kernel through a template.</para></listitem>
        <listitem><para>Applying patches to the kernel without using a template.</para></listitem>
        <listitem><para>Updating patches and configurations for a BSP.</para></listitem>
        <listitem><para>Using guilt to add and export patches.</para></listitem>
        <listitem><para>Using scc.</para></listitem>
        <listitem><para>Building a 'dirty' image.</para></listitem>
        <listitem><para>Temporarily using a different base kernel.</para></listitem>
        <listitem><para>Creating a custom kernel layer.</para></listitem>
        <listitem><para>Creating the git repository of the transition kernel.</para></listitem>
    </itemizedlist> -->


</section>

</article>
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