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 that are performed 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.
</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 BitBake 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 them.</para></listitem>
                <listitem><para>"<ulink url='&YOCTO_DOCS_DEV_URL;#get-your-layer-setup-for-the-build'>Get Your Layer Setup 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;#get-your-layer-setup-for-the-build'>Get Your Layer Setup 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_3.4.bb</filename>
                recipe, the append file will typical be located as follows
                within your custom layer:
                <literallayout class='monospaced'>
     &lt;your-layer&gt;/recipes-kernel/linux/linux-yocto_3.4.bbappend
                </literallayout>
                The append file should initially contain the following text:
                <literallayout class='monospaced'>
     <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'>FILESEXTRAPATHS</ulink> := "${THISDIR}/${PN}"
                </literallayout>
                The path <filename>${THISDIR}/${</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,
                you need to place them in your layer in the following area:
                <literallayout class='monospaced'>
     &lt;your-layer&gt;/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-first-change.patch"
     SRC_URI += "file://0003-first-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 Linux
                kernel <filename>.config</filename> file by including a
                <filename>defconfig</filename> or by specifying
                configuration fragments in the <filename>SRC_URI</filename>.
            </para>

            <para>
                If you have a complete Linux kernel <filename>.config</filename>
                file you want to use, copy it to the
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-FILES'><filename>FILES</filename></ulink><filename>}</filename>
                directory within your layer and name it "defconfig".
                Then, add the following line to your linux-yocto
                <filename>.bbappend</filename> file in your layer:
                <literallayout class='monospaced'>
     SRC_URI += "file://defconfig"
                </literallayout>
            </para>

            <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>${FILES}</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 in a
                <filename>SRC_URI</filename> statement in your
                <filename>.bbappend</filename> file:
                <literallayout class='monospaced'>
     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>

    <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='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-MACHINE'><filename>MACHINE</filename></ulink><filename>}-${KTYPE}-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.
            </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></para></listitem>
                    <listitem><para>Copy and rename the resulting
                        <filename>.config</filename> file (e.g.
                        <filename>config.orig</filename>).
                        </para></listitem>
                    <listitem><para>Run the <filename>menuconfig</filename>
                        command:
                        <literallayout class='monospaced'>
     $ bitbake linux-yocto -c menuconfig
                        </literallayout></para></listitem>
                    <listitem><para>Prepare a configuration fragment based on
                        the differences between the two files.
                        </para></listitem>
                </orderedlist>
            </para>

            <para>
                Ultimately, the configuration fragment file needs to be a
                list of Linux kernel <filename>CONFIG_</filename> assignments.
                It cannot be in <filename>diff</filename> format.
                Here is an example of a command that creates your
                configuration fragment file.
                Regardless of the exact command you use, plan on reviewing
                the output as you can usually remove some of the defaults:
                <literallayout class='monospaced'>
     $ diff -Nurp config.orig .config | sed -n "s/^\+//p" > frag.cfg
                </literallayout>
                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 aren't 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 <filename>fetch</filename> task for the recipe.
                You can avoid triggering this task by not issuing BitBake's
                <filename>cleanall</filename>, <filename>cleansstate</filename>,
                or forced <filename>fetch</filename> 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 <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>:
                <literallayout class='monospaced'>
     ${WORKDIR}/linux-${MACHINE}-${KTYPE}-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 section
                "<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_3.5.bb</filename>,
                    where "3.5" 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>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 be 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 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.
                            "3.4").</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'>
     3.4.11+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='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 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 appropriate 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
            <filename>KERNEL_SRC</filename> variable.
            The module <filename>bbclass</filename> sets this variable,
            as well as the <filename>KERNEL_PATH</filename> variable
            to <filename>${STAGING_KERNEL_DIR}</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
            <filename>do_compile()</filename> 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 as
            appropriate in 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>
            Where 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 <filename>RRECOMMENDS</filename> and not
            a <filename>RDEPENDS</filename> variable, the build will not fail
            if this module is not available to include in the image.
        </para>
    </section>
</chapter>
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