554 lines
29 KiB
XML
554 lines
29 KiB
XML
<!DOCTYPE appendix 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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<appendix id='dev-manual-kernel-appendix'>
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<title>Kernel Modification Example</title>
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<para>
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Kernel modification involves changing or adding configurations to an existing kernel,
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changing or adding recipes to the kernel that are needed to support specific hardware features,
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or even altering the source code itself.
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This appendix presents simple examples that modify the kernel source code,
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change the kernel configuration, and add a kernel source recipe.
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<note>
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You can use the <filename>yocto-kernel</filename> script
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found in the <link linkend='source-directory'>Source Directory</link>
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under <filename>scripts</filename> to manage kernel patches and configuration.
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See the "<ulink url='&YOCTO_DOCS_BSP_URL;#managing-kernel-patches-and-config-items-with-yocto-kernel'>Managing kernel Patches and Config Items with yocto-kernel</ulink>"
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section in the Yocto Project Board Support Packages (BSP) Developer's Guide for
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more information.</note>
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</para>
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<section id='modifying-the-kernel-source-code'>
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<title>Modifying the Kernel Source Code</title>
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<para>
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This example adds some simple QEMU emulator console output at boot time by
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adding <filename>printk</filename> statements to the kernel's
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<filename>calibrate.c</filename> source code file.
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Booting the modified image causes the added messages to appear on the emulator's
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console.
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</para>
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<section id='understanding-the-files-you-need'>
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<title>Understanding the Files You Need</title>
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<para>
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Before you modify the kernel, you need to know what Git repositories and file
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structures you need.
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Briefly, you need the following:
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<itemizedlist>
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<listitem><para>A local
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<link linkend='source-directory'>Source Directory</link> for the
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poky Git repository</para></listitem>
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<listitem><para>Local copies of the
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<link linkend='poky-extras-repo'><filename>poky-extras</filename></link>
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Git repository placed within the Source Directory.</para></listitem>
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<listitem><para>A bare clone of the
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<link linkend='local-kernel-files'>Yocto Project Kernel</link> upstream Git
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repository to which you want to push your modifications.
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</para></listitem>
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<listitem><para>A copy of that bare clone in which you make your source
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modifications</para></listitem>
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</itemizedlist>
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</para>
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<para>
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The following figure summarizes these four areas.
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Within each rectangular that represents a data structure, a
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host development directory pathname appears at the
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lower left-hand corner of the box.
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These pathnames are the locations used in this example.
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The figure also provides key statements and commands used during the kernel
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modification process:
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</para>
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<para>
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<imagedata fileref="figures/kernel-example-repos-generic.png" width="7in" depth="5in"
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align="center" scale="100" />
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</para>
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<para>
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Here is a brief description of the four areas:
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<itemizedlist>
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<listitem><para><emphasis>Local Source Directory:</emphasis>
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This area contains all the metadata that supports building images
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using the OpenEmbedded build system.
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In this example, the
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<link linkend='source-directory'>Source Directory</link> also
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contains the
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<link linkend='build-directory'>Build Directory</link>,
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which contains the configuration directory
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that lets you control the build.
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Also in this example, the Source Directory contains local copies of the
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<filename>poky-extras</filename> Git repository.</para>
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<para>See the bulleted item
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"<link linkend='local-yp-release'>Yocto Project Release</link>"
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for information on how to get these files on your local system.</para></listitem>
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<listitem><para><emphasis>Local copies of the <filename>poky-extras</filename> Git Repository:</emphasis>
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This area contains the <filename>meta-kernel-dev</filename> layer,
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which is where you make changes that append the kernel build recipes.
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You edit <filename>.bbappend</filename> files to locate your
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local kernel source files and to identify the kernel being built.
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This Git repository is a gathering place for extensions to the Yocto Project
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(or really any) kernel recipes that faciliate the creation and development
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of kernel features, BSPs or configurations.</para>
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<para>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>"
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for information on how to get these files.</para></listitem>
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<listitem><para><emphasis>Bare Clone of the Yocto Project kernel:</emphasis>
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This bare Git repository tracks the upstream Git repository of the Linux
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Yocto kernel source code you are changing.
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When you modify the kernel you must work through a bare clone.
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All source code changes you make to the kernel must be committed and
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pushed to the bare clone using Git commands.
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As mentioned, the <filename>.bbappend</filename> file in the
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<filename>poky-extras</filename> repository points to the bare clone
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so that the build process can locate the locally changed source files.</para>
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<para>See the bulleted item
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"<link linkend='local-kernel-files'>Yocto Project Kernel</link>"
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for information on how to set up the bare clone.
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</para></listitem>
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<listitem><para><emphasis>Copy of the Yocto Project Kernel Bare Clone:</emphasis>
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This Git repository contains the actual source files that you modify.
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Any changes you make to files in this location need to ultimately be pushed
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to the bare clone using the <filename>git push</filename> command.</para>
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<para>See the bulleted item
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"<link linkend='local-kernel-files'>Yocto Project Kernel</link>"
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for information on how to set up the bare clone.
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<note>Typically, Git workflows follow a scheme where changes made to a local area
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are pulled into a Git repository.
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However, because the <filename>git pull</filename> command does not work
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with bare clones, this workflow pushes changes to the
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repository even though you could use other more complicated methods to
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get changes into the bare clone.</note>
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</para></listitem>
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</itemizedlist>
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</para>
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</section>
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<section id='setting-up-the-local-yocto-project-files-git-repository'>
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<title>Setting Up the Local Source Directory</title>
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<para>
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You can set up the
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<link linkend='source-directory'>Source Directory</link>
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through tarball extraction or by
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cloning the <filename>poky</filename> Git repository.
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This example uses <filename>poky</filename> as the root directory of the
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local Source Directory.
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See the bulleted item
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"<link linkend='local-yp-release'>Yocto Project Release</link>"
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for information on how to get these files.
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</para>
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<para>
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Once you have Source Directory set up,
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you have many development branches from which you can work.
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From inside the local repository you can see the branch names and the tag names used
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in the upstream Git repository by using either of the following commands:
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<literallayout class='monospaced'>
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$ cd poky
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$ git branch -a
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$ git tag -l
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</literallayout>
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This example uses the Yocto Project &DISTRO; Release code named "&DISTRO_NAME;",
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which maps to the <filename>&DISTRO_NAME;</filename> branch in the repository.
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The following commands create and checkout the local <filename>&DISTRO_NAME;</filename>
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branch:
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<literallayout class='monospaced'>
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$ git checkout -b &DISTRO_NAME; origin/&DISTRO_NAME;
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Branch &DISTRO_NAME; set up to track remote branch &DISTRO_NAME; from origin.
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Switched to a new branch '&DISTRO_NAME;'
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</literallayout>
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</para>
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</section>
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<section id='setting-up-the-poky-extras-git-repository'>
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<title>Setting Up the Local poky-extras Git Repository</title>
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<para>
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This example creates a local copy of the <filename>poky-extras</filename> Git
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repository inside the <filename>poky</filename> Source Directory.
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See the bulleted item "<link linkend='poky-extras-repo'>The
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<filename>poky-extras</filename> Git Repository</link>"
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for information on how to set up a local copy of the
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<filename>poky-extras</filename> repository.
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</para>
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<para>
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Because this example uses the Yocto Project &DISTRO; Release code
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named "&DISTRO_NAME;", which maps to the <filename>&DISTRO_NAME;</filename>
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branch in the repository, you need to be sure you are using that
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branch for <filename>poky-extras</filename>.
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The following commands create and checkout the local
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branch you are using for the <filename>&DISTRO_NAME;</filename>
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branch:
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<literallayout class='monospaced'>
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$ cd ~/poky/poky-extras
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$ git checkout -b &DISTRO_NAME; origin/&DISTRO_NAME;
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Branch &DISTRO_NAME; set up to track remote branch &DISTRO_NAME; from origin.
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Switched to a new branch '&DISTRO_NAME;'
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</literallayout>
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</para>
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</section>
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<section id='setting-up-the-bare-clone-and-its-copy'>
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<title>Setting Up the Bare Clone and its Copy</title>
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<para>
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This example modifies the <filename>linux-yocto-3.4</filename> kernel.
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Thus, you need to create a bare clone of that kernel and then make a copy of the
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bare clone.
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See the bulleted item
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"<link linkend='local-kernel-files'>Yocto Project Kernel</link>"
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for information on how to do that.
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</para>
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<para>
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The bare clone exists for the kernel build tools and simply as the receiving end
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of <filename>git push</filename>
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commands after you make edits and commits inside the copy of the clone.
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The copy (<filename>my-linux-yocto-3.4-work</filename> in this example) has to have
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a local branch created and checked out for your work.
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This example uses <filename>common-pc-base</filename> as the local branch.
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The following commands create and checkout the branch:
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<literallayout class='monospaced'>
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$ cd ~/my-linux-yocto-3.4-work
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$ git checkout -b standard-common-pc-base origin/standard/common-pc/base
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Branch standard-common-pc-base set up to track remote branch
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standard/common-pc/base from origin.
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Switched to a new branch 'standard-common-pc-base'
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</literallayout>
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</para>
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</section>
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<section id='building-and-booting-the-default-qemu-kernel-image'>
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<title>Building and Booting the Default QEMU Kernel Image</title>
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<para>
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Before we make changes to the kernel source files, this example first builds the
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default image and then boots it inside the QEMU emulator.
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<note>
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Because a full build can take hours, you should check two variables in the
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<filename>build</filename> directory that is created after you source the
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<filename>&OE_INIT_FILE;</filename> script.
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You can find these variables
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<filename>BB_NUMBER_THREADS</filename> and <filename>PARALLEL_MAKE</filename>
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in the <filename>build/conf</filename> directory in the
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<filename>local.conf</filename> configuration file.
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By default, these variables are commented out.
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If your host development system supports multi-core and multi-thread capabilities,
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you can uncomment these statements and set the variables to significantly shorten
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the full build time.
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As a guideline, set both <filename>BB_NUMBER_THREADS</filename> and
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<filename>PARALLEL_MAKE</filename> to twice the number
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of cores your machine supports.
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</note>
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The following two commands <filename>source</filename> the build environment setup script
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and build the default <filename>qemux86</filename> image.
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If necessary, the script creates the build directory:
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<literallayout class='monospaced'>
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$ cd ~/poky
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$ source &OE_INIT_FILE;
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You had no conf/local.conf file. This configuration file has therefore been
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created for you with some default values. You may wish to edit it to use a
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different MACHINE (target hardware) or enable parallel build options to take
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advantage of multiple cores for example. See the file for more information as
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common configuration options are commented.
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The Yocto Project has extensive documentation about OE including a reference manual
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which can be found at:
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http://yoctoproject.org/documentation
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For more information about OpenEmbedded see their website:
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http://www.openembedded.org/
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You had no conf/bblayers.conf file. The configuration file has been created for
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you with some default values. To add additional metadata layers into your
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configuration please add entries to this file.
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The Yocto Project has extensive documentation about OE including a reference manual
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which can be found at:
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http://yoctoproject.org/documentation
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For more information about OpenEmbedded see their website:
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http://www.openembedded.org/
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### Shell environment set up for builds. ###
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You can now run 'bitbake <target>>'
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Common targets are:
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core-image-minimal
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core-image-sato
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meta-toolchain
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meta-toolchain-sdk
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adt-installer
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meta-ide-support
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You can also run generated qemu images with a command like 'runqemu qemux86'
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</literallayout>
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</para>
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<para>
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The following <filename>bitbake</filename> command starts the build:
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<literallayout class='monospaced'>
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$ bitbake -k core-image-minimal
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</literallayout>
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<note>Be sure to check the settings in the <filename>local.conf</filename>
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before starting the build.</note>
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</para>
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<para>
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After the build completes, you can start the QEMU emulator using the resulting image
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<filename>qemux86</filename> as follows:
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<literallayout class='monospaced'>
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$ runqemu qemux86
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</literallayout>
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</para>
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<para>
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As the image boots in the emulator, console message and status output appears
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across the terminal window.
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Because the output scrolls by quickly, it is difficult to read.
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To examine the output, you log into the system using the
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login <filename>root</filename> with no password.
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Once you are logged in, issue the following command to scroll through the
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console output:
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<literallayout class='monospaced'>
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# dmesg | less
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</literallayout>
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</para>
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<para>
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Take note of the output as you will want to look for your inserted print command output
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later in the example.
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</para>
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</section>
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<section id='changing-the-source-code-and-pushing-it-to-the-bare-clone'>
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<title>Changing the Source Code and Pushing it to the Bare Clone</title>
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<para>
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The file you change in this example is named <filename>calibrate.c</filename>
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and is located in the <filename>my-linux-yocto-3.4-work</filename> Git repository
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(the copy of the bare clone) in <filename>init</filename>.
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This example simply inserts several <filename>printk</filename> statements
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at the beginning of the <filename>calibrate_delay</filename> function.
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</para>
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<para>
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Here is the unaltered code at the start of this function:
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<literallayout class='monospaced'>
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void __cpuinit calibrate_delay(void)
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{
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unsigned long lpj;
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static bool printed;
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int this_cpu = smp_processor_id();
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if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
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.
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.
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.
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</literallayout>
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</para>
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<para>
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Here is the altered code showing five new <filename>printk</filename> statements
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near the top of the function:
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<literallayout class='monospaced'>
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void __cpuinit calibrate_delay(void)
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{
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unsigned long lpj;
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static bool printed;
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int this_cpu = smp_processor_id();
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printk("*************************************\n");
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printk("* *\n");
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printk("* HELLO YOCTO KERNEL *\n");
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printk("* *\n");
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printk("*************************************\n");
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if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
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.
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.
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.
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</literallayout>
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</para>
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<para>
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After making and saving your changes, you need to stage them for the push.
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The following Git commands are one method of staging and committing your changes:
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<literallayout class='monospaced'>
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$ git add calibrate.c
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$ git commit --signoff
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</literallayout>
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</para>
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<para>
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Once the source code has been modified, you need to use Git to push the changes to
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the bare clone.
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If you do not push the changes, then the OpenEmbedded build system will not pick
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up the changed source files.
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</para>
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<para>
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The following command pushes the changes to the bare clone:
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<literallayout class='monospaced'>
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$ git push origin standard-common-pc-base:standard/default/common-pc/base
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</literallayout>
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</para>
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</section>
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<section id='changing-build-parameters-for-your-build'>
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<title>Changing Build Parameters for Your Build</title>
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<para>
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At this point, the source has been changed and pushed.
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The example now defines some variables used by the OpenEmbedded build system
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to locate your kernel source.
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You essentially need to identify where to find the kernel recipe and the changed source code.
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You also need to be sure some basic configurations are in place that identify the
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type of machine you are building and to help speed up the build should your host support
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multiple-core and thread capabilities.
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</para>
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<para>
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Do the following to make sure the build parameters are set up for the example.
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Once you set up these build parameters, they do not have to change unless you
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change the target architecture of the machine you are building or you move
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the bare clone, copy of the clone, or the <filename>poky-extras</filename> repository:
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<itemizedlist>
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<listitem><para><emphasis>Build for the Correct Target Architecture:</emphasis> The
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<filename>local.conf</filename> file in the build directory defines the build's
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target architecture.
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By default, <filename>MACHINE</filename> is set to
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<filename>qemux86</filename>, which specifies a 32-bit
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<trademark class='registered'>Intel</trademark> Architecture
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target machine suitable for the QEMU emulator.
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In this example, <filename>MACHINE</filename> is correctly configured.
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</para></listitem>
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<listitem><para><emphasis>Optimize Build Time:</emphasis> Also in the
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<filename>local.conf</filename> file are two variables that can speed your
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build time if your host supports multi-core and multi-thread capabilities:
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<filename>BB_NUMBER_THREADS</filename> and <filename>PARALLEL_MAKE</filename>.
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If the host system has multiple cores then you can optimize build time
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by setting both these variables to twice the number of
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cores.</para></listitem>
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<listitem><para><emphasis>Identify Your <filename>meta-kernel-dev</filename>
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Layer:</emphasis> The <filename>BBLAYERS</filename> variable in the
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<filename>bblayers.conf</filename> file found in the
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<filename>poky/build/conf</filename> directory needs to have the path to your local
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<filename>meta-kernel-dev</filename> layer.
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By default, the <filename>BBLAYERS</filename> variable contains paths to
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<filename>meta</filename> and <filename>meta-yocto</filename> in the
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<filename>poky</filename> Git repository.
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Add the path to your <filename>meta-kernel-dev</filename> location.
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Be sure to substitute your user information in the statement.
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Here is an example:
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<literallayout class='monospaced'>
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BBLAYERS = " \
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/home/scottrif/poky/meta \
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/home/scottrif/poky/meta-yocto \
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/home/scottrif/poky/meta-yocto-bsp \
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/home/scottrif/poky/poky-extras/meta-kernel-dev \
|
|
"
|
|
</literallayout></para></listitem>
|
|
<listitem><para><emphasis>Identify Your Source Files:</emphasis> In the
|
|
<filename>linux-yocto_3.4.bbappend</filename> file located in the
|
|
<filename>poky-extras/meta-kernel-dev/recipes-kernel/linux</filename>
|
|
directory, you need to identify the location of the
|
|
local source code, which in this example is the bare clone named
|
|
<filename>linux-yocto-3.4.git</filename>.
|
|
To do this, set the <filename>KSRC_linux_yocto</filename> variable to point to your
|
|
local <filename>linux-yocto-3.4.git</filename> Git repository by adding the
|
|
following statement.
|
|
Also, be sure the <filename>SRC_URI</filename> variable is pointing to
|
|
your kernel source files by removing the comment.
|
|
Finally, be sure to substitute your user information in the statement:
|
|
<literallayout class='monospaced'>
|
|
KSRC_linux_yocto_3_4 ?= "/home/scottrif/linux-yocto-3.4.git"
|
|
SRC_URI = "git://${KSRC_linux_yocto_3_4};protocol=file;nocheckout=1;branch=${KBRANCH},meta;name=machine,meta"
|
|
</literallayout></para></listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<note>
|
|
<para>Before attempting to build the modified kernel, there is one more set of changes you
|
|
need to make in the <filename>meta-kernel-dev</filename> layer.
|
|
Because all the kernel <filename>.bbappend</filename> files are parsed during the
|
|
build process regardless of whether you are using them or not, you should either
|
|
comment out the <filename>COMPATIBLE_MACHINE</filename> statements in all
|
|
unused <filename>.bbappend</filename> files, or simply remove (or rename) all the files
|
|
except the one your are using for the build
|
|
(i.e. <filename>linux-yocto_3.4.bbappend</filename> in this example).</para>
|
|
<para>If you do not make one of these two adjustments, your machine will be compatible
|
|
with all the kernel recipes in the <filename>meta-kernel-dev</filename> layer.
|
|
When your machine is comapatible with all the kernel recipes, the build attempts
|
|
to build all kernels in the layer.
|
|
You could end up with build errors blocking your work.</para>
|
|
</note>
|
|
</section>
|
|
|
|
<section id='building-and-booting-the-modified-qemu-kernel-image'>
|
|
<title>Building and Booting the Modified QEMU Kernel Image</title>
|
|
|
|
<para>
|
|
Next, you need to build the modified image.
|
|
Do the following:
|
|
<orderedlist>
|
|
<listitem><para>Your environment should be set up since you previously sourced
|
|
the <filename>&OE_INIT_FILE;</filename> script.
|
|
If it isn't, source the script again from <filename>poky</filename>.
|
|
<literallayout class='monospaced'>
|
|
$ cd ~/poky
|
|
$ source &OE_INIT_FILE;
|
|
</literallayout>
|
|
</para></listitem>
|
|
<listitem><para>Be sure old images are cleaned out by running the
|
|
<filename>cleanall</filename> BitBake task as follows from your build directory:
|
|
<literallayout class='monospaced'>
|
|
$ bitbake -c cleanall linux-yocto
|
|
</literallayout></para>
|
|
<para><note>Never remove any files by hand from the <filename>tmp/deploy</filename>
|
|
directory insided the build directory.
|
|
Always use the BitBake <filename>cleanall</filename> task to clear
|
|
out previous builds.</note></para></listitem>
|
|
<listitem><para>Next, build the kernel image using this command:
|
|
<literallayout class='monospaced'>
|
|
$ bitbake -k core-image-minimal
|
|
</literallayout></para></listitem>
|
|
<listitem><para>Finally, boot the modified image in the QEMU emulator
|
|
using this command:
|
|
<literallayout class='monospaced'>
|
|
$ runqemu qemux86
|
|
</literallayout></para></listitem>
|
|
</orderedlist>
|
|
</para>
|
|
|
|
<para>
|
|
Log into the machine using <filename>root</filename> with no password and then
|
|
use the following shell command to scroll through the console's boot output.
|
|
<literallayout class='monospaced'>
|
|
# dmesg | less
|
|
</literallayout>
|
|
</para>
|
|
|
|
<para>
|
|
You should see the results of your <filename>printk</filename> statements
|
|
as part of the output.
|
|
</para>
|
|
</section>
|
|
</section>
|
|
</appendix>
|
|
|
|
<!--
|
|
vim: expandtab tw=80 ts=4
|
|
-->
|