path: root/documentation/dev-manual/dev-manual-kernel-appendix.xml

<!DOCTYPE appendix PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >

<appendix id='dev-manual-kernel-appendix'>

<title>Kernel Modification Example</title>

    <para>
        Kernel modification involves changing or adding configurations to an existing kernel,  
        changing or adding recipes to the kernel that are needed to support specific hardware features, 
        or even altering the source code itself.  
        This appendix presents simple examples that modify the kernel source code, 
        change the kernel configuration, and add a kernel source recipe.
        <note>
            You can use the <filename>yocto-kernel</filename> script
            found in the <link linkend='source-directory'>Source Directory</link>
            under <filename>scripts</filename> to manage kernel patches and configuration.
            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>" 
            section in the Yocto Project Board Support Packages (BSP) Developer's Guide for
            more information.</note>
    </para>

    <section id='modifying-the-kernel-source-code'>
        <title>Modifying the Kernel Source Code</title>

        <para>
            This example adds some simple QEMU emulator console output at boot time by 
            adding <filename>printk</filename> statements to the kernel's
            <filename>calibrate.c</filename> source code file.
            Booting the modified image causes the added messages to appear on the emulator's
            console.
        </para> 

        <section id='understanding-the-files-you-need'>
            <title>Understanding the Files You Need</title>
          
            <para>
                Before you modify the kernel, you need to know what Git repositories and file 
                structures you need.
                Briefly, you need the following: 
                <itemizedlist>
                    <listitem><para>A local 
                        <link linkend='source-directory'>Source Directory</link> for the 
                        poky Git repository</para></listitem>
                    <listitem><para>Local copies of the
                        <link linkend='poky-extras-repo'><filename>poky-extras</filename></link>
                        Git repository placed within the Source Directory.</para></listitem>  
                    <listitem><para>A bare clone of the 
                        <link linkend='local-kernel-files'>Yocto Project Kernel</link> upstream Git 
                        repository to which you want to push your modifications.
                        </para></listitem>
                    <listitem><para>A copy of that bare clone in which you make your source 
                        modifications</para></listitem>
                </itemizedlist>
            </para>

            <para> 
                The following figure summarizes these four areas.  
                Within each rectangular that represents a data structure, a  
                host development directory pathname appears at the 
                lower left-hand corner of the box.  
                These pathnames are the locations used in this example.
                The figure also provides key statements and commands used during the kernel
                modification process:
            </para>

            <para>
                <imagedata fileref="figures/kernel-example-repos-generic.png" width="7in" depth="5in" 
                    align="center" scale="100" />
            </para>

            <para>
                Here is a brief description of the four areas:
                <itemizedlist>
                    <listitem><para><emphasis>Local Source Directory:</emphasis> 
                        This area contains all the metadata that supports building images 
                        using the OpenEmbedded build system.
                        In this example, the 
                        <link linkend='source-directory'>Source Directory</link> also 
                        contains the 
                        <link linkend='build-directory'>Build Directory</link>, 
                        which contains the configuration directory 
                        that lets you control the build.
                        Also in this example, the Source Directory contains local copies of the 
                        <filename>poky-extras</filename> Git repository.</para>
                        <para>See the bulleted item
                        "<link linkend='local-yp-release'>Yocto Project Release</link>"
                        for information on how to get these files on your local system.</para></listitem>
                    <listitem><para><emphasis>Local copies of the&nbsp;<filename>poky-extras</filename>&nbsp;Git Repository:</emphasis> 
                        This area contains the <filename>meta-kernel-dev</filename> layer, 
                        which is where you make changes that append the kernel build recipes.
                        You edit <filename>.bbappend</filename> files to locate your 
                        local kernel source files and to identify the kernel being built.
                        This Git repository is a gathering place for extensions to the Yocto Project
                        (or really any) kernel recipes that faciliate the creation and development
                        of kernel features, BSPs or configurations.</para>
                        <para>See the bulleted item
                        "<link linkend='poky-extras-repo'>The 
                        <filename>poky-extras</filename> Git Repository</link>" 
                        for information on how to get these files.</para></listitem>                
                    <listitem><para><emphasis>Bare Clone of the Yocto Project kernel:</emphasis> 
                        This bare Git repository tracks the upstream Git repository of the Linux 
                        Yocto kernel source code you are changing.
                        When you modify the kernel you must work through a bare clone.
                        All source code changes you make to the kernel must be committed and 
                        pushed to the bare clone using Git commands.
                        As mentioned, the <filename>.bbappend</filename> file in the 
                        <filename>poky-extras</filename> repository points to the bare clone
                        so that the build process can locate the locally changed source files.</para>
                        <para>See the bulleted item
                        "<link linkend='local-kernel-files'>Yocto Project Kernel</link>"
                        for information on how to set up the bare clone.
                        </para></listitem>
                    <listitem><para><emphasis>Copy of the Yocto Project Kernel Bare Clone:</emphasis>
                        This Git repository contains the actual source files that you modify.  
                        Any changes you make to files in this location need to ultimately be pushed
                        to the bare clone using the <filename>git push</filename> command.</para>
                        <para>See the bulleted item
                        "<link linkend='local-kernel-files'>Yocto Project Kernel</link>"
                        for information on how to set up the bare clone.
                        <note>Typically, Git workflows follow a scheme where changes made to a local area 
                        are pulled into a Git repository.
                        However, because the <filename>git pull</filename> command does not work 
                        with bare clones, this workflow pushes changes to the 
                        repository even though you could use other more complicated methods to
                        get changes into the bare clone.</note>
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='setting-up-the-local-yocto-project-files-git-repository'>
            <title>Setting Up the Local Source Directory</title>

            <para>
                You can set up the 
                <link linkend='source-directory'>Source Directory</link>
                through tarball extraction or by 
                cloning the <filename>poky</filename> Git repository.  
                This example uses <filename>poky</filename> as the root directory of the 
                local Source Directory.
                See the bulleted item
                "<link linkend='local-yp-release'>Yocto Project Release</link>"
                for information on how to get these files.
            </para>

            <para>
                Once you have Source Directory set up, 
                you have many development branches from which you can work. 
                From inside the local repository you can see the branch names and the tag names used 
                in the upstream Git repository by using either of the following commands:
                <literallayout class='monospaced'>
     $ cd poky
     $ git branch -a
     $ git tag -l
                </literallayout> 
                This example uses the Yocto Project &DISTRO; Release code named "&DISTRO_NAME;", 
                which maps to the <filename>&DISTRO_NAME;</filename> branch in the repository. 
                The following commands create and checkout the local <filename>&DISTRO_NAME;</filename>
                branch:
                <literallayout class='monospaced'>
     $ git checkout -b &DISTRO_NAME; origin/&DISTRO_NAME;
     Branch &DISTRO_NAME; set up to track remote branch &DISTRO_NAME; from origin.
     Switched to a new branch '&DISTRO_NAME;'
                </literallayout>
            </para>
        </section>

        <section id='setting-up-the-poky-extras-git-repository'>
            <title>Setting Up the Local poky-extras Git Repository</title>

            <para>
                This example creates a local copy of the <filename>poky-extras</filename> Git 
                repository inside the <filename>poky</filename> Source Directory.
                See the bulleted item "<link linkend='poky-extras-repo'>The 
                <filename>poky-extras</filename> Git Repository</link>" 
                for information on how to set up a local copy of the
                <filename>poky-extras</filename> repository.
            </para>

            <para>
                Because this example uses the Yocto Project &DISTRO; Release code 
                named "&DISTRO_NAME;", which maps to the <filename>&DISTRO_NAME;</filename> 
                branch in the repository, you need to be sure you are using that 
                branch for <filename>poky-extras</filename>.
                The following commands create and checkout the local 
                branch you are using for the <filename>&DISTRO_NAME;</filename>
                branch:
                <literallayout class='monospaced'>
     $ cd ~/poky/poky-extras
     $ git checkout -b &DISTRO_NAME; origin/&DISTRO_NAME;
     Branch &DISTRO_NAME; set up to track remote branch &DISTRO_NAME; from origin.
     Switched to a new branch '&DISTRO_NAME;'
                </literallayout>
            </para>
        </section>

        <section id='setting-up-the-bare-clone-and-its-copy'>
            <title>Setting Up the Bare Clone and its Copy</title>

            <para>
                This example modifies the <filename>linux-yocto-3.4</filename> kernel.
                Thus, you need to create a bare clone of that kernel and then make a copy of the 
                bare clone.
                See the bulleted item
                "<link linkend='local-kernel-files'>Yocto Project Kernel</link>" 
                for information on how to do that.
            </para>

            <para>
                The bare clone exists for the kernel build tools and simply as the receiving end 
                of <filename>git push</filename>
                commands after you make edits and commits inside the copy of the clone.
                The copy (<filename>my-linux-yocto-3.4-work</filename> in this example) has to have 
                a local branch created and checked out for your work.
                This example uses <filename>common-pc-base</filename> as the local branch.
                The following commands create and checkout the branch:
                <literallayout class='monospaced'>
     $ cd ~/my-linux-yocto-3.4-work
     $ git checkout -b standard-common-pc-base origin/standard/common-pc/base
     Branch standard-common-pc-base set up to track remote branch 
        standard/common-pc/base from origin.
     Switched to a new branch 'standard-common-pc-base'
                </literallayout>
            </para>
        </section>

        <section id='building-and-booting-the-default-qemu-kernel-image'>
            <title>Building and Booting the Default QEMU Kernel Image</title>

            <para>
                Before we make changes to the kernel source files, this example first builds the 
                default image and then boots it inside the QEMU emulator.
                <note>
                    Because a full build can take hours, you should check two variables in the 
                    <filename>build</filename> directory that is created after you source the 
                    <filename>&OE_INIT_FILE;</filename> script.
                    You can find these variables
                    <filename>BB_NUMBER_THREADS</filename> and <filename>PARALLEL_MAKE</filename>
                    in the <filename>build/conf</filename> directory in the 
                    <filename>local.conf</filename> configuration file.
                    By default, these variables are commented out.
                    If your host development system supports multi-core and multi-thread capabilities,
                    you can uncomment these statements and set the variables to significantly shorten
                    the full build time.
                    As a guideline, set both <filename>BB_NUMBER_THREADS</filename> and 
                    <filename>PARALLEL_MAKE</filename> to twice the number 
                    of cores your machine supports.
                </note>
                The following two commands <filename>source</filename> the build environment setup script
                and build the default <filename>qemux86</filename> image.
                If necessary, the script creates the build directory:
                <literallayout class='monospaced'>
     $ cd ~/poky
     $ source &OE_INIT_FILE;
     You had no conf/local.conf file. This configuration file has therefore been
     created for you with some default values. You may wish to edit it to use a 
     different MACHINE (target hardware) or enable parallel build options to take 
     advantage of multiple cores for example. See the file for more information as 
     common configuration options are commented.

     The Yocto Project has extensive documentation about OE including a reference manual
     which can be found at:
         http://yoctoproject.org/documentation

     For more information about OpenEmbedded see their website:
         http://www.openembedded.org/

     You had no conf/bblayers.conf file. The configuration file has been created for
     you with some default values. To add additional metadata layers into your
     configuration please add entries to this file.

     The Yocto Project has extensive documentation about OE including a reference manual
     which can be found at:
         http://yoctoproject.org/documentation

     For more information about OpenEmbedded see their website:
         http://www.openembedded.org/



     ### Shell environment set up for builds. ###

     You can now run 'bitbake &lt;target&gt;>'

     Common targets are:
         core-image-minimal
         core-image-sato
         meta-toolchain
         meta-toolchain-sdk
         adt-installer
         meta-ide-support

     You can also run generated qemu images with a command like 'runqemu qemux86'
                </literallayout>
            </para>

            <para>
                The following <filename>bitbake</filename> command starts the build:
                <literallayout class='monospaced'>
     $ bitbake -k core-image-minimal
                </literallayout>
                <note>Be sure to check the settings in the <filename>local.conf</filename>
                before starting the build.</note>
            </para>

            <para>  
                After the build completes, you can start the QEMU emulator using the resulting image
                <filename>qemux86</filename> as follows:
                <literallayout class='monospaced'>
     $ runqemu qemux86
                </literallayout>
            </para>

            <para>
                As the image boots in the emulator, console message and status output appears
                across the terminal window.
                Because the output scrolls by quickly, it is difficult to read.
                To examine the output, you log into the system using the 
                login <filename>root</filename> with no password.
                Once you are logged in, issue the following command to scroll through the 
                console output:
                <literallayout class='monospaced'>
     # dmesg | less
                </literallayout>
            </para>

            <para>
                Take note of the output as you will want to look for your inserted print command output
                later in the example.
            </para>
        </section>

        <section id='changing-the-source-code-and-pushing-it-to-the-bare-clone'>
            <title>Changing the Source Code and Pushing it to the Bare Clone</title>

            <para>
                The file you change in this example is named <filename>calibrate.c</filename>
                and is located in the <filename>my-linux-yocto-3.4-work</filename> Git repository
                (the copy of the bare clone) in <filename>init</filename>.
                This example simply inserts several <filename>printk</filename> statements
                at the beginning of the <filename>calibrate_delay</filename> function.
            </para>

            <para>
                Here is the unaltered code at the start of this function:
                <literallayout class='monospaced'>
     void __cpuinit calibrate_delay(void)
     {
     	unsigned long lpj;
     	static bool printed;
     	int this_cpu = smp_processor_id();

     	if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
               .
               .
               .
               </literallayout>
           </para>

           <para>
               Here is the altered code showing five new <filename>printk</filename> statements 
               near the top of the function:
                <literallayout class='monospaced'>
     void __cpuinit calibrate_delay(void)
     {
         unsigned long lpj;
         static bool printed;
         int this_cpu = smp_processor_id();

         printk("*************************************\n");
         printk("*                                   *\n");
         printk("*        HELLO YOCTO KERNEL         *\n");
         printk("*                                   *\n");
         printk("*************************************\n");

     	if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
               .
               .
               .
               </literallayout>
           </para>

           <para>
               After making and saving your changes, you need to stage them for the push.
               The following Git commands are one method of staging and committing your changes:
               <literallayout class='monospaced'>
     $ git add calibrate.c
     $ git commit --signoff
               </literallayout>
           </para>

           <para>
               Once the source code has been modified, you need to use Git to push the changes to 
               the bare clone.  
               If you do not push the changes, then the OpenEmbedded build system will not pick 
               up the changed source files.
           </para>

           <para>
               The following command pushes the changes to the bare clone:
               <literallayout class='monospaced'>
     $ git push origin standard-common-pc-base:standard/default/common-pc/base
               </literallayout>
           </para>
       </section>

       <section id='changing-build-parameters-for-your-build'>
           <title>Changing Build Parameters for Your Build</title>

           <para>
               At this point, the source has been changed and pushed.
               The example now defines some variables used by the OpenEmbedded build system 
               to locate your kernel source.
               You essentially need to identify where to find the kernel recipe and the changed source code.
               You also need to be sure some basic configurations are in place that identify the 
               type of machine you are building and to help speed up the build should your host support
               multiple-core and thread capabilities.
           </para>

           <para>
               Do the following to make sure the build parameters are set up for the example.
               Once you set up these build parameters, they do not have to change unless you 
               change the target architecture of the machine you are building or you move
               the bare clone, copy of the clone, or the <filename>poky-extras</filename> repository:
               <itemizedlist>
                   <listitem><para><emphasis>Build for the Correct Target Architecture:</emphasis> The 
                       <filename>local.conf</filename> file in the build directory defines the build's 
                       target architecture.
                       By default, <filename>MACHINE</filename> is set to 
                       <filename>qemux86</filename>, which specifies a 32-bit 
                       <trademark class='registered'>Intel</trademark> Architecture 
                       target machine suitable for the QEMU emulator.  
                       In this example, <filename>MACHINE</filename> is correctly configured.
                       </para></listitem>
                   <listitem><para><emphasis>Optimize Build Time:</emphasis> Also in the 
                       <filename>local.conf</filename> file are two variables that can speed your 
                       build time if your host supports multi-core and multi-thread capabilities:
                       <filename>BB_NUMBER_THREADS</filename> and <filename>PARALLEL_MAKE</filename>.
                       If the host system has multiple cores then you can optimize build time 
                       by setting both these variables to twice the number of 
                       cores.</para></listitem>
                   <listitem><para><emphasis>Identify Your <filename>meta-kernel-dev</filename>
                       Layer:</emphasis> The <filename>BBLAYERS</filename> variable in the 
                       <filename>bblayers.conf</filename> file found in the 
                       <filename>poky/build/conf</filename> directory needs to have the path to your local
                       <filename>meta-kernel-dev</filename> layer.  
                       By default, the <filename>BBLAYERS</filename> variable contains paths to  
                       <filename>meta</filename> and <filename>meta-yocto</filename> in the 
                       <filename>poky</filename> Git repository.
                       Add the path to your <filename>meta-kernel-dev</filename> location.
                       Be sure to substitute your user information in the statement.
                       Here is an example:
                       <literallayout class='monospaced'>
     BBLAYERS = " \
       /home/scottrif/poky/meta \
       /home/scottrif/poky/meta-yocto \
       /home/scottrif/poky/meta-yocto-bsp \
       /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.
                       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"
                       </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>

    <section id='changing-the-kernel-configuration'>
        <title>Changing the Kernel Configuration</title>

        <para>
            This example changes the default behavior, which is "on", of the Symmetric 
            Multi-processing Support (<filename>CONFIG_SMP</filename>) to "off".  
            It is a simple example that demonstrates how to reconfigure the kernel.
        </para>

        <section id='getting-set-up-to-run-this-example'>
            <title>Getting Set Up to Run this Example</title>

            <para>
                If you took the time to work through the example that modifies the kernel source code 
                in "<link linkend='modifying-the-kernel-source-code'>Modifying the Kernel Source
                Code</link>" you should already have the Source Directory set up on your 
                host machine.
                If this is the case, go to the next section, which is titled
                "<link linkend='examining-the-default-config-smp-behavior'>Examining the Default
                <filename>CONFIG_SMP</filename> Behavior</link>", and continue with the 
                example.
            </para>

            <para>
                If you don't have the Source Directory established on your system, 
                you can get them through tarball extraction or by 
                cloning the <filename>poky</filename> Git repository.  
                This example uses <filename>poky</filename> as the root directory of the 
                <link linkend='source-directory'>Source Directory</link>.
                See the bulleted item
                "<link linkend='local-yp-release'>Yocto Project Release</link>"
                for information on how to get these files.
            </para>

            <para>
                Once you have the local copy of the repository set up, 
                you have many development branches from which you can work. 
                From inside the repository you can see the branch names and the tag names used 
                in the upstream Git repository using either of the following commands:
                <literallayout class='monospaced'>
     $ cd poky
     $ git branch -a
     $ git tag -l
                </literallayout> 
                This example uses the Yocto Project &DISTRO; Release code named "&DISTRO_NAME;", 
                which maps to the <filename>&DISTRO_NAME;</filename> branch in the repository. 
                The following commands create and checkout the local <filename>&DISTRO_NAME;</filename>
                branch:
                <literallayout class='monospaced'>
     $ git checkout -b &DISTRO_NAME; origin/&DISTRO_NAME;
     Branch &DISTRO_NAME; set up to track remote branch &DISTRO_NAME; from origin.
     Switched to a new branch '&DISTRO_NAME;'
                </literallayout>
            </para>

            <para>
                Next, you need to build the default <filename>qemux86</filename> image that you 
                can boot using QEMU.
                <note>
                    Because a full build can take hours, you should check two variables in the 
                    <filename>build</filename> directory that is created after you source the 
                    <filename>&OE_INIT_FILE;</filename> script.
                    You can find these variables
                    <filename>BB_NUMBER_THREADS</filename> and <filename>PARALLEL_MAKE</filename>
                    in the <filename>build/conf</filename> directory in the 
                    <filename>local.conf</filename> configuration file.
                    By default, these variables are commented out.
                    If your host development system supports multi-core and multi-thread capabilities,
                    you can uncomment these statements and set the variables to significantly shorten
                    the full build time.
                    As a guideline, set both the <filename>BB_NUMBER_THREADS</filename> and the 
                    <filename>PARALLEL_MAKE</filename> variables to twice the number 
                    of cores your machine supports.
                </note>
                The following two commands <filename>source</filename> the build environment setup script
                and build the default <filename>qemux86</filename> image.
                If necessary, the script creates the build directory:
                <literallayout class='monospaced'>
     $ cd ~/poky
     $ source &OE_INIT_FILE;

          ### Shell environment set up for builds. ###

     You can now run 'bitbake &lt;target&gt;'

     Common targets are:
         core-image-minimal
         core-image-sato
         meta-toolchain
         meta-toolchain-sdk
         adt-installer
         meta-ide-support

     You can also run generated qemu images with a command like 'runqemu qemux86'
                </literallayout>
            </para>

            <para>
                The following <filename>bitbake</filename> command starts the build:
                <literallayout class='monospaced'>
     $ bitbake -k core-image-minimal
                </literallayout>
                <note>Be sure to check the settings in the <filename>local.conf</filename>
                before starting the build.</note>
            </para>
        </section>

        <section id='examining-the-default-config-smp-behavior'>
            <title>Examining the Default&nbsp;&nbsp;<filename>CONFIG_SMP</filename> Behavior</title>

            <para>
                By default, <filename>CONFIG_SMP</filename> supports multiple processor machines.
                To see this default setting from within the QEMU emulator, boot your image using 
                the emulator as follows:
                <literallayout class='monospaced'>
     $ runqemu qemux86 qemuparams="-smp 4"
                </literallayout>
            </para>

            <para>
                Login to the machine using <filename>root</filename> with no password.
                After logging in, enter the following command to see how many processors are 
                being supported in the emulator.
                The emulator reports support for the number of processors you specified using 
                the <filename>-smp</filename> option, four in this case:
                <literallayout class='monospaced'>
     # cat /proc/cpuinfo | grep processor 
     processor       : 0
     processor       : 1
     processor       : 2
     processor       : 3
     #
                </literallayout>
                To check the setting for <filename>CONFIG_SMP</filename>, you can use the 
                following command:
                <literallayout class='monospaced'>
     zcat /proc/config.gz | grep CONFIG_SMP
                </literallayout>
                The console returns the following showing that multi-processor machine support
                is set:
                <literallayout class='monospaced'>
     CONFIG_SMP=y
                </literallayout>
                Logout of the emulator using the <filename>exit</filename> command and 
                then close it down.
            </para>
        </section>

        <section id='changing-the-config-smp-configuration-using-menuconfig'>
            <title>Changing the&nbsp;&nbsp;<filename>CONFIG_SMP</filename> Configuration Using&nbsp;&nbsp;<filename>menuconfig</filename></title>

            <para>
                The <filename>menuconfig</filename> tool provides an interactive method with which
                to set kernel configurations.
                You need to run <filename>menuconfig</filename> inside the Yocto BitBake environment.
                Thus, the environment must be set up using the <filename>&OE_INIT_FILE;</filename>
                script found in the build directory.
                If you have not sourced this script do so with the following commands:
                <literallayout class='monospaced'>
     $ cd ~/poky
     $ source &OE_INIT_FILE;
                </literallayout>
            </para>

            <para>
                After setting up the environment to run <filename>menuconfig</filename>, you are ready 
                to use the tool to interactively change the kernel configuration.
                In this example, we are basing our changes on the <filename>linux-yocto-3.4</filename>
                kernel.
                The OpenEmbedded build system recognizes this kernel as 
                <filename>linux-yocto</filename>.
                Thus, the following commands from the shell in which you previously sourced the 
                environment initialization script cleans the shared state cache and the 
                <ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink>
                directory and then builds and launches <filename>menuconfig</filename>:
                <literallayout class='monospaced'>
     $ bitbake linux-yocto -c menuconfig
                </literallayout>
            </para>
                  
            <para>
                Once <filename>menuconfig</filename> launches, navigate through the user interface
                to find the <filename>CONFIG_SMP</filename> configuration setting.
                You can find it at <filename>Processor Type and Features</filename>.
                The configuration selection is 
                <filename>Symmetric Multi-processing Support</filename>.
                After using the arrow keys to highlight this selection, press "n" to turn it off.
                Then, exit out and save your selections.
            </para>

            <para>
                Once you save the selection, the <filename>.config</filename> configuration file
                is updated. 
                This is the file that the build system uses to configure the Yocto Project kernel
                when it is built. 
                You can find and examine this file in the build directory. 
                This example uses the following:
                <literallayout class='monospaced'>
     ~/poky/build/tmp/work/qemux86-poky-linux/linux-yocto-3.4.11+git1+84f...
        ...656ed30-r1/linux-qemux86-standard-build
                </literallayout>
                <note>
                    The previous example directory is artificially split and many of the characters 
                    in the actual filename are omitted in order to make it more readable.
                    Also, depending on the kernel you are using, the exact pathname might differ 
                    slightly.
                </note>
            </para>

            <para>
                Within the <filename>.config</filename> file, you can see the following setting:
                <literallayout class='monospaced'>
     # CONFIG_SMP is not set
                </literallayout>
            </para>

            <para>
                A good method to isolate changed configurations is to use a combination of the 
                <filename>menuconfig</filename> tool and simple shell commands.
                Before changing configurations with <filename>menuconfig</filename>, copy the 
                existing <filename>.config</filename> and rename it to something else,
                use <filename>menuconfig</filename> to make 
                as many changes an you want and save them, then compare the renamed configuration 
                file against the newly created file.
                You can use the resulting differences as your base to create configuration fragments
                to permanently save in your kernel layer.
                <note>
                    Be sure to make a copy of the <filename>.config</filename> and don't just
                    rename it. 
                    The build system needs an existing <filename>.config</filename>
                    from which to work.
                </note>
            </para>
        </section>

        <section id='recompiling-the-kernel-and-testing-the-new-configuration'>
            <title>Recompiling the Kernel and Testing the New Configuration</title>

            <para>
                At this point, you are ready to recompile your kernel image with 
                the new setting in effect using the BitBake command below:
                <literallayout class='monospaced'>
     $ bitbake linux-yocto
                </literallayout>
            </para>
 
            <para>
                Now run the QEMU emulator and pass it the same multi-processor option as before:
                <literallayout class='monospaced'>
     $ runqemu qemux86 qemuparams="-smp 4"
                </literallayout>
            </para>

            <para>
                Login to the machine using <filename>root</filename> with no password
                and test for the number of processors the kernel supports:
                <literallayout class='monospaced'>
     # cat /proc/cpuinfo | grep processor 
     processor       : 0
     #
                </literallayout>
            </para>

            <para>
                From the output, you can see that the kernel no longer supports multi-processor systems.
                The output indicates support for a single processor.  You can verify the 
                <filename>CONFIG_SMP</filename> setting by using this command:
                <literallayout class='monospaced'>
     zcat /proc/config.gz | grep CONFIG_SMP
                </literallayout>
                The console returns the following output:
                <literallayout class='monospaced'>
     # CONFIG_SMP is not set
                </literallayout>
                You have successfully reconfigured the kernel.
            </para>
        </section>
    </section>

    <section id='adding-kernel-recipes'>
        <title>Adding Kernel Recipes</title>

        <para>
            A future release of this manual will present an example that adds kernel recipes, which provide 
            new functionality to the kernel.
        </para>

        <para>
            <imagedata fileref="figures/wip.png" 
                width="2in" depth="3in" align="center" scalefit="1" />
        </para> 
    </section> 
</appendix>

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