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<chapter id='kernel-how-to'>

<title>Working with the Yocto Project Kernel</title>


<section id='actions-org'>
    <title>Introduction</title>
    <para>
        This chapter describes how to accomplish tasks involving the kernel's tree structure. 
        The information is designed to help the developer that wants to modify the Yocto 
        Project kernel and contribute changes upstream to the Yocto Project.
        The information covers the following:
        <itemizedlist> 
            <listitem><para>Tree construction</para></listitem>
            <listitem><para>Build strategies</para></listitem>
            <listitem><para>Workflow examples</para></listitem>
        </itemizedlist>
    </para>
</section>

    <section id='tree-construction'>
        <title>Tree Construction</title>
        <para>
            This section describes construction of the Yocto Project kernel repositories 
            as accomplished by the Yocto Project team to create kernel repositories.
            These kernel repositories are found at
            <ulink url='http://git.yoctoproject.org/cgit.cgi'>http://git.yoctoproject.org/cgit.cgi</ulink>
            and can be shipped as part of a Yocto Project release. 
            The team creates these repositories by
            compiling and executing the set of feature descriptions for every BSP/feature
            in the product. 
            Those feature descriptions list all necessary patches,
            configuration, branching, tagging and feature divisions found in the kernel.
            Thus, the Yocto Project kernel repository (or tree) is built.
        </para>
        <para>
            The existence of this tree allows you to access and clone a particular 
            Linux Yocto kernel repository and use it to build images based on their configurations
            and features.
        </para>
        <para>
            You can find the files used to describe all the valid features and BSPs 
            in the Yocto Project kernel in any clone of the Linux Yocto kernel source repository Git tree.
            For example, the following command clones the Yocto Project baseline kernel that 
            branched off of <filename>linux.org</filename> version 3.0:
            <literallayout class='monospaced'>
     $ git clone git://git.yoctoproject.org/linux-yocto-3.0
            </literallayout>
            For another example of how to set up a local Git repository of the Linux Yocto 
            kernel files, see the 
            "<ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#local-kernel-files'>Linux Yocto Kernel</ulink>" bulleted item in The Yocto Project Development Manual.
        </para>
        <para>  
            Once you have cloned the kernel Git repository on your local machine, you can 
            switch to the <filename>meta</filename> branch within the repository.
            Here is an example that assumes the local Git repository for the kernel is in 
            a top-level directory named <filename>linux-yocto-3.0</filename>:
            <literallayout class='monospaced'>
     $ cd ~/linux-yocto-3.0
     $ git checkout -b meta origin/meta
            </literallayout>
            Once you have checked out and switched to the <filename>meta</filename> branch,
            you can see a snapshot of all the kernel configuration and feature descriptions that are 
            used to build that particular kernel repository.
            These descriptions are in the form of <filename>.scc</filename> files. 
        </para>
        <para>
            You should realize, however, that browsing your local kernel repository
            for feature descriptions and patches is not an effective way to determine what is in a
            particular kernel branch. 
            Instead, you should use Git directly to discover the changes in a branch.
            Using Git is an efficient and flexible way to inspect changes to the kernel.  
            For examples showing how to use Git to inspect kernel commits, see the following sections
            in this chapter.
            <note>
                Ground up reconstruction of the complete kernel tree is an action only taken by the 
                Yocto Project team during an active development cycle.  
                When you create a clone of the kernel Git repository, you are simply making it 
                efficiently available for building and development.
            </note>
        </para>
        <para>
            The following steps describe what happens when the Yocto Project Team constructs
            the Yocto Linux kernel source Git repository (or tree) found at 
            <ulink url='http://git.yoctoproject.org/cgit.cgi'></ulink> given the 
            introduction of a new top-level kernel feature or BSP. 
            These are the actions that effectively create the tree 
            that includes the new feature, patch or BSP:
            <orderedlist>
                <listitem><para>A top-level kernel feature is passed to the kernel build subsystem.
                    Normally, this feature is a BSP for a particular kernel type.</para></listitem>
                <listitem><para>The file that describes the top-level feature is located by searching
                    these system directories:
                    <itemizedlist>
                        <listitem><para>The in-tree kernel-cache directories, which are located
                            in <filename>meta/cfg/kernel-cache</filename></para></listitem>
                        <listitem><para>Areas pointed to by <filename>SRC_URI</filename> statements
                            found in recipes</para></listitem>
                    </itemizedlist>
                    For a typical build, the target of the search is a 
                    feature description in an <filename>.scc</filename> file
                    whose name follows this format:
                    <literallayout class='monospaced'>
     &lt;bsp_name&gt;-&lt;kernel_type&gt;.scc
                    </literallayout>
                </para></listitem>
                <listitem><para>Once located, the feature description is either compiled into a simple script
                    of actions, or into an existing equivalent script that is already part of the
                    shipped kernel.</para></listitem>
                <listitem><para>Extra features are appended to the top-level feature description. 
                    These features can come from the <filename>KERNEL_FEATURES</filename> variable in 
                    recipes.</para></listitem>
                <listitem><para>Each extra feature is located, compiled and appended to the script 
                    as described in step three.</para></listitem>
                <listitem><para>The script is executed to produce a series of <filename>meta-*</filename>
                    directories. 
                    These directories are descriptions of all the branches, tags, patches and configurations that
                    need to be applied to the base Git repository to completely create the
                    source (build) branch for the new BSP or feature.</para></listitem>
                <listitem><para>The base repository is cloned, and the actions
                    listed in the <filename>meta-*</filename> directories are applied to the 
                    tree.</para></listitem>
                <listitem><para>The Git repository is left with the desired branch checked out and any
                    required branching, patching and tagging has been performed.</para></listitem>
            </orderedlist>
        </para>
        <para>
            The kernel tree is now ready for developer consumption to be locally cloned,
            configured, and built into a Linux Yocto kernel specific to some target hardware. 
            <note><para>The generated <filename>meta-*</filename> directories add to the kernel 
                as shipped with the Yocto Project release. 
                Any add-ons and configuration data are applied to the end of an existing branch. 
                The full repository generation that is found in the 
                official Yocto Project kernel repositories at
                <ulink url='http://git.yoctoproject.org/cgit.cgi'>http://git.yoctoproject.org/cgit.cgi</ulink>
                is the combination of all supported boards and configurations.</para>
                <para>The technique the Yocto Project team uses is flexible and allows for seamless 
                blending of an immutable history with additional patches specific to a 
                deployment. 
                Any additions to the kernel become an integrated part of the branches.</para>
            </note>
        </para>
    </section>

    <section id='build-strategy'>
        <title>Build Strategy</title>
        <para>
            Once a local Git repository of the Linux Yocto kernel exists on a development system, 
            you can consider the compilation phase of kernel development - building a kernel image.
            Some prerequisites exist that are validated by the build process before compilation
            starts:
        </para>

        <itemizedlist>
            <listitem><para>The <filename>SRC_URI</filename> points to the kernel Git 
                repository.</para></listitem>
            <listitem><para>A BSP build branch exists.
                This branch has the following form:
                <literallayout class='monospaced'>
     &lt;kernel_type&gt;/&lt;bsp_name&gt;
                </literallayout></para></listitem>
        </itemizedlist>

        <para>
            The Yocto Project makes sure these conditions exist before attempting compilation.
            Other means, however, do exist, such as as bootstrapping a BSP, see 
            the "<link linkend='workflow-examples'>Workflow Examples</link>".
        </para>

        <para>
            Before building a kernel, the build process verifies the tree 
            and configures the kernel by processing all of the
            configuration "fragments" specified by feature descriptions in the <filename>.scc</filename>
            files. 
            As the features are compiled, associated kernel configuration fragments are noted
            and recorded in the <filename>meta-*</filename> series of directories in their compilation order. 
            The fragments are migrated, pre-processed and passed to the Linux Kernel
            Configuration subsystem (<filename>lkc</filename>) as raw input in the form 
            of a <filename>.config</filename> file.
            The <filename>lkc</filename> uses its own internal dependency constraints to do the final
            processing of that information and generates the final <filename>.config</filename> file 
            that is used during compilation.
        </para>

        <para>
            Using the board's architecture and other relevant values from the board's template,
            kernel compilation is started and a kernel image is produced.
        </para>

        <para>
            The other thing that you notice once you configure a kernel is that
            the build process generates a build tree that is separate from your kernel's local Git 
            source repository tree.
            This build tree has a name that uses the following form, where 
            <filename>${MACHINE}</filename> is the metadata name of the machine (BSP) and "kernel_type" is one
            of the Yocto Project supported kernel types (e.g. "standard"):
        <literallayout class='monospaced'>
     linux-${MACHINE}-&lt;kernel_type&gt;-build
        </literallayout>
        </para>

        <para>
            The existing support in the <filename>kernel.org</filename> tree achieves this 
            default functionality.
        </para>

        <para>
            What this means, is that all the generated files for a particular machine or BSP are now in 
            the build tree directory.  
            The files include the final <filename>.config</filename> file, all the <filename>.o</filename>
            files, the <filename>.a</filename> files, and so forth.
            Since each machine or BSP has its own separate build directory in its own separate branch
            of the Git repository, you can easily switch between different builds.
        </para>
    </section>

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

        <para>
            As previously noted, the Yocto Project kernel has built in Git integration.
            However, these utilities are not the only way to work with the kernel repository.
            The Yocto Project has not made changes to Git or to other tools that
            would invalidate alternate workflows. 
            Additionally, the way the kernel repository is constructed results in using 
            only core Git functionality, thus allowing any number of tools or front ends to use the 
            resulting tree.
        </para>

        <para>
            This section contains several workflow examples.
            Many of the examples use Git commands. 
            You can find Git documentation at 
            <ulink url='http://git-scm.com/documentation'></ulink>.
            You can find a simple overview of using Git with the Yocto Project in the 
            "<ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#git'>Git</ulink>"
            section of The Yocto Project Development Manual.
        </para>

        <section id='change-inspection-kernel-changes-commits'>
            <title>Change Inspection: Kernel Changes/Commits</title>

            <para>
                A common question when working with a BSP or kernel is: 
                "What changes have been applied to this tree?"
            </para>

            <para>
                In projects that have a collection of directories that
                contain patches to the kernel, it is possible to inspect or "grep" the contents 
                of the directories to get a general feel for the changes. 
                This sort of patch inspection is not an efficient way to determine what has been done to the
                kernel.
                The reason it is inefficient is because there are many optional patches that are 
                selected based on the kernel type and the feature description.
                Additionally, patches could exist in directories that are not included in the search.
            </para>

            <para>
                A more efficient way to determine what has changed in the kernel is to use
                Git and inspect or search the kernel tree. 
                This method gives you a full view of not only the source code modifications, 
                but also provides the reasons for the changes.
            </para>

            <section id='what-changed-in-a-bsp'>
                <title>What Changed in a BSP?</title>

                <para>
                    Following are a few examples that show how to use Git to examine changes.
                    Because the Yocto Project Git repository does not break existing Git
                    functionality and because there exists many permutations of these types of 
                    commands, there are many more methods to discover changes.
                    <note>
                        Unless you provide a commit range 
                        (&lt;kernel-type&gt;..&lt;bsp&gt;-&lt;kernel-type&gt;), <filename>kernel.org</filename> history 
                        is blended with Yocto Project changes.
                    </note>
                    <literallayout class='monospaced'>
     # full description of the changes
     &gt; git whatchanged &lt;kernel type&gt;..&lt;kernel type&gt;/&lt;bsp&gt;
        &gt; eg: git whatchanged yocto/standard/base..yocto/standard/common-pc/base

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

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

     # dump individual patches per commit
     &gt; git format-patch -o &lt;dir&gt; &lt;kernel type&gt;..&lt;kernel type&gt;/&lt;bsp&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>
                </para>
            </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>
                        Because BSP branch, <filename>kernel.org</filename>, and feature tags are all 
                        present, there could be many tags.
                    </note>
                    <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 --contains &lt;tag&gt;

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

                <para>
                    You can use many other comparisons to isolate BSP changes.
                    For example, you can compare against <filename>kernel.org</filename> tags 
                    (e.g. v2.6.27.18, etc), or
                    you can compare against subsystems (e.g. <filename>git whatchanged mm</filename>).
                </para>
            </section>
        </section>

        <section id='development-saving-kernel-modifications'>
            <title>Development: Saving Kernel Modifications</title>

            <para>
                Another common operation is to build a BSP supplied by the Yocto Project, make some
                changes, rebuild, and then test. 
                Those local changes often need to be exported, shared or otherwise maintained.
            </para>

            <para>
                Since the Yocto Project kernel source tree is backed by Git, this activity is
                much easier as compared to with previous releases. 
                Because Git tracks file modifications, additions and deletions, it is easy 
                to modify the code and later realize that you need to save the changes.
                It is also easy to determine what has changed. 
                This method also provides many tools to commit, undo and export those modifications.
            </para>

            <para>
                This section and its sub-sections, describe general application of Git's 
                <filename>push</filename> and <filename>pull</filename> commands, which are used to 
                get your changes upstream or source your code from an upstream repository.  
                The Yocto Project provides scripts that help you work in a collaborative development 
                environment.  
                For information on these scripts, see the
                "<ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#pushing-a-change-upstream'>Pushing a Change Upstream and Requesting a Pull</ulink>" and   
                "<ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#submitting-a-patch'>Submitting a Patch Through Email</ulink>" sections in The Yocto Project Development 
                Manual.
            </para>

            <para>
                There are many ways to save kernel modifications.
                The technique employed
                depends on the destination for the patches:
   
                <itemizedlist>
                    <listitem><para>Bulk storage</para></listitem>
                    <listitem><para>Internal sharing either through patches or by using Git</para></listitem>
                    <listitem><para>External submissions</para></listitem>
                    <listitem><para>Exporting for integration into another Source Code
                        Manager (SCM)</para></listitem>
                </itemizedlist>
            </para>

            <para>
                Because of the following list of issues, the destination of the patches also influences 
                the method for gathering them:

                <itemizedlist>
                    <listitem><para>Bisectability</para></listitem>
                    <listitem><para>Commit headers</para></listitem>
                    <listitem><para>Division of subsystems for separate submission or review</para></listitem>
                </itemizedlist>
            </para>

            <section id='bulk-export'>
                <title>Bulk Export</title>

                <para>
                    This section describes how you can "bulk" export changes that have not 
                    been separated or divided. 
                    This situation works well when you are simply storing patches outside of the kernel 
                    source repository, either permanently or temporarily, and you are not committing 
                    incremental changes during development.
                    <note>
                        This technique is not appropriate for full integration of upstream submission
                        because changes are not properly divided and do not provide an avenue for per-change
                        commit messages.  
                        Therefore, this example assumes that changes have not been committed incrementally 
                        during development and that you simply must gather and export them.
                    </note>
                    <literallayout class='monospaced'>
     # bulk export of ALL modifications without separation or division
     # of the changes 

     &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>
                    The previous operations capture all the local changes in the project source
                    tree in a single Git commit.
                    And, that commit is also stored in the project's source tree.
                </para>

                <para>
                    Once the changes are exported, you can restore them manually using a template
                    or through integration with the <filename>default_kernel</filename>.
                </para>

            </section>

            <section id='incremental-planned-sharing'>
                <title>Incremental/Planned Sharing</title>

                <para>
                    This section describes how to save modifications when you are making incremental
                    commits or practicing planned sharing.
                    The examples in this section assume that you have incrementally committed 
                    changes to the tree during development and now need to export them.  
                    The sections that follow
                    describe how you can export your changes internally through either patches or by
                    using Git commands.
                </para>

                <para>
                    During development, the following commands are of interest.
                    For full Git documentation, refer to the Git documentation at  
                    <ulink url='http://github.com'></ulink>.

                    <literallayout class='monospaced'>
     # edit a file
     &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 when you use a universally 
                    agreed-upon unique commit identifier (set by the creator of the commit) that maps to a
                    specific change set with a specific parent.  
                    This identifier is created for you when
                    you create a commit, and is re-created when you amend, alter or re-apply
                    a commit.  
                    As an individual in isolation, this is of no interest.
                    However, if you
                    intend to share your tree with normal Git <filename>push</filename> and 
                    <filename>pull</filename> operations for
                    distributed development, you should consider the ramifications of changing a
                    commit that you have already shared with others.
                </para>

                <para>
                    Assuming that the changes have not been pushed upstream, or pulled into
                    another repository, you can update both the commit content and commit messages 
                    associated with development by using the following commands:

                    <literallayout class='monospaced'>
     &gt; Git add &gt;path&lt;/file
     &gt; Git commit --amend
     &gt; Git rebase or Git rebase -i
                    </literallayout>
                </para>

                <para>   
                    Again, assuming that the changes have not been pushed upstream, and that
                    no pending works-in-progress exists (use <filename>git status</filename> to check), then
                    you can revert (undo) commits by using the following commands:

                    <literallayout class='monospaced'>
     # remove the commit, update working tree and remove all
     # traces of the change
     &gt; git reset --hard HEAD^
     # remove the commit, but leave the files changed and staged for re-commit
     &gt; git reset --soft HEAD^
     # remove the commit, leave file change, but not staged for commit
     &gt; git reset --mixed HEAD^
                    </literallayout>
                </para>

                <para>
                    You can create branches, "cherry-pick" changes, or perform any number of Git
                    operations until the commits are in good order for pushing upstream
                    or for pull requests. 
                    After a <filename>push</filename> or <filename>pull</filename> command, 
                    commits are normally considered
                    "permanent" and you should not modify them.
                    If the commits need to be changed, you can incrementally do so with new commits.
                    These practices follow standard Git workflow and the <filename>kernel.org</filename> best 
                    practices, which Yocto Project recommends.
                    <note>
                        It is recommended to tag or branch before adding changes to a Yocto Project
                        BSP or before creating a new one.
                        The reason for this recommendation is because the branch or tag provides a
                        reference point to facilitate locating and exporting local changes.
                    </note>
                </para>
          
                <section id='export-internally-via-patches'>
                    <title>Exporting Changes Internally by Using Patches</title>

                    <para>
                        This section describes how you can extract committed changes from a working directory
                        by exporting them as patches.
                        Once extracted, you can use the patches for upstream submission, 
                        place them in a Yocto Project template for automatic kernel patching,
                        or apply them in many other common uses.
                    </para>

                    <para>
                        This example shows how to create a directory with sequentially numbered patches.
                        Once the directory is created, you can apply it to a repository using the 
                        <filename>git am</filename> command to reproduce the original commit and all 
                        the related information such as author, date, commit log, and so forth.
                        <note>
                            The new commit identifiers (ID) will be generated upon re-application.  
                            This action reflects that the commit is now applied to an underlying commit 
                            with a different ID.
                        </note>
                        <literallayout class='monospaced'>
     # &lt;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> 
                </section> 

                <section id='export-internally-via-git'>
                    <title>Exporting Changes Internally by Using Git</title>

                    <para>
                        This section describes how you can export changes from a working directory 
                        by pushing the changes into a master repository or by making a pull request.
                        Once you have pushed the changes in the master repository, you can then 
                        pull those same changes into a new kernel build at a later time.
                    </para>
 
                    <para>
                        Use this command form to push the changes:
                        <literallayout class='monospaced'>
     &gt; git push ssh://&lt;master_server&gt;/&lt;path_to_repo&gt;
        &lt;local_branch&gt;:&lt;remote_branch&gt;
                        </literallayout>
                    </para>

                    <para>
                        For example, the following command pushes the changes from your local branch
                        <filename>yocto/standard/common-pc/base</filename> to the remote branch with the same name 
                        in the master repository <filename>//git.mycompany.com/pub/git/kernel-3.0</filename>.
                        <literallayout class='monospaced'>
     &gt; git push ssh://git.mycompany.com/pub/git/kernel-3.0 \
        yocto/standard/common-pc/base:yocto/standard/common-pc/base
                        </literallayout>
                    </para>

                    <para>
                        A pull request entails using <filename>git request-pull</filename> to compose 
                        an email to the
                        maintainer requesting that a branch be pulled into the master repository, see
                        <ulink url='http://github.com/guides/pull-requests'></ulink> for an example.
                        <note>
                            Other commands such as <filename>git stash</filename> or branching can also be used to save
                            changes, but are not covered in this document.
                        </note>
                    </para>
                </section> 
            </section>

            <section id='export-for-external-upstream-submission'>
                <title>Exporting Changes for External (Upstream) Submission</title>

                <para>
                    This section describes how to export changes for external upstream submission.
                    If the patch series is large or the maintainer prefers to pull
                    changes, you can submit these changes by using a pull request.
                    However, it is common to send patches as an email series.
                    This method allows easy review and integration of the changes.
                    <note>
                        Before sending patches for review be sure you understand the
                        community standards for submitting and documenting changes and follow their best practices.  
                        For example, kernel patches should follow standards such as:
                        <itemizedlist>
                            <listitem><para>
                                <ulink url='http://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>
                    </note>
                </para>

                <para>
                    The messages used to commit changes are a large part of these standards.
                    Consequently, be sure that the headers for each commit have the required information.
                    For information on how to follow the Yocto Project commit message standards, see the
                    "<ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#how-to-submit-a-change'>How to Submit a Change</ulink>" section in The Yocto Project Development Manual.
                </para>

                <para>
                    If the initial commits were not properly documented or do not meet those standards,
                    you can re-base by using the <filename>git rebase -i</filename> command to 
                    manipulate the commits and
                    get them into the required format. 
                    Other techniques such as branching and cherry-picking commits are also viable options.
                </para>

                <para>
                    Once you complete the commits, you can generate the email that sends the patches 
                    to the maintainer(s) or lists that review and integrate changes. 
                    The command <filename>git send-email</filename> is commonly used to ensure 
                    that patches are properly 
                    formatted for easy application and avoid mailer-induced patch damage.
                </para>

                <para>
                    The following is an example of dumping patches for external submission:
                    <literallayout class='monospaced'>
     # dump the last 4 commits 
     &gt; git format-patch --thread -n -o ~/rr/ HEAD^^^^
     &gt; git send-email --compose --subject '[RFC 0/N] &lt;patch series summary&gt;' \
      --to foo@yoctoproject.org --to bar@yoctoproject.org \
      --cc list@yoctoproject.org  ~/rr
     # the editor is invoked for the 0/N patch, and when complete the entire
     # series is sent via email for review
                    </literallayout>
                </para>
            </section>

            <section id='export-for-import-into-other-scm'>
                <title>Exporting Changes for Import into Another SCM</title>

                <para>
                    When you want to export changes for import into another
                    Source Code Manager (SCM), you can use any of the previously discussed 
                    techniques.
                    However, if the patches are manually applied to a secondary tree and then 
                    that tree is checked into the SCM, you can lose change information such as 
                    commit logs.
                    The Yocto Project does not recommend this process.
                </para>

                <para>
                    Many SCMs can directly import Git commits, or can translate Git patches so that
                    information is not lost. 
                    Those facilities are SCM-dependent and you should use them whenever possible.
                </para>
            </section>
        </section>

        <section id='scm-working-with-the-yocto-project-kernel-in-another-scm'>
            <title>Working with the Yocto Project Kernel in Another SCM</title>

            <para>
                This section describes kernel development in an SCM other than Git, 
                which is not the same as exporting changes to another SCM described earlier.
                For this scenario, you use the Yocto Project build system to 
                develop the kernel in a different SCM.
                The following must be true for you to accomplish this:
                <itemizedlist>
                    <listitem><para>The delivered Yocto Project kernel must be exported into the second
                        SCM.</para></listitem>
                    <listitem><para>Development must be exported from that secondary SCM into a 
                        format that can be used by the Yocto Project build system.</para></listitem>
                </itemizedlist>
            </para>

            <section id='exporting-delivered-kernel-to-scm'>
                <title>Exporting the Delivered Kernel to the SCM</title>

                <para>
                    Depending on the SCM, it might be possible to export the entire Yocto Project
                    kernel Git repository, branches and all, into a new environment. 
                    This method is preferred because it has the most flexibility and potential to maintain
                    the meta data associated with each commit.
                </para>

                <para>
                    When a direct import mechanism is not available, it is still possible to
                    export a branch (or series of branches) and check them into a new repository.
                </para>

                <para>
                    The following commands illustrate some of the steps you could use to
                    import the <filename>yocto/standard/common-pc/base</filename>
                    kernel into a secondary SCM:
                    <literallayout class='monospaced'>
     &gt; git checkout yocto/standard/common-pc/base
     &gt; cd .. ; echo linux/.git &gt; .cvsignore
     &gt; cvs import -m "initial import" linux MY_COMPANY start
                    </literallayout>
                </para>

                <para>
                    You could now relocate the CVS repository and use it in a centralized manner. 
                </para>

                <para>
                    The following commands illustrate how you can condense and merge two BSPs into a second SCM:
                    <literallayout class='monospaced'>
     &gt; git checkout yocto/standard/common-pc/base
     &gt; git merge yocto/standard/common-pc-64/base
     # 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 the Build</title>
 
                <para>
                    Once development has reached a suitable point in the second development
                    environment, you need to export the changes as patches.
                    To export them, place the changes in a recipe and
                    automatically apply them to the kernel during patching.
                </para>
            </section>
        </section>

        <section id='bsp-creating'>
            <title>Creating a BSP Based on an Existing Similar BSP</title>

            <para>
                This section overviews the process of creating a BSP based on an 
                existing similar BSP.
                The information is introductory in nature and does not provide step-by-step examples. 
                For detailed information on how to create a BSP given an existing similar BSP, see
                the "<ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#dev-manual-bsp-appendix'>BSP Development Example</ulink>" appendix in The 
                Yocto Project Development Manual, or see the 
                <ulink url='https://wiki.yoctoproject.org/wiki/Transcript:_creating_one_generic_Atom_BSP_from_another'>Transcript:_creating_one_generic_Atom_BSP_from_another</ulink>
                wiki page.
            </para>

            <para>
                The basic steps you need to follow are:
                <orderedlist>
                    <listitem><para><emphasis>Make sure you have the Yocto Project source tree available:</emphasis>
                        You should either create a Yocto Project Git repository (recommended), or 
                        you should get the Yocto Project release tarball and extract it.</para></listitem>
                    <listitem><para><emphasis>Choose an existing BSP available with the Yocto Project:</emphasis>
                        Try to map your board features as closely to the features of a BSP that is  
                        already supported and exists in the Yocto Project.  
                        Starting with something as close as possible to your board makes developing 
                        your BSP easier.  
                        You can find all the BSPs that are supported and ship with the Yocto Project 
                        on the Yocto Project's Download page at 
                        <ulink url='http://www.yoctoproject.org/download'></ulink>.</para></listitem>
                    <listitem><para><emphasis>Be sure you have the Base BSP:</emphasis>
                        You need to either have the Yocto Project Git repository set up or download
                        the tarball of the base BSP.
                        Either method gives you access to the BSP source files.</para></listitem>
                    <listitem><para><emphasis>Make a copy of the existing BSP, thus isolating your new 
                        BSP work:</emphasis>
                        Copying the existing BSP structure gives you a new area in which to work.</para></listitem>
                    <listitem><para><emphasis>Make configuration and recipe changes to your new BSP:</emphasis>
                        Configuration changes involve the files in the BSP's <filename>conf</filename>
                        directory. 
                        Changes include creating a machine-specific configuration file and editing the 
                        <filename>layer.conf</filename> file.  
                        The configuration changes identify the kernel you will be using.
                        Recipe changes include removing, modifying, or adding new recipe files that 
                        instruct the build process on what features to include in the image.</para></listitem>
                    <listitem><para><emphasis>Prepare for the build:</emphasis>
                        Before you actually initiate the build, you need to set up the build environment
                        by sourcing the environment initialization script.  
                        After setting up the environment, you need to make some build configuration 
                        changes to the <filename>local.conf</filename> and <filename>bblayers.conf</filename>
                        files.</para></listitem>
                    <listitem><para><emphasis>Build the image:</emphasis>
                        The Yocto Project uses the BitBake tool to create the image. 
                        You need to decide on the type of image you are going to build (e.g. minimal, base, 
                        core, sato, and so forth) and then start the build using the <filename>bitbake</filename>
                        command.</para></listitem>
                </orderedlist>
            </para>
        </section>

        <section id='tip-dirty-string'>
            <title>"-dirty" String</title>

            <para>
                If kernel images are being built with "-dirty" on the end of the version
                string, this simply means that modifications in the source
                directory have not been committed.
                <literallayout class='monospaced'>
     &gt; git status
                </literallayout>
            </para>

            <para>
                You can use the above Git command to report modified, removed, or added files. 
                You should commit those changes to the tree regardless of whether they will be saved, 
                exported, or used.
                Once you commit the changes you need to rebuild the kernel.
            </para>

            <para>
                To brute force pickup and commit all such pending changes, enter the following:
                <literallayout class='monospaced'>
     &gt; git add .
     &gt; git commit -s -a -m "getting rid of -dirty"
                </literallayout>
            </para>

            <para>
                Next, rebuild the kernel.
            </para>
        </section>
    </section>
</chapter>
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