path: root/documentation/kernel-dev/kernel-dev-common.xml

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<chapter id='kernel-dev-common'>

<title>Common Tasks</title>

<para>
    This chapter presents several common tasks you perform when you
    work with the Yocto Project Linux kernel.
    These tasks include preparing a layer, modifying an existing recipe,
    iterative development, working with your own sources, and incorporating
    out-of-tree modules.
    <note>
        The examples presented in this chapter work with the Yocto Project
        1.2.2 Release and forward.
    </note>
</para>

    <section id='creating-and-preparing-a-layer'>
        <title>Creating and Preparing a Layer</title>

        <para>
            If you are going to be modifying kernel recipes, it is recommended
            that you create and prepare your own layer in which to do your
            work.
            Your layer contains its own
            <ulink url='&YOCTO_DOCS_DEV_URL;#bitbake-term'>BitBake</ulink>
            append files
            (<filename>.bbappend</filename>) and provides a convenient
            mechanism to create your own recipe files
            (<filename>.bb</filename>).
            For details on how to create and work with layers, see the following
            sections in the Yocto Project Development Manual:
            <itemizedlist>
                <listitem><para>"<ulink url='&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers'>Understanding and Creating Layers</ulink>" for
                    general information on layers and how to create layers.</para></listitem>
                <listitem><para>"<ulink url='&YOCTO_DOCS_DEV_URL;#set-up-your-layer-for-the-build'>Set Up Your Layer for the Build</ulink>" for
                    specific instructions on setting up a layer for kernel
                    development.</para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='modifying-an-existing-recipe'>
        <title>Modifying an Existing Recipe</title>

        <para>
            In many cases, you can customize an existing linux-yocto recipe to
            meet the needs of your project.
            Each release of the Yocto Project provides a few Linux
            kernel recipes from which you can choose.
            These are located in the
            <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>
            in <filename>meta/recipes-kernel/linux</filename>.
        </para>

        <para>
            Modifying an existing recipe can consist of the following:
            <itemizedlist>
                <listitem><para>Creating the append file</para></listitem>
                <listitem><para>Applying patches</para></listitem>
                <listitem><para>Changing the configuration</para></listitem>
            </itemizedlist>
        </para>

        <para>
            Before modifying an existing recipe, be sure that you have created
            a minimal, custom layer from which you can work.
            See the "<link linkend='creating-and-preparing-a-layer'>Creating and Preparing a Layer</link>"
            section for some general resources.
            You can also see the
            "<ulink url='&YOCTO_DOCS_DEV_URL;#set-up-your-layer-for-the-build'>Set Up Your Layer for the Build</ulink>" section
            of the Yocto Project Development Manual for a detailed
            example.
        </para>

        <section id='creating-the-append-file'>
            <title>Creating the Append File</title>

            <para>
                You create this file in your custom layer.
                You also name it accordingly based on the linux-yocto recipe
                you are using.
                For example, if you are modifying the
                <filename>meta/recipes-kernel/linux/linux-yocto_3.19.bb</filename>
                recipe, the append file will typically be located as follows
                within your custom layer:
                <literallayout class='monospaced'>
     <replaceable>your-layer</replaceable>/recipes-kernel/linux/linux-yocto_3.19.bbappend
                </literallayout>
                The append file should initially extend the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESPATH'><filename>FILESPATH</filename></ulink>
                search path by prepending the directory that contains your
                files to the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>
                variable as follows:
                <literallayout class='monospaced'>
     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
                </literallayout>
                The path <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-THISDIR'><filename>THISDIR</filename></ulink><filename>}/${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink><filename>}</filename>
                expands to "linux-yocto" in the current directory for this
                example.
                If you add any new files that modify the kernel recipe and you
                have extended <filename>FILESPATH</filename> as
                described above, you must place the files in your layer in the
                following area:
                <literallayout class='monospaced'>
     <replaceable>your-layer</replaceable>/recipes-kernel/linux/linux-yocto/
                </literallayout>
                <note>If you are working on a new machine Board Support Package
                    (BSP), be sure to refer to the
                    <ulink url='&YOCTO_DOCS_BSP_URL;'>Yocto Project Board Support Package (BSP) Developer's Guide</ulink>.
                </note>
            </para>
        </section>

        <section id='applying-patches'>
            <title>Applying Patches</title>

            <para>
                If you have a single patch or a small series of patches
                that you want to apply to the Linux kernel source, you
                can do so just as you would with any other recipe.
                You first copy the patches to the path added to
                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>
                in your <filename>.bbappend</filename> file as described in
                the previous section, and then reference them in
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                statements.
            </para>

            <para>
                For example, you can apply a three-patch series by adding the
                following lines to your linux-yocto
                <filename>.bbappend</filename> file in your layer:
                <literallayout class='monospaced'>
     SRC_URI += "file://0001-first-change.patch"
     SRC_URI += "file://0002-second-change.patch"
     SRC_URI += "file://0003-third-change.patch"
                </literallayout>
                The next time you run BitBake to build the Linux kernel,
                BitBake detects the change in the recipe and fetches and
                applies the patches before building the kernel.
            </para>

            <para>
                For a detailed example showing how to patch the kernel, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#patching-the-kernel'>Patching the Kernel</ulink>"
                section in the Yocto Project Development Manual.
            </para>
        </section>

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

            <para>
                You can make wholesale or incremental changes to the final
                <filename>.config</filename> file used for the eventual
                Linux kernel configuration by including a
                <filename>defconfig</filename> file and by specifying
                configuration fragments in the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                to be applied to that file.
            </para>

            <para>
                If you have a complete, working Linux kernel
                <filename>.config</filename>
                file you want to use for the configuration, as before, copy
                that file to the appropriate <filename>${PN}</filename>
                directory in your layer's
                <filename>recipes-kernel/linux</filename> directory,
                and rename the copied file to "defconfig".
                Then, add the following lines to the linux-yocto
                <filename>.bbappend</filename> file in your layer:
                <literallayout class='monospaced'>
     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
     SRC_URI += "file://defconfig"
                </literallayout>
                The <filename>SRC_URI</filename> tells the build system how to
                search for the file, while the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>
                extends the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESPATH'><filename>FILESPATH</filename></ulink>
                variable (search directories) to include the
                <filename>${PN}</filename> directory you created to hold the
                configuration changes.
            </para>

            <note>
                The build system applies the configurations from the
                <filename>defconfig</filename> file before applying any
                subsequent configuration fragments.
                The final kernel configuration is a combination of the
                configurations in the <filename>defconfig</filename> file and
                any configuration fragments you provide.
                You need to realize that if you have any configuration
                fragments, the build system applies these on top of and
                after applying the existing <filename>defconfig</filename>
                file configurations.
            </note>

            <para>
                Generally speaking, the preferred approach is to determine the
                incremental change you want to make and add that as a
                configuration fragment.
                For example, if you want to add support for a basic serial
                console, create a file named <filename>8250.cfg</filename> in
                the <filename>${PN}</filename> directory with the following
                content (without indentation):
                <literallayout class='monospaced'>
     CONFIG_SERIAL_8250=y
     CONFIG_SERIAL_8250_CONSOLE=y
     CONFIG_SERIAL_8250_PCI=y
     CONFIG_SERIAL_8250_NR_UARTS=4
     CONFIG_SERIAL_8250_RUNTIME_UARTS=4
     CONFIG_SERIAL_CORE=y
     CONFIG_SERIAL_CORE_CONSOLE=y
                </literallayout>
                Next, include this configuration fragment and extend the
                <filename>FILESPATH</filename> variable in your
                <filename>.bbappend</filename> file:
                <literallayout class='monospaced'>
     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
     SRC_URI += "file://8250.cfg"
                </literallayout>
                The next time you run BitBake to build the Linux kernel, BitBake
                detects the change in the recipe and fetches and applies the
                new configuration before building the kernel.
            </para>

            <para>
                For a detailed example showing how to configure the kernel,
                see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#configuring-the-kernel'>Configuring the Kernel</ulink>"
                section in the Yocto Project Development Manual.
            </para>
        </section>

        <section id='using-an-in-tree-defconfig-file'>
            <title>Using an "In-Tree"&nbsp;&nbsp;<filename>defconfig</filename> File</title>

            <para>
                It might be desirable to have kernel configuration fragment
                support through a <filename>defconfig</filename> file that
                is pulled from the kernel source tree for the configured
                machine.
                By default, the OpenEmbedded build system looks for
                <filename>defconfig</filename> files in the layer used for
                Metadata, which is "out-of-tree", and then configures them
                using the following:
                <literallayout class='monospaced'>
     SRC_URI += "file://defconfig"
                </literallayout>
                If you do not want to maintain copies of
                <filename>defconfig</filename> files in your layer but would
                rather allow users to use the default configuration from the
                kernel tree and still be able to add configuration fragments
                to the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                through, for example, append files, you can direct the
                OpenEmbedded build system to use a
                <filename>defconfig</filename> file that is "in-tree".
            </para>

            <para>
                To specify an "in-tree" <filename>defconfig</filename> file,
                edit the recipe that builds your kernel so that it has the
                following command form:
                <literallayout class='monospaced'>
     KBUILD_DEFCONFIG_<ulink url='&YOCTO_DOCS_REF_URL;#var-KMACHINE'>KMACHINE</ulink> ?= <replaceable>defconfig_file</replaceable>
                </literallayout>
                You need to append the variable with
                <filename>KMACHINE</filename> and then supply the path to
                your "in-tree" <filename>defconfig</filename> file.
            </para>

            <para>
                Aside from modifying your kernel recipe and providing your own
                <filename>defconfig</filename> file, you need to be sure no
                files or statements set <filename>SRC_URI</filename> to use a
                <filename>defconfig</filename> other than your "in-tree"
                file (e.g. a kernel's <filename>linux-</filename><replaceable>machine</replaceable><filename>.inc</filename>
                file).
                In other words, if the build system detects a statement
                that identifies an "out-of-tree"
                <filename>defconfig</filename> file, that statement
                will override your
                <filename>KBUILD_DEFCONFIG</filename> variable.
            </para>

            <para>
                See the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-KBUILD_DEFCONFIG'><filename>KBUILD_DEFCONFIG</filename></ulink>
                variable description for more information.
            </para>
        </section>
    </section>

    <section id='using-an-iterative-development-process'>
        <title>Using an Iterative Development Process</title>

        <para>
            If you do not have existing patches or configuration files,
            you can iteratively generate them from within the BitBake build
            environment as described within this section.
            During an iterative workflow, running a previously completed BitBake
            task causes BitBake to invalidate the tasks that follow the
            completed task in the build sequence.
            Invalidated tasks rebuild the next time you run the build using
            BitBake.
        </para>

        <para>
            As you read this section, be sure to substitute the name
            of your Linux kernel recipe for the term
            "linux-yocto".
        </para>

        <section id='tip-dirty-string'>
            <title>"-dirty" String</title>

<!--
            <para>
                <emphasis>AR - Darren Hart:</emphasis>  This section
                originated from the old Yocto Project Kernel Architecture
                and Use Manual.
                It was decided we need to put it in this section here.
                Darren needs to figure out where we want it and what part
                of it we want (all, revision???)
            </para>
-->

            <para>
                If kernel images are being built with "-dirty" on the
                end of the version string, this simply means that
                modifications in the source directory have not been committed.
                <literallayout class='monospaced'>
     $ git status
                </literallayout>
            </para>

            <para>
                You can use the above Git command to report modified,
                removed, or added files.
                You should commit those changes to the tree regardless of
                whether they will be saved, exported, or used.
                Once you commit the changes, you need to rebuild the kernel.
            </para>

            <para>
                To force a pickup and commit of all such pending changes,
                enter the following:
                <literallayout class='monospaced'>
     $ git add .
     $ git commit -s -a -m "getting rid of -dirty"
                </literallayout>
            </para>

            <para>
                Next, rebuild the kernel.
            </para>
        </section>

        <section id='generating-configuration-files'>
            <title>Generating Configuration Files</title>

            <para>
                You can manipulate the <filename>.config</filename> file
                used to build a linux-yocto recipe with the
                <filename>menuconfig</filename> command as follows:
                <literallayout class='monospaced'>
     $ bitbake linux-yocto -c menuconfig
                </literallayout>
                This command starts the Linux kernel configuration tool,
                which allows you to prepare a new
                <filename>.config</filename> file for the build.
                When you exit the tool, be sure to save your changes
                at the prompt.
            </para>

            <para>
                The resulting <filename>.config</filename> file is
                located in
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink><filename>}</filename> under the
                <filename>linux-${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGE_ARCH'><filename>PACKAGE_ARCH</filename></ulink><filename>}-${<ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_KERNEL_TYPE'><filename>LINUX_KERNEL_TYPE</filename></ulink>}-build</filename> directory.
                You can use the entire <filename>.config</filename> file as the
                <filename>defconfig</filename> file as described in the
                "<link linkend='changing-the-configuration'>Changing the Configuration</link>" section.
                For more information on the <filename>.config</filename> file,
                see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#using-menuconfig'>Using <filename>menuconfig</filename></ulink>"
                section in the Yocto Project Development Manual.
            </para>

            <para>
                A better method is to create a configuration fragment using the
                differences between two configuration files: one previously
                created and saved, and one freshly created using the
                <filename>menuconfig</filename> tool.
            </para>

            <para>
                To create a configuration fragment using this method, follow
                these steps:
                <orderedlist>
                    <listitem><para>Complete a build at least through the kernel
                        configuration task as follows:
                        <literallayout class='monospaced'>
     $ bitbake linux-yocto -c kernel_configme -f
                        </literallayout>
                        This step ensures that you will be creating a
                        <filename>.config</filename> file from a known state.
                        Because situations exist where your build state might
                        become unknown, it is best to run the previous
                        command prior to starting up
                        <filename>menuconfig</filename>.
                        </para></listitem>
                    <listitem><para>Run the <filename>menuconfig</filename>
                        command:
                        <literallayout class='monospaced'>
     $ bitbake linux-yocto -c menuconfig
                        </literallayout></para></listitem>
                    <listitem><para>Run the <filename>diffconfig</filename>
                        command to prepare a configuration fragment.
                        The resulting file <filename>fragment.cfg</filename>
                        will be placed in the
                        <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink><filename>}</filename> directory:
                        <literallayout class='monospaced'>
     $ bitbake linux-yocto -c diffconfig
                        </literallayout></para></listitem>
                </orderedlist>
            </para>

            <para>
                The <filename>diffconfig</filename> command creates a file that is a
                list of Linux kernel <filename>CONFIG_</filename> assignments.
                See the "<link linkend='changing-the-configuration'>Changing the Configuration</link>"
                section for information on how to use the output as a
                configuration fragment.
                <note>
                    You can also use this method to create configuration
                    fragments for a BSP.
                    See the "<link linkend='bsp-descriptions'>BSP Descriptions</link>"
                    section for more information.
                </note>
            </para>

            <para>
                The kernel tools also provide configuration validation.
                You can use these tools to produce warnings for when a
                requested configuration does not appear in the final
                <filename>.config</filename> file or when you override a
                policy configuration in a hardware configuration fragment.
                Here is an example with some sample output of the command
                that runs these tools:
                <literallayout class='monospaced'>
     $ bitbake linux-yocto -c kernel_configcheck -f

     ...

     NOTE: validating kernel configuration
     This BSP sets 3 invalid/obsolete kernel options.
     These config options are not offered anywhere within this kernel.
     The full list can be found in your kernel src dir at:
     meta/cfg/standard/mybsp/invalid.cfg

     This BSP sets 21 kernel options that are possibly non-hardware related.
     The full list can be found in your kernel src dir at:
     meta/cfg/standard/mybsp/specified_non_hdw.cfg

     WARNING: There were 2 hardware options requested that do not
              have a corresponding value present in the final ".config" file.
              This probably means you are not getting the config you wanted.
              The full list can be found in your kernel src dir at:
              meta/cfg/standard/mybsp/mismatch.cfg
                </literallayout>
            </para>

            <para>
                The output describes the various problems that you can
                encounter along with where to find the offending configuration
                items.
                You can use the information in the logs to adjust your
                configuration files and then repeat the
                <filename>kernel_configme</filename> and
                <filename>kernel_configcheck</filename> commands until
                they produce no warnings.
            </para>

            <para>
                For more information on how to use the
                <filename>menuconfig</filename> tool, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#using-menuconfig'>Using <filename>menuconfig</filename></ulink>"
                section in the Yocto Project Development Manual.
            </para>
        </section>

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

            <para>
                You can experiment with source code changes and create a
                simple patch without leaving the BitBake environment.
                To get started, be sure to complete a build at
                least through the kernel configuration task:
                <literallayout class='monospaced'>
     $ bitbake linux-yocto -c kernel_configme -f
                </literallayout>
                Taking this step ensures you have the sources prepared
                and the configuration completed.
                You can find the sources in the
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink><filename>}/linux</filename> directory.
            </para>

            <para>
                You can edit the sources as you would any other Linux source
                tree.
                However, keep in mind that you will lose changes if you
                trigger the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-fetch'><filename>do_fetch</filename></ulink>
                task for the recipe.
                You can avoid triggering this task by not using BitBake to
                run the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-cleanall'><filename>cleanall</filename></ulink>,
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-cleansstate'><filename>cleansstate</filename></ulink>,
                or forced
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-fetch'><filename>fetch</filename></ulink>
                commands.
                Also, do not modify the recipe itself while working
                with temporary changes or BitBake might run the
                <filename>fetch</filename> command depending on the
                changes to the recipe.
            </para>

            <para>
                To test your temporary changes, instruct BitBake to run the
                <filename>compile</filename> again.
                The <filename>-f</filename> option forces the command to run
                even though BitBake might think it has already done so:
                <literallayout class='monospaced'>
     $ bitbake linux-yocto -c compile -f
                </literallayout>
                If the compile fails, you can update the sources and repeat
                the <filename>compile</filename>.
                Once compilation is successful, you can inspect and test
                the resulting build (i.e. kernel, modules, and so forth) from
                the following build directory:
                <literallayout class='monospaced'>
     ${WORKDIR}/linux-${PACKAGE_ARCH}-${LINUX_KERNEL_TYPE}-build
                </literallayout>
                Alternatively, you can run the <filename>deploy</filename>
                command to place the kernel image in the
                <filename>tmp/deploy/images</filename> directory:
                <literallayout class='monospaced'>
	$ bitbake linux-yocto -c deploy
                </literallayout>
                And, of course, you can perform the remaining installation and
                packaging steps by issuing:
                <literallayout class='monospaced'>
	$ bitbake linux-yocto
                </literallayout>
            </para>

            <para>
                For rapid iterative development, the edit-compile-repeat loop
                described in this section is preferable to rebuilding the
                entire recipe because the installation and packaging tasks
                are very time consuming.
            </para>

            <para>
                Once you are satisfied with your source code modifications,
                you can make them permanent by generating patches and
                applying them to the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                statement as described in the
                "<link linkend='applying-patches'>Applying Patches</link>"
                section.
                If you are not familiar with generating patches, refer to the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#creating-the-patch'>Creating the Patch</ulink>"
                section in the Yocto Project Development Manual.
            </para>
        </section>
    </section>

    <section id='working-with-your-own-sources'>
        <title>Working With Your Own Sources</title>

        <para>
            If you cannot work with one of the Linux kernel
            versions supported by existing linux-yocto recipes, you can
            still make use of the Yocto Project Linux kernel tooling by
            working with your own sources.
            When you use your own sources, you will not be able to
            leverage the existing kernel
            <ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink> and
            stabilization work of the linux-yocto sources.
            However, you will be able to manage your own Metadata in the same
            format as the linux-yocto sources.
            Maintaining format compatibility facilitates converging with
            linux-yocto on a future, mutually-supported kernel version.
        </para>

        <para>
            To help you use your own sources, the Yocto Project provides a
            linux-yocto custom recipe
            (<filename>linux-yocto-custom.bb</filename>) that uses
            <filename>kernel.org</filename> sources
            and the Yocto Project Linux kernel tools for managing
            kernel Metadata.
            You can find this recipe in the
            <filename>poky</filename> Git repository of the
            Yocto Project <ulink url='&YOCTO_GIT_URL;'>Source Repository</ulink>
            at:
            <literallayout class="monospaced">
     poky/meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb
            </literallayout>
        </para>

        <para>
            Here are some basic steps you can use to work with your own sources:
            <orderedlist>
                <listitem><para>Copy the <filename>linux-yocto-custom.bb</filename>
                    recipe to your layer and give it a meaningful name.
                    The name should include the version of the Linux kernel you
                    are using (e.g.
                    <filename>linux-yocto-myproject_3.19.bb</filename>,
                    where "3.19" is the base version of the Linux kernel
                    with which you would be working).</para></listitem>
                <listitem><para>In the same directory inside your layer,
                    create a matching directory
                    to store your patches and configuration files (e.g.
                    <filename>linux-yocto-myproject</filename>).
                    </para></listitem>
                <listitem><para>Make sure you have either a
                    <filename>defconfig</filename> file or configuration
                    fragment files.
                    When you use the <filename>linux-yocto-custom.bb</filename>
                    recipe, you must specify a configuration.
                    If you do not have a <filename>defconfig</filename> file,
                    you can run the following:
                    <literallayout class='monospaced'>
     $ make defconfig
                    </literallayout>
                    After running the command, copy the resulting
                    <filename>.config</filename> to the
                    <filename>files</filename> directory as "defconfig" and
                    then add it to the
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                    variable in the recipe.</para>
                    <para>Running the <filename>make defconfig</filename>
                    command results in the default configuration for your
                    architecture as defined by your kernel.
                    However, no guarantee exists that this configuration is
                    valid for your use case, or that your board will even boot.
                    This is particularly true for non-x86 architectures.
                    To use non-x86 <filename>defconfig</filename> files, you
                    need to be more specific and find one that matches your
                    board (i.e. for arm, you look in
                    <filename>arch/arm/configs</filename> and use the one that
                    is the best starting point for your board).
                    </para></listitem>
                <listitem><para>Edit the following variables in your recipe
                    as appropriate for your project:
                    <itemizedlist>
                        <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>:
                            The <filename>SRC_URI</filename> should specify
                            a Git repository that uses one of the supported Git
                            fetcher protocols (i.e. <filename>file</filename>,
                            <filename>git</filename>, <filename>http</filename>,
                            and so forth).
                            The <filename>SRC_URI</filename> variable should
                            also specify either a <filename>defconfig</filename>
                            file or some configuration fragment files.
                            The skeleton recipe provides an example
                            <filename>SRC_URI</filename> as a syntax reference.
                            </para></listitem>
                        <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_VERSION'><filename>LINUX_VERSION</filename></ulink>:
                            The Linux kernel version you are using (e.g.
                            "3.19").</para></listitem>
                        <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_VERSION_EXTENSION'><filename>LINUX_VERSION_EXTENSION</filename></ulink>:
                            The Linux kernel <filename>CONFIG_LOCALVERSION</filename>
                            that is compiled into the resulting kernel and visible
                            through the <filename>uname</filename> command.
                            </para></listitem>
                        <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-SRCREV'><filename>SRCREV</filename></ulink>:
                            The commit ID from which you want to build.
                            </para></listitem>
                        <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-PR'><filename>PR</filename></ulink>:
                            Treat this variable the same as you would in any other
                            recipe.
                            Increment the variable to indicate to the OpenEmbedded
                            build system that the recipe has changed.
                            </para></listitem>
                        <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>:
                            The default <filename>PV</filename> assignment is
                            typically adequate.
                            It combines the <filename>LINUX_VERSION</filename>
                            with the Source Control Manager (SCM) revision
                            as derived from the
                            <ulink url='&YOCTO_DOCS_REF_URL;#var-SRCPV'><filename>SRCPV</filename></ulink>
                            variable.
                            The combined results are a string with
                            the following form:
                            <literallayout class='monospaced'>
     3.19.11+git1+68a635bf8dfb64b02263c1ac80c948647cc76d5f_1+218bd8d2022b9852c60d32f0d770931e3cf343e2
                            </literallayout>
                            While lengthy, the extra verbosity in <filename>PV</filename>
                            helps ensure you are using the exact
                            sources from which you intend to build.
                            </para></listitem>
                        <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-COMPATIBLE_MACHINE'><filename>COMPATIBLE_MACHINE</filename></ulink>:
                            A list of the machines supported by your new recipe.
                            This variable in the example recipe is set
                            by default to a regular expression that matches
                            only the empty string, "(^$)".
                            This default setting triggers an explicit build
                            failure.
                            You must change it to match a list of the machines
                            that your new recipe supports.
                            For example, to support the <filename>qemux86</filename>
                            and <filename>qemux86-64</filename> machines, use
                            the following form:
                            <literallayout class='monospaced'>
     COMPATIBLE_MACHINE = "qemux86|qemux86-64"
                            </literallayout></para></listitem>
                    </itemizedlist></para></listitem>
                <listitem><para>Provide further customizations to your recipe
                    as needed just as you would customize an existing
                    linux-yocto recipe.
                    See the "<link linkend='modifying-an-existing-recipe'>Modifying
                    an Existing Recipe</link>" section for information.
                    </para></listitem>
            </orderedlist>
        </para>
    </section>

    <section id='working-with-out-of-tree-modules'>
        <title>Working with Out-of-Tree Modules</title>

        <para>
            This section describes steps to build out-of-tree modules on
            your target and describes how to incorporate out-of-tree modules
            in the build.
        </para>

        <section id='building-out-of-tree-modules-on-the-target'>
            <title>Building Out-of-Tree Modules on the Target</title>

            <para>
                While the traditional Yocto Project development model would be
                to include kernel modules as part of the normal build
                process, you might find it useful to build modules on the
                target.
                This could be the case if your target system is capable
                and powerful enough to handle the necessary compilation.
                Before deciding to build on your target, however, you should
                consider the benefits of using a proper cross-development
                environment from your build host.
            </para>

            <para>
                If you want to be able to build out-of-tree modules on
                the target, there are some steps you need to take
                on the target that is running your SDK image.
                Briefly, the <filename>kernel-dev</filename> package
                is installed by default on all
                <filename>*.sdk</filename> images and the
                <filename>kernel-devsrc</filename> package is installed
                on many of the <filename>*.sdk</filename> images.
                However, you need to create some scripts prior to
                attempting to build the out-of-tree modules on the target
                that is running that image.
            </para>

            <para>
                Prior to attempting to build the out-of-tree modules,
                you need to be on the target as root and you need to
                change to the <filename>/usr/src/kernel</filename> directory.
                Next, <filename>make</filename> the scripts:
                <literallayout class='monospaced'>
     # cd /usr/src/kernel
     # make scripts
                </literallayout>
                Because all SDK image recipes include
                <filename>dev-pkgs</filename>, the
                <filename>kernel-dev</filename> packages will be installed
                as part of the SDK image and the
                <filename>kernel-devsrc</filename> packages will be installed
                as part of applicable SDK images.
                The SDK uses the scripts when building out-of-tree
                modules.
                Once you have switched to that directory and created the
                scripts, you should be able to build your out-of-tree modules
                on the target.
            </para>
        </section>

        <section id='incorporating-out-of-tree-modules'>
            <title>Incorporating Out-of-Tree Modules</title>

            <para>
                While it is always preferable to work with sources integrated
                into the Linux kernel sources, if you need an external kernel
                module, the <filename>hello-mod.bb</filename> recipe is
                available as a template from which you can create your
                own out-of-tree Linux kernel module recipe.
            </para>

            <para>
                This template recipe is located in the
                <filename>poky</filename> Git repository of the
                Yocto Project <ulink url='&YOCTO_GIT_URL;'>Source Repository</ulink>
                at:
                <literallayout class="monospaced">
     poky/meta-skeleton/recipes-kernel/hello-mod/hello-mod_0.1.bb
                </literallayout>
            </para>

            <para>
                To get started, copy this recipe to your layer and give it a
                meaningful name (e.g. <filename>mymodule_1.0.bb</filename>).
                In the same directory, create a new directory named
                <filename>files</filename> where you can store any source files,
                patches, or other files necessary for building
                the module that do not come with the sources.
                Finally, update the recipe as needed for the module.
                Typically, you will need to set the following variables:
                <itemizedlist>
                    <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-DESCRIPTION'><filename>DESCRIPTION</filename></ulink>
                        </para></listitem>
                    <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-LICENSE'><filename>LICENSE*</filename></ulink>
                        </para></listitem>
                    <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                        </para></listitem>
                    <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>
                        </para></listitem>
                </itemizedlist>
            </para>

            <para>
                Depending on the build system used by the module sources,
                you might need to make some adjustments.
                For example, a typical module <filename>Makefile</filename>
                looks much like the one provided with the
                <filename>hello-mod</filename> template:
                <literallayout class='monospaced'>
     obj-m := hello.o

     SRC := $(shell pwd)

     all:
         $(MAKE) -C $(KERNEL_SRC) M=$(SRC)

     modules_install:
         $(MAKE) -C $(KERNEL_SRC) M=$(SRC) modules_install
     ...
                </literallayout>
            </para>

            <para>
                The important point to note here is the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_SRC'><filename>KERNEL_SRC</filename></ulink>
                variable.
                The
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-module'><filename>module</filename></ulink>
                class sets this variable and the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_PATH'><filename>KERNEL_PATH</filename></ulink>
                variable to
                <filename>${<ulink url='&YOCTO_DOCS_REF_URL;#var-STAGING_KERNEL_DIR'><filename>STAGING_KERNEL_DIR</filename></ulink>}</filename>
                with the necessary Linux kernel build information to build
                modules.
                If your module <filename>Makefile</filename> uses a different
                variable, you might want to override the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-compile'><filename>do_compile()</filename></ulink>
                step, or create a patch to
                the <filename>Makefile</filename> to work with the more typical
                <filename>KERNEL_SRC</filename> or
                <filename>KERNEL_PATH</filename> variables.
            </para>

            <para>
                After you have prepared your recipe, you will likely want to
                include the module in your images.
                To do this, see the documentation for the following variables in
                the Yocto Project Reference Manual and set one of them
                appropriately for your machine configuration file:
                <itemizedlist>
                    <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_ESSENTIAL_EXTRA_RDEPENDS'><filename>MACHINE_ESSENTIAL_EXTRA_RDEPENDS</filename></ulink>
                        </para></listitem>
                    <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS'><filename>MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS</filename></ulink>
                        </para></listitem>
                    <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_EXTRA_RDEPENDS'><filename>MACHINE_EXTRA_RDEPENDS</filename></ulink>
                        </para></listitem>
                    <listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_EXTRA_RRECOMMENDS'><filename>MACHINE_EXTRA_RRECOMMENDS</filename></ulink>
                        </para></listitem>
                </itemizedlist>
            </para>

            <para>
                Modules are often not required for boot and can be excluded from
                certain build configurations.
                The following allows for the most flexibility:
                <literallayout class='monospaced'>
     MACHINE_EXTRA_RRECOMMENDS += "kernel-module-mymodule"
                </literallayout>
                The value is derived by appending the module filename without
                the <filename>.ko</filename> extension to the string
                "kernel-module-".
            </para>

            <para>
                Because the variable is
                <ulink url='&YOCTO_DOCS_REF_URL;#var-RRECOMMENDS'><filename>RRECOMMENDS</filename></ulink>
                and not a
                <ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'><filename>RDEPENDS</filename></ulink>
                variable, the build will not fail if this module is not
                available to include in the image.
            </para>
        </section>
    </section>


    <section id='inspecting-changes-and-commits'>
        <title>Inspecting Changes and Commits</title>

        <para>
            A common question when working with a kernel is:
            "What changes have been applied to this tree?"
            Rather than using "grep" across directories to see what has
            changed, you can use Git to inspect or search the kernel tree.
            Using Git is an efficient way to see what has changed in the tree.
        </para>

        <section id='what-changed-in-a-kernel'>
            <title>What Changed in a Kernel?</title>

            <para>
                Following are a few examples that show how to use Git
                commands to examine changes.
                These examples are by no means the only way to see changes.
                <note>
                    In the following examples, unless you provide a commit
                    range, <filename>kernel.org</filename> history is blended
                    with Yocto Project kernel changes.
                    You can form ranges by using branch names from the
                    kernel tree as the upper and lower commit markers with
                    the Git commands.
                    You can see the branch names through the web interface
                    to the Yocto Project source repositories at
                    <ulink url='http://git.yoctoproject.org/cgit.cgi'></ulink>.
                </note>
                To see a full range of the changes, use the
                <filename>git whatchanged</filename> command and specify a
                commit range for the branch
                (<replaceable>commit</replaceable><filename>..</filename><replaceable>commit</replaceable>).
            </para>

            <para>
                Here is an example that looks at what has changed in the
                <filename>emenlow</filename> branch of the
                <filename>linux-yocto-3.19</filename> kernel.
                The lower commit range is the commit associated with the
                <filename>standard/base</filename> branch, while
                the upper commit range is the commit associated with the
                <filename>standard/emenlow</filename> branch.
                <literallayout class='monospaced'>
     $ git whatchanged origin/standard/base..origin/standard/emenlow
                </literallayout>
            </para>

            <para>
                To see short, one line summaries of changes use the
                <filename>git log</filename> command:
                <literallayout class='monospaced'>
     $ git log --oneline origin/standard/base..origin/standard/emenlow
                </literallayout>
            </para>

            <para>
                Use this command to see code differences for the changes:
                <literallayout class='monospaced'>
     $ git diff origin/standard/base..origin/standard/emenlow
                </literallayout>
            </para>

            <para>
                Use this command to see the commit log messages and the
                text differences:
                <literallayout class='monospaced'>
     $ git show origin/standard/base..origin/standard/emenlow
                </literallayout>
            </para>

            <para>
                Use this command to create individual patches for
                each change.
                Here is an example that that creates patch files for each
                commit and places them in your <filename>Documents</filename>
                directory:
                <literallayout class='monospaced'>
     $ git format-patch -o $HOME/Documents origin/standard/base..origin/standard/emenlow
                </literallayout>
            </para>
        </section>

        <section id='showing-a-particular-feature-or-branch-change'>
            <title>Showing a Particular Feature or Branch Change</title>

            <para>
                Tags in the Yocto Project kernel tree divide changes for
                significant features or branches.
                The <filename>git show</filename>&nbsp;<replaceable>tag</replaceable>
                command shows changes based on a tag.
                Here is an example that shows <filename>systemtap</filename>
                changes:
                <literallayout class='monospaced'>
     $ git show systemtap
                </literallayout>
                You can use the
                <filename>git branch --contains</filename>&nbsp;<replaceable>tag</replaceable>
                command to show the branches that contain a particular feature.
                This command shows the branches that contain the
                <filename>systemtap</filename> feature:
                <literallayout class='monospaced'>
     $ git branch --contains systemtap
                </literallayout>
            </para>
        </section>
    </section>
</chapter>
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