408 lines
27 KiB
XML
408 lines
27 KiB
XML
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<chapter id='dev-manual-model'>
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<title>Common Development Models</title>
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<para>
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Many development models exist for which you can use the Yocto Project.
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However, for the purposes of this manual we are going to focus on two common ones:
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System Development and User Application Development.
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System Development covers Board Support Package (BSP) development and kernel modification.
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User Application Development covers development of applications that you intend to run on some
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target hardware.
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</para>
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<para>
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This chapter presents overviews of both system and application models.
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If you want to reference specific examples of these development models,
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see <xref linkend='dev-manual-bsp-appendix'>BSP Development Example</xref> and
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<xref linkend='dev-manual-kernel-appendix'>Kernel Modification Example</xref>.
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</para>
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<section id='system-development-model'>
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<title>System Development</title>
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<para>
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System development involves modification or creation of an image that you want to run on
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a specific hardware target.
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Usually when you want to create an image that runs on embedded hardware the image does
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not require the same amount of features that a full-fledged Linux distribution provides.
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Thus, you can create a much smaller image that is designed to just use the hardware
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features for your particular hardware.
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</para>
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<para>
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To help you understand how system development works in the Yocto Project, this section
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covers two types of image development: BSP creation and kernel modification
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(see <xref linkend='kernel-spot'></xref>).
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</para>
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<section id='developing-a-board-support-package-bsp'>
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<title>Developing a Board Support Package (BSP)</title>
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<para>
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A BSP is a package of recipes that when applied during a build results in
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an image you can run on a particular board.
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Thus, the package, when compiled into the new image, supports the operation of the board.
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</para>
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<note>
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For a brief list of terms used when describing the development process in the Yocto Project,
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see <xref linkend='yocto-project-terms'>Yocto Project Terms</xref> in this manual.
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</note>
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<para>
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The remainder of this section presents the basic steps to create a BSP basing it on an
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existing BSP that ships with the Yocto Project.
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You can reference <xref linkend='dev-manual-bsp-appendix'>BSP Development Example</xref>
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for a detailed example that uses the Crown Bay BSP as a base BSP from which to start.
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</para>
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<para>
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This illustration and the following list summarizes the BSP creation general workflow.
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</para>
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<para>
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<imagedata fileref="figures/bsp-dev-flow.png" width="6in" depth="7in" align="center" scalefit="1" />
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</para>
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<para>
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<orderedlist>
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<listitem><para><emphasis>Set up your host development system to support
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development using the Yocto Project</emphasis>: See
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<ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#the-linux-distro'>
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The Linux Distributions</ulink> section and
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<ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#packages'>
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The Packages</ulink> section both
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in the Yocto Project Quick Start for requirements.</para></listitem>
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<listitem><para><emphasis>Establish a local copy of the Yocto Project files on your
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system</emphasis>: You need to have the Yocto Project files available on your host system.
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Having the Yocto Project files on your system gives you access to the build
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process and tools you need.
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For information on how to get these files, see the
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<xref linkend='getting-setup'>Getting Setup</xref> section in this manual.</para></listitem>
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<listitem><para><emphasis>Establish a local copy of the base BSP files</emphasis>: Having
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the BSP files on your system gives you access to the build
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process and tools you need.
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For information on how to get these files, see
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<xref linkend='getting-setup'>Getting Setup</xref> earlier in this manual.</para></listitem>
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<listitem><para><emphasis>Choose a Yocto Project-supported BSP as your base BSP</emphasis>:
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The Yocto Project ships with several BSPs that support various hardware.
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It is best to base your new BSP on an existing BSP rather than create all the
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recipes and configuration files from scratch.
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While it is possible to create everything from scratch, basing your new BSP
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on something that is close is much easier.
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Or, at a minimum, it gives you some structure with which to start.</para>
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<para>At this point you need to understand your target hardware well enough to determine which
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existing BSP it most closely matches.
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Things to consider are your hardware’s on-board features such as CPU type and graphics support.
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You should look at the README files for supported BSPs to get an idea of which one
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you could use.
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A generic Atom-based BSP to consider is the Crown Bay that does not support
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the Intel® Embedded Media Graphics Driver (EMGD).
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The remainder of this example uses that base BSP.</para>
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<para>To see the supported BSPs, go to the Yocto Project
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<ulink url='http://www.yoctoproject.org/download'>download page</ulink> and click
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on “BSP Downloads.”</para></listitem>
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<listitem><para><emphasis>Create your own BSP layer</emphasis>: Layers are ideal for
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isolating and storing work for a given piece of hardware.
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A layer is really just a location or area in which you place the recipes for your BSP.
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In fact, a BSP is, in itself, a special type of layer.
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Consider an application as another example that illustrates a layer.
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Suppose you are creating an application that has library or other dependencies in
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order for it to compile and run.
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The layer, in this case, would be where all the recipes that define those dependencies
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are kept. The key point for a layer is that it is an isolated area that contains
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all the relevant information for the project that the Yocto Project build
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system knows about.</para>
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<note>The Yocto Project supports four BSPs that are part of the
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Yocto Project release: <filename>atom-pc</filename>, <filename>beagleboard</filename>,
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<filename>mpc8315e</filename>, and <filename>routerstationpro</filename>.
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The recipes and configurations for these four BSPs are located and dispersed
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within local Yocto Project files.
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Consequently, they are not totally isolated in the spirit of layers unless you think
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of <filename>meta-yocto</filename> as a layer itself.
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On the other hand, BSP layers for Crown Bay, Emenlow, Jasper Forest,
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N450, and Sugar Bay are isolated.</note>
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<para>When you set up a layer for a new BSP you should follow a standard layout.
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This layout is described in the
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<ulink url='http://www.yoctoproject.org/docs/1.1/bsp-guide/bsp-guide.html#bsp-filelayout'>
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Example Filesystem Layout</ulink> section of the Board Support Package (BSP) Development
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Guide.
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In the standard layout you will notice a suggested structure for recipes and
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configuration information.
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You can see the standard layout for the Crown Bay BSP in this example by examining the
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directory structure of the <filename>meta-crownbay</filename> layer inside the
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local Yocto Project files.</para></listitem>
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<listitem><para><emphasis>Make configuration changes to your new BSP
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layer</emphasis>: The standard BSP layer structure organizes the files you need to edit in
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<filename>conf</filename> and several <filename>recipes-*</filename> within the
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BSP layer.
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Configuration changes identify where your new layer is on the local system
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and identify which kernel you are going to use.
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</para></listitem>
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<listitem><para><emphasis>Make recipe changes to your new BSP layer</emphasis>: Recipe
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changes include altering recipes (<filename>.bb</filename> files), removing
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recipes you don't use, and adding new recipes that you need to support your hardware.
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</para></listitem>
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<listitem><para><emphasis>Prepare for the build</emphasis>: Once you have made all the
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changes to your BSP layer there remains a few things
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you need to do for the Yocto Project build system in order for it to create your image.
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You need to get the build environment ready by sourcing an environment setup script
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and you need to be sure two key configuration files are configured appropriately.</para>
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<para>The entire process for building an image is overviewed in the
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<ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#building-image'>
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Building an Image</ulink> section of the Yocto Project Quick Start.
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You might want to reference this information.</para></listitem>
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<listitem><para><emphasis>Build the image</emphasis>: The Yocto Project uses the BitBake
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tool to build images based on the type of image you want to create.
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You can find more information on BitBake
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<ulink url='http://bitbake.berlios.de/manual/'>here</ulink>.</para>
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<para>The build process supports several types of images to satisfy different needs.
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See
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<ulink url='http://www.yoctoproject.org/docs/1.1/poky-ref-manual/poky-ref-manual.html#ref-images'>
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Reference: Images</ulink> in the
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<ulink url='http://www.yoctoproject.org/docs/1.1/poky-ref-manual/poky-ref-manual.html'>
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Yocto Project Reference Manual</ulink>for information on supported images.</para></listitem>
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</orderedlist>
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</para>
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<para>
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You can view a video presentation on "Building Custom Embedded Images with Yocto"
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at <ulink url='http://free-electrons.com/blog/elc-2011-videos'>Free Electrons</ulink>.
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You can also find supplemental information in the
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<ulink url='http://yoctoproject.org/docs/1.1/bsp-guide/bsp-guide.html'>
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Board Support Package (BSP) Development Guide</ulink>.
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Finally, there is wiki page write up of the example located
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<ulink url='https://wiki.yoctoproject.org/wiki/Transcript:_creating_one_generic_Atom_BSP_from_another'>
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here</ulink> you might find helpful.
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</para>
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</section>
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<section id='modifying-the-kernel'>
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<title><anchor id='kernel-spot' />Modifying the Kernel</title>
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<para>
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Kernel modification involves changing the Linux Yocto kernel, which could involve changing
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configuration variables as well as adding new kernel recipes.
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Configuration changes can be added in the form of configuration fragments, while recipe
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modification comes through the kernel's <filename>recipes-kernel</filename> area
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in a kernel layer you create.
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</para>
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<para>
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The remainder of this section presents a high-level overview of the Linux Yocto
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kernel architecture and the steps to modify the Linux Yocto kernel.
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For a complete discussion of the kernel, see the
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<ulink url='http://www.yoctoproject.org/docs/1.1/kernel-manual/kernel-manual.html'>
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Yocto Project Kernel Architecture and Use Manual</ulink>.
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You can reference <xref linkend='dev-manual-kernel-appendix'>Kernel Modification Example</xref>
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for a detailed example that changes the configuration of a kernel.
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</para>
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<section id='kernel-overview'>
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<title>Kernel Overview</title>
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<para>
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When one thinks of the source files for a kernel they usually think of a fixed structure
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of files that contain kernel patches.
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The Yocto Project, however, employs mechanisims that in a sense result in a kernel source
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generator.
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</para>
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<para>
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The Yocto Project uses the source code management (SCM) tool Git to manage and track Yocto
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Project files.
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Git employs branching strategies that effectively produce a tree-like structure whose
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branches represent diversions from more general code.
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For example, suppose two kernels are basically identical with the exception of a couple
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different features in each.
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In the Yocto Project source repositories managed by Git a main branch can contain the
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common or shared
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parts of the kernel source and two branches that diverge from that common branch can
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each contain the features specific to the respective kernel.
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The result is a managed tree whose "leaves" represent the end of a specific path that yields
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a set of kernel source files necessary for a specific piece of hardware and its features.
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</para>
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<para>
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A big advantage to this scheme is the sharing of common features by keeping them in
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"larger" branches that are further up the tree.
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This practice eliminates redundant storage of similar features shared among kernels.
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</para>
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<para>
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When you build the kernel on your development system all files needed for the build
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are taken from the Yocto Project source repositories pointed to by the
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<filename>SRC_URI</filename> variable and gathered in a temporary work area
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where they are subsequently used to create the unique kernel.
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Thus, in a sense, the process constructs a local source tree specific to your
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kernel to generate the new kernel image - a source generator if you will.
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</para>
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<para>
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You can find a web interface to the Yocto Project source repository at
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<ulink url='http://git.yoctoproject.org/'></ulink>.
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Within the interface you will see groups of related source code, each of which can
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be cloned using Git to result in a working Git repository on your local system
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(referred to as the "local Yocto Project files" in this manual).
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</para>
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<para>
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The Yocto Project supports four types of kernels in its source repositories at
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<ulink url='http://git.yoctoproject.org/'></ulink>:
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<itemizedlist>
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<listitem><para><emphasis><filename>linux-yocto-2.6.34</filename></emphasis> - The
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stable Linux Yocto kernel that is based on the Linux 2.6.34 release.</para></listitem>
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<listitem><para><emphasis><filename>linux-yocto-2.6.37</filename></emphasis> - The
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stable Linux Yocto kernel that is based on the Linux 2.6.37 release.</para></listitem>
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<listitem><para><emphasis><filename>linux-yocto-3.0</filename></emphasis> - The current
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Linux Yocto kernel that is based on the Linux 3.0 release.</para></listitem>
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<listitem><para><emphasis><filename>linux-yocto-dev</filename></emphasis> - A development
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kernel based on the latest upstream release candidate available.</para></listitem>
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<listitem><para><emphasis><filename>linux-2.6</filename></emphasis> - A kernel based on
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minimal Linux mainline tracking.
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You can find the <filename>.bbappend</filename> for this kernel in the
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<filename>meta-kernel-dev</filename> Git repository.</para></listitem>
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</itemizedlist>
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</para>
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<para>
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For a complete discussion of the Yocto Project kernel's architcture and its branching strategy,
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see the <ulink url='http://www.yoctoproject.org/docs/1.1/kernel-manual/kernel-manual.html'>
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The Yocto Project Kernel Architecture and Use Manual</ulink>.
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</para>
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</section>
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<section id='kernel-modification-workflow'>
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<title>Kernel Modification Workflow</title>
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<para>
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This illustration and the following list summarizes the kernel modification general workflow.
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</para>
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<para>
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<imagedata fileref="figures/kernel-dev-flow.png" width="6in" depth="7in" align="center" scalefit="1" />
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</para>
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<para>
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<orderedlist>
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<listitem><para><emphasis>Set up your host development system to support
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development using the Yocto Project</emphasis>: See
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<ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#the-linux-distro'>
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The Linux Distributions</ulink> section and
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<ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#packages'>
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The Packages</ulink> section both
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in the Yocto Project Quick Start for requirements.</para></listitem>
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<listitem><para><emphasis>Establish a local copy of the Yocto Project files on your
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system</emphasis>: Having the Yocto Project files on your system gives you access to
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the build process and tools you need.
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For information on how to get these files, see the bulleted item
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<link linkend='local-yp-release'>Yocto Project Release</link> in
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<xref linkend='getting-setup'>Getting Setup</xref> earlier in this manual.
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</para></listitem>
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<listitem><para><emphasis>Set up the <filename>poky-extras</filename> Git
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repository</emphasis>: This repository is the area for your configuration
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fragments, new kernel recipes, and the kernel <filename>.bbappend</filename>
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file used during the build.
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It is good practice to set this repository up inside the local Yocto
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Project files Git repository.
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For information on how to get these files, see the bulleted item
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<link linkend='poky-extras-repo'>The
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<filename>poky-extras</filename> Git Repository</link> in
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<xref linkend='getting-setup'>Getting Setup</xref> earlier in this manual.
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</para></listitem>
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<listitem><para><emphasis>Establish a local copy of the Linux Yocto kernel files on your
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system</emphasis>: In order to make modifications to the kernel you need two things:
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a bare clone of the Linux Yocto kernel you are modifying and a copy of that
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bare clone.
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The bare clone is required by the build process and is the area to which you
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push your kernel source changes.
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The copy of the bare clone is a local Git repository that contains all the kernel's
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source files.
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You make your changes to the files in this copy of the bare clone.
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For information on how to set these two items up, see the bulleted item
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<link linkend='local-kernel-files'>Linux Yocto Kernel</link> in
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<xref linkend='getting-setup'>Getting Setup</xref> earlier in this manual.
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</para></listitem>
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<listitem><para><emphasis>Make changes to the kernel source code if
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applicable</emphasis>: Modifying the kernel does not always mean directly
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changing source files.
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However, if you have to do this then you make the changes in the local
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Git repository you set up to hold the source files (i.e. the copy of the
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bare clone).
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Once the changes are made you need to use Git commands to commit the changes
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and then push them to the bare clone.</para></listitem>
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<listitem><para><emphasis>Make kernel configuration changes
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to your local kernel layer if applicable</emphasis>:
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If your situation calls for changing the kernel's configuration you can
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use <filename>menuconfig</filename>
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to enable and disable kernel configurations.
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Using <filename>menuconfig</filename> allows you to develop and test the
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||
configuration changes you are making to the kernel.</para></listitem>
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<listitem><para><emphasis>Add new kernel recipes if applicable</emphasis>: The standard
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layer structure organizes recipe files inside the
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<filename>meta-kernel-dev</filename> layer that is within the
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<filename>poky-extras</filename> Git repository.
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If you need to add new kernel recipes you add them within this layer.
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Also within this area you will find the <filename>.bbappend</filename>
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file that appends information to the kernel's recipe file used during the
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build.
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</para></listitem>
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<listitem><para><emphasis>Prepare for the build</emphasis>: Once you have made all the
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changes to your kernel (configurations, source code changes, recipe additions,
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||
or recipe changes) there remains a few things
|
||
you need to do for the Yocto Project build system in order for it to create your image.
|
||
If you have not done so you need to get the build environment ready by sourcing
|
||
the environment setup script described earlier.
|
||
You also 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.
|
||
Also, you should look at the detailed examples found in the appendices at
|
||
end of this manual.</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.
|
||
See
|
||
<ulink url='http://www.yoctoproject.org/docs/1.1/poky-ref-manual/poky-ref-manual.html#ref-images'>
|
||
Reference: Images</ulink> in the
|
||
<ulink url='http://www.yoctoproject.org/docs/1.1/poky-ref-manual/poky-ref-manual.html'>
|
||
Yocto Project Reference Manual</ulink> for information on supported
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images.</para></listitem>
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<listitem><para><emphasis>Make your configuration changes available
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in the kernel layer</emphasis>: Up to this point all the configuration changes to the
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kernel have been done and tested iteratively.
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Once they are tested and ready to go you can move them into the kernel layer,
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which allows you to distribute the layer.
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[WRITER'S NOTE: Not sure if the layer is meta-kernel-dev or if it would be
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a new layer copied from the work done there.]</para></listitem>
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<listitem><para><emphasis>Push your configuration and recipe changes upstream to the
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linux Yocto Git repository (in-tree changes)</emphasis>: If the changes you made
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||
are suited for all Linux Yocto users you might want to push the changes up into
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the Linux Yocto Git repository so that they become part of the kernel tree
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and available to everyone using the kernel.</para></listitem>
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</orderedlist>
|
||
</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>
|
||
<!--
|
||
vim: expandtab tw=80 ts=4
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||
-->
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