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(From yocto-docs rev: d5880262ab74499fbeef620af7907ee6e5d328fd) Signed-off-by: Scott Rifenbark <srifenbark@gmail.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
543 lines
28 KiB
XML
543 lines
28 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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[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >
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<chapter id='kernel-dev-intro'>
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<title>Introduction</title>
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<section id='kernel-dev-overview'>
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<title>Overview</title>
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<para>
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Regardless of how you intend to make use of the Yocto Project,
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chances are you will work with the Linux kernel.
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This manual describes how to set up your build host to support
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kernel development, introduces the kernel development process,
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provides background information on the Yocto Linux kernel
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<ulink url='&YOCTO_DOCS_REF_URL;#metadata'>Metadata</ulink>,
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describes common tasks you can perform using the kernel tools,
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and shows you how to use the kernel Metadata needed to work with
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the kernel inside the Yocto Project.
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</para>
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<para>
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Each Yocto Project release has a set of Yocto Linux kernel recipes,
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whose Git repositories you can view in the Yocto
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<ulink url='&YOCTO_GIT_URL;'>Source Repositories</ulink> under
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the "Yocto Linux Kernel" heading.
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New recipes for the release track the latest Linux kernel
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upstream developments from
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<ulink url='http://www.kernel.org'></ulink> and introduce
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newly-supported platforms.
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Previous recipes in the release are refreshed and supported for at
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least one additional Yocto Project release.
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As they align, these previous releases are updated to include the
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latest from the
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<ulink url='&YOCTO_HOME_URL;/organization/long-term-support-initiative-ltsi'>Long Term Support Initiative</ulink>
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(LTSI) project.
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Also included is a Yocto Linux kernel development recipe
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(<filename>linux-yocto-dev.bb</filename>) should you want to work
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with the very latest in upstream Yocto Linux kernel development and
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kernel Metadata development.
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<note>
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For more on Yocto Linux kernels, see the
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"<link linkend='kernel-big-picture'>Yocto Project Kernel Development and Maintenance</link>
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section.
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</note>
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</para>
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<para>
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The Yocto Project also provides a powerful set of kernel
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tools for managing Yocto Linux kernel sources and configuration data.
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You can use these tools to make a single configuration change,
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apply multiple patches, or work with your own kernel sources.
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</para>
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<para>
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In particular, the kernel tools allow you to generate configuration
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fragments that specify only what you must, and nothing more.
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Configuration fragments only need to contain the highest level
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visible <filename>CONFIG</filename> options as presented by the
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Yocto Linux kernel <filename>menuconfig</filename> system.
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Contrast this against a complete Yocto Linux kernel
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<filename>.config</filename> file, which includes all the automatically
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selected <filename>CONFIG</filename> options.
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This efficiency reduces your maintenance effort and allows you
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to further separate your configuration in ways that make sense for
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your project.
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A common split separates policy and hardware.
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For example, all your kernels might support the
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<filename>proc</filename> and <filename>sys</filename> filesystems,
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but only specific boards require sound, USB, or specific drivers.
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Specifying these configurations individually allows you to aggregate
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them together as needed, but maintains them in only one place.
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Similar logic applies to separating source changes.
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</para>
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<para>
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If you do not maintain your own kernel sources and need to make
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only minimal changes to the sources, the released recipes provide a
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vetted base upon which to layer your changes.
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Doing so allows you to benefit from the continual kernel
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integration and testing performed during development of the
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Yocto Project.
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</para>
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<para>
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If, instead, you have a very specific Linux kernel source tree
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and are unable to align with one of the official Yocto Linux kernel
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recipes, an alternative exists by which you can use the Yocto
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Project Linux kernel tools with your own kernel sources.
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</para>
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</section>
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<section id='preparing-the-build-host-to-work-on-the-kernel'>
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<title>Preparing the Build Host to Work on the Kernel</title>
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<para>
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Before you can do any kernel development, you need to be
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sure your build host is set up to use the Yocto Project.
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For information on how to get set up, see the
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"<ulink url='&YOCTO_DOCS_DEV_URL;#setting-up-the-development-host-to-use-the-yocto-project'>Setting Up to Use the Yocto Project</ulink>"
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section in the Yocto Project Development Manual.
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Part of preparing the system is creating a local Git
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repository of the
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<ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>
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(<filename>poky</filename>) on your system.
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Follow the steps in the
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"<ulink url='&YOCTO_DOCS_DEV_URL;#cloning-the-poky-repository'>Cloning the <filename>poky</filename> Repository</ulink>"
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section in the Yocto Project Development Manual to set up your
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Source Directory.
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<note>
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Be sure you check out the appropriate development branch or
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by tag to get the version of Yocto Project you want.
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See the
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"<ulink url='&YOCTO_DOCS_DEV_URL;#checking-out-by-branch-in-poky'>Checking Out by Branch in Poky</ulink>"
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and
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"<ulink url='&YOCTO_DOCS_DEV_URL;#checkout-out-by-tag-in-poky'>Checking Out by Tag in Poky</ulink>"
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sections in the Yocto Project Development Manual for more
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information.
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</note>
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</para>
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<para>
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Kernel development is best accomplished using
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<ulink url='&YOCTO_DOCS_SDK_URL;#using-devtool-in-your-sdk-workflow'><filename>devtool</filename></ulink>
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and not through traditional kernel workflow methods.
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The remainder of this section provides information for both scenarios.
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</para>
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<section id='getting-ready-to-develop-using-devtool'>
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<title>Getting Ready to Develop using <filename>devtool</filename></title>
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<para>
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Follow these steps to prepare to update the kernel image using
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<filename>devtool</filename>.
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Completing this procedure leaves you with a clean kernel image
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and ready to make modifications as described in the
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"<link linkend='using-devtool-to-patch-the-kernel'>Using <filename>devtool</filename> to Patch the Kernel</link>"
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section:
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<orderedlist>
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<listitem><para>
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<emphasis>Initialize the BitBake Environment:</emphasis>
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Before building an extensible SDK, you need to
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initialize the BitBake build environment by sourcing a
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build environment script
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(i.e. <ulink url='&YOCTO_DOCS_REF_URL;#structure-core-script'><filename>oe-init-build-env</filename></ulink>
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or
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<ulink url='&YOCTO_DOCS_REF_URL;#structure-memres-core-script'><filename>oe-init-build-env-memres</filename></ulink>):
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<literallayout class='monospaced'>
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$ cd ~/poky
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$ source oe-init-build-env
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</literallayout>
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<note>
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The previous commands assume the
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<ulink url='&YOCTO_DOCS_REF_URL;#source-repositories'>Source Repositories</ulink>
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(i.e. <filename>poky</filename>) have been cloned
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using Git and the local repository is named
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"poky".
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</note>
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</para></listitem>
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<listitem><para>
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<emphasis>Prepare Your <filename>local.conf</filename> File:</emphasis>
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By default, the
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<ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>
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variable is set to "qemux86", which is fine if you are
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building for the QEMU emulator in 32-bit mode.
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However, if you are not, you need to set the
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<filename>MACHINE</filename> variable appropriately in
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your <filename>conf/local.conf</filename> file found in the
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<ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>
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(i.e. <filename>~/poky/build</filename> in this example).
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</para>
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<para>Also, since you are preparing to work on the kernel
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image, you need to set the
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<ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS'><filename>MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS</filename></ulink>
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variable to include kernel modules.</para>
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<para>This example uses the default "qemux86" for the
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<filename>MACHINE</filename> variable but needs to
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add the "kernel-modules":
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<literallayout class='monospaced'>
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MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS += "kernel-modules"
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</literallayout>
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</para></listitem>
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<listitem><para>
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<emphasis>Create a Layer for Patches:</emphasis>
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You need to create a layer to hold patches created
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for the kernel image.
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You can use the <filename>yocto-layer</filename> command
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as follows:
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<literallayout class='monospaced'>
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$ cd ~/poky
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$ yocto-layer create my-kernel -o ../meta-my-kernel
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Please enter the layer priority you'd like to use for the layer: [default: 6]
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Would you like to have an example recipe created? (y/n) [default: n]
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Would you like to have an example bbappend file created? (y/n) [default: n]
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New layer created in ../meta-my-kernel.
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Don't forget to add it to your BBLAYERS (for details see ../meta-my-kernel/README).
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</literallayout>
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</para></listitem>
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<listitem><para>
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<emphasis>Inform the BitBake Build Environment About Your Layer:</emphasis>
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As directed when you created your layer, you need to add
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the layer to the
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<ulink url='&YOCTO_DOCS_REF_URL;#var-BBLAYERS'><filename>BBLAYERS</filename></ulink>
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variable in the <filename>bblayers.conf</filename> file
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as follows:
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<literallayout class='monospaced'>
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$ cd ~/poky/build
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$ bitbake-layers add-layer ../../meta-my-kernel
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</literallayout>
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</para></listitem>
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<listitem><para>
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<emphasis>Build the Extensible SDK:</emphasis>
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Use BitBake to build the extensible SDK specifically for
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the Minnowboard:
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<literallayout class='monospaced'>
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$ cd ~/poky/build
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$ bitbake core-image-minimal -c populate_sdk_ext
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</literallayout>
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Once the build finishes, you can find the SDK installer
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file (i.e. <filename>*.sh</filename> file) in the
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following directory:
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<literallayout class='monospaced'>
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~/poky/build/tmp/deploy/sdk
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</literallayout>
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For this example, the installer file is named
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<filename>poky-glibc-x86_64-core-image-minimal-i586-toolchain-ext-&DISTRO;.sh</filename>
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</para></listitem>
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<listitem><para>
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<emphasis>Install the Extensible SDK:</emphasis>
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Use the following command to install the SDK.
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For this example, install the SDK in the default
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<filename>~/poky_sdk</filename> directory:
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<literallayout class='monospaced'>
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$ cd ~/poky/build/tmp/deploy/sdk
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$ ./poky-glibc-x86_64-core-image-minimal-i586-toolchain-ext-&DISTRO;.sh
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Poky (Yocto Project Reference Distro) Extensible SDK installer version &DISTRO;
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============================================================================
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Enter target directory for SDK (default: ~/poky_sdk):
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You are about to install the SDK to "/home/scottrif/poky_sdk". Proceed[Y/n]? Y
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Extracting SDK......................................done
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Setting it up...
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Extracting buildtools...
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Preparing build system...
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Parsing recipes: 100% |#################################################################| Time: 0:00:52
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Initializing tasks: 100% |############## ###############################################| Time: 0:00:04
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Checking sstate mirror object availability: 100% |######################################| Time: 0:00:00
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Parsing recipes: 100% |#################################################################| Time: 0:00:33
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Initializing tasks: 100% |##############################################################| Time: 0:00:00
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done
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SDK has been successfully set up and is ready to be used.
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Each time you wish to use the SDK in a new shell session, you need to source the environment setup script e.g.
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$ . /home/scottrif/poky_sdk/environment-setup-i586-poky-linux
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</literallayout>
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</para></listitem>
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<listitem><para id='setting-up-the-esdk-terminal'>
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<emphasis>Set Up a New Terminal to Work With the Extensible SDK:</emphasis>
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You must set up a new terminal to work with the SDK.
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You cannot use the same BitBake shell used to build the
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installer.</para>
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<para>After opening a new shell, run the SDK environment
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setup script as directed by the output from installing
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the SDK:
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<literallayout class='monospaced'>
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$ source ~/poky_sdk/environment-setup-i586-poky-linux
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"SDK environment now set up; additionally you may now run devtool to perform development tasks.
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Run devtool --help for further details.
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</literallayout>
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<note>
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If you get a warning about attempting to use the
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extensible SDK in an environment set up to run
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BitBake, you did not use a new shell.
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</note>
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</para></listitem>
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<listitem><para>
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<emphasis>Build the Clean Image:</emphasis>
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The final step in preparing to work on the kernel is to
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build an initial image using <filename>devtool</filename>
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in the new terminal you just set up and initialized for
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SDK work:
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<literallayout class='monospaced'>
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$ devtool build-image
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Parsing recipes: 100% |##########################################| Time: 0:00:05
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Parsing of 830 .bb files complete (0 cached, 830 parsed). 1299 targets, 47 skipped, 0 masked, 0 errors.
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WARNING: No packages to add, building image core-image-minimal unmodified
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Loading cache: 100% |############################################| Time: 0:00:00
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Loaded 1299 entries from dependency cache.
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NOTE: Resolving any missing task queue dependencies
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Initializing tasks: 100% |#######################################| Time: 0:00:07
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Checking sstate mirror object availability: 100% |###############| Time: 0:00:00
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NOTE: Executing SetScene Tasks
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NOTE: Executing RunQueue Tasks
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NOTE: Tasks Summary: Attempted 2866 tasks of which 2604 didn't need to be rerun and all succeeded.
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NOTE: Successfully built core-image-minimal. You can find output files in /home/scottrif/poky_sdk/tmp/deploy/images/qemux86
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</literallayout>
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If you were building for actual hardware and not for
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emulation, you could flash the image to a USB stick
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on <filename>/dev/sdd</filename> and boot your device.
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For an example that uses a Minnowboard, see the
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<ulink url='https://wiki.yoctoproject.org/wiki/TipsAndTricks/KernelDevelopmentWithEsdk'>TipsAndTricks/KernelDevelopmentWithEsdk</ulink>
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Wiki page.
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</para></listitem>
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</orderedlist>
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</para>
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<para>
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At this point you have set up to start making modifications to the
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kernel by using the extensible SDK.
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For a continued example, see the
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"<link linkend='using-devtool-to-patch-the-kernel'>Using <filename>devtool</filename> to Patch the Kernel</link>"
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section.
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</para>
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</section>
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<section id='getting-ready-for-traditional-kernel-development'>
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<title>Getting Ready for Traditional Kernel Development</title>
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<para>
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For traditional kernel development using the Yocto
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Project, you need to establish a local copy of the
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kernel source.
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You can find Git repositories of supported Yocto Project
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kernels organized under "Yocto Linux Kernel" in the Yocto
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Project Source Repositories at
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<ulink url='&YOCTO_GIT_URL;/cgit.cgi'></ulink>.
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</para>
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<para>
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For simplicity, it is recommended that you create your copy
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of the kernel Git repository outside of the
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<ulink url='&YOCTO_DOCS_REF_URL;source-directory'>Source Directory</ulink>,
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which is usually named <filename>poky</filename>.
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</para>
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<para>
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The following command shows how to create a local copy of the
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<filename>linux-yocto-4.9</filename> kernel:
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<literallayout class='monospaced'>
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$ git clone git://git.yoctoproject.org/linux-yocto-4.9 linux-yocto-4.9.git
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Cloning into 'linux-yocto-4.9.git'...
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remote: Counting objects: 5094108, done.
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remote: Compressing objects: 100% (765113/765113), done.
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remote: Total 5094108 (delta 4294009), reused 5088388 (delta 4288312)
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Receiving objects: 100% (5094108/5094108), 1.02 GiB | 7.82 MiB/s, done.
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Resolving deltas: 100% (4294009/4294009), done.
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Checking connectivity... done.
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Checking out files: 100% (56233/56233), done.
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</literallayout>
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</para>
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</section>
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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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Kernel modification involves changing the Yocto Project kernel,
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which could involve changing configuration options as well as adding
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new kernel recipes.
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Configuration changes can be added in the form of configuration
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fragments, while recipe modification comes through the kernel's
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<filename>recipes-kernel</filename> area in a kernel layer you create.
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</para>
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<para>
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This section presents a high-level overview of the Yocto Project
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kernel modification workflow.
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You can find additional information here:
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<itemizedlist>
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<listitem><para>
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The
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"<link linkend='using-devtool-to-patch-the-kernel'>Using <filename>devtool</filename> to Patch the Kernel</link>"
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section.
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</para></listitem>
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<listitem><para>
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The
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"<ulink url='&YOCTO_DOCS_DEV_URL;#configuring-the-kernel'>Configuring the Kernel</ulink>"
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section in the Yocto Project Development Manual.
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</para></listitem>
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</itemizedlist>
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This illustration and the following list summarizes the kernel
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modification general workflow.
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<imagedata fileref="figures/kernel-dev-flow.png"
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width="9in" depth="5in" 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>
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<emphasis>Set up Your Host Development System to Support
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Development Using the Yocto Project</emphasis>:
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See the
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"<ulink url='&YOCTO_DOCS_QS_URL;#yp-resources'>Setting Up to Use the Yocto Project</ulink>"
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section in the Yocto Project Quick Start for options on how
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to get a build host ready to use the Yocto Project.
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</para></listitem>
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<listitem><para>
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<emphasis>Establish the Temporary Kernel Source Files</emphasis>:
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Temporary kernel source files are kept in the
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<ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>
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created by the OpenEmbedded build system when you run BitBake.
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If you have never built the kernel in which you are
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interested, you need to run an initial build to
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establish local kernel source files.</para>
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<para>If you are building an image for the first time, you
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need to get the build environment ready by sourcing an
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environment setup script
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(i.e. <filename>oe-init-build-env</filename> or
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<filename>oe-init-build-env-memres</filename>).
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You also need to be sure two key configuration files
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(<filename>local.conf</filename> and
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<filename>bblayers.conf</filename>) are configured
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appropriately.</para>
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<para>The entire process for building an image is overviewed
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in the
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|
"<ulink url='&YOCTO_DOCS_QS_URL;#qs-building-images'>Building Images</ulink>"
|
|
section of the Yocto Project Quick Start.
|
|
You might want to reference this information.
|
|
You can find more information on BitBake in the
|
|
<ulink url='&YOCTO_DOCS_BB_URL;'>BitBake User Manual</ulink>.
|
|
</para>
|
|
|
|
<para>The build process supports several types of images to
|
|
satisfy different needs.
|
|
See the "<ulink url='&YOCTO_DOCS_REF_URL;#ref-images'>Images</ulink>"
|
|
chapter in the Yocto Project Reference Manual for information
|
|
on supported images.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
<emphasis>Make Changes to the Kernel Source Code if
|
|
applicable</emphasis>:
|
|
Modifying the kernel does not always mean directly
|
|
changing source files.
|
|
However, if you have to do this, you make the changes to the
|
|
files in the Build Directory.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
<emphasis>Make Kernel Configuration Changes if
|
|
Applicable</emphasis>:
|
|
If your situation calls for changing the kernel's
|
|
configuration, you can use
|
|
<link linkend='generating-configuration-files'><filename>menuconfig</filename></link>,
|
|
which allows you to interactively develop and test the
|
|
configuration changes you are making to the kernel.
|
|
Saving changes you make with <filename>menuconfig</filename>
|
|
updates the kernel's <filename>.config</filename> file.
|
|
<note><title>Warning</title>
|
|
Try to resist the temptation to directly edit an
|
|
existing <filename>.config</filename> file, which is
|
|
found in the Build Directory among the source code
|
|
used for the build (e.g. see the workflow illustration
|
|
in the
|
|
"<link linkend='kernel-modification-workflow'>Kernel Modification Workflow</link>"
|
|
section).
|
|
Doing so, can produce unexpected results when the
|
|
OpenEmbedded build system regenerates the configuration
|
|
file.
|
|
</note>
|
|
Once you are satisfied with the configuration
|
|
changes made using <filename>menuconfig</filename>
|
|
and you have saved them, you can directly compare the
|
|
resulting <filename>.config</filename> file against an
|
|
existing original and gather those changes into a
|
|
<ulink url='&YOCTO_DOCS_DEV_URL;#creating-config-fragments'>configuration fragment file</ulink>
|
|
to be referenced from within the kernel's
|
|
<filename>.bbappend</filename> file.</para>
|
|
|
|
<para>Additionally, if you are working in a BSP layer
|
|
and need to modify the BSP's kernel's configuration,
|
|
you can use the
|
|
<ulink url='&YOCTO_DOCS_BSP_URL;#managing-kernel-patches-and-config-items-with-yocto-kernel'><filename>yocto-kernel</filename></ulink>
|
|
script as well as <filename>menuconfig</filename>.
|
|
The <filename>yocto-kernel</filename> script lets
|
|
you interactively set up kernel configurations.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
<emphasis>Rebuild the Kernel Image With Your Changes</emphasis>:
|
|
Rebuilding the kernel image applies your changes.
|
|
</para></listitem>
|
|
</orderedlist>
|
|
</para>
|
|
</section>
|
|
|
|
<section id='kernel-dev-other-resources'>
|
|
<title>Other Resources</title>
|
|
|
|
<para>
|
|
The remainder of this manual provides instructions for completing
|
|
specific Linux kernel development tasks.
|
|
These instructions assume you are comfortable working with
|
|
<ulink url='http://openembedded.org/wiki/Bitbake'>BitBake</ulink>
|
|
recipes and basic open-source development tools.
|
|
Understanding these concepts will facilitate the process of working
|
|
with the kernel recipes.
|
|
If you find you need some additional background, please be sure to
|
|
review and understand the following documentation:
|
|
<itemizedlist>
|
|
<listitem><para>
|
|
<ulink url='&YOCTO_DOCS_QS_URL;'>Yocto Project Quick Start</ulink>
|
|
</para></listitem>
|
|
<listitem><para>
|
|
<ulink url='&YOCTO_DOCS_SDK_URL;#using-devtool-in-your-sdk-workflow'><filename>devtool</filename> workflow</ulink>
|
|
as described in the Yocto Project Software Development Kit
|
|
(SDK) Developer's Guide.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
The
|
|
"<ulink url='&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers'>Understanding and Creating Layers</ulink>" section
|
|
in the Yocto Project Development Manual
|
|
</para></listitem>
|
|
<listitem><para>
|
|
The
|
|
"<link linkend='kernel-modification-workflow'>Kernel Modification Workflow</link>"
|
|
section.
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<para>
|
|
Finally, while this document focuses on the manual creation of
|
|
recipes, patches, and configuration files, the Yocto Project
|
|
Board Support Package (BSP) tools are available to automate
|
|
this process with existing content and work well to create the
|
|
initial framework and boilerplate code.
|
|
For details on these tools, see the
|
|
"<ulink url='&YOCTO_DOCS_BSP_URL;#using-the-yocto-projects-bsp-tools'>Using the Yocto Project's BSP Tools</ulink>"
|
|
section in the Yocto Project Board Support Package (BSP) Developer's
|
|
Guide.
|
|
</para>
|
|
</section>
|
|
|
|
</chapter>
|
|
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|
|
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|
|
-->
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