generic-poky/documentation/dev-manual/dev-manual-model.xml

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

<title>Common Development Models</title>

<para>
    Many development models exist for which you can use the Yocto Project. 
    However, for the purposes of this manual we are going to focus on two common ones:  
    System Development and User Application Development.  
    System Development covers Board Support Package (BSP) development and kernel modification.  
    User Application Development covers development of applications that you intend to run on some 
    target hardware.  
</para>

<para>
    This chapter presents overviews of both system and application models.
    If you want to reference specific examples of these development models, 
    see <xref linkend='dev-manual-bsp-appendix'>BSP Development Example</xref> and 
    <xref linkend='dev-manual-kernel-appendix'>Kernel Modification Example</xref>.
</para>

<section id='system-development-model'>
    <title>System Development</title>

    <para>
        System development involves modification or creation of an image that you want to run on 
        a specific hardware target.  
        Usually when you want to create an image that runs on embedded hardware the image does 
        not require the same amount of features that a full-fledged Linux distribution provides.  
        Thus, you can create a much smaller image that is designed to just use the hardware 
        features for your particular hardware.
    </para>

    <para>
        To help you understand how system development works in the Yocto Project, this section 
        covers two types of image development:  BSP creation and kernel modification
        (see <xref linkend='kernel-spot'></xref>).
    </para>

    <section id='developing-a-board-support-package-bsp'>
        <title>Developing a Board Support Package (BSP)</title>

        <para>
            A BSP is a package of recipes that when applied during a build results in 
            an image you can run on a particular board.  
            Thus, the package, when compiled into the new image, supports the operation of the board.  
        </para>

        <note>
            For a brief list of terms used when describing the development process in the Yocto Project, 
            see <xref linkend='yocto-project-terms'>Yocto Project Terms</xref> in this manual.
        </note>

        <para>
            The remainder of this section presents the basic steps to create a BSP basing it on an 
            existing BSP that ships with the Yocto Project.
            You can reference <xref linkend='dev-manual-bsp-appendix'>BSP Development Example</xref>
            for a detailed example that uses the Crown Bay BSP as a base BSP from which to start.
        </para>

        <para>
            This illustration and the following list summarizes the BSP creation general workflow.
        </para>

        <para>
            <imagedata fileref="figures/bsp-dev-flow.png" width="6in" depth="8.5in" align="left" scale="100" />
        </para>

        <para>
            <orderedlist>
                <listitem><para><emphasis>Set up your host development system to support 
                    development using the Yocto Project</emphasis>:  See  
                    <ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#the-linux-distro'>
                    The Linux Distributions</ulink> section and  
                    <ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#packages'>
                    The Packages</ulink> section both
                    in the Yocto Project Quick Start for requirements.
                    You will also need a release of Yocto Project installed on the host.</para></listitem>
                <listitem><para><emphasis>Establish a local copy of the Yocto Project files on your 
                    system</emphasis>:  You need to have the Yocto Project files available on your host system. 
                    Having the Yocto Project files on your system gives you access to the build
                    process and tools you need.
                    For information on how to get these files, see the
                    <xref linkend='getting-setup'>Getting Setup</xref> section in this manual.</para></listitem>
                <listitem><para><emphasis>Establish a local copy of the base BSP files</emphasis>:  Having    
                    the BSP files on your system gives you access to the build
                    process and tools you need.
                    For information on how to get these files, see 
                    <xref linkend='getting-setup'>Getting Setup</xref> earlier in this manual.</para></listitem>    
                <listitem><para><emphasis>Choose a Yocto Project-supported BSP as your base BSP</emphasis>:  
                    The Yocto Project ships with several BSPs that support various hardware.  
                    It is best to base your new BSP on an existing BSP rather than create all the 
                    recipes and configuration files from scratch.  
                    While it is possible to create everything from scratch, basing your new BSP 
                    on something that is close is much easier.  
                    Or, at a minimum, it gives you some structure with which to start.</para>
                    <para>At this point you need to understand your target hardware well enough to determine which 
                    existing BSP it most closely matches.  
                    Things to consider are your hardware’s on-board features such as CPU type and graphics support.  
                    You should look at the README files for supported BSPs to get an idea of which one 
                    you could use.  
                    A generic Atom-based BSP to consider is the Crown Bay that does not support
                    the Intel® Embedded Media Graphics Driver (EMGD).  
                    The remainder of this example uses that base BSP.</para>
                    <para>To see the supported BSPs, go to the Yocto Project
                    <ulink url='http://www.yoctoproject.org/download'>download page</ulink> and click 
                    on “BSP Downloads.”</para></listitem>
                <listitem><para><emphasis>Create your own BSP layer</emphasis>:  Layers are ideal for 
                    isolating and storing work for a given piece of hardware.  
                    A layer is really just a location or area in which you place the recipes for your BSP.  
                    In fact, a BSP is, in itself, a special type of layer.   
                    Consider an application as another example that illustrates a layer.  
                    Suppose you are creating an application that has library or other dependencies in 
                    order for it to compile and run.  
                    The layer, in this case, would be where all the recipes that define those dependencies 
                    are kept.  The key point for a layer is that it is an isolated area that contains 
                    all the relevant information for the project that the Yocto Project build 
                    system knows about.</para>
                    <note>The Yocto Project supports four BSPs that are part of the 
                    Yocto Project release: <filename>atom-pc</filename>, <filename>beagleboard</filename>,
                    <filename>mpc8315e</filename>, and <filename>routerstationpro</filename>.  
                    The recipes and configurations for these four BSPs are located and dispersed 
                    within local Yocto Project files.
                    Consequently, they are not totally isolated in the spirit of layers unless you think 
                    of <filename>meta-yocto</filename> as a layer itself.  
                    On the other hand, BSP layers for Crown Bay, Emenlow, Jasper Forest, 
                    N450, and Sugar Bay are isolated.</note>
                    <para>When you set up a layer for a new BSP you should follow a standard layout.  
                    This layout is described in the
                    <ulink url='http://www.yoctoproject.org/docs/1.1/bsp-guide/bsp-guide.html#bsp-filelayout'>
                    Example Filesystem Layout</ulink> section of the Board Support Package (BSP) Development
                    Guide.  
                    In the standard layout you will notice a suggested structure for recipes and  
                    configuration information.  
                    You can see the standard layout for the Crown Bay BSP in this example by examining the 
                    directory structure of the <filename>meta-crownbay</filename> layer inside the 
                    local Yocto Project files.</para></listitem>
                <listitem><para><emphasis>Make configuration changes to your new BSP 
                    layer</emphasis>:  The standard BSP layer structure organizes the files you need to edit in 
                    <filename>conf</filename> and several <filename>recipes-*</filename> within the 
                    BSP layer.
                    Configuration changes identify where your new layer is on the local system
                    and identify which kernel you are going to use.
                    </para></listitem>
                <listitem><para><emphasis>Make recipe changes to your new BSP layer</emphasis>:  Recipe 
                    changes include altering recipes (<filename>.bb</filename> files), removing 
                    recipes you don't use, and adding new recipes that you need to support your hardware.
                    </para></listitem>
                <listitem><para><emphasis>Prepare for the build</emphasis>:  Once you have made all the 
                    changes to your BSP layer there remains a few things 
                    you need to do for the Yocto Project build system in order for it to create your image.  
                    You need to get the build environment ready by sourcing an environment setup script 
                    and you need to be sure two key configuration files are configured appropriately.</para>
                    <para>The entire process for building an image is overviewed in the 
                    <ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#building-image'>
                    Building an Image</ulink> section of the Yocto Project Quick Start.    
                    You might want to reference this information.</para></listitem>
                <listitem><para><emphasis>Build the image</emphasis>:  The Yocto Project uses the BitBake 
                    tool to build images based on the type of image 
                    you want to create.  
                    You can find more information on BitBake 
                    <ulink url='http://bitbake.berlios.de/manual/'>here</ulink>.</para>
                    <para>The build process supports several types of images to satisfy different needs.  
                    When you issue the BitBake command you provide a “top-level” recipe that essentially 
                    starts the process off of building the type of image you want.</para>
                    <para>[WRITER'S NOTE: Consider moving this to the Poky Reference Manual.]</para>
                    <para>You can find these recipes in the <filename>meta/recipes-core/images</filename> and 
                    <filename>meta/recipes-sato/images</filename> directories of your local Yocto Project 
                    file structure (Git repository or extracted release tarball).  
                    Although the recipe names are somewhat explanatory, here is a list that describes them:
                    <itemizedlist>
                        <listitem><para><emphasis>Base</emphasis> – A foundational basic image without support 
                            for X that can be reasonably used for customization.</para></listitem>
                        <listitem><para><emphasis>Core</emphasis> – A foundational basic image with support for 
                            X that can be reasonably used for customization.</para></listitem>
                        <listitem><para><emphasis>Direct Disk</emphasis> – An image that you can copy directory to 
                            the disk of the target device.</para></listitem>
                        <listitem><para><emphasis>Live</emphasis> – An image you can run from a USB device or from 
                            a CD without having to first install something.</para></listitem>
                        <listitem><para><emphasis>Minimal</emphasis> – A small image without a GUI.  
                            This image is not much more than a kernel with a shell.</para></listitem>
                        <listitem><para><emphasis>Minimal Development</emphasis> – A Minimal image suitable for 
                            development work.</para></listitem>
                        <listitem><para><emphasis>Minimal Direct Disk</emphasis> – A Minimal Direct 
                            Disk image.</para></listitem>
                        <listitem><para><emphasis>Minimal RAM-based Initial Root Filesystem</emphasis> – 
                            A minimal image 
                           that has the <filename>initramfs</filename> as part of the kernel, which allows the 
                            system to find the first “init” program more efficiently.</para></listitem>
                        <listitem><para><emphasis>Minimal Live</emphasis> – A Minimal Live image.</para></listitem>
                       <listitem><para><emphasis>Minimal MTD Utilities</emphasis> – A minimal image that has support 
                            for the MTD utilities, which let the user interact with the MTD subsystem in 
                            the kernel to perform operations on flash devices.</para></listitem>
                        <listitem><para><emphasis>Sato</emphasis> – An image with Sato support, a mobile environment 
                            and visual style that works well with mobile devices.</para></listitem>
                        <listitem><para><emphasis>Sato Development</emphasis> – A Sato image suitable for 
                            development work.</para></listitem>
                        <listitem><para><emphasis>Sato Direct Disk</emphasis> – A Sato Direct 
                            Disk image.</para></listitem>
                        <listitem><para><emphasis>Sato Live</emphasis> – A Sato Live image.</para></listitem>
                        <listitem><para><emphasis>Sato SDK</emphasis> – A Sato image that includes the Yocto Project 
                            toolchain and development libraries.</para></listitem>
                        <listitem><para><emphasis>Sato SDK Direct Disk</emphasis> – A Sato SDK Direct 
                            Disk image.</para></listitem>
                        <listitem><para><emphasis>Sato SDK Live</emphasis> – A Sato SDK Live 
                           image.</para></listitem>
                    </itemizedlist>
                </para></listitem>
            </orderedlist>
        </para>

        <para>
            You can view a video presentation of the BSP creation process 
            <ulink url='http://free-electrons.com/blog/elc-2011-videos'>here</ulink>.  
            You can also find supplemental information in the 
            <ulink url='http://yoctoproject.org/docs/1.1/bsp-guide/bsp-guide.html'>
            Board Support Package (BSP) Development Guide</ulink>.
            Finally, there is wiki page write up of the example located 
            <ulink url='https://wiki.yoctoproject.org/wiki/Transcript:_creating_one_generic_Atom_BSP_from_another'>
            here</ulink> you might find helpful.
       </para>
    </section>

    <section id='modifying-the-kernel'>
        <title><anchor id='kernel-spot' />Modifying the Kernel</title>

        <para>
            Kernel modification involves changing the Linux Yocto kernel, which could involve changing
            configuration variables as well as adding new kernel recipes.  
            Configuration changes can be added in the form of configuration fragments, while recipe
            modification comes through the kernel's <filename>recipes-kernel</filename> area
            in a kernel layer you create.
        </para>

        <para>
            The remainder of this section presents a high-level overview of the Linux Yocto 
            kernel architecture and the steps to modify the Linux Yocto kernel.
            For a complete discussion of the kernel, see the 
            <ulink url='http://www.yoctoproject.org/docs/1.1/kernel-manual/kernel-manual.html'>
            Yocto Project Kernel Architecture and Use Manual</ulink>.
            You can reference <xref linkend='dev-manual-kernel-appendix'>Kernel Modification Example</xref>
            for a detailed example that changes the configuration of a kernel.
        </para>

        <section id='kernel-overview'>
            <title>Kernel Overview</title>

            <para>
                When one thinks of the source files for a kernel they usually think of a fixed structure 
                of files that contain kernel patches.
                The Yocto Project, however, employs mechanisims that in a sense result in a kernel source
                generator.
            </para>

            <para>
                The Yocto Project uses the source code management (SCM) tool Git to manage and track Yocto 
                Project files.
                Git employs branching strategies that effectively produce a tree-like structure whose 
                branches represent diversions from more general code. 
                For example, suppose two kernels are basically identical with the exception of a couple
                different features in each.
                In the Yocto Project source repositories managed by Git a main branch can contain the 
                common or shared
                parts of the kernel source and two branches that diverge from that common branch can 
                each contain the features specific to the respective kernel.
                The result is a managed tree whose "leaves" represent the end of a specific path that yields
                a set of kernel source files necessary for a specific piece of hardware and its features.
            </para>
         
            <para>
                A big advantage to this scheme is the sharing of common features by keeping them in 
                "larger" branches that are further up the tree.  
                This practice eliminates redundant storage of similar features shared among kernels.
            </para>

            <para>
                When you build the kernel on your development system all files needed for the build
                are taken from the Yocto Project source repositories pointed to by the 
                <filename>SRC_URI</filename> variable and gathered in a temporary work area
                where they are subsequently used to create the unique kernel.
                Thus, in a sense, the process constructs a local source tree specific to your 
                kernel to generate the new kernel image - a source generator if you will.
            </para>

            <para>
                For a complete discussion of the Yocto Project kernel's architcture and its branching strategy,
                see the <ulink url='http://www.yoctoproject.org/docs/1.1/kernel-manual/kernel-manual.html'>
                The Yocto Project Kernel Architecture and Use Manual</ulink>.
            </para>
        </section> 

        <section id='kernel-modification-workflow'>
            <title>Kernel Modification Workflow</title>

            <para>
                This illustration and the following list summarizes the kernel modification general workflow.
            </para>

<!--            <para>
                <imagedata fileref="figures/bsp-dev-flow.png" width="6in" depth="8.5in" align="left" scale="100" />
            </para> -->

            <para>
                [WRITER'S NOTE:  Need new flow illustration here]
            </para>

            <para>
                <orderedlist>
                    <listitem><para><emphasis>Set up your host development system to support 
                        development using the Yocto Project</emphasis>:  See  
                        <ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#the-linux-distro'>
                        The Linux Distributions</ulink> section and  
                        <ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#packages'>
                        The Packages</ulink> section both
                        in the Yocto Project Quick Start for requirements.
                        You will also need a release of Yocto Project installed on the host.</para></listitem>
                    <listitem><para><emphasis>Establish a local copy of the Yocto Project files on your 
                        system</emphasis>:  You need to have the Yocto Project files available on your host system. 
                        Having the Yocto Project files on your system gives you access to the build
                        process and tools you need.
                        For information on how to get these files, see the bulleted item
                        <link linkend='local-yp-release'>Yocto Project Release</link> in  
                        <xref linkend='getting-setup'>Getting Setup</xref> earlier in this manual.
                        </para></listitem>
                    <listitem><para><emphasis>Establish a local kernel layer by copying the 
                        <filename>meta-skeleton</filename> layer</emphasis>:  When you set up a kernel layer 
                        for your changes you should follow a standard layout.  
                        For kernel layers you can start with <filename>meta-skeleton</filename>, which 
                        is a minimal, base kernel layer in the local Yocto Project files.
                        You can examine <filename>meta-skeleton</filename>
                        in the <filename>poky</filename> Git repository.</para>
                        <para>A layer is really just a location or area in which you place configuration 
                        fragments and recipes that modify your kernel.  
                        The layer, in this case, would be where all the recipes that define those dependencies 
                        are kept.  
                        The key point for a layer is that it is an isolated area that contains 
                        all the relevant information for the project that the Yocto Project build 
                        system knows about.</para>
                        <para></para></listitem>
                    <listitem><para><emphasis>Iteratively make kernel configuration changes
                        to your local kernel layer</emphasis>:  Use <filename>menuconfig</filename>
                        to enable and disable the configurations to the Linux Yocto kernel.  
                        Using <filename>menuconfig</filename> allows you to develop and test the 
                        configuration changes you are making to the kernel.</para></listitem>                      
                    <listitem><para><emphasis>Iteratively make kernel recipe changes to your new kernel 
                        layer</emphasis>:  The standard layer structure organizes the files you need to edit in 
                        <filename>conf</filename> and several <filename>recipes-*</filename> within the 
                        layer.
                        Recipe changes include altering recipes (<filename>.bb</filename> files), removing 
                        recipes you don't use, and adding new recipes that you need to support your hardware.
                        </para></listitem>
                    <listitem><para><emphasis>Prepare for the build</emphasis>:  Once you have made all the 
                        changes to your kernel layer there remains a few things 
                        you need to do for the Yocto Project build system in order for it to create your image.  
                        You need to get the build environment ready by sourcing an environment setup script 
                        and you need to be sure two key configuration files are configured appropriately.</para>
                        <para>The entire process for building an image is overviewed in the 
                        <ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#building-image'>
                        Building an Image</ulink> section of the Yocto Project Quick Start.    
                        You might want to reference this information.</para></listitem>
                    <listitem><para><emphasis>Build the image</emphasis>:  The Yocto Project uses the BitBake 
                        tool to build images based on the type of image 
                        you want to create.  
                        You can find more information on BitBake 
                        <ulink url='http://bitbake.berlios.de/manual/'>here</ulink>.</para>
                        <para>The build process supports several types of images to satisfy different needs.  
                        When you issue the BitBake command you provide a “top-level” recipe that essentially 
                        starts the process off of building the type of image you want.</para>
                        <para>[WRITER'S NOTE: Consider moving this to the Poky Reference Manual.]</para>
                        <para>You can find these recipes in the <filename>meta/recipes-core/images</filename> and 
                        <filename>meta/recipes-sato/images</filename> directories of your local Yocto Project 
                        file structure (Git repository or extracted release tarball).  
                        Although the recipe names are somewhat explanatory, here is a list that describes them:
                        <itemizedlist>
                            <listitem><para><emphasis>Base</emphasis> – A foundational basic image without support 
                                for X that can be reasonably used for customization.</para></listitem>
                            <listitem><para><emphasis>Core</emphasis> – A foundational basic image with support for 
                                X that can be reasonably used for customization.</para></listitem>
                            <listitem><para><emphasis>Direct Disk</emphasis> – An image that you can copy directory to 
                                the disk of the target device.</para></listitem>
                            <listitem><para><emphasis>Live</emphasis> – An image you can run from a USB device or from 
                                a CD without having to first install something.</para></listitem>
                            <listitem><para><emphasis>Minimal</emphasis> – A small image without a GUI.  
                                This image is not much more than a kernel with a shell.</para></listitem>
                            <listitem><para><emphasis>Minimal Development</emphasis> – A Minimal image suitable for 
                                development work.</para></listitem>
                            <listitem><para><emphasis>Minimal Direct Disk</emphasis> – A Minimal Direct 
                                Disk image.</para></listitem>
                            <listitem><para><emphasis>Minimal RAM-based Initial Root Filesystem</emphasis> – 
                                A minimal image 
                                that has the <filename>initramfs</filename> as part of the kernel, which allows the 
                                system to find the first “init” program more efficiently.</para></listitem>
                            <listitem><para><emphasis>Minimal Live</emphasis> – A Minimal Live image.</para></listitem>
                            <listitem><para><emphasis>Minimal MTD Utilities</emphasis> – A minimal image that has support 
                                for the MTD utilities, which let the user interact with the MTD subsystem in 
                                the kernel to perform operations on flash devices.</para></listitem>
                            <listitem><para><emphasis>Sato</emphasis> – An image with Sato support, a mobile environment 
                                and visual style that works well with mobile devices.</para></listitem>
                            <listitem><para><emphasis>Sato Development</emphasis> – A Sato image suitable for 
                                development work.</para></listitem>
                            <listitem><para><emphasis>Sato Direct Disk</emphasis> – A Sato Direct 
                                Disk image.</para></listitem>
                            <listitem><para><emphasis>Sato Live</emphasis> – A Sato Live image.</para></listitem>
                            <listitem><para><emphasis>Sato SDK</emphasis> – A Sato image that includes the Yocto Project 
                                toolchain and development libraries.</para></listitem>
                            <listitem><para><emphasis>Sato SDK Direct Disk</emphasis> – A Sato SDK Direct 
                                Disk image.</para></listitem>
                            <listitem><para><emphasis>Sato SDK Live</emphasis> – A Sato SDK Live 
                               image.</para></listitem>
                        </itemizedlist>
                    </para></listitem>
                    <listitem><para><emphasis>Make your configuration and recipe changes available
                        in the kernel layer</emphasis>:  Up to this point all the changes to the 
                        kernel have been done and tested iteratively.  
                        Once they are tested and ready to go you can move them into the kernel layer, 
                        which allows you to distribute the layer.</para></listitem>
                    <listitem><para><emphasis>Make your configuration and recipe changes to the 
                        linux Yocto Git repository (in-tree changes)</emphasis>:  If the changes you made
                        are suited for all Linux Yocto users you might want to push the changes up into 
                        the Linux Yocto Git repository so that they become part of the kernel tree
                        and available to everyone using the kernel.</para></listitem> 
                </orderedlist>
            </para>

            <para>  
                You can find a web interface to the Yocto Project source repository at 
                <ulink url='http://git.yoctoproject.org/'></ulink>.  
                Within the interface you will see groups of related source code, each of which can 
                be cloned using Git to result in a working Git repository on your local system 
                (referred to as the "local Yocto Project files" in this manual).
                The Yocto Project supports four types of kernels in its source repositories at 
                <ulink url='http://git.yoctoproject.org/'></ulink>:
                <itemizedlist>
                    <listitem><para><emphasis><filename>linux-yocto-2.6.34</filename></emphasis> - The 
                    stable Linux Yocto kernel that is based on the Linux 2.6.34 release.</para></listitem>
                    <listitem><para><emphasis><filename>linux-yocto-2.6.37</filename></emphasis> - The current
                    Linux Yocto kernel that is based on the Linux 2.6.37 release.</para></listitem>
                    <listitem><para><emphasis><filename>linux-yocto-dev</filename></emphasis> - A development
                    kernel based on the Linux 2.6.39-rc1 release.</para></listitem>
                    <listitem><para><emphasis><filename>linux-2.6</filename></emphasis> - A kernel based on 
                    minimal Linux mainline tracking.
                    [WRITER'S NOTE: I don't know which Git repository the user needs to clone to get this
                    repository on their development system.]</para></listitem>
                </itemizedlist>
            </para>
        </section>
    </section>
</section>

<section id='place-holder-section-two'>
    <title>Place-Holder Section For Application Development</title>

    <para>
        Text needed here.
    </para>
</section>

</chapter>
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