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">

<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.
<!--        [WRITER'S NOTE:  I might want to work in information about applying a local
            change to a kernel layer and also pushing a change upstream into the tree]
        <orderedlist>
            <listitem><para>Iteratively determine and set kernel configurations and make 
                kernel recipe changes.</para></listitem>
            <listitem><para>Apply your configuration changes to your local kernel layer.
                </para></listitem>
            <listitem><para>Push your configuration and recipe changes upstream into the 
                Yocto Project source repositories to make them available to the community.
                </para></listitem>
        </orderedlist> -->
    </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 Yocto Project files Git repository</para></listitem>
                    <listitem><para>The <filename>poky-extras</filename> Git repository placed 
                        within the local Yocto Project files Git repository</para></listitem>  
                    <listitem><para>A bare clone of the Linux Yocto kernel 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 
                        modifcations</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.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 Yocto Project Files Git Repository:</emphasis> 
                        This area contains all the metadata that supports building images in the 
                        Yocto Project build environment - the local Yocto Project files.
                        The local Yocto Project files Git repository also contains the build directory
                        and a configuration directory that let you control the build.
                        Note also that in this example, the repository also contains 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.</para></listitem>
                    <listitem><para><emphasis><filename>poky-extras</filename> 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 Linux Yocto 
                        (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 Linux Yocto 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'>Linux Yocto Kernel</link>"
                        for information on how to set up the bare clone.
                        </para></listitem>
                    <listitem><para><emphasis>Copy of the Linux Yocto 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'>Linux Yocto 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 Yocto Project Files Git Repository</title>

            <para>
                You can get the local Yocto Project files 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 Yocto Project files Git repository.
                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 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 Git repository using either of the following two commands:
                <literallayout class='monospaced'>
     $ cd poky
     $ git branch -a
     $ git tag -l
                </literallayout> 
                This example uses the Yocto Project 1.1 Release code named "edison", 
                which maps to the <filename>edison</filename> branch in the repository. 
                The following commands create and checkout the local <filename>edison</filename>
                branch:
                <literallayout class='monospaced'>
     $ git checkout -b edison origin/edison
     Branch edison set up to track remote branch edison from origin.
     Switched to a new branch 'edison'
                </literallayout>
            </para>
        </section>

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

            <para>
                This example places the <filename>poky-extras</filename> Git repository inside
                of <filename>poky</filename>.
                See the bulleted item
                "<link linkend='poky-extras-repo'>The 
                <filename>poky-extras</filename> Git Repository</link>" 
                for information on how to get the <filename>poky-extras</filename> repository.
            </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-3.0</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'>Linux Yocto 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>linux-yocto-3.0</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 ~/linux-yocto-3.0
     $ git checkout -b common-pc-base origin/yocto/standard/common-pc/base
     Branch common-pc-base set up to track remote branch 
        yocto/standard/common-pc/base from origin.
     Switched to a new branch '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-build-env</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 <filename>BB_NUMBER_THREADS</filename> to twice the number 
                    of cores your machine supports and set <filename>PARALLEL_MAKE</filename> to one and 
                    a half times the number of cores your machine supports.
                </note>
                The following two commands build the default <filename>qemux86</filename> image and 
                <filename>source</filename> build environment setup script.
                If necessary, the script creates the build directory:
                <literallayout class='monospaced'>
     $ cd ~/poky
     $ source oe-init-build-env

          ### 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>linux-yocto-3.0</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;

         if (preset_lpj) {
               .
               .
               .
               </literallayout>
           </para>

           <para>
               Here is the altered code showing five new <filename>printk</filename> statements 
               just after initializing <filename>lps_precision</filename>:
                <literallayout class='monospaced'>
     void __cpuinit calibrate_delay(void)
     {
	     unsigned long lpj;
	     static bool printed;
         printk("*************************************\n");
         printk("*                                   *\n");
         printk("*        HELLO YOCTO KERNEL         *\n");
         printk("*                                   *\n");
         printk("*************************************\n");

         if (preset_lpj) {
               .
               .
               .
               </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 Yocto Project 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 common-pc-base:yocto/standard/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 Yocto Project 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 <filename>BB_NUMBER_THREADS</filename> to twice the number of 
                       cores and setting <filename>PARALLEL_MAKE</filename> to one and a half times 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/poky-extras/meta-kernel-dev \
       "
                       </literallayout></para></listitem>
                   <listitem><para><emphasis>Identify Your Source Files:</emphasis> In the 
                       <filename>linux-yocto_3.0.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-3.0.git</filename>.
                       To do this, set the <filename>KSRC_linux_yocto</filename> variable to point to your
                       local <filename>linux-yocto-3.0.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 ?= /home/scottrif/linux-yocto-3.0.git
                       </literallayout></para></listitem>
                   <listitem><para><emphasis>Specify the Kernel Machine:</emphasis> Also in the 
                       <filename>linux-yocto_3.0.bbappend</filename> file, you need to specify
                       the kernel machine with the following statement:
                       <literallayout class='monospaced'>
     KMACHINE_qemux86 = "yocto/standard/common-pc/base"
                       </literallayout></para></listitem>
               </itemizedlist>
           </para>

           <note>
               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 
               <filename>.bbappend</filename> files, or you should simply remove all the files 
               except the one your are using for the build 
               (i.e. <filename>linux-yocto_3.0.bbappend</filename> in this example).
           </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-build-env</filename> script.
                        If it isn't, source the script again from <filename>poky</filename>.
                        </para></listitem>
                    <listitem><para>Be sure old images are cleaned out by running the 
                        <filename>cleanall</filename> BitBake task as follows:
                        <literallayout class='monospaced'>
     $ cd ~/poky
     $ bitbake -c cleanall linux-yocto
                        </literallayout></para>
                        <para><note>Never remove any files by hand from the <filename>tmp/deploy</filename>
                        directory insided the local Yocto Project files build directory.
                        Always use the BitBake <filename>cleanall</filename> task to clear
                        out previous builds.</note></para></listitem>
                    <listitem><para>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 "off", of the Symmetric 
            Multi-processing Support (<filename>CONFIG_SMP</filename>) to "on".  
            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 Yocto Project files set up on your 
                host machine.
            </para>

            <para>
                If you don't have the Yocto Project files established on your system, 
                See "<link linkend='setting-up-the-local-yocto-project-files-git-repository'>Setting 
                Up the Local Yocto Project Files Git Repository</link>" for 
                information.
                To reconfigure the kernel, this is the only Git repository you need to have set up. 
            </para>

<!--

            <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 are already set up to quickly work through this example.
                If not, then work through the following list to prepare:
                <itemizedlist>
                    <listitem><para><emphasis>Understand the development environment:</emphasis>
                        See "<link linkend='understanding-the-files-you-need'>Understanding 
                        the Files You Need</link>" for information.</para></listitem>
                    <listitem><para><emphasis>Set up the local Yocto Project files Git 
                        repository:</emphasis>
                        See "<link linkend='setting-up-the-local-yocto-project-files-git-repository'>Setting 
                        Up the Local Yocto Project Files Git Repository</link>" for 
                        information.</para></listitem>
                    <listitem><para><emphasis>Set up the <filename>poky-extras</filename> Git 
                        repository:</emphasis>
                        See "<link linkend='setting-up-the-poky-extras-git-repository'>Setting 
                        Up <filename>poky-extras</filename> Git repository</link>" for
                        information.</para></listitem>
                    <listitem><para><emphasis>Set up the the bare clone and its copy:</emphasis>
                        See "<link linkend='setting-up-the-bare-clone-and-its-copy'>Setting Up the 
                        Bare Clone and its Copy</link>" for information.</para></listitem>
                    <listitem><para><emphasis>Build the default QEMU kernel image:</emphasis>
                        See "<link linkend='building-and-booting-the-default-qemu-kernel-image'>Building 
                        and Booting the Default QEMU Kernel image</link>" for information.
                        Do not boot the image in the QEMU emulator at this point.</para></listitem>
                </itemizedlist>
            </para> -->
        </section>

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

            <para>
                By default, <filename>CONFIG_SMP</filename> supports single 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 2"
                </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 a single processor:
                <literallayout class='monospaced'>
     # cat /proc/cpuinfo | grep processor 
     processor       : 0
     #
                </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 <filename>CONFIG_SMP</filename> Configuration Using <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-build-env</filename>
                script found in the Yocto Project files Git repository build directory.
                If you have not sourced this script do so with the following commands:
                <literallayout class='monospaced'>
     $ cd ~/poky
     $ source oe-init-build-env
                </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-3.0</filename>
                kernel.
                The Yocto Project build environment recognizes this kernel as 
                <filename>linux-yocto</filename>.
                Thus, the following command from the shell in which you previously sourced the 
                environment initialization script 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 "y" to select it.
                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 Linux Yocto kernel
                when it is built. 
                You can find and examine this file in the Yocto Project files Git repository in 
                the build directory. 
                This example uses the following.
                Note that this example directory is artificially split and many of the characters 
                in the actually filename are omitted in order to make it more
                readable:
                <literallayout class='monospaced'>
     ~/poky/build/tmp/work/qemux86-poky-linux/linux-yocto-2.6.37+git1+84f...
        ...r20/linux-qemux86-standard-build
                </literallayout>
            </para>

            <para>
                Within the <filename>.config</filename> file, you can see the following setting:
                <literallayout class='monospaced'>
     CONFIG_SMP=y
                </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 Yocto Project 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 commands below:
                <literallayout class='monospaced'>
     $ bitbake linux-yocto -c compile -f
     $ bitbake linux-yocto
                </literallayout>
            </para>
 
            <para>
                Now run the QEMU emulator:
                <literallayout class='monospaced'>
     $ runqemu qemux86 qemuparams="-smp 2"
                </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
     processor       : 1
     #
                </literallayout>
            </para>

            <para>
                From the output, you can see that 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> 



<!--        <section id='is-vfat-supported'>
            <title>Is VFAT Supported?</title>
            
            <para>
                <literallayout class='monospaced'>
I entered runqemu qemux86 and it fires upthis fires up the emulator and uses the
image and filesystem in the build area created in the previous section.

Then I copied over a pre-created and formated 5.2MB VFAT file named vfat.img.
I did this with scp vfat.img root@192.168.7.2:
The file is in the root directory.
I had to do this because the mkfs.vfat vfat.img command does not work.
mkfs is not recognized in the qemu terminal session.

when I try mount -o loop -t vfat vfat.img mnt/ I get the error
mount: can't set up loop device: No space left on device.
This error is because the loop module is not currently in the kernel image.
However, this module is available in the 
build area in the tarball modules-2.6.37.6-yocto-starndard+-20-qemux86.tgz.
You can add this to the kernel image by adding the 
IMAGE_INSTALL += " kernel-module-loop" statement at the top of the local.conf
file in the build area and then rebuilding the kernel using bitbake.
It should just build whatever is necessary and not go through an entire build again.




                The <filename>menuconfig</filename> tool provides an interactive method with which
                to set kernel configurations.
                In order to use <filename>menuconfig</filename> from within the BitBake environment
                you need to source an environment setup script.
                This script is located in the local Yocto Project file structure and is called
                <filename>oe-init-build-env</filename>.
            </para>

            <para>
                The following command sets up the environment:
                <literallayout class='monospaced'>
     $ cd ~/poky
     $ source oe-init-build-env
     $ runqemu qemux86
     Continuing with the following parameters:
     KERNEL: [/home/scottrif/poky/build/tmp/deploy/images/bzImage-qemux86.bin]
     ROOTFS: [/home/scottrif/poky/build/tmp/deploy/images/core-image-sato-qemux86.ext3]
     FSTYPE: [ext3]
     Setting up tap interface under sudo
     Acquiring lockfile for tap0...
     WARNING: distccd not present, no distcc support loaded.
     Running qemu...
     /home/scottrif/poky/build/tmp/sysroots/x86_64-linux/usr/bin/qemu 
        -kernel /home/scottrif/poky/build/tmp/deploy/images/bzImage-qemux86.bin 
        -net nic,vlan=0 -net tap,vlan=0,ifname=tap0,script=no,downscript=no 
        -hda /home/scottrif/poky/build/tmp/deploy/images/core-image-sato-qemux86.ext3 
        -show-cursor -usb -usbdevice wacom-tablet -vga vmware -enable-gl -no-reboot 
        -m 128 &dash;&dash;append "vga=0 root=/dev/hda rw mem=128M ip=192.168.7.2::192.168.7.1:255.255.255.0 oprofile.timer=1 "
     Enabling opengl
     vmsvga_value_write: guest runs Linux.
                </literallayout>
            </para>
        </section> 

        <section id='prepare-to-use-menuconfig'>
            <title>Prepare to use <filename>menuconfig</filename></title>
            

            <para>
                [WRITER'S NOTE: Stuff from here down are crib notes]
            </para>

            <para>
                Once menuconfig fires up you see all kinds of categories that you can interactively
                investigate.
                If they have an "M" in it then the feature is "modularized".
                I guess that means that means that it needs to be manually linked in when the 
                kernel is booted??? (Not sure).
                If they have an "*" then the feature is automatically part of the kernel.]
            </para>

            <para>
                So the tmp/work/ area was created in poky and there is a .config file in there and
                a .config.old file.  
                The old one must have been created when I exited from menuconfig after poking around 
                a bit.
                Nope - appears to just be created automatically.
            </para>

            <para>
                A good practice is to first determine what configurations you have for the kernel.
                You can see the results by looking in the .config file in the build/tmp/work/qemux86-poky-linux area
                of the local YP files. 
                There is a directory named linux-yocto-2.6.37* in the directory.
                In that directory is a directory named linux-qemux86-standard-build.
                In that directory you will find a file named .config that is the configuration file
                for the kernel that will be used when you build the kernel.
                You can open that file up and examine it. 
                If you do a search for "VFAT" you will see that that particular configuration is not 
                enabled for the kernel.
                This means that you cannot print a VFAT text file, or for that matter, even mount one
                from the image if you were to build it at this point.
            </para>

            <para>
                You can prove the point by actually trying it at this point.
                Here are the commands:
                <literallayout class='monospaced'>
     $ mkdir ~/vfat-test
     $ cd ~/vfat-test
     $ dd if=/dev/zero of=vfat.img bs=1024 count=5000  [creates a 5MB disk image]
     5+0 records in
     5+0 records out
     5242880 bytes (5.2 MB) copied, 0.00798912 s, 656 MB/s
     $ ls -lah   [lists the contents of the new image. l=long, a=all, h=human readable]
     total 5.1M
     drwxr-xr-x  2 srifenbark scottrif 4.0K 2011-08-01 08:18 .
     drwxr-xr-x 66 srifenbark scottrif 4.0K 2011-08-01 08:14 ..
     -rw-r&dash;&dash;r&dash;&dash;  1 srifenbark scottrif 5.0M 2011-08-01 08:18 vfat.img
     $ mkfs.vfat vfat.img                      [formats the disk image]
     mkfs.vfat 3.0.7 (24 Dec 2009)
     $ mkdir mnt                               [mounts the disk image]
     $ sudo su                                 [gives you root privilege]
     # mount -o loop vfat.img mnt              [mounts it as a loop device]
     # ls mnt                                  [shows nothing in mnt]
     # mount                                   [lists the mounted filesystems - note/dev/loop0]
     /dev/sda1 on / type ext4 (rw,errors=remount-ro)
     proc on /proc type proc (rw,noexec,nosuid,nodev)
     none on /sys type sysfs (rw,noexec,nosuid,nodev)
     none on /sys/fs/fuse/connections type fusectl (rw)
     none on /sys/kernel/debug type debugfs (rw)
     none on /sys/kernel/security type securityfs (rw)
     none on /dev type devtmpfs (rw,mode=0755)
     none on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=0620)
     none on /dev/shm type tmpfs (rw,nosuid,nodev)
     none on /var/run type tmpfs (rw,nosuid,mode=0755)
     none on /var/lock type tmpfs (rw,noexec,nosuid,nodev)
     none on /lib/init/rw type tmpfs (rw,nosuid,mode=0755)
     binfmt_misc on /proc/sys/fs/binfmt_misc type binfmt_misc (rw,noexec,nosuid,nodev)
     gvfs-fuse-daemon on /home/scottrif/.gvfs type fuse.gvfs-fuse-daemon (rw,nosuid,nodev,user=srifenbark)
     /dev/loop0 on /home/scottrif/vfat-test/mnt type vfat (rw)
     # echo "hello world" > mnt/hello.txt    [creates a text file in the mounted VFAT system]
     # ls mnt                                [verifies the file is there]
     hello.txt
     # cat mnt/hello.txt                     [displays the contents of the file created]
     hello world 
     # umount mnt                            [unmounts the system and destroys the loop]
     # exit                                  [gets out of privileged user mode]
     exit

     $ lsmod                                 [this stuff Darren did to show me ]
     Module                  Size  Used by   [the status of modules in the regular linux kernel]
     nls_iso8859_1           4633  0 
     nls_cp437               6351  0 
     vfat                   10866  0 
     fat                    55350  1 vfat
     snd_hda_codec_atihdmi     3023  1 
     binfmt_misc             7960  1 
     snd_hda_codec_realtek   279008  1 
     ppdev                   6375  0 
     snd_hda_intel          25805  2 
     fbcon                  39270  71 
     tileblit                2487  1 fbcon
     font                    8053  1 fbcon
     bitblit                 5811  1 fbcon
     snd_hda_codec          85759  3 snd_hda_codec_atihdmi,snd_hda_codec_realtek,snd_hda_intel
     softcursor              1565  1 bitblit
     snd_seq_dummy           1782  0 
     snd_hwdep               6924  1 snd_hda_codec
     vga16fb                12757  0 
     snd_pcm_oss            41394  0 
     snd_mixer_oss          16299  1 snd_pcm_oss
     snd_pcm                87946  3 snd_hda_intel,snd_hda_codec,snd_pcm_oss
     vgastate                9857  1 vga16fb
     snd_seq_oss            31191  0 
     snd_seq_midi            5829  0 
     snd_rawmidi            23420  1 snd_seq_midi
     radeon                744506  3 
     snd_seq_midi_event      7267  2 snd_seq_oss,snd_seq_midi
     ttm                    61007  1 radeon
     snd_seq                57481  6 snd_seq_dummy,snd_seq_oss,snd_seq_midi,snd_seq_midi_event
     drm_kms_helper         30742  1 radeon
     snd_timer              23649  2 snd_pcm,snd_seq
     snd_seq_device          6888  5 snd_seq_dummy,snd_seq_oss,snd_seq_midi,snd_rawmidi,snd_seq
     usb_storage            50377  0 
     snd                    71283  16 \
                                      snd_hda_codec_realtek,snd_hda_intel,snd_hda_codec, \
                                      snd_hwdep,snd_pcm_oss,snd_mixer_oss,snd_pcm, \
                                      snd_seq_oss,snd_rawmidi,snd_seq,snd_timer,snd_seq_device
     soundcore               8052  1 snd
     psmouse                65040  0 
     drm                   198886  5 radeon,ttm,drm_kms_helper
     i2c_algo_bit            6024  1 radeon
     serio_raw               4918  0 
     snd_page_alloc          8500  2 snd_hda_intel,snd_pcm
     dell_wmi                2177  0 
     dcdbas                  6886  0 
     lp                      9336  0 
     parport                37160  2 ppdev,lp
     usbhid                 41116  0 
     ohci1394               30260  0 
     hid                    83888  1 usbhid
     ieee1394               94771  1 ohci1394
     tg3                   122382  0
                </literallayout>
            </para>
        </section>
    </section> -->
</appendix>

<!--


EXTRA STUFF I MIGHT NEED BUT NOW SURE RIGHT NOW.

In the standard layer structure you have several areas that you need to examine or 
                    modify.
                    For this example the layer contains four areas:
                    <itemizedlist>
                        <listitem><para><emphasis><filename>conf</filename></emphasis> - Contains the 
                        <filename>layer.conf</filename> that identifies the location of the recipe files. 
                        </para></listitem>
                        <listitem><para><emphasis><filename>images</filename></emphasis> - Contains the 
                        image recipe file. 
                        This recipe includes the base image you will be using and specifies other 
                        packages the image might need.</para></listitem>
                        <listitem><para><emphasis><filename>recipes-bsp</filename></emphasis> - Contains 
                        recipes specific to the hardware for which you are developing the kernel.
                        </para></listitem>
                        <listitem><para><emphasis><filename>recipes-kernel</filename></emphasis> - Contains the 
                        "append" files that add information to the main recipe kernel.  
                        </para></listitem>
                    </itemizedlist>
                </para>

                <para>
                    Let's take a look at the <filename>layer.conf</filename> in the 
                    <filename>conf</filename> directory first.
                    This configuration file enables the Yocto Project build system to locate and 
                    use the information in your new layer.
                </para>

                <para>
                    The variable <filename>BBPATH</filename> needs to include the path to your layer
                    as follows:
                    <literallayout class='monospaced'>
     BBPATH := "${BBPATH}:${LAYERDIR}"
                    </literallayout>
                    And, the variable <filename>BBFILES</filename> needs to be modified to include your 
                    recipe and append files:
                    <literallayout class='monospaced'>
     BBFILES := "${BBFILES} ${LAYERDIR}/images/*.bb \ 
        ${LAYERDIR}/images/*.bbappend \
        ${LAYERDIR}/recipes-*/*/*.bb \
        ${LAYERDIR}/recipes-*/*/*.bbappend"
                    </literallayout>
                    Finally, you need to be sure to use your layer name in these variables at the 
                    end of the file:
                    <literallayout class='monospaced'>
     BBFILE_COLLECTIONS += "elc"
     BBFILE_PATTERN_elc := "^${LAYERDIR}/"
     BBFILE_PRIORITY_elc = "9"
                   </literallayout>
                </para>

                <para>
                    The <filename>images</filename> directory contains an append file that helps 
                    further define the image.
                    In our example, the base image is <filename>core-image-minimal</filename>.  
                    The image does, however, need some additional modules that we are using
                    for this example.
                    These modules support the amixer functionality.
                    Here is the append file:
                    <literallayout class='monospaced'>
     require recipes-core/images/poky-image-minimal.bb

     IMAGE_INSTALL += "dropbear alsa-utils-aplay alsa-utils-alsamixer"
     IMAGE_INSTALL_append_qemux86 += " kernel-module-snd-ens1370 \
        kernel-module-snd-rawmidi kernel-module-loop kernel-module-nls-cp437 \
        kernel-module-nls-iso8859-1 qemux86-audio alsa-utils-amixer"

     LICENSE = "MIT"
                   </literallayout>
               </para>

               <para>
                   While the focus of this example is not on the BSP, it is worth mentioning that the
                   <filename>recipes-bsp</filename> directory has the recipes and append files for 
                   features that the hardware requires. 
                   In this example, there is a script and a recipe to support the 
                   <filename>amixer</filename> functionality in QEMU.
                   It is beyond the scope of this manual to go too deeply into the script.
                   Suffice it to say that the script tests for the presence of the mixer, sets up 
                   default mixer values, enables the mixer, unmutes master and then 
                   sets the volume to 100.
               </para>

               <para>
                   The recipe <filename>qemu86-audio.bb</filename> installs and runs the 
                   <filename>amixer</filename> when the system boots.
                   Here is the recipe:
                   <literallayout class='monospaced'>
     SUMMARY = "Provide a basic init script to enable audio"
     DESCRIPTION = "Set the volume and unmute the Front mixer setting during boot."
     SECTION = "base"
     LICENSE = "MIT"
     LIC_FILES_CHKSUM = "file://${POKYBASE}/LICENSE;md5=3f40d7994397109285ec7b81fdeb3b58"

     PR = "r4"

     inherit update-rc.d

     RDEPENDS = "alsa-utils-amixer"

     SRC_URI = "file://qemux86-audio"

     INITSCRIPT_NAME = "qemux86-audio"
     INITSCRIPT_PARAMS = "defaults 90"

     do_install() {
	     install -d ${D}${sysconfdir} \
     	           ${D}${sysconfdir}/init.d
     	install -m 0755 ${WORKDIR}/qemux86-audio ${D}${sysconfdir}/init.d
             cat ${WORKDIR}/${INITSCRIPT_NAME} | \
                 sed -e 's,/etc,${sysconfdir},g' \
                     -e 's,/usr/sbin,${sbindir},g' \
                     -e 's,/var,${localstatedir},g' \
                     -e 's,/usr/bin,${bindir},g' \
                     -e 's,/usr,${prefix},g' > ${D}${sysconfdir}/init.d/${INITSCRIPT_NAME}
             chmod 755 ${D}${sysconfdir}/init.d/${INITSCRIPT_NAME}
     }
                </literallayout>
                </para>

                <para>
                    The last area to look at is <filename>recipes-kernel</filename>.
                    This area holds configuration fragments and kernel append files.
                    The append file must have the same name as the kernel recipe, which is 
                    <filename>linux-yocto-2.6.37</filename> in this example.
                    The file can <filename>SRC_URI</filename> statements to point to configuration 
                    fragments you might have in the layer. 
                    The file can also contain <filename>KERNEL_FEATURES</filename> statements that specify
                    included kernel configurations that ship with the Yocto Project.
                </para>
-->

<!--
vim: expandtab tw=80 ts=4
-->