path: root/documentation/kernel-dev/kernel-dev-advanced.xml

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

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


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

    <section id='tree-construction'>
        <title>Tree Construction</title>
        <para>
            This section describes construction of the Yocto Project kernel source repositories
            as accomplished by the Yocto Project team to create kernel repositories.
            These kernel repositories are found under the heading "Yocto Linux Kernel" at
            <ulink url='&YOCTO_GIT_URL;/cgit.cgi'>&YOCTO_GIT_URL;/cgit.cgi</ulink>
            and can be shipped as part of a Yocto Project release.
            The team creates these repositories by
            compiling and executing the set of feature descriptions for every BSP/feature
            in the product.
            Those feature descriptions list all necessary patches,
            configuration, branching, tagging and feature divisions found in a kernel.
            Thus, the Yocto Project kernel repository (or tree) is built.
        </para>
        <para>
            The existence of this tree allows you to access and clone a particular
            Yocto Project kernel repository and use it to build images based on their configurations
            and features.
        </para>
        <para>
            You can find the files used to describe all the valid features and BSPs
            in the Yocto Project kernel in any clone of the Yocto Project kernel source repository
            Git tree.
            For example, the following command clones the Yocto Project baseline kernel that
            branched off of <filename>linux.org</filename> version 3.4:
            <literallayout class='monospaced'>
     $ git clone git://git.yoctoproject.org/linux-yocto-3.4
            </literallayout>
            For another example of how to set up a local Git repository of the Yocto Project
            kernel files, see the
            "<ulink url='&YOCTO_DOCS_DEV_URL;#local-kernel-files'>Yocto Project Kernel</ulink>" bulleted
            item in the Yocto Project Development Manual.
        </para>
        <para>
            Once you have cloned the kernel Git repository on your local machine, you can
            switch to the <filename>meta</filename> branch within the repository.
            Here is an example that assumes the local Git repository for the kernel is in
            a top-level directory named <filename>linux-yocto-3.4</filename>:
            <literallayout class='monospaced'>
     $ cd ~/linux-yocto-3.4
     $ git checkout -b meta origin/meta
            </literallayout>
            Once you have checked out and switched to the <filename>meta</filename> branch,
            you can see a snapshot of all the kernel configuration and feature descriptions that are
            used to build that particular kernel repository.
            These descriptions are in the form of <filename>.scc</filename> files.
        </para>
        <para>
            You should realize, however, that browsing your local kernel repository
            for feature descriptions and patches is not an effective way to determine what is in a
            particular kernel branch.
            Instead, you should use Git directly to discover the changes in a branch.
            Using Git is an efficient and flexible way to inspect changes to the kernel.
            For examples showing how to use Git to inspect kernel commits, see the following sections
            in this chapter.
            <note>
                Ground up reconstruction of the complete kernel tree is an action only taken by the
                Yocto Project team during an active development cycle.
                When you create a clone of the kernel Git repository, you are simply making it
                efficiently available for building and development.
            </note>
        </para>
        <para>
            The following steps describe what happens when the Yocto Project Team constructs
            the Yocto Project kernel source Git repository (or tree) found at
            <ulink url='&YOCTO_GIT_URL;/cgit.cgi'></ulink> given the
            introduction of a new top-level kernel feature or BSP.
            These are the actions that effectively create the tree
            that includes the new feature, patch or BSP:
            <orderedlist>
                <listitem><para>A top-level kernel feature is passed to the kernel build subsystem.
                    Normally, this feature is a BSP for a particular kernel type.</para></listitem>
                <listitem><para>The file that describes the top-level feature is located by searching
                    these system directories:
                    <itemizedlist>
                        <listitem><para>The in-tree kernel-cache directories, which are located
                            in <filename>meta/cfg/kernel-cache</filename></para></listitem>
                        <listitem><para>Areas pointed to by <filename>SRC_URI</filename> statements
                            found in recipes</para></listitem>
                    </itemizedlist>
                    For a typical build, the target of the search is a
                    feature description in an <filename>.scc</filename> file
                    whose name follows this format:
                    <literallayout class='monospaced'>
     &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
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'><filename>KERNEL_FEATURES</filename></ulink>
                    variable in recipes.</para></listitem>
                <listitem><para>Each extra feature is located, compiled and appended to the script
                    as described in step three.</para></listitem>
                <listitem><para>The script is executed to produce a series of <filename>meta-*</filename>
                    directories.
                    These directories are descriptions of all the branches, tags, patches and configurations that
                    need to be applied to the base Git repository to completely create the
                    source (build) branch for the new BSP or feature.</para></listitem>
                <listitem><para>The base repository is cloned, and the actions
                    listed in the <filename>meta-*</filename> directories are applied to the
                    tree.</para></listitem>
                <listitem><para>The Git repository is left with the desired branch checked out and any
                    required branching, patching and tagging has been performed.</para></listitem>
            </orderedlist>
        </para>
        <para>
            The kernel tree is now ready for developer consumption to be locally cloned,
            configured, and built into a Yocto Project kernel specific to some target hardware.
            <note><para>The generated <filename>meta-*</filename> directories add to the kernel
                as shipped with the Yocto Project release.
                Any add-ons and configuration data are applied to the end of an existing branch.
                The full repository generation that is found in the
                official Yocto Project kernel repositories at
                <ulink url='&YOCTO_GIT_URL;/cgit.cgi'>http://git.yoctoproject.org/cgit.cgi</ulink>
                is the combination of all supported boards and configurations.</para>
                <para>The technique the Yocto Project team uses is flexible and allows for seamless
                blending of an immutable history with additional patches specific to a
                deployment.
                Any additions to the kernel become an integrated part of the branches.</para>
            </note>
        </para>
    </section>

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

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

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

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

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

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

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

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

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

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

        <para>
            This section contains several workflow examples.
            Many of the examples use Git commands.
            You can find Git documentation at
            <ulink url='http://git-scm.com/documentation'></ulink>.
            You can find a simple overview of using Git with the Yocto Project in the
            "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>"
            section of the Yocto Project Development Manual.
        </para>

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

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

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

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

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

                <para>
                    Following are a few examples that show how to use Git commands to examine changes.
                    Because Git repositories in the Yocto Project do not break existing Git
                    functionality, and because there exists many permutations of these types of
                    Git commands, many methods exist by which you can discover changes.
                    <note>
                        In the following examples, unless you provide a commit range,
                        <filename>kernel.org</filename> history is blended with Yocto Project
                        kernel changes.
                        You can form ranges by using branch names from the kernel tree as the
                        upper and lower commit markers with the Git commands.
                        You can see the branch names through the web interface to the
                        Yocto Project source repositories at
                        <ulink url='http://git.yoctoproject.org/cgit.cgi'></ulink>.
                        For example, the branch names for the <filename>linux-yocto-3.4</filename>
                        kernel repository can be seen at
                        <ulink url='http://git.yoctoproject.org/cgit.cgi/linux-yocto-3.4/refs/heads'></ulink>.
                    </note>
                    To see a full range of the changes, use the
                    <filename>git whatchanged</filename> command and specify a commit range
                    for the branch (<filename>&lt;commit&gt;..&lt;commit&gt;</filename>).
                </para>

                <para>
                    Here is an example that looks at what has changed in the
                    <filename>emenlow</filename> branch of the
                    <filename>linux-yocto-3.4</filename> kernel.
                    The lower commit range is the commit associated with the
                    <filename>standard/base</filename> branch, while
                    the upper commit range is the commit associated with the
                    <filename>standard/emenlow</filename> branch.
                    <literallayout class='monospaced'>
     $ git whatchanged origin/standard/base..origin/standard/emenlow
                    </literallayout>
                </para>

                <para>
                    To see a summary of changes use the <filename>git log</filename> command.
                    Here is an example using the same branches:
                    <literallayout class='monospaced'>
     $ git log --oneline origin/standard/base..origin/standard/emenlow
                    </literallayout>
                    The <filename>git log</filename> output might be more useful than
                    the <filename>git whatchanged</filename> as you get
                    a short, one-line summary of each change and not the entire commit.
                </para>

                <para>
                    If you want to see code differences associated with all the changes, use
                    the <filename>git diff</filename> command.
                    Here is an example:
                    <literallayout class='monospaced'>
     $ git diff origin/standard/base..origin/standard/emenlow
                    </literallayout>
                </para>

                <para>
                    You can see the commit log messages and the text differences using the
                    <filename>git show</filename> command:
                    Here is an example:
                    <literallayout class='monospaced'>
     $ git show origin/standard/base..origin/standard/emenlow
                    </literallayout>
                </para>

                <para>
                    You can create individual patches for each change by using the
                    <filename>git format-patch</filename> command.
                    Here is an example that that creates patch files for each commit and
                    places them in your <filename>Documents</filename> directory:
                    <literallayout class='monospaced'>
     $ git format-patch -o $HOME/Documents origin/standard/base..origin/standard/emenlow
                    </literallayout>
                </para>
            </section>

            <section id='show-a-particular-feature-or-branch-change'>
                <title>Show a Particular Feature or Branch Change</title>

                <para>
                    Developers use tags in the Yocto Project kernel tree to divide changes for significant
                    features or branches.
                    Once you know a particular tag, you can use Git commands
                    to show changes associated with the tag and find the branches that contain
                    the feature.
                    <note>
                        Because BSP branch, <filename>kernel.org</filename>, and feature tags are all
                        present, there could be many tags.
                    </note>
                    The <filename>git show &lt;tag&gt;</filename> command shows changes that are tagged by
                    a feature.
                    Here is an example that shows changes tagged by the <filename>systemtap</filename>
                    feature:
                    <literallayout class='monospaced'>
     $ git show systemtap
                    </literallayout>
                    You can use the <filename>git branch --contains &lt;tag&gt;</filename> command
                    to show the branches that contain a particular feature.
                    This command shows the branches that contain the <filename>systemtap</filename>
                    feature:
                    <literallayout class='monospaced'>
     $ git branch --contains systemtap
                    </literallayout>
                </para>

                <para>
                    You can use many other comparisons to isolate BSP and kernel changes.
                    For example, you can compare against <filename>kernel.org</filename> tags
                    such as the <filename>v3.4</filename> tag.
                </para>
            </section>
        </section>

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

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

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

            <para>
                This section and its sub-sections, describe general application of Git's
                <filename>push</filename> and <filename>pull</filename> commands, which are used to
                get your changes upstream or source your code from an upstream repository.
                The Yocto Project provides scripts that help you work in a collaborative development
                environment.
                For information on these scripts, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#pushing-a-change-upstream'>Using Scripts to Push a Change
                Upstream and Request a Pull</ulink>" and
                "<ulink url='&YOCTO_DOCS_DEV_URL;#submitting-a-patch'>Using Email to Submit a Patch</ulink>"
                sections in the Yocto Project Development Manual.
            </para>

            <para>
                There are many ways to save kernel modifications.
                The technique employed
                depends on the destination for the patches:

                <itemizedlist>
                    <listitem><para>Bulk storage</para></listitem>
                    <listitem><para>Internal sharing either through patches or by using Git</para></listitem>
                    <listitem><para>External submissions</para></listitem>
                    <listitem><para>Exporting for integration into another Source Code
                        Manager (SCM)</para></listitem>
                </itemizedlist>
            </para>

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

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

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

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

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

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

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

            </section>

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

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

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

                    <literallayout class='monospaced'>
     # edit a file
     $ vi &lt;path&gt;/file
     # stage the change
     $ git add &lt;path&gt;/file
     # commit the change
     $ git commit -s
     # remove a file
     $ git rm &lt;path&gt;/file
     # commit the change
     $ git commit -s

     ... etc.
                    </literallayout>
                </para>

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

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

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

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

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

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

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

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

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

     $ 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
     $ git format-patch -o &lt;save dir&gt; &lt;tag&gt;

     # If no tags are available
     $ git format-patch -o &lt;save dir&gt; HEAD^  # last commit
     $ git format-patch -o &lt;save dir&gt; HEAD^^ # last 2 commits
     $ git whatchanged # identify last commit
     $ git format-patch -o &lt;save dir&gt; &lt;commit id&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 to the master repository, you can then
                        pull those same changes into a new kernel build at a later time.
                    </para>

                    <para>
                        Use this command form to push the changes:
                        <literallayout class='monospaced'>
     $ git push ssh://&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-3.4</filename>.
                        <literallayout class='monospaced'>
     $ git push ssh://git.mycompany.com/pub/git/kernel-3.4 \
        yocto/standard/common-pc/base:yocto/standard/common-pc/base
                        </literallayout>
                    </para>

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

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

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

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

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

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

                <para>
                    The following is an example of dumping patches for external submission:
                    <literallayout class='monospaced'>
     # dump the last 4 commits
     $ git format-patch --thread -n -o ~/rr/ HEAD^^^^
     $ git send-email --compose --subject '[RFC 0/N] &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.
                    This process is not recommended.
                </para>

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

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

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

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

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

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

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

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

                <para>
                    The following commands illustrate how you can condense and merge two BSPs into a
                    second SCM:
                    <literallayout class='monospaced'>
     $ git checkout yocto/standard/common-pc/base
     $ git merge yocto/standard/common-pc-64/base
     # resolve any conflicts and commit them
     $ cd .. ; echo linux/.git &gt; .cvsignore
     $ cvs import -m "initial import" linux MY_COMPANY start
                    </literallayout>
                </para>
            </section>

            <section id='importing-changes-for-build'>
                <title>Importing Changes for the Build</title>

                <para>
                    Once development has reached a suitable point in the second development
                    environment, you need to export the changes as patches.
                    To export them, place the changes in a recipe and
                    automatically apply them to the kernel during patching.
                </para>
            </section>
        </section>

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

            <para>
                This section overviews the process of creating a BSP based on an
                existing similar BSP.
                The information is introductory in nature and does not provide step-by-step examples.
                For detailed information on how to create a new BSP, see
                the "<ulink url='&YOCTO_DOCS_BSP_URL;#creating-a-new-bsp-layer-using-the-yocto-bsp-script'>Creating a New BSP Layer Using the yocto-bsp Script</ulink>" section in the
                Yocto Project Board Support Package (BSP) Developer's Guide, or see the
                <ulink url='&YOCTO_WIKI_URL;/wiki/Transcript:_creating_one_generic_Atom_BSP_from_another'>Transcript:_creating_one_generic_Atom_BSP_from_another</ulink>
                wiki page.
            </para>

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

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

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

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

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

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