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

<!DOCTYPE appendix PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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<appendix id='dev-manual-kernel-appendix'>

<title>Kernel Modification Example</title>

        <para>
            Kernel modification involves changing or adding configurations to an existing kernel, or 
            adding recipes to the kernel that are needed to support specific hardware features.  
            The process is similar to creating a Board Support Package (BSP) except that it does not
            involve a BSP layer.
        </para>

        <para>
            This section presents a brief overview of the kernel structure and then provides a simple 
            example that shows how to modify the kernel.
        </para>

        <section id='yocto-project-kernel'>
            <title>Yocto Project Kernel Overview</title>

            <para>
                When one thinks of the source files for a kernel they usually think of a fixed structure 
                of files that contain kernel patches.
                The Yocto Project, however, employs mechanisims that in a sense result in a kernel source
                generator.
            </para>

            <para>
                The Yocto Project uses the source code management (SCM) tool Git to manage and track Yocto 
                Project files.
                Git employs branching strategies that effectively produce a tree-like structure whose 
                branches represent diversions from more general code. 
                For example, suppose two kernels are basically identical with the exception of a couple
                different features in each.
                In the Yocto Project source repositories managed by Git a main branch can contain the 
                common or shared
                parts of the kernel source and two branches that diverge from that common branch can 
                each contain the features specific to the respective kernel.
                The result is a managed tree whose "leaves" represent the end of a specific path that yields
                a set of kernel source files necessary for a specific piece of hardware and its features.
            </para>
         
            <para>
                A big advantage to this scheme is the sharing of common features by keeping them in 
                "larger" branches that are further up the tree.  
                This practice eliminates redundant storage of similar features shared among kernels.
            </para>

            <para>
                When you build the kernel on your development system all files needed for the build
                are taken from the Yocto Project source repositories pointed to by the 
                <filename>SRC_URI</filename> variable and gathered in a temporary work area
                where they are subsequently used to create the unique kernel.
                Thus, in a sense, the process constructs a local source tree specific to your 
                kernel to generate the new kernel image - a source generator if you will.
            </para>

            <para>
                For a complete discussion of the Yocto Project kernel's architcture and its branching strategy,
                see the <ulink url='http://www.yoctoproject.org/docs/1.1/kernel-manual/kernel-manual.html'>
                The Yocto Project Kernel Architecture and Use Manual</ulink>.
            </para>

            <para>  
                You can find a web interface to the Yocto Project source repository at 
                <ulink url='http://git.yoctoproject.org/'></ulink>.  
                Within the interface you will see groups of related source code, each of which can 
                be cloned using Git to result in a working Git repository on your local system 
                (referred to as the "local Yocto Project files" in this manual).
                The Yocto Project supports four types of kernels in its source repositories at 
                <ulink url='http://git.yoctoproject.org/'></ulink>:
                <itemizedlist>
                    <listitem><para><emphasis><filename>linux-yocto-2.6.34</filename></emphasis> - The 
                    stable Linux Yocto kernel that is based on the Linux 2.6.34 release.</para></listitem>
                    <listitem><para><emphasis><filename>linux-yocto-2.6.37</filename></emphasis> - The current
                    Linux Yocto kernel that is based on the Linux 2.6.37 release.</para></listitem>
                    <listitem><para><emphasis><filename>linux-yocto-dev</filename></emphasis> - A development
                    kernel based on the Linux 2.6.39-rc1 release.</para></listitem>
                    <listitem><para><emphasis><filename>linux-2.6</filename></emphasis> - A kernel based on 
                    minimal Linux mainline tracking.
                    [WRITER'S NOTE: I don't know which Git repository the user needs to clone to get this
                    repository on their development system.]</para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='modifying-a-kernel-example'>
            <title>Modifying a Kernel Example</title>

            <para>
                This section presents a simple example that illustrates kernel modification
                based on the <filename>linux-yocto-2.6.37</filename> kernel.
                The example uses the audio and mixer capabilities supported by the 
                <ulink url='http://www.alsa-project.org/main/index.php/Main_Page'>Advanced Linux 
                Sound Architecture (ALSA) Project</ulink>.
                As the example progresses you will see how to do the following:
                <itemizedlist>
                    <listitem><para>Iteratively modify a base kernel locally.</para></listitem>      
                    <listitem><para>Provide a recipe-based solution for your modified kernel.
                    </para></listitem>
                    <listitem><para>Proved an "in-tree" solution for your modified kernel
                    (i.e. make the modifcations part of the Yocto Project).</para></listitem>
                </itemizedlist>
            </para>

            <para>
                The example flows as follows:
            </para>

            <para>
                <itemizedlist>
                    <listitem><para>Be sure your host development system is set up to support 
                    development using the Yocto Project. 
                    See  
                    <ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#the-linux-distro'>
                    The Linux Distributions</ulink> section and  
                    <ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#packages'>
                    The Packages</ulink> section both
                    in the Yocto Project Quick Start for requirements.
                    You will also need a release of Yocto Project installed on the host.</para></listitem>
                    <listitem><para>Set up your environment for optimal local kernel development.
                    </para></listitem>
                    <listitem><para>Create a layer to isolate your kernel work.</para></listitem>
                    <listitem><para>Next item.</para></listitem>
                    <listitem><para>Next item.</para></listitem>
                    <listitem><para>Next item.</para></listitem>
                    <listitem><para>Next item.</para></listitem>
                </itemizedlist>        
            </para>

            <section id='setting-up-yocto-project-kernel-example'>
                <title>Setting Up Yocto Project</title>

                <para>
                    You need to have the Yocto Project files available on your host system.  
                    The process is identical to that described in the 
                    <xref linkend='getting-setup'>"Getting Setup"</xref> section earlier in this 
                    manual.
                    Be sure to either set up a local Git repository for <filename>poky</filename>
                    or download and unpack the Yocto Project release tarball.  
                </para>
            </section>

            <section id='create-a-git-repository-of-poky-extras'>
                <title>Create a Git Repository of <filename>poky-extras</filename></title>

                <para>
                    Everytime you change a configuration or add a recipe to the kernel you need to 
                    do a fetch from the Linux Yocto kernel source repositories.
                    This can get tedious and time consuming if you need to fetch the entire 
                    Linux Yocto 2.6.37 Git repository down from the Internet everytime you make a change
                    to the kernel.  
                </para>

                <para>
                    You can get around this by setting up a <filename>meta-kernel-dev</filename>
                    area on your local system.  
                    This area contains "append" files for every kernel recipe, which also include
                    a <filename>KSRC</filename> statement that points to the kernel source files.
                    You can set up the environment so that the <filename>KSRC</filename> points to the
                    <filename>meta-kernel-dev</filename>, thus pulling source from a local area.
                    This setup can speed up development time. 
                 </para>

                 <para>
                    To get set up you need to do two things: create a local Git repository 
                    of the <filename>poky-extras</filename> repository, and create a bare clone of the 
                    Linux Yocto 2.6.37 kernel Git repository.
                 </para>

                 <para>
                    The following transcript shows how to clone the <filename>poky-extras</filename> 
                    Git repository into the current working directory, which is <filename>poky</filename>
                    in this example.  
                    The command creates the repository in a directory named <filename>poky-extras</filename>:
                    <literallayout class='monospaced'>
     $ git clone git://git.yoctoproject.org/poky-extras
     Initialized empty Git repository in /home/scottrif/poky/poky-extras/.git/
     remote: Counting objects: 532, done.
     remote: Compressing objects: 100% (472/472), done.
     remote: Total 532 (delta 138), reused 307 (delta 39)
     Receiving objects: 100% (532/532), 534.28 KiB | 362 KiB/s, done.
     Resolving deltas: 100% (138/138), done.
                    </literallayout>
                </para>

                <para>
                    This transcript shows how to clone a bare Git repository of the Linux Yocto 
                    2.6.37 kernel:
                    <literallayout class='monospaced'>
     $ git clone --bare git://git.yoctoproject.org/linux-yocto-2.6.37
     Initialized empty Git repository in /home/scottrif/linux-yocto-2.6.37.git/
     remote: Counting objects: 1886034, done.
     remote: Compressing objects: 100% (314326/314326), done.
     remote: Total 1886034 (delta 1570202), reused 1870335 (delta 1554798)
     Receiving objects: 100% (1886034/1886034), 401.51 MiB | 714 KiB/s, done.
     Resolving deltas: 100% (1570202/1570202), done.
                    </literallayout>
                </para>

                <para>
                    The bare clone of the Linux Yocto 2.6.37 kernel on your local system mirrors
                    the upstream repository of the kernel.
                    You can effectively point to this local clone now during development to avoid 
                    having to fetch the entire Linux Yocto 2.6.37 kernel every time you make a 
                    kernel change.
                </para>
            </section>

            <section id='create-a-layer-for-your-kernel-work'>
                <title>Create a Layer for Your Kernel Work</title>

                <para>
                    It is always good to isolate your work using your own layer.
                    Doing so allows you to experiment and easily start over should things go wrong. 
                    This example uses a layer named <filename>meta-amixer</filename>.
                </para>

                <para>
                    When you set up a layer for kernel work you should follow the general layout
                    guidelines as described for BSP layers. 
                    This layout is described in the
                    <ulink url='http://www.yoctoproject.org/docs/1.1/bsp-guide/bsp-guide.html#bsp-filelayout'>
                    Example Filesystem Layout</ulink> section of the Board Support Package (BSP) Development
                    Guide.  
                    In the standard layout you will notice a suggested structure for recipes and  
                    configuration information.  
                    [WRITER'S NOTE: The <filename>meta-elc</filename> example uses an 
                    <filename>images</filename> directory.
                    Currently, <filename>images</filename> is not part of the standard BSP layout.
                    I need to find out from Darren if this directory is required for kernel work.]
                </para>

                <para>
                    [WRITER'S NOTE:  I need a paragraph here describing how to set up the layer. 
                    I am not sure if you should copy an existing BSP layer and modify from there.
                    Or, if you should just look at a BSP layer and then create your own files.
                    Email to Darren on this but no answer yet.]
                </para>
            </section>

            <section id='making-changes-to-your-kernel-layer'>
                <title>Making Changes to Your Kernel Layer</title>
            
                <para>
                    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>
            </section>
        </section>

</appendix>
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