import-layers/yocto-poky/documentation/dev-manual/dev-manual-newbie.xml

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[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >

<chapter id='dev-manual-newbie'>

<title>The Yocto Project Open Source Development Environment</title>

<para>
    This chapter helps you understand the Yocto Project as an open source development project.
    In general, working in an open source environment is very different from working in a
    closed, proprietary environment.
    Additionally, the Yocto Project uses specific tools and constructs as part of its development
    environment.
    This chapter specifically addresses open source philosophy, using the
    Yocto Project in a team environment, source repositories, Yocto Project
    terms, licensing, the open source distributed version control system Git,
    workflows, bug tracking, and how to submit changes.
</para>

<section id='open-source-philosophy'>
    <title>Open Source Philosophy</title>

    <para>
        Open source philosophy is characterized by software development directed by peer production
        and collaboration through an active community of developers.
        Contrast this to the more standard centralized development models used by commercial software
        companies where a finite set of developers produces a product for sale using a defined set
        of procedures that ultimately result in an end product whose architecture and source material
        are closed to the public.
    </para>

    <para>
        Open source projects conceptually have differing concurrent agendas, approaches, and production.
        These facets of the development process can come from anyone in the public (community) that has a
        stake in the software project.
        The open source environment contains new copyright, licensing, domain, and consumer issues
        that differ from the more traditional development environment.
        In an open source environment, the end product, source material, and documentation are
        all available to the public at no cost.
    </para>

    <para>
        A benchmark example of an open source project is the Linux kernel, which was initially conceived
        and created by Finnish computer science student Linus Torvalds in 1991.
        Conversely, a good example of a non-open source project is the
        <trademark class='registered'>Windows</trademark> family of operating
        systems developed by <trademark class='registered'>Microsoft</trademark> Corporation.
    </para>

    <para>
        Wikipedia has a good historical description of the Open Source Philosophy
        <ulink url='http://en.wikipedia.org/wiki/Open_source'>here</ulink>.
        You can also find helpful information on how to participate in the Linux Community
        <ulink url='http://ldn.linuxfoundation.org/book/how-participate-linux-community'>here</ulink>.
    </para>
</section>

<section id="usingpoky-changes-collaborate">
    <title>Using the Yocto Project in a Team Environment</title>

    <para>
        It might not be immediately clear how you can use the Yocto
        Project in a team environment, or scale it for a large team of
        developers.
        One of the strengths of the Yocto Project is that it is extremely
        flexible.
        Thus, you can adapt it to many different use cases and scenarios.
        However, these characteristics can cause a struggle if you are trying
        to create a working setup that scales across a large team.
    </para>

    <para>
        To help with these types of situations, this section presents
        some of the project's most successful experiences,
        practices, solutions, and available technologies that work well.
        Keep in mind, the information here is a starting point.
        You can build off it and customize it to fit any
        particular working environment and set of practices.
    </para>

    <section id='best-practices-system-configurations'>
        <title>System Configurations</title>

        <para>
            Systems across a large team should meet the needs of
            two types of developers: those working on the contents of the
            operating system image itself and those developing applications.
            Regardless of the type of developer, their workstations must
            be both reasonably powerful and run Linux.
        </para>

        <section id='best-practices-application-development'>
            <title>Application Development</title>

            <para>
                For developers who mainly do application level work
                on top of an existing software stack,
                the following list shows practices that work best.
                For information on using a Software Development Kit (SDK), see
                the
                <ulink url='&YOCTO_DOCS_SDK_URL;#sdk-intro'>Yocto Project Software Development Kit (SDK) Developer's Guide</ulink>:
                <itemizedlist>
                    <listitem><para>Use a pre-built toolchain that
                        contains the software stack itself.
                        Then, develop the application code on top of the
                        stack.
                        This method works well for small numbers of relatively
                        isolated applications.</para></listitem>
                    <listitem><para>When possible, use the Yocto Project
                        plug-in for the <trademark class='trade'>Eclipse</trademark> IDE
                        and SDK development practices.
                        For more information, see the
                        "<ulink url='&YOCTO_DOCS_SDK_URL;'>Yocto Project Software Development Kit (SDK) Developer's Guide</ulink>".
                        </para></listitem>
                    <listitem><para>Keep your cross-development toolchains
                        updated.
                        You can do this through provisioning either as new
                        toolchain downloads or as updates through a package
                        update mechanism using <filename>opkg</filename>
                        to provide updates to an existing toolchain.
                        The exact mechanics of how and when to do this are a
                        question for local policy.</para></listitem>
                    <listitem><para>Use multiple toolchains installed locally
                        into different locations to allow development across
                        versions.</para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='best-practices-core-system-development'>
            <title>Core System Development</title>

            <para>
                For core system development, it is often best to have the
                build system itself available on the developer workstations
                so developers can run their own builds and directly
                rebuild the software stack.
                You should keep the core system unchanged as much as
                possible and do your work in layers on top of the core system.
                Doing so gives you a greater level of portability when
                upgrading to new versions of the core system or Board
                Support Packages (BSPs).
                You can share layers amongst the developers of a particular
                project and contain the policy configuration that defines
                the project.
            </para>

            <para>
                Aside from the previous best practices, there exists a number
                of tips and tricks that can help speed up core development
                projects:
                <itemizedlist>
                    <listitem><para>Use a
                        <ulink url='&YOCTO_DOCS_REF_URL;#shared-state-cache'>Shared State Cache</ulink>
                        (sstate) among groups of developers who are on a
                        fast network.
                        The best way to share sstate is through a
                        Network File System (NFS) share.
                        The first user to build a given component for the
                        first time contributes that object to the sstate,
                        while subsequent builds from other developers then
                        reuse the object rather than rebuild it themselves.
                        </para>
                        <para>Although it is possible to use other protocols for the
                        sstate such as HTTP and FTP, you should avoid these.
                        Using HTTP limits the sstate to read-only and
                        FTP provides poor performance.
                        </para></listitem>
                    <listitem><para>Have autobuilders contribute to the sstate
                        pool similarly to how the developer workstations
                        contribute.
                        For information, see the
                        "<link linkend='best-practices-autobuilders'>Autobuilders</link>"
                        section.</para></listitem>
                    <listitem><para>Build stand-alone tarballs that contain
                        "missing" system requirements if for some reason
                        developer workstations do not meet minimum system
                        requirements such as latest Python versions,
                        <filename>chrpath</filename>, or other tools.
                        You can install and relocate the tarball exactly as you
                        would the usual cross-development toolchain so that
                        all developers can meet minimum version requirements
                        on most distributions.</para></listitem>
                    <listitem><para>Use a small number of shared,
                        high performance systems for testing purposes
                        (e.g. dual, six-core Xeons with 24 Gbytes of RAM
                        and plenty of disk space).
                        Developers can use these systems for wider, more
                        extensive testing while they continue to develop
                        locally using their primary development system.
                        </para></listitem>
                    <listitem><para>Enable the PR Service when package feeds
                        need to be incremental with continually increasing
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-PR'>PR</ulink>
                        values.
                        Typically, this situation occurs when you use or
                        publish package feeds and use a shared state.
                        You should enable the PR Service for all users who
                        use the shared state pool.
                        For more information on the PR Service, see the
                        "<link linkend='working-with-a-pr-service'>Working With a PR Service</link>".
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>
    </section>

    <section id='best-practices-source-control-management'>
        <title>Source Control Management (SCM)</title>

        <para>
            Keeping your
            <ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink>
            and any software you are developing under the
            control of an SCM system that is compatible
            with the OpenEmbedded build system is advisable.
            Of the SCMs BitBake supports, the
            Yocto Project team strongly recommends using
            <link linkend='git'>Git</link>.
            Git is a distributed system that is easy to backup,
            allows you to work remotely, and then connects back to the
            infrastructure.
            <note>
                For information about BitBake, see the
                <ulink url='&YOCTO_DOCS_BB_URL;'>BitBake User Manual</ulink>.
            </note>
        </para>

        <para>
            It is relatively easy to set up Git services and create
            infrastructure like
            <ulink url='&YOCTO_GIT_URL;'>http://git.yoctoproject.org</ulink>,
            which is based on server software called
            <filename>gitolite</filename> with <filename>cgit</filename>
            being used to generate the web interface that lets you view the
            repositories.
            The <filename>gitolite</filename> software identifies users
            using SSH keys and allows branch-based
            access controls to repositories that you can control as little
            or as much as necessary.
        </para>

        <note>
            The setup of these services is beyond the scope of this manual.
            However, sites such as these exist that describe how to perform
            setup:
            <itemizedlist>
                <listitem><para><ulink url='http://git-scm.com/book/ch4-8.html'>Git documentation</ulink>:
                    Describes how to install <filename>gitolite</filename>
                    on the server.</para></listitem>
                <listitem><para><ulink url='http://sitaramc.github.com/gitolite/master-toc.html'>The <filename>gitolite</filename> master index</ulink>:
                    All topics for <filename>gitolite</filename>.
                    </para></listitem>
                <listitem><para><ulink url='https://git.wiki.kernel.org/index.php/Interfaces,_frontends,_and_tools'>Interfaces, frontends, and tools</ulink>:
                    Documentation on how to create interfaces and frontends
                    for Git.</para></listitem>
            </itemizedlist>
        </note>
    </section>

    <section id='best-practices-autobuilders'>
        <title>Autobuilders</title>

        <para>
            Autobuilders are often the core of a development project.
            It is here that changes from individual developers are brought
            together and centrally tested and subsequent decisions about
            releases can be made.
            Autobuilders also allow for "continuous integration" style
            testing of software components and regression identification
            and tracking.
        </para>

        <para>
            See "<ulink url='http://autobuilder.yoctoproject.org'>Yocto Project Autobuilder</ulink>"
            for more information and links to buildbot.
            The Yocto Project team has found this implementation
            works well in this role.
            A public example of this is the Yocto Project
            Autobuilders, which we use to test the overall health of the
            project.
        </para>

        <para>
            The features of this system are:
            <itemizedlist>
                <listitem><para>Highlights when commits break the build.
                    </para></listitem>
                <listitem><para>Populates an sstate cache from which
                    developers can pull rather than requiring local
                    builds.</para></listitem>
                <listitem><para>Allows commit hook triggers,
                    which trigger builds when commits are made.
                    </para></listitem>
                <listitem><para>Allows triggering of automated image booting
                    and testing under the QuickEMUlator (QEMU).
                    </para></listitem>
                <listitem><para>Supports incremental build testing and
                    from-scratch builds.</para></listitem>
                <listitem><para>Shares output that allows developer
                    testing and historical regression investigation.
                    </para></listitem>
                <listitem><para>Creates output that can be used for releases.
                    </para></listitem>
                <listitem><para>Allows scheduling of builds so that resources
                    can be used efficiently.</para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='best-practices-policies-and-change-flow'>
        <title>Policies and Change Flow</title>

        <para>
            The Yocto Project itself uses a hierarchical structure and a
            pull model.
            Scripts exist to create and send pull requests
            (i.e. <filename>create-pull-request</filename> and
            <filename>send-pull-request</filename>).
            This model is in line with other open source projects where
            maintainers are responsible for specific areas of the project
            and a single maintainer handles the final "top-of-tree" merges.
        </para>

        <note>
            You can also use a more collective push model.
            The <filename>gitolite</filename> software supports both the
            push and pull models quite easily.
        </note>

        <para>
            As with any development environment, it is important
            to document the policy used as well as any main project
            guidelines so they are understood by everyone.
            It is also a good idea to have well structured
            commit messages, which are usually a part of a project's
            guidelines.
            Good commit messages are essential when looking back in time and
            trying to understand why changes were made.
        </para>

        <para>
            If you discover that changes are needed to the core layer of the
            project, it is worth sharing those with the community as soon
            as possible.
            Chances are if you have discovered the need for changes, someone
            else in the community needs them also.
        </para>
    </section>

    <section id='best-practices-summary'>
        <title>Summary</title>

        <para>
            This section summarizes the key recommendations described in the
            previous sections:
            <itemizedlist>
                <listitem><para>Use <link linkend='git'>Git</link>
                    as the source control system.</para></listitem>
                <listitem><para>Maintain your Metadata in layers that make sense
                    for your situation.
                    See the "<link linkend='understanding-and-creating-layers'>Understanding
                    and Creating Layers</link>" section for more information on
                    layers.</para></listitem>
                <listitem><para>
                    Separate the project's Metadata and code by using
                    separate Git repositories.
                    See the
                    "<link linkend='yocto-project-repositories'>Yocto Project Source Repositories</link>"
                    section for information on these repositories.
                    See the
                    "<link linkend='getting-setup'>Getting Set Up</link>"
                    section for information on how to set up local Git
                    repositories for related upstream Yocto Project
                    Git repositories.
                    </para></listitem>
                <listitem><para>Set up the directory for the shared state cache
                    (<ulink url='&YOCTO_DOCS_REF_URL;#var-SSTATE_DIR'><filename>SSTATE_DIR</filename></ulink>)
                    where it makes sense.
                    For example, set up the sstate cache on a system used
                    by developers in the same organization and share the
                    same source directories on their machines.
                    </para></listitem>
                <listitem><para>Set up an Autobuilder and have it populate the
                    sstate cache and source directories.</para></listitem>
                <listitem><para>The Yocto Project community encourages you
                    to send patches to the project to fix bugs or add features.
                    If you do submit patches, follow the project commit
                    guidelines for writing good commit messages.
                    See the "<link linkend='how-to-submit-a-change'>How to Submit a Change</link>"
                    section.</para></listitem>
                <listitem><para>Send changes to the core sooner than later
                    as others are likely to run into the same issues.
                    For some guidance on mailing lists to use, see the list in the
                    "<link linkend='how-to-submit-a-change'>How to Submit a Change</link>"
                    section.
                    For a description of the available mailing lists, see the
                    "<ulink url='&YOCTO_DOCS_REF_URL;#resources-mailinglist'>Mailing Lists</ulink>"
                    section in the Yocto Project Reference Manual.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>
</section>

<section id='yocto-project-repositories'>
    <title>Yocto Project Source Repositories</title>

    <para>
        The Yocto Project team maintains complete source repositories for all
        Yocto Project files at
        <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi'></ulink>.
        This web-based source code browser is organized into categories by
        function such as IDE Plugins, Matchbox, Poky, Yocto Linux Kernel, and
        so forth.
        From the interface, you can click on any particular item in the "Name"
        column and see the URL at the bottom of the page that you need to clone
        a Git repository for that particular item.
        Having a local Git repository of the
        <link linkend='source-directory'>Source Directory</link>, which is
        usually named "poky", allows
        you to make changes, contribute to the history, and ultimately enhance
        the Yocto Project's tools, Board Support Packages, and so forth.
    </para>

    <para>
        For any supported release of Yocto Project, you can also go to the
        <ulink url='&YOCTO_HOME_URL;'>Yocto Project Website</ulink> and
        select the "Downloads" tab and get a released tarball of the
        <filename>poky</filename> repository or any supported BSP tarballs.
        Unpacking these tarballs gives you a snapshot of the released
        files.
        <note><title>Notes</title>
            <itemizedlist>
                <listitem><para>
                    The recommended method for setting up the Yocto Project
                    <link linkend='source-directory'>Source Directory</link>
                    and the files for supported BSPs
                    (e.g., <filename>meta-intel</filename>) is to use
                    <link linkend='git'>Git</link> to create a local copy of
                    the upstream repositories.
                    </para></listitem>
                <listitem><para>
                    Be sure to always work in matching branches for both
                    the selected BSP repository and the
                    <link linkend='source-directory'>Source Directory</link>
                    (i.e. <filename>poky</filename>) repository.
                    For example, if you have checked out the "master" branch
                    of <filename>poky</filename> and you are going to use
                    <filename>meta-intel</filename>, be sure to checkout the
                    "master" branch of <filename>meta-intel</filename>.
                    </para></listitem>
            </itemizedlist>
        </note>
    </para>

    <para>
        In summary, here is where you can get the project files needed for development:
        <itemizedlist>
            <listitem><para id='source-repositories'><emphasis><ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi'>Source Repositories:</ulink></emphasis>
                This area contains IDE Plugins, Matchbox, Poky, Poky Support, Tools, Yocto Linux Kernel, and Yocto
                Metadata Layers.
                You can create local copies of Git repositories for each of these areas.</para>
                <para>
                <imagedata fileref="figures/source-repos.png" align="center" width="6in" depth="4in" />
                </para></listitem>
            <listitem><para><anchor id='index-downloads' /><emphasis><ulink url='&YOCTO_DL_URL;/releases/'>Index of /releases:</ulink></emphasis>
                This is an index of releases such as
                the <trademark class='trade'>Eclipse</trademark>
                Yocto Plug-in, miscellaneous support, Poky, Pseudo, installers for cross-development toolchains,
                and all released versions of Yocto Project in the form of images or tarballs.
                Downloading and extracting these files does not produce a local copy of the
                Git repository but rather a snapshot of a particular release or image.</para>
                <para>
                <imagedata fileref="figures/index-downloads.png" align="center" width="6in" depth="3.5in" />
                </para></listitem>
            <listitem><para><emphasis>"Downloads" page for the
                <ulink url='&YOCTO_HOME_URL;'>Yocto Project Website</ulink>:</emphasis>
                Access this page by going to the website and then selecting
                the "Downloads" tab.
                This page allows you to download any Yocto Project
                release or Board Support Package (BSP) in tarball form.
                The tarballs are similar to those found in the
                <ulink url='&YOCTO_DL_URL;/releases/'>Index of /releases:</ulink> area.</para>
                <para>
                <imagedata fileref="figures/yp-download.png" align="center" width="6in" depth="4in" />
            </para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='yocto-project-terms'>
    <title>Yocto Project Terms</title>

    <para>
        Following is a list of terms and definitions users new to the Yocto Project development
        environment might find helpful.
        While some of these terms are universal, the list includes them just in case:
        <itemizedlist>
            <listitem><para><emphasis>Append Files:</emphasis> Files that append build information to
                a recipe file.
                Append files are known as BitBake append files and <filename>.bbappend</filename> files.
                The OpenEmbedded build system expects every append file to have a corresponding
                recipe (<filename>.bb</filename>) file.
                Furthermore, the append file and corresponding recipe file
                must use the same root filename.
                The filenames can differ only in the file type suffix used (e.g.
                <filename>formfactor_0.0.bb</filename> and <filename>formfactor_0.0.bbappend</filename>).
                </para>
                <para>Information in append files extends or overrides the
                information in the similarly-named recipe file.
                For an example of an append file in use, see the
                "<link linkend='using-bbappend-files'>Using .bbappend Files</link>" section.
                <note>
                    Append files can also use wildcard patterns in their version numbers
                    so they can be applied to more than one version of the underlying recipe file.
                </note>
                </para></listitem>
            <listitem><para id='bitbake-term'><emphasis>BitBake:</emphasis>
                The task executor and scheduler used by the OpenEmbedded build
                system to build images.
                For more information on BitBake, see the
                <ulink url='&YOCTO_DOCS_BB_URL;'>BitBake User Manual</ulink>.
                </para></listitem>
            <listitem>
                <para id='build-directory'><emphasis>Build Directory:</emphasis>
                This term refers to the area used by the OpenEmbedded build
                system for builds.
                The area is created when you <filename>source</filename> the
                setup environment script that is found in the Source Directory
                (i.e. <ulink url='&YOCTO_DOCS_REF_URL;#structure-core-script'><filename>&OE_INIT_FILE;</filename></ulink>
                or
                <ulink url='&YOCTO_DOCS_REF_URL;#structure-memres-core-script'><filename>oe-init-build-env-memres</filename></ulink>).
                The <ulink url='&YOCTO_DOCS_REF_URL;#var-TOPDIR'><filename>TOPDIR</filename></ulink>
                variable points to the Build Directory.</para>

                <para>
                    You have a lot of flexibility when creating the Build
                    Directory.
                    Following are some examples that show how to create the
                    directory.
                    The examples assume your
                    <link linkend='source-directory'>Source Directory</link> is
                    named <filename>poky</filename>:
                   <itemizedlist>
                        <listitem><para>Create the Build Directory inside your
                            Source Directory and let the name of the Build
                            Directory default to <filename>build</filename>:
                            <literallayout class='monospaced'>
     $ cd $HOME/poky
     $ source &OE_INIT_FILE;
                            </literallayout></para></listitem>
                        <listitem><para>Create the Build Directory inside your
                            home directory and specifically name it
                            <filename>test-builds</filename>:
                            <literallayout class='monospaced'>
     $ cd $HOME
     $ source poky/&OE_INIT_FILE; test-builds
                            </literallayout></para></listitem>
                        <listitem><para>
                            Provide a directory path and
                            specifically name the Build Directory.
                            Any intermediate folders in the pathname must
                            exist.
                            This next example creates a Build Directory named
                            <filename>YP-&POKYVERSION;</filename>
                            in your home directory within the existing
                            directory <filename>mybuilds</filename>:
                            <literallayout class='monospaced'>
     $cd $HOME
     $ source $HOME/poky/&OE_INIT_FILE; $HOME/mybuilds/YP-&POKYVERSION;
                            </literallayout></para></listitem>
                    </itemizedlist>
                    <note>
                        By default, the Build Directory contains
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-TMPDIR'><filename>TMPDIR</filename></ulink>,
                        which is a temporary directory the build system uses for
                        its work.
                        <filename>TMPDIR</filename> cannot be under NFS.
                        Thus, by default, the Build Directory cannot be under NFS.
                        However, if you need the Build Directory to be under NFS,
                        you can set this up by setting <filename>TMPDIR</filename>
                        in your <filename>local.conf</filename> file
                        to use a local drive.
                        Doing so effectively separates <filename>TMPDIR</filename>
                        from <filename>TOPDIR</filename>, which is the Build
                        Directory.
                    </note>
                    </para></listitem>
            <listitem><para><emphasis>Classes:</emphasis> Files that provide for logic encapsulation
                and inheritance so that commonly used patterns can be defined once and then easily used
                in multiple recipes.
                For reference information on the Yocto Project classes, see the
                "<ulink url='&YOCTO_DOCS_REF_URL;#ref-classes'>Classes</ulink>" chapter of the
                Yocto Project Reference Manual.
                Class files end with the <filename>.bbclass</filename> filename extension.
                </para></listitem>
            <listitem><para><emphasis>Configuration File:</emphasis>
                Configuration information in various <filename>.conf</filename>
                files provides global definitions of variables.
                The <filename>conf/local.conf</filename> configuration file in
                the
                <link linkend='build-directory'>Build Directory</link>
                contains user-defined variables that affect every build.
                The <filename>meta-poky/conf/distro/poky.conf</filename>
                configuration file defines Yocto "distro" configuration
                variables used only when building with this policy.
                Machine configuration files, which
                are located throughout the
                <link linkend='source-directory'>Source Directory</link>, define
                variables for specific hardware and are only used when building
                for that target (e.g. the
                <filename>machine/beaglebone.conf</filename> configuration
                file defines variables for the Texas Instruments ARM Cortex-A8
                development board).
                Configuration files end with a <filename>.conf</filename>
                filename extension.
                </para></listitem>
            <listitem><para id='cross-development-toolchain'>
                <emphasis>Cross-Development Toolchain:</emphasis>
                    In general, a cross-development toolchain is a collection of
                    software development tools and utilities that run on one
                    architecture and allow you to develop software for a
                    different, or targeted, architecture.
                    These toolchains contain cross-compilers, linkers, and
                    debuggers that are specific to the target architecture.
                </para>

                <para>The Yocto Project supports two different cross-development
                    toolchains:
                    <itemizedlist>
                        <listitem><para>A toolchain only used by and within
                            BitBake when building an image for a target
                            architecture.</para></listitem>
                        <listitem><para>A relocatable toolchain used outside of
                            BitBake by developers when developing applications
                            that will run on a targeted device.
                            </para></listitem>
                    </itemizedlist>
                </para>

                <para>
                    Creation of these toolchains is simple and automated.
                    For information on toolchain concepts as they apply to the
                    Yocto Project, see the
                    "<ulink url='&YOCTO_DOCS_REF_URL;#cross-development-toolchain-generation'>Cross-Development Toolchain Generation</ulink>"
                    section in the Yocto Project Reference Manual.
                    You can also find more information on using the
                    relocatable toolchain in the
                    <ulink url='&YOCTO_DOCS_SDK_URL;'>Yocto Project Software Development Kit (SDK) Developer's Guide</ulink>.
                </para></listitem>
            <listitem><para><emphasis>Image:</emphasis>
                An image is an artifact of the BitBake build process given
                a collection of recipes and related Metadata.
                Images are the binary output that run on specific hardware or
                QEMU and are used for specific use-cases.
                For a list of the supported image types that the Yocto Project provides, see the
                "<ulink url='&YOCTO_DOCS_REF_URL;#ref-images'>Images</ulink>"
                chapter in the Yocto Project Reference Manual.</para></listitem>
            <listitem><para id='layer'><emphasis>Layer:</emphasis> A collection of recipes representing the core,
                a BSP, or an application stack.
                For a discussion specifically on BSP Layers, see the
                "<ulink url='&YOCTO_DOCS_BSP_URL;#bsp-layers'>BSP Layers</ulink>"
                section in the Yocto Project Board Support Packages (BSP)
                Developer's Guide.</para></listitem>
            <listitem><para id='metadata'><emphasis>Metadata:</emphasis>
                The files that BitBake parses when building an image.
                In general, Metadata includes recipes, classes, and
                configuration files.
                In the context of the kernel ("kernel Metadata"),
                it refers to Metadata in the <filename>meta</filename>
                branches of the kernel source Git repositories.
                </para></listitem>
            <listitem><para id='oe-core'><emphasis>OE-Core:</emphasis> A core set of Metadata originating
                with OpenEmbedded (OE) that is shared between OE and the Yocto Project.
                This Metadata is found in the <filename>meta</filename> directory of the
                <link linkend='source-directory'>Source Directory</link>.</para></listitem>
            <listitem><para id='build-system-term'><emphasis>OpenEmbedded Build System:</emphasis>
                The build system specific to the Yocto Project.
                The OpenEmbedded build system is based on another project known
                as "Poky", which uses
                <link linkend='bitbake-term'>BitBake</link> as the task
                executor.
                Throughout the Yocto Project documentation set, the
                OpenEmbedded build system is sometimes referred to simply
                as "the build system".
                If other build systems, such as a host or target build system
                are referenced, the documentation clearly states the
                difference.
                <note>
                    For some historical information about Poky, see the
                    <link linkend='poky'>Poky</link> term.
                </note>
                </para></listitem>
            <listitem><para><emphasis>Package:</emphasis>
                In the context of the Yocto Project, this term refers to a
                recipe's packaged output produced by BitBake (i.e. a
                "baked recipe").
                A package is generally the compiled binaries produced from the
                recipe's sources.
                You "bake" something by running it through BitBake.</para>
                <para>It is worth noting that the term "package" can, in general, have subtle
                meanings.  For example, the packages referred to in the
                "<ulink url='&YOCTO_DOCS_QS_URL;#packages'>The Build Host Packages</ulink>" section are
                compiled binaries that, when installed, add functionality to your Linux
                distribution.</para>
                <para>Another point worth noting is that historically within the Yocto Project,
                recipes were referred to as packages - thus, the existence of several BitBake
                variables that are seemingly mis-named,
                (e.g. <ulink url='&YOCTO_DOCS_REF_URL;#var-PR'><filename>PR</filename></ulink>,
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>, and
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PE'><filename>PE</filename></ulink>).
                </para></listitem>
            <listitem><para><emphasis>Package Groups:</emphasis>
                Arbitrary groups of software Recipes.
                You use package groups to hold recipes that, when built,
                usually accomplish a single task.
                For example, a package group could contain the recipes for a
                company’s proprietary or value-add software.
                Or, the package group could contain the recipes that enable
                graphics.
                A package group is really just another recipe.
                Because package group files are recipes, they end with the
                <filename>.bb</filename> filename extension.</para></listitem>
            <listitem><para id='poky'><emphasis>Poky:</emphasis>
                The term "poky" can mean several things.
                In its most general sense, it is an open-source
                project that was initially developed by OpenedHand.
                With OpenedHand, poky was developed off of the existing
                OpenEmbedded build system becoming a commercially
                supportable build system for embedded Linux.
                After Intel Corporation acquired OpenedHand, the
                project poky became the basis for the Yocto Project's
                build system.</para>
                <para>Within the Yocto Project source repositories,
                <filename>poky</filename> exists as a separate Git
                repository you can clone to yield a local copy on your
                host system.
                Thus, "poky" can refer to the local copy of the Source
                Directory used for development within the Yocto
                Project.</para>
                <para>Finally, "poky" can refer to the default
                <ulink url='&YOCTO_DOCS_REF_URL;#var-DISTRO'><filename>DISTRO</filename></ulink>
                (i.e. distribution) created when you use the Yocto
                Project in conjunction with the
                <filename>poky</filename> repository to build an image.
                </para></listitem>
            <listitem><para><emphasis>Recipe:</emphasis>
                A set of instructions for building packages.
                A recipe describes where you get source code, which patches
                to apply, how to configure the source, how to compile it and so on.
                Recipes also describe dependencies for libraries or for other
                recipes.
                Recipes represent the logical unit of execution, the software
                to build, the images to build, and use the
                <filename>.bb</filename> file extension.
                </para></listitem>
            <listitem>
                <para id='source-directory'><emphasis>Source Directory:</emphasis>
                This term refers to the directory structure created as a result
                of creating a local copy of the <filename>poky</filename> Git
                repository <filename>git://git.yoctoproject.org/poky</filename>
                or expanding a released <filename>poky</filename> tarball.
                <note>
                    Creating a local copy of the <filename>poky</filename>
                    Git repository is the recommended method for setting up
                    your Source Directory.
                </note>
                Sometimes you might hear the term "poky directory" used to refer
                to this directory structure.
                <note>
                    The OpenEmbedded build system does not support file or
                    directory names that contain spaces.
                    Be sure that the Source Directory you use does not contain
                    these types of names.
                </note></para>

                <para>The Source Directory contains BitBake, Documentation,
                Metadata and other files that all support the Yocto Project.
                Consequently, you must have the Source Directory in place on
                your development system in order to do any development using
                the Yocto Project.</para>

                <para>When you create a local copy of the Git repository, you
                can name the repository anything you like.
                Throughout much of the documentation, "poky"
                is used as the name of the top-level folder of the local copy of
                the poky Git repository.
                So, for example, cloning the <filename>poky</filename> Git
                repository results in a local Git repository whose top-level
                folder is also named "poky".</para>

                <para>While it is not recommended that you use tarball expansion
                to set up the Source Directory, if you do, the top-level
                directory name of the Source Directory is derived from the
                Yocto Project release tarball.
                For example, downloading and unpacking
                <filename>&YOCTO_POKY_TARBALL;</filename> results in a
                Source Directory whose root folder is named
                <filename>&YOCTO_POKY;</filename>.</para>

                <para>It is important to understand the differences between the
                Source Directory created by unpacking a released tarball as
                compared to cloning
                <filename>git://git.yoctoproject.org/poky</filename>.
                When you unpack a tarball, you have an exact copy of the files
                based on the time of release - a fixed release point.
                Any changes you make to your local files in the Source Directory
                are on top of the release and will remain local only.
                On the other hand, when you clone the <filename>poky</filename>
                Git repository, you have an active development repository with
                access to the upstream repository's branches and tags.
                In this case, any local changes you make to the local
                Source Directory can be later applied to active development
                branches of the upstream <filename>poky</filename> Git
                repository.</para>

                <para>For more information on concepts related to Git
                repositories, branches, and tags, see the
                "<link linkend='repositories-tags-and-branches'>Repositories, Tags, and Branches</link>"
                section.</para></listitem>
            <listitem><para><emphasis>Task:</emphasis>
                A unit of execution for BitBake (e.g.
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-compile'><filename>do_compile</filename></ulink>,
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-fetch'><filename>do_fetch</filename></ulink>,
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-patch'><filename>do_patch</filename></ulink>,
                and so forth).
                </para></listitem>
            <listitem><para><emphasis>Upstream:</emphasis> A reference to source code or repositories
                that are not local to the development system but located in a master area that is controlled
                by the maintainer of the source code.
                For example, in order for a developer to work on a particular piece of code, they need to
                first get a copy of it from an "upstream" source.</para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='licensing'>
    <title>Licensing</title>

    <para>
        Because open source projects are open to the public, they have different licensing structures in place.
        License evolution for both Open Source and Free Software has an interesting history.
        If you are interested in this history, you can find basic information here:
    <itemizedlist>
        <listitem><para><ulink url='http://en.wikipedia.org/wiki/Open-source_license'>Open source license history</ulink>
            </para></listitem>
        <listitem><para><ulink url='http://en.wikipedia.org/wiki/Free_software_license'>Free software license
            history</ulink></para></listitem>
    </itemizedlist>
    </para>

    <para>
        In general, the Yocto Project is broadly licensed under the Massachusetts Institute of Technology
        (MIT) License.
        MIT licensing permits the reuse of software within proprietary software as long as the
        license is distributed with that software.
        MIT is also compatible with the GNU General Public License (GPL).
        Patches to the Yocto Project follow the upstream licensing scheme.
        You can find information on the MIT license
        <ulink url='http://www.opensource.org/licenses/mit-license.php'>here</ulink>.
        You can find information on the GNU GPL <ulink url='http://www.opensource.org/licenses/LGPL-3.0'>
        here</ulink>.
    </para>

    <para>
        When you build an image using the Yocto Project, the build process uses a
        known list of licenses to ensure compliance.
        You can find this list in the
        <link linkend='source-directory'>Source Directory</link> at
        <filename>meta/files/common-licenses</filename>.
        Once the build completes, the list of all licenses found and used during that build are
        kept in the
        <link linkend='build-directory'>Build Directory</link> at
        <filename>tmp/deploy/licenses</filename>.
    </para>

    <para>
        If a module requires a license that is not in the base list, the build process
        generates a warning during the build.
        These tools make it easier for a developer to be certain of the licenses with which
        their shipped products must comply.
        However, even with these tools it is still up to the developer to resolve potential licensing issues.
    </para>

    <para>
        The base list of licenses used by the build process is a combination of the Software Package
        Data Exchange (SPDX) list and the Open Source Initiative (OSI) projects.
        <ulink url='http://spdx.org'>SPDX Group</ulink> is a working group of the Linux Foundation
        that maintains a specification
        for a standard format for communicating the components, licenses, and copyrights
        associated with a software package.
        <ulink url='http://opensource.org'>OSI</ulink> is a corporation dedicated to the Open Source
        Definition and the effort for reviewing and approving licenses that
        conform to the Open Source Definition (OSD).
    </para>

    <para>
        You can find a list of the combined SPDX and OSI licenses that the
        Yocto Project uses in the
        <filename>meta/files/common-licenses</filename> directory in your
        <link linkend='source-directory'>Source Directory</link>.
    </para>

    <para>
        For information that can help you maintain compliance with various
        open source licensing during the lifecycle of a product created using
        the Yocto Project, see the
        "<link linkend='maintaining-open-source-license-compliance-during-your-products-lifecycle'>Maintaining Open Source License Compliance During Your Product's Lifecycle</link>"
        section.
    </para>
</section>

<section id='git'>
    <title>Git</title>

    <para>
        The Yocto Project makes extensive use of Git,
        which is a free, open source distributed version control system.
        Git supports distributed development, non-linear development, and can handle large projects.
        It is best that you have some fundamental understanding of how Git tracks projects and
        how to work with Git if you are going to use the Yocto Project for development.
        This section provides a quick overview of how Git works and provides you with a summary
        of some essential Git commands.
    </para>

    <para>
        For more information on Git, see
        <ulink url='http://git-scm.com/documentation'></ulink>.
        If you need to download Git, go to <ulink url='http://git-scm.com/download'></ulink>.
    </para>

    <section id='repositories-tags-and-branches'>
        <title>Repositories, Tags, and Branches</title>

        <para>
            As mentioned earlier in the section
            "<link linkend='yocto-project-repositories'>Yocto Project Source Repositories</link>",
            the Yocto Project maintains source repositories at
            <ulink url='&YOCTO_GIT_URL;/cgit.cgi'></ulink>.
            If you look at this web-interface of the repositories, each item is a separate
            Git repository.
        </para>

        <para>
            Git repositories use branching techniques that track content change (not files)
            within a project (e.g. a new feature or updated documentation).
            Creating a tree-like structure based on project divergence allows for excellent historical
            information over the life of a project.
            This methodology also allows for an environment from which you can do lots of
            local experimentation on projects as you develop changes or new features.
        </para>

        <para>
            A Git repository represents all development efforts for a given project.
            For example, the Git repository <filename>poky</filename> contains all changes
            and developments for Poky over the course of its entire life.
            That means that all changes that make up all releases are captured.
            The repository maintains a complete history of changes.
        </para>

        <para>
            You can create a local copy of any repository by "cloning" it with the Git
            <filename>clone</filename> command.
            When you clone a Git repository, you end up with an identical copy of the
            repository on your development system.
            Once you have a local copy of a repository, you can take steps to develop locally.
            For examples on how to clone Git repositories, see the
            "<link linkend='getting-setup'>Getting Set Up</link>" section.
        </para>

        <para>
            It is important to understand that Git tracks content change and
            not files.
            Git uses "branches" to organize different development efforts.
            For example, the <filename>poky</filename> repository has
            several branches that include the current
            <filename>&DISTRO_NAME_NO_CAP;</filename> branch, the
            <filename>master</filename> branch, and many branches for past
            Yocto Project releases.
            You can see all the branches by going to
            <ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/'></ulink> and
            clicking on the
            <filename><ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/refs/heads'>[...]</ulink></filename>
            link beneath the "Branch" heading.
        </para>

        <para>
            Each of these branches represents a specific area of development.
            The <filename>master</filename> branch represents the current or most recent
            development.
            All other branches represent offshoots of the <filename>master</filename>
            branch.
        </para>

        <para>
            When you create a local copy of a Git repository, the copy has the same set
            of branches as the original.
            This means you can use Git to create a local working area (also called a branch)
            that tracks a specific development branch from the source Git repository.
            in other words, you can define your local Git environment to work on any development
            branch in the repository.
            To help illustrate, here is a set of commands that creates a local copy of the
            <filename>poky</filename> Git repository and then creates and checks out a local
            Git branch that tracks the Yocto Project &DISTRO; Release (&DISTRO_NAME;) development:
            <literallayout class='monospaced'>
     $ cd ~
     $ git clone git://git.yoctoproject.org/poky
     $ cd poky
     $ git checkout -b &DISTRO_NAME_NO_CAP; origin/&DISTRO_NAME_NO_CAP;
            </literallayout>
            In this example, the name of the top-level directory of your local
            <link linkend='source-directory'>Source Directory</link>
            is "poky" and the name of that local working area (local branch)
            you just created and checked out is "&DISTRO_NAME_NO_CAP;".
            The files in your local repository now reflect the same files that
            are in the "&DISTRO_NAME_NO_CAP;" development branch of the
            Yocto Project's "poky" upstream repository.
            It is important to understand that when you create and checkout a
            local working branch based on a branch name,
            your local environment matches the "tip" of that development branch
            at the time you created your local branch, which could be
            different from the files at the time of a similarly named release.
            In other words, creating and checking out a local branch based on
            the "&DISTRO_NAME_NO_CAP;" branch name is not the same as
            cloning and checking out the "master" branch.
            Keep reading to see how you create a local snapshot of a Yocto
            Project Release.
        </para>

        <para>
            Git uses "tags" to mark specific changes in a repository.
            Typically, a tag is used to mark a special point such as the final
            change before a project is released.
            You can see the tags used with the <filename>poky</filename> Git
            repository by going to
            <ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/'></ulink> and
            clicking on the
            <filename><ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/refs/tags'>[...]</ulink></filename>
            link beneath the "Tag" heading.
        </para>

        <para>
            Some key tags are
            <filename>dizzy-12.0.0</filename>,
            <filename>fido-13.0.0</filename>,
            <filename>jethro-14.0.0</filename>, and
            <filename>&DISTRO_NAME_NO_CAP;-&POKYVERSION;</filename>.
            These tags represent Yocto Project releases.
        </para>

        <para>
            When you create a local copy of the Git repository, you also have access to all the
            tags.
            Similar to branches, you can create and checkout a local working Git branch based
            on a tag name.
            When you do this, you get a snapshot of the Git repository that reflects
            the state of the files when the change was made associated with that tag.
            The most common use is to checkout a working branch that matches a specific
            Yocto Project release.
            Here is an example:
            <literallayout class='monospaced'>
     $ cd ~
     $ git clone git://git.yoctoproject.org/poky
     $ cd poky
     $ git checkout -b my-&DISTRO_NAME_NO_CAP;-&POKYVERSION; &DISTRO_NAME_NO_CAP;-&POKYVERSION;
            </literallayout>
            In this example, the name of the top-level directory of your local Yocto Project
            Files Git repository is <filename>poky</filename>.
            And, the name of the local branch you have created and checked out is
            <filename>my-&DISTRO_NAME_NO_CAP;-&POKYVERSION;</filename>.
            The files in your repository now exactly match the Yocto Project &DISTRO;
            Release tag (<filename>&DISTRO_NAME_NO_CAP;-&POKYVERSION;</filename>).
            It is important to understand that when you create and checkout a local
            working branch based on a tag, your environment matches a specific point
            in time and not the entire development branch.
        </para>
    </section>

    <section id='basic-commands'>
        <title>Basic Commands</title>

        <para>
            Git has an extensive set of commands that lets you manage changes and perform
            collaboration over the life of a project.
            Conveniently though, you can manage with a small set of basic operations and workflows
            once you understand the basic philosophy behind Git.
            You do not have to be an expert in Git to be functional.
            A good place to look for instruction on a minimal set of Git commands is
            <ulink url='http://git-scm.com/documentation'>here</ulink>.
            If you need to download Git, you can do so
            <ulink url='http://git-scm.com/download'>here</ulink>, although
            any reasonably current Linux distribution should already have an
            installable package for Git.
        </para>

        <para>
            If you do not know much about Git, you should educate
            yourself by visiting the links previously mentioned.
        </para>

        <para>
            The following list briefly describes some basic Git operations as a way to get started.
            As with any set of commands, this list (in most cases) simply shows the base command and
            omits the many arguments they support.
            See the Git documentation for complete descriptions and strategies on how to use these commands:
            <itemizedlist>
                <listitem><para><emphasis><filename>git init</filename>:</emphasis> Initializes an empty Git repository.
                    You cannot use Git commands unless you have a <filename>.git</filename> repository.</para></listitem>
                <listitem><para><emphasis><filename>git clone</filename>:</emphasis>
                    Creates a local clone of a Git repository.
                    During collaboration, this command allows you to create a
                    local Git repository that is on equal footing with a fellow
                    developer’s Git repository.
                    </para></listitem>
                <listitem><para><emphasis><filename>git add</filename>:</emphasis> Stages updated file contents
                    to the index that
                    Git uses to track changes.
                    You must stage all files that have changed before you can commit them.</para></listitem>
                <listitem><para><emphasis><filename>git commit</filename>:</emphasis> Creates a "commit" that documents
                    the changes you made.
                    Commits are used for historical purposes, for determining if a maintainer of a project
                    will allow the change, and for ultimately pushing the change from your local Git repository
                    into the project’s upstream (or master) repository.</para></listitem>
                <listitem><para><emphasis><filename>git status</filename>:</emphasis> Reports any modified files that
                    possibly need to be staged and committed.</para></listitem>
                <listitem><para><emphasis><filename>git checkout</filename> <replaceable>branch-name</replaceable>:</emphasis> Changes
                    your working branch.
                    This command is analogous to "cd".</para></listitem>
                <listitem><para><emphasis><filename>git checkout –b</filename> <replaceable>working-branch</replaceable>:</emphasis> Creates
                    a working branch on your local machine where you can isolate work.
                    It is a good idea to use local branches when adding specific features or changes.
                    This way if you do not like what you have done you can easily get rid of the work.</para></listitem>
                <listitem><para><emphasis><filename>git branch</filename>:</emphasis> Reports
                    existing local branches and
                    tells you the branch in which you are currently working.</para></listitem>
                <listitem><para><emphasis><filename>git branch -D</filename> <replaceable>branch-name</replaceable>:</emphasis>
                    Deletes an existing local branch.
                    You need to be in a local branch other than the one you are deleting
                    in order to delete <replaceable>branch-name</replaceable>.</para></listitem>
                <listitem><para><emphasis><filename>git pull</filename>:</emphasis> Retrieves information
                    from an upstream Git
                    repository and places it in your local Git repository.
                    You use this command to make sure you are synchronized with the repository
                    from which you are basing changes (.e.g. the master branch).</para></listitem>
                <listitem><para><emphasis><filename>git push</filename>:</emphasis>
                    Sends all your committed local changes to an upstream Git
                    repository (e.g. a contribution repository).
                    The maintainer of the project draws from these repositories
                    when adding changes to the project’s master repository or
                    other development branch.
                    </para></listitem>
                <listitem><para><emphasis><filename>git merge</filename>:</emphasis> Combines or adds changes from one
                    local branch of your repository with another branch.
                    When you create a local Git repository, the default branch is named "master".
                    A typical workflow is to create a temporary branch for isolated work, make and commit your
                    changes, switch to your local master branch, merge the changes from the temporary branch into the
                    local master branch, and then delete the temporary branch.</para></listitem>
                <listitem><para><emphasis><filename>git cherry-pick</filename>:</emphasis> Choose and apply specific
                    commits from one branch into another branch.
                    There are times when you might not be able to merge all the changes in one branch with
                    another but need to pick out certain ones.</para></listitem>
                <listitem><para><emphasis><filename>gitk</filename>:</emphasis> Provides a GUI view of the branches
                    and changes in your local Git repository.
                    This command is a good way to graphically see where things have diverged in your
                    local repository.</para></listitem>
                <listitem><para><emphasis><filename>git log</filename>:</emphasis> Reports a history of your changes to the
                    repository.</para></listitem>
                <listitem><para><emphasis><filename>git diff</filename>:</emphasis> Displays line-by-line differences
                    between your local working files and the same files in the upstream Git repository that your
                    branch currently tracks.</para></listitem>
            </itemizedlist>
        </para>
    </section>
</section>

<section id='workflows'>
    <title>Workflows</title>

    <para>
        This section provides some overview on workflows using Git.
        In particular, the information covers basic practices that describe roles and actions in a
        collaborative development environment.
        Again, if you are familiar with this type of development environment, you might want to just
        skip this section.
    </para>

    <para>
        The Yocto Project files are maintained using Git in a "master" branch whose Git history
        tracks every change and whose structure provides branches for all diverging functionality.
        Although there is no need to use Git, many open source projects do so.
        For the Yocto Project, a key individual called the "maintainer" is responsible for the "master"
        branch of a given Git repository.
        The "master" branch is the “upstream” repository where the final builds of the project occur.
        The maintainer is responsible for accepting changes from other developers and for
        organizing the underlying branch structure to reflect release strategies and so forth.
        <note>For information on finding out who is responsible for (maintains)
            a particular area of code, see the
            "<link linkend='how-to-submit-a-change'>How to Submit a Change</link>"
            section.
        </note>
    </para>

    <para>
        The project also has an upstream contribution Git repository named
        <filename>poky-contrib</filename>.
        You can see all the branches in this repository using the web interface
        of the
        <ulink url='&YOCTO_GIT_URL;'>Source Repositories</ulink> organized
        within the "Poky Support" area.
        These branches temporarily hold changes to the project that have been
        submitted or committed by the Yocto Project development team and by
        community members who contribute to the project.
        The maintainer determines if the changes are qualified to be moved
        from the "contrib" branches into the "master" branch of the Git
        repository.
    </para>

    <para>
        Developers (including contributing community members) create and maintain cloned repositories
        of the upstream "master" branch.
        These repositories are local to their development platforms and are used to develop changes.
        When a developer is satisfied with a particular feature or change, they "push" the changes
        to the appropriate "contrib" repository.
    </para>

    <para>
        Developers are responsible for keeping their local repository up-to-date with "master".
        They are also responsible for straightening out any conflicts that might arise within files
        that are being worked on simultaneously by more than one person.
        All this work is done locally on the developer’s machines before anything is pushed to a
        "contrib" area and examined at the maintainer’s level.
    </para>

    <para>
        A somewhat formal method exists by which developers commit changes and push them into the
        "contrib" area and subsequently request that the maintainer include them into "master"
        This process is called “submitting a patch” or "submitting a change."
        For information on submitting patches and changes, see the
        "<link linkend='how-to-submit-a-change'>How to Submit a Change</link>" section.
    </para>

    <para>
        To summarize the environment:  a single point of entry exists for
        changes into the project’s "master" branch of the Git repository,
        which is controlled by the project’s maintainer.
        And, a set of developers exist who independently develop, test, and
        submit changes to "contrib" areas for the maintainer to examine.
        The maintainer then chooses which changes are going to become a
        permanent part of the project.
    </para>

    <para>
        <imagedata fileref="figures/git-workflow.png" width="6in" depth="3in" align="left" scalefit="1" />
    </para>

    <para>
        While each development environment is unique, there are some best practices or methods
        that help development run smoothly.
        The following list describes some of these practices.
        For more information about Git workflows, see the workflow topics in the
        <ulink url='http://book.git-scm.com'>Git Community Book</ulink>.
        <itemizedlist>
            <listitem><para><emphasis>Make Small Changes:</emphasis> It is best to keep the changes you commit
                small as compared to bundling many disparate changes into a single commit.
                This practice not only keeps things manageable but also allows the maintainer
                to more easily include or refuse changes.</para>
                <para>It is also good practice to leave the repository in a state that allows you to
                still successfully build your project.  In other words, do not commit half of a feature,
                then add the other half as a separate, later commit.
                Each commit should take you from one buildable project state to another
                buildable state.</para></listitem>
            <listitem><para><emphasis>Use Branches Liberally:</emphasis> It is very easy to create, use, and
                delete local branches in your working Git repository.
                You can name these branches anything you like.
                It is helpful to give them names associated with the particular feature or change
                on which you are working.
                Once you are done with a feature or change and have merged it
                into your local master branch, simply discard the temporary
                branch.</para></listitem>
            <listitem><para><emphasis>Merge Changes:</emphasis> The <filename>git merge</filename>
                command allows you to take the
                changes from one branch and fold them into another branch.
                This process is especially helpful when more than a single developer might be working
                on different parts of the same feature.
                Merging changes also automatically identifies any collisions or "conflicts"
                that might happen as a result of the same lines of code being altered by two different
                developers.</para></listitem>
            <listitem><para><emphasis>Manage Branches:</emphasis> Because branches are easy to use, you should
                use a system where branches indicate varying levels of code readiness.
                For example, you can have a "work" branch to develop in, a "test" branch where the code or
                change is tested, a "stage" branch where changes are ready to be committed, and so forth.
                As your project develops, you can merge code across the branches to reflect ever-increasing
                stable states of the development.</para></listitem>
            <listitem><para><emphasis>Use Push and Pull:</emphasis> The push-pull workflow is based on the
                concept of developers "pushing" local commits to a remote repository, which is
                usually a contribution repository.
                This workflow is also based on developers "pulling" known states of the project down into their
                local development repositories.
                The workflow easily allows you to pull changes submitted by other developers from the
                upstream repository into your work area ensuring that you have the most recent software
                on which to develop.
                The Yocto Project has two scripts named <filename>create-pull-request</filename> and
                <filename>send-pull-request</filename> that ship with the release to facilitate this
                workflow.
                You can find these scripts in the <filename>scripts</filename>
                folder of the
                <link linkend='source-directory'>Source Directory</link>.
                For information on how to use these scripts, see the
                "<link linkend='pushing-a-change-upstream'>Using Scripts to Push a Change Upstream and Request a Pull</link>" section.
                </para></listitem>
            <listitem><para><emphasis>Patch Workflow:</emphasis> This workflow allows you to notify the
                maintainer through an email that you have a change (or patch) you would like considered
                for the "master" branch of the Git repository.
                To send this type of change, you format the patch and then send the email using the Git commands
                <filename>git format-patch</filename> and <filename>git send-email</filename>.
                For information on how to use these scripts, see the
                "<link linkend='how-to-submit-a-change'>How to Submit a Change</link>"
                section.
                </para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='tracking-bugs'>
    <title>Tracking Bugs</title>

    <para>
        The Yocto Project uses its own implementation of
        <ulink url='http://www.bugzilla.org/about/'>Bugzilla</ulink> to track bugs.
        Implementations of Bugzilla work well for group development because they track bugs and code
        changes, can be used to communicate changes and problems with developers, can be used to
        submit and review patches, and can be used to manage quality assurance.
        The home page for the Yocto Project implementation of Bugzilla is
        <ulink url='&YOCTO_BUGZILLA_URL;'>&YOCTO_BUGZILLA_URL;</ulink>.
    </para>

    <para>
        Sometimes it is helpful to submit, investigate, or track a bug against the Yocto Project itself
        such as when discovering an issue with some component of the build system that acts contrary
        to the documentation or your expectations.
        Following is the general procedure for submitting a new bug using the Yocto Project
        Bugzilla.
        You can find more information on defect management, bug tracking, and feature request
        processes all accomplished through the Yocto Project Bugzilla on the
        <ulink url='&YOCTO_WIKI_URL;/wiki/Bugzilla_Configuration_and_Bug_Tracking'>wiki page</ulink>.
        <orderedlist>
            <listitem><para>Always use the Yocto Project implementation of Bugzilla to submit
                a bug.</para></listitem>
            <listitem><para>When submitting a new bug, be sure to choose the appropriate
                Classification, Product, and Component for which the issue was found.
                Defects for the Yocto Project fall into one of seven classifications:
                Yocto Project Components, Infrastructure, Build System &amp; Metadata,
                Documentation, QA/Testing, Runtime and Hardware.
                Each of these Classifications break down into multiple Products and, in some
                cases, multiple Components.</para></listitem>
            <listitem><para>Use the bug form to choose the correct Hardware and Architecture
                for which the bug applies.</para></listitem>
            <listitem><para>Indicate the Yocto Project version you were using when the issue
                occurred.</para></listitem>
            <listitem><para>Be sure to indicate the Severity of the bug.
                Severity communicates how the bug impacted your work.</para></listitem>
            <listitem><para>Select the appropriate "Documentation change" item
                for the bug.
                Fixing a bug may or may not affect the Yocto Project
                documentation.</para></listitem>
            <listitem><para>Provide a brief summary of the issue.
                Try to limit your summary to just a line or two and be sure to capture the
                essence of the issue.</para></listitem>
            <listitem><para>Provide a detailed description of the issue.
                You should provide as much detail as you can about the context, behavior, output,
                and so forth that surrounds the issue.
                You can even attach supporting files for output from logs by
                using the "Add an attachment" button.</para></listitem>
            <listitem><para>Be sure to copy the appropriate people in the
                "CC List" for the bug.
                See the "<link linkend='how-to-submit-a-change'>How to Submit a Change</link>"
                section for information about finding out who is responsible
                for code.</para></listitem>
            <listitem><para>Submit the bug by clicking the "Submit Bug" button.</para></listitem>
        </orderedlist>
    </para>
</section>

<section id='how-to-submit-a-change'>
    <title>How to Submit a Change</title>

    <para>
        Contributions to the Yocto Project and OpenEmbedded are very welcome.
        Because the system is extremely configurable and flexible, we recognize that developers
        will want to extend, configure or optimize it for their specific uses.
        You should send patches to the appropriate mailing list so that they
        can be reviewed and merged by the appropriate maintainer.
    </para>

    <para>
        Before submitting any change, be sure to find out who you should be
        notifying.
        Several methods exist through which you find out who you should be copying
        or notifying:
        <itemizedlist>
            <listitem><para><emphasis>Maintenance File:</emphasis>
                Examine the <filename>maintainers.inc</filename> file, which is
                located in the
                <link linkend='source-directory'>Source Directory</link>
                at <filename>meta-poky/conf/distro/include</filename>, to
                see who is responsible for code.
                </para></listitem>
            <listitem><para><emphasis>Board Support Package (BSP) README Files:</emphasis>
                For BSP maintainers of supported BSPs, you can examine
                individual BSP <filename>README</filename> files.
                In addition, some layers (such as the <filename>meta-intel</filename> layer),
                include a <filename>MAINTAINERS</filename> file which contains
                a list of all supported BSP maintainers for that layer.
                </para></listitem>
            <listitem><para><emphasis>Search by File:</emphasis>
                Using <link linkend='git'>Git</link>, you can enter the
                following command to bring up a short list of all commits
                against a specific file:
                <literallayout class='monospaced'>
     git shortlog -- <replaceable>filename</replaceable>
                </literallayout>
                Just provide the name of the file for which you are interested.
                The information returned is not ordered by history but does
                include a list of all committers grouped by name.
                From the list, you can see who is responsible for the bulk of
                the changes against the file.
                </para></listitem>
        </itemizedlist>
    </para>

    <para>
        For a list of the Yocto Project and related mailing lists, see the
        "<ulink url='&YOCTO_DOCS_REF_URL;#resources-mailinglist'>Mailing lists</ulink>" section in
        the Yocto Project Reference Manual.
    </para>

    <para>
        Here is some guidance on which mailing list to use for what type of change:
        <itemizedlist>
            <listitem><para>For changes to the core
                <link linkend='metadata'>Metadata</link>, send your patch to the
                <ulink url='&OE_LISTS_URL;/listinfo/openembedded-core'>openembedded-core</ulink> mailing list.
                For example, a change to anything under the <filename>meta</filename> or
                <filename>scripts</filename> directories
                should be sent to this mailing list.</para></listitem>
            <listitem><para>For changes to BitBake (anything under the <filename>bitbake</filename>
                directory), send your patch to the
                <ulink url='&OE_LISTS_URL;/listinfo/bitbake-devel'>bitbake-devel</ulink> mailing list.</para></listitem>
            <listitem><para>For changes to <filename>meta-poky</filename>, send your patch to the
                <ulink url='&YOCTO_LISTS_URL;/listinfo/poky'>poky</ulink> mailing list.</para></listitem>
            <listitem><para>For changes to other layers hosted on
                <filename>yoctoproject.org</filename> (unless the
                layer's documentation specifies otherwise), tools, and Yocto Project
                documentation, use the
                <ulink url='&YOCTO_LISTS_URL;/listinfo/yocto'>yocto</ulink> mailing list.</para></listitem>
            <listitem><para>For additional recipes that do not fit into the core Metadata,
                you should determine which layer the recipe should go into and submit the
                change in the manner recommended by the documentation (e.g. README) supplied
                with the layer. If in doubt, please ask on the
                <ulink url='&YOCTO_LISTS_URL;/listinfo/yocto'>yocto</ulink> or
                <ulink url='&OE_LISTS_URL;/listinfo/openembedded-devel'>openembedded-devel</ulink>
                mailing lists.</para></listitem>
        </itemizedlist>
    </para>

    <para>
        When you send a patch, be sure to include a "Signed-off-by:"
        line in the same style as required by the Linux kernel.
        Adding this line signifies that you, the submitter, have agreed to the Developer's Certificate of Origin 1.1
        as follows:
        <literallayout class='monospaced'>
     Developer's Certificate of Origin 1.1

     By making a contribution to this project, I certify that:

     (a) The contribution was created in whole or in part by me and I
         have the right to submit it under the open source license
         indicated in the file; or

     (b) The contribution is based upon previous work that, to the best
         of my knowledge, is covered under an appropriate open source
         license and I have the right under that license to submit that
         work with modifications, whether created in whole or in part
         by me, under the same open source license (unless I am
         permitted to submit under a different license), as indicated
         in the file; or

     (c) The contribution was provided directly to me by some other
         person who certified (a), (b) or (c) and I have not modified
         it.

     (d) I understand and agree that this project and the contribution
         are public and that a record of the contribution (including all
         personal information I submit with it, including my sign-off) is
         maintained indefinitely and may be redistributed consistent with
         this project or the open source license(s) involved.
        </literallayout>
    </para>

    <para>
        In a collaborative environment, it is necessary to have some sort of standard
        or method through which you submit changes.
        Otherwise, things could get quite chaotic.
        One general practice to follow is to make small, controlled changes.
        Keeping changes small and isolated aids review, makes merging/rebasing easier
        and keeps the change history clean when anyone needs to refer to it in future.
    </para>

    <para>
        When you make a commit, you must follow certain standards established by the
        OpenEmbedded and Yocto Project development teams.
        For each commit, you must provide a single-line summary of the change and you
        should almost always provide a more detailed description of what you did (i.e.
        the body of the commit message).
        The only exceptions for not providing a detailed description would be if your
        change is a simple, self-explanatory change that needs no further description
        beyond the summary.
        Here are the guidelines for composing a commit message:
        <itemizedlist>
            <listitem><para>Provide a single-line, short summary of the change.
                This summary is typically viewable in the "shortlist" of changes.
                Thus, providing something short and descriptive that gives the reader
                a summary of the change is useful when viewing a list of many commits.
                This short description should be prefixed by the recipe name (if changing a recipe), or
                else the short form path to the file being changed.
                </para></listitem>
            <listitem><para>For the body of the commit message, provide detailed information
                that describes what you changed, why you made the change, and the approach
                you used. It may also be helpful if you mention how you tested the change.
                Provide as much detail as you can in the body of the commit message.
                </para></listitem>
            <listitem><para>
                If the change addresses a specific bug or issue that is
                associated with a bug-tracking ID, include a reference to that
                ID in your detailed description.
                For example, the Yocto Project uses a specific convention for
                bug references - any commit that addresses a specific bug should
                use the following form for the detailed description:
                <literallayout class='monospaced'>
     Fixes [YOCTO #<replaceable>bug-id</replaceable>]

     <replaceable>detailed description of change</replaceable>
                </literallayout></para></listitem>
                Where <replaceable>bug-id</replaceable> is replaced with the
                specific bug ID from the Yocto Project Bugzilla instance.
        </itemizedlist>
    </para>

    <para>
        You can find more guidance on creating well-formed commit messages at this OpenEmbedded
        wiki page:
        <ulink url='&OE_HOME_URL;/wiki/Commit_Patch_Message_Guidelines'></ulink>.
    </para>

    <para>
        The next two sections describe general instructions for both pushing
        changes upstream and for submitting changes as patches.
    </para>

    <section id='pushing-a-change-upstream'>
        <title>Using Scripts to Push a Change Upstream and Request a Pull</title>

        <para>
            The basic flow for pushing a change to an upstream "contrib" Git repository is as follows:
            <itemizedlist>
                <listitem><para>Make your changes in your local Git repository.</para></listitem>
                <listitem><para>Stage your changes by using the <filename>git add</filename>
                    command on each file you changed.</para></listitem>
                <listitem><para>
                    Commit the change by using the
                    <filename>git commit</filename> command.
                    Be sure to provide a commit message that follows the
                    project’s commit message standards as described earlier.
                    </para></listitem>
                <listitem><para>
                    Push the change to the upstream "contrib" repository by
                    using the <filename>git push</filename> command.
                    </para></listitem>
                <listitem><para>Notify the maintainer that you have pushed a change by making a pull
                    request.
                    The Yocto Project provides two scripts that conveniently let you generate and send
                    pull requests to the Yocto Project.
                    These scripts are <filename>create-pull-request</filename> and
                    <filename>send-pull-request</filename>.
                    You can find these scripts in the <filename>scripts</filename> directory
                    within the <link linkend='source-directory'>Source Directory</link>.</para>
                    <para>Using these scripts correctly formats the requests without introducing any
                    whitespace or HTML formatting.
                    The maintainer that receives your patches needs to be able to save and apply them
                    directly from your emails.
                    Using these scripts is the preferred method for sending patches.</para>
                    <para>For help on using these scripts, simply provide the
                    <filename>-h</filename> argument as follows:
                    <literallayout class='monospaced'>
     $ poky/scripts/create-pull-request -h
     $ poky/scripts/send-pull-request -h
                    </literallayout></para></listitem>
            </itemizedlist>
        </para>

        <para>
            You can find general Git information on how to push a change upstream in the
            <ulink url='http://git-scm.com/book/en/v2/Distributed-Git-Distributed-Workflows'>Git Community Book</ulink>.
        </para>
    </section>

    <section id='submitting-a-patch'>
        <title>Using Email to Submit a Patch</title>

        <para>
            You can submit patches without using the <filename>create-pull-request</filename> and
            <filename>send-pull-request</filename> scripts described in the previous section.
            However, keep in mind, the preferred method is to use the scripts.
        </para>

        <para>
            Depending on the components changed, you need to submit the email to a specific
            mailing list.
            For some guidance on which mailing list to use, see the list in the
            "<link linkend='how-to-submit-a-change'>How to Submit a Change</link>"
            section.
            For a description of the available mailing lists, see the
            "<ulink url='&YOCTO_DOCS_REF_URL;#resources-mailinglist'>Mailing Lists</ulink>"
            section in the Yocto Project Reference Manual.
        </para>

        <para>
            Here is the general procedure on how to submit a patch through email without using the
            scripts:
            <itemizedlist>
                <listitem><para>Make your changes in your local Git repository.</para></listitem>
                <listitem><para>Stage your changes by using the <filename>git add</filename>
                    command on each file you changed.</para></listitem>
                <listitem><para>Commit the change by using the
                    <filename>git commit --signoff</filename> command.
                    Using the <filename>--signoff</filename> option identifies you as the person
                    making the change and also satisfies the Developer's Certificate of
                    Origin (DCO) shown earlier.</para>
                    <para>When you form a commit, you must follow certain standards established by the
                    Yocto Project development team.
                    See the earlier section
                    "<link linkend='how-to-submit-a-change'>How to Submit a Change</link>"
                    for Yocto Project commit message standards.</para></listitem>
                <listitem><para>Format the commit into an email message.
                    To format commits, use the <filename>git format-patch</filename> command.
                    When you provide the command, you must include a revision list or a number of patches
                    as part of the command.
                    For example, either of these two commands takes your most
                    recent single commit and formats it as an email message in
                    the current directory:
                    <literallayout class='monospaced'>
     $ git format-patch -1
                    </literallayout>
                    or
                    <literallayout class='monospaced'>
     $ git format-patch HEAD~
                    </literallayout></para>
                    <para>After the command is run, the current directory contains a
                    numbered <filename>.patch</filename> file for the commit.</para>
                    <para>If you provide several commits as part of the command,
                    the <filename>git format-patch</filename> command produces a
                    series of numbered files in the current directory – one for each commit.
                    If you have more than one patch, you should also use the
                    <filename>--cover</filename> option with the command, which generates a
                    cover letter as the first "patch" in the series.
                    You can then edit the cover letter to provide a description for
                    the series of patches.
                    For information on the <filename>git format-patch</filename> command,
                    see <filename>GIT_FORMAT_PATCH(1)</filename> displayed using the
                    <filename>man git-format-patch</filename> command.</para>
                    <note>If you are or will be a frequent contributor to the Yocto Project
                    or to OpenEmbedded, you might consider requesting a contrib area and the
                    necessary associated rights.</note></listitem>
                <listitem><para>Import the files into your mail client by using the
                    <filename>git send-email</filename> command.
                    <note>In order to use <filename>git send-email</filename>, you must have the
                    the proper Git packages installed.
                    For Ubuntu, Debian, and Fedora the package is <filename>git-email</filename>.</note></para>
                    <para>The <filename>git send-email</filename> command sends email by using a local
                    or remote Mail Transport Agent (MTA) such as
                    <filename>msmtp</filename>, <filename>sendmail</filename>, or through a direct
                    <filename>smtp</filename> configuration in your Git <filename>config</filename>
                    file.
                    If you are submitting patches through email only, it is very important
                    that you submit them without any whitespace or HTML formatting that
                    either you or your mailer introduces.
                    The maintainer that receives your patches needs to be able to save and
                    apply them directly from your emails.
                    A good way to verify that what you are sending will be applicable by the
                    maintainer is to do a dry run and send them to yourself and then
                    save and apply them as the maintainer would.</para>
                    <para>The <filename>git send-email</filename> command is the preferred method
                    for sending your patches since there is no risk of compromising whitespace
                    in the body of the message, which can occur when you use your own mail client.
                    The command also has several options that let you
                    specify recipients and perform further editing of the email message.
                    For information on how to use the <filename>git send-email</filename> command,
                    see <filename>GIT-SEND-EMAIL(1)</filename> displayed using
                    the <filename>man git-send-email</filename> command.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>
</section>
</chapter>
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