poky/documentation/kernel-dev/kernel-dev-maint-appx.xml - openbmc/openbmc - Gitiles

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 <appendix id='kernel-dev-maint-appx'>
 <title>Kernel Maintenance</title>

     <section id='tree-construction'>
         <title>Tree Construction</title>

         <para>
             This section describes construction of the Yocto Project kernel
             source repositories as accomplished by the Yocto Project team to
             create Yocto Linux kernel repositories.
             These kernel repositories are found under the heading "Yocto Linux
             Kernel" at
             <ulink url='&YOCTO_GIT_URL;'>&YOCTO_GIT_URL;</ulink>
             and are shipped as part of a Yocto Project release.
             The team creates these repositories by compiling and executing the
             set of feature descriptions for every BSP and feature in the
             product.
             Those feature descriptions list all necessary patches,
             configurations, branches, tags, and feature divisions found in a
             Yocto Linux kernel.
             Thus, the Yocto Project Linux kernel repository (or tree) and
             accompanying Metadata in the
             <filename>yocto-kernel-cache</filename> are built.
         </para>

         <para>
             The existence of these repositories allow you to access and clone a
             particular Yocto Project Linux kernel repository and use it to
             build images based on their configurations and features.
         </para>

         <para>
             You can find the files used to describe all the valid features and
             BSPs in the Yocto Project Linux kernel in any clone of the Yocto
             Project Linux kernel source repository and
             <filename>yocto-kernel-cache</filename> Git trees.
             For example, the following commands clone the Yocto Project
             baseline Linux kernel that branches off
             <filename>linux.org</filename> version 4.12 and the
             <filename>yocto-kernel-cache</filename>, which contains stores of
             kernel Metadata:
             <literallayout class='monospaced'>
      $ git clone git://git.yoctoproject.org/linux-yocto-4.12
      $ git clone git://git.yoctoproject.org/linux-kernel-cache
             </literallayout>
             For more information on how to set up a local Git repository of
             the Yocto Project Linux kernel files, see the
             "<link linkend='preparing-the-build-host-to-work-on-the-kernel'>Preparing the Build Host to Work on the Kernel</link>"
             section.
         </para>

         <para>
             Once you have cloned the kernel Git repository and the
             cache of Metadata on your local machine, you can discover the
             branches that are available in the repository using the following
             Git command:
             <literallayout class='monospaced'>
      $ git branch -a
             </literallayout>
             Checking out a branch allows you to work with a particular
             Yocto Linux kernel.
             For example, the following commands check out the
             "standard/beagleboard" branch of the Yocto Linux kernel repository
             and the "yocto-4.12" branch of the
             <filename>yocto-kernel-cache</filename> repository:
             <literallayout class='monospaced'>
      $ cd ~/linux-yocto-4.12
      $ git checkout -b my-kernel-4.12 remotes/origin/standard/beagleboard
      $ cd ~/linux-kernel-cache
      $ git checkout -b my-4.12-metadata remotes/origin/yocto-4.12
             </literallayout>
             <note>
                 Branches in the <filename>yocto-kernel-cache</filename>
                 repository correspond to Yocto Linux kernel versions
                 (e.g. "yocto-4.12", "yocto-4.10", "yocto-4.9", and so forth).
             </note>
             Once you have checked out and switched to appropriate branches,
             you can see a snapshot of all the kernel source files used to
             used to build that particular Yocto Linux kernel for a
             particular board.
         </para>

         <para>
             To see the features and configurations for a particular Yocto
             Linux kernel, you need to examine the
             <filename>yocto-kernel-cache</filename> Git repository.
             As mentioned, branches in the
             <filename>yocto-kernel-cache</filename> repository correspond to
             Yocto Linux kernel versions (e.g. <filename>yocto-4.12</filename>).
             Branches contain descriptions in the form of
             <filename>.scc</filename> and <filename>.cfg</filename> files.
         </para>

         <para>
             You should realize, however, that browsing your local
             <filename>yocto-kernel-cache</filename> repository for feature
             descriptions and patches is not an effective way to determine what
             is in a particular kernel branch.
             Instead, you should use Git directly to discover the changes in
             a branch.
             Using Git is an efficient and flexible way to inspect changes to
             the kernel.
             <note>
                 Ground up reconstruction of the complete kernel tree is an
                 action only taken by the Yocto Project team during an active
                 development cycle.
                 When you create a clone of the kernel Git repository, you are
                 simply making it efficiently available for building and
                 development.
             </note>
         </para>

         <para>
             The following steps describe what happens when the Yocto Project
             Team constructs the Yocto Project kernel source Git repository
             (or tree) found at
             <ulink url='&YOCTO_GIT_URL;'></ulink> given the
             introduction of a new top-level kernel feature or BSP.
             The following actions effectively provide the Metadata
             and create the tree that includes the new feature, patch, or BSP:
             <orderedlist>
                 <listitem><para>
                     <emphasis>Pass Feature to the OpenEmbedded Build System:</emphasis>
                     A top-level kernel feature is passed to the kernel build
                     subsystem.
                     Normally, this feature is a BSP for a particular kernel
                     type.
                     </para></listitem>
                 <listitem><para>
                     <emphasis>Locate Feature:</emphasis>
                     The file that describes the top-level feature is located
                     by searching these system directories:
                     <itemizedlist>
                         <listitem><para>
                             The in-tree kernel-cache directories, which are
                             located in the
                             <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/yocto-kernel-cache/tree/bsp'><filename>yocto-kernel-cache</filename></ulink>
                             repository organized under the "Yocto Linux Kernel"
                             heading in the
                             <ulink url='http://git.yoctoproject.org/cgit/cgit.cgi'>Yocto Project Source Repositories</ulink>.
                             </para></listitem>
                         <listitem><para>
                             Areas pointed to by <filename>SRC_URI</filename>
                             statements found in kernel recipes
                             </para></listitem>
                     </itemizedlist>
                     For a typical build, the target of the search is a
                     feature description in an <filename>.scc</filename> file
                     whose name follows this format (e.g.
                     <filename>beaglebone-standard.scc</filename> and
                     <filename>beaglebone-preempt-rt.scc</filename>):
                     <literallayout class='monospaced'>
      <replaceable>bsp_root_name</replaceable>-<replaceable>kernel_type</replaceable>.scc
                     </literallayout>
                 </para></listitem>
                 <listitem><para>
                     <emphasis>Expand Feature:</emphasis>
                     Once located, the feature description is either expanded
                     into a simple script of actions, or into an existing
                     equivalent script that is already part of the shipped
                     kernel.
                     </para></listitem>
                 <listitem><para>
                     <emphasis>Append Extra Features:</emphasis>
                     Extra features are appended to the top-level feature
                     description.
                     These features can come from the
                     <ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'><filename>KERNEL_FEATURES</filename></ulink>
                     variable in recipes.
                     </para></listitem>
                 <listitem><para>
                     <emphasis>Locate, Expand, and Append Each Feature:</emphasis>
                     Each extra feature is located, expanded and appended to
                     the script as described in step three.
                     </para></listitem>
                 <listitem><para>
                     <emphasis>Execute the Script:</emphasis>
                     The script is executed to produce files
                     <filename>.scc</filename> and <filename>.cfg</filename>
                     files in appropriate directories of the
                     <filename>yocto-kernel-cache</filename> repository.
                     These files are descriptions of all the branches, tags,
                     patches and configurations that need to be applied to the
                     base Git repository to completely create the
                     source (build) branch for the new BSP or feature.
                     </para></listitem>
                 <listitem><para>
                     <emphasis>Clone Base Repository:</emphasis>
                     The base repository is cloned, and the actions
                     listed in the <filename>yocto-kernel-cache</filename>
                     directories are applied to the tree.
                     </para></listitem>
                 <listitem><para>
                     <emphasis>Perform Cleanup:</emphasis>
                     The Git repositories are left with the desired branches
                     checked out and any required branching, patching and
                     tagging has been performed.
                     </para></listitem>
             </orderedlist>
         </para>

         <para>
             The kernel tree and cache are ready for developer consumption to
             be locally cloned, configured, and built into a Yocto Project
             kernel specific to some target hardware.
             <note><title>Notes</title>
                 <itemizedlist>
                     <listitem><para>
                         The generated <filename>yocto-kernel-cache</filename>
                         repository adds to the kernel as shipped with the Yocto
                         Project release.
                         Any add-ons and configuration data are applied to the
                         end of an existing branch.
                         The full repository generation that is found in the
                         official Yocto Project kernel repositories at
                         <ulink url='&YOCTO_GIT_URL;'>http://git.yoctoproject.org</ulink>
                         is the combination of all supported boards and
                         configurations.
                         </para></listitem>
                     <listitem><para>
                         The technique the Yocto Project team uses is flexible
                         and allows for seamless blending of an immutable
                         history with additional patches specific to a
                         deployment.
                         Any additions to the kernel become an integrated part
                         of the branches.
                         </para></listitem>
                     <listitem><para>
                         The full kernel tree that you see on
                         <ulink url='&YOCTO_GIT_URL;'></ulink> is
                         generated through repeating the above steps for all
                         valid BSPs.
                         The end result is a branched, clean history tree that
                         makes up the kernel for a given release.
                         You can see the script (<filename>kgit-scc</filename>)
                         responsible for this in the
                         <ulink url='&YOCTO_GIT_URL;/cgit.cgi/yocto-kernel-tools/tree/tools'><filename>yocto-kernel-tools</filename></ulink>
                         repository.
                         </para></listitem>
                     <listitem><para>
                         The steps used to construct the full kernel tree are
                         the same steps that BitBake uses when it builds a
                         kernel image.
                         </para></listitem>
                 </itemizedlist>
             </note>
         </para>
     </section>

     <section id='build-strategy'>
         <title>Build Strategy</title>

         <para>
             Once you have cloned a Yocto Linux kernel repository and the
             cache repository (<filename>yocto-kernel-cache</filename>) onto
             your development system, you can consider the compilation phase
             of kernel development, which is building a kernel image.
             Some prerequisites exist that are validated by the build process
             before compilation starts:
         </para>

         <itemizedlist>
             <listitem><para>
                 The
                 <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                 points to the kernel Git repository.
                 </para></listitem>
             <listitem><para>
                 A BSP build branch with Metadata exists in the
                 <filename>yocto-kernel-cache</filename> repository.
                 The branch is based on the Yocto Linux kernel version and
                 has configurations and features grouped under the
                 <filename>yocto-kernel-cache/bsp</filename> directory.
                 For example, features and configurations for the
                 BeagleBone Board assuming a
                 <filename>linux-yocto_4.12</filename> kernel reside in the
                 following area of the <filename>yocto-kernel-cache</filename>
                 repository:
                 <literallayout class='monospaced'>
      yocto-kernel-cache/bsp/beaglebone
                 </literallayout>
                 <note>
                     In the previous example, the "yocto-4.12" branch is
                     checked out in the <filename>yocto-kernel-cache</filename>
                     repository.
                 </note>
                 </para></listitem>
         </itemizedlist>

         <para>
             The OpenEmbedded build system makes sure these conditions exist
             before attempting compilation.
             Other means, however, do exist, such as as bootstrapping a BSP.
         </para>

         <para>
             Before building a kernel, the build process verifies the tree
             and configures the kernel by processing all of the
             configuration "fragments" specified by feature descriptions
             in the <filename>.scc</filename> files.
             As the features are compiled, associated kernel configuration
             fragments are noted and recorded in the series of directories
             in their compilation order.
             The fragments are migrated, pre-processed and passed to the
             Linux Kernel Configuration subsystem (<filename>lkc</filename>) as
             raw input in the form of a <filename>.config</filename> file.
             The <filename>lkc</filename> uses its own internal dependency
             constraints to do the final processing of that information and
             generates the final <filename>.config</filename> file that is used
             during compilation.
         </para>

         <para>
             Using the board's architecture and other relevant values from
             the board's template, kernel compilation is started and a kernel
             image is produced.
         </para>

         <para>
             The other thing that you notice once you configure a kernel is that
             the build process generates a build tree that is separate from
             your kernel's local Git source repository tree.
             This build tree has a name that uses the following form, where
             <filename>${MACHINE}</filename> is the metadata name of the
             machine (BSP) and "kernel_type" is one of the Yocto Project
             supported kernel types (e.g. "standard"):
         <literallayout class='monospaced'>
      linux-${MACHINE}-<replaceable>kernel_type</replaceable>-build
         </literallayout>
         </para>

         <para>
             The existing support in the <filename>kernel.org</filename> tree
             achieves this default functionality.
         </para>

         <para>
             This behavior means that all the generated files for a particular
             machine or BSP are now in the build tree directory.
             The files include the final <filename>.config</filename> file,
             all the <filename>.o</filename> files, the <filename>.a</filename>
             files, and so forth.
             Since each machine or BSP has its own separate
             <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>
             in its own separate branch of the Git repository, you can easily
             switch between different builds.
         </para>
     </section>
 </appendix>
 <!--
 vim: expandtab tw=80 ts=4
 -->
	<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
	"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
	[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >
	<!--SPDX-License-Identifier: CC-BY-2.0-UK-->

	<appendix id='kernel-dev-maint-appx'>
	<title>Kernel Maintenance</title>

	<section id='tree-construction'>
	<title>Tree Construction</title>

	<para>
	This section describes construction of the Yocto Project kernel
	source repositories as accomplished by the Yocto Project team to
	create Yocto Linux kernel repositories.
	These kernel repositories are found under the heading "Yocto Linux
	Kernel" at
	<ulink url='&YOCTO_GIT_URL;'>&YOCTO_GIT_URL;</ulink>
	and are shipped as part of a Yocto Project release.
	The team creates these repositories by compiling and executing the
	set of feature descriptions for every BSP and feature in the
	product.
	Those feature descriptions list all necessary patches,
	configurations, branches, tags, and feature divisions found in a
	Yocto Linux kernel.
	Thus, the Yocto Project Linux kernel repository (or tree) and
	accompanying Metadata in the
	<filename>yocto-kernel-cache</filename> are built.
	</para>

	<para>
	The existence of these repositories allow you to access and clone a
	particular Yocto Project Linux kernel repository and use it to
	build images based on their configurations and features.
	</para>

	<para>
	You can find the files used to describe all the valid features and
	BSPs in the Yocto Project Linux kernel in any clone of the Yocto
	Project Linux kernel source repository and
	<filename>yocto-kernel-cache</filename> Git trees.
	For example, the following commands clone the Yocto Project
	baseline Linux kernel that branches off
	<filename>linux.org</filename> version 4.12 and the
	<filename>yocto-kernel-cache</filename>, which contains stores of
	kernel Metadata:
	<literallayout class='monospaced'>
	$ git clone git://git.yoctoproject.org/linux-yocto-4.12
	$ git clone git://git.yoctoproject.org/linux-kernel-cache
	</literallayout>
	For more information on how to set up a local Git repository of
	the Yocto Project Linux kernel files, see the
	"<link linkend='preparing-the-build-host-to-work-on-the-kernel'>Preparing the Build Host to Work on the Kernel</link>"
	section.
	</para>

	<para>
	Once you have cloned the kernel Git repository and the
	cache of Metadata on your local machine, you can discover the
	branches that are available in the repository using the following
	Git command:
	<literallayout class='monospaced'>
	$ git branch -a
	</literallayout>
	Checking out a branch allows you to work with a particular
	Yocto Linux kernel.
	For example, the following commands check out the
	"standard/beagleboard" branch of the Yocto Linux kernel repository
	and the "yocto-4.12" branch of the
	<filename>yocto-kernel-cache</filename> repository:
	<literallayout class='monospaced'>
	$ cd ~/linux-yocto-4.12
	$ git checkout -b my-kernel-4.12 remotes/origin/standard/beagleboard
	$ cd ~/linux-kernel-cache
	$ git checkout -b my-4.12-metadata remotes/origin/yocto-4.12
	</literallayout>
	<note>
	Branches in the <filename>yocto-kernel-cache</filename>
	repository correspond to Yocto Linux kernel versions
	(e.g. "yocto-4.12", "yocto-4.10", "yocto-4.9", and so forth).
	</note>
	Once you have checked out and switched to appropriate branches,
	you can see a snapshot of all the kernel source files used to
	used to build that particular Yocto Linux kernel for a
	particular board.
	</para>

	<para>
	To see the features and configurations for a particular Yocto
	Linux kernel, you need to examine the
	<filename>yocto-kernel-cache</filename> Git repository.
	As mentioned, branches in the
	<filename>yocto-kernel-cache</filename> repository correspond to
	Yocto Linux kernel versions (e.g. <filename>yocto-4.12</filename>).
	Branches contain descriptions in the form of
	<filename>.scc</filename> and <filename>.cfg</filename> files.
	</para>

	<para>
	You should realize, however, that browsing your local
	<filename>yocto-kernel-cache</filename> repository for feature
	descriptions and patches is not an effective way to determine what
	is in a particular kernel branch.
	Instead, you should use Git directly to discover the changes in
	a branch.
	Using Git is an efficient and flexible way to inspect changes to
	the kernel.
	<note>
	Ground up reconstruction of the complete kernel tree is an
	action only taken by the Yocto Project team during an active
	development cycle.
	When you create a clone of the kernel Git repository, you are
	simply making it efficiently available for building and
	development.
	</note>
	</para>

	<para>
	The following steps describe what happens when the Yocto Project
	Team constructs the Yocto Project kernel source Git repository
	(or tree) found at
	<ulink url='&YOCTO_GIT_URL;'></ulink> given the
	introduction of a new top-level kernel feature or BSP.
	The following actions effectively provide the Metadata
	and create the tree that includes the new feature, patch, or BSP:
	<orderedlist>
	<listitem><para>
	<emphasis>Pass Feature to the OpenEmbedded Build System:</emphasis>
	A top-level kernel feature is passed to the kernel build
	subsystem.
	Normally, this feature is a BSP for a particular kernel
	type.
	</para></listitem>
	<listitem><para>
	<emphasis>Locate Feature:</emphasis>
	The file that describes the top-level feature is located
	by searching these system directories:
	<itemizedlist>
	<listitem><para>
	The in-tree kernel-cache directories, which are
	located in the
	<ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/yocto-kernel-cache/tree/bsp'><filename>yocto-kernel-cache</filename></ulink>
	repository organized under the "Yocto Linux Kernel"
	heading in the
	<ulink url='http://git.yoctoproject.org/cgit/cgit.cgi'>Yocto Project Source Repositories</ulink>.
	</para></listitem>
	<listitem><para>
	Areas pointed to by <filename>SRC_URI</filename>
	statements found in kernel recipes
	</para></listitem>
	</itemizedlist>
	For a typical build, the target of the search is a
	feature description in an <filename>.scc</filename> file
	whose name follows this format (e.g.
	<filename>beaglebone-standard.scc</filename> and
	<filename>beaglebone-preempt-rt.scc</filename>):
	<literallayout class='monospaced'>
	<replaceable>bsp_root_name</replaceable>-<replaceable>kernel_type</replaceable>.scc
	</literallayout>
	</para></listitem>
	<listitem><para>
	<emphasis>Expand Feature:</emphasis>
	Once located, the feature description is either expanded
	into a simple script of actions, or into an existing
	equivalent script that is already part of the shipped
	kernel.
	</para></listitem>
	<listitem><para>
	<emphasis>Append Extra Features:</emphasis>
	Extra features are appended to the top-level feature
	description.
	These features can come from the
	<ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'><filename>KERNEL_FEATURES</filename></ulink>
	variable in recipes.
	</para></listitem>
	<listitem><para>
	<emphasis>Locate, Expand, and Append Each Feature:</emphasis>
	Each extra feature is located, expanded and appended to
	the script as described in step three.
	</para></listitem>
	<listitem><para>
	<emphasis>Execute the Script:</emphasis>
	The script is executed to produce files
	<filename>.scc</filename> and <filename>.cfg</filename>
	files in appropriate directories of the
	<filename>yocto-kernel-cache</filename> repository.
	These files are descriptions of all the branches, tags,
	patches and configurations that need to be applied to the
	base Git repository to completely create the
	source (build) branch for the new BSP or feature.
	</para></listitem>
	<listitem><para>
	<emphasis>Clone Base Repository:</emphasis>
	The base repository is cloned, and the actions
	listed in the <filename>yocto-kernel-cache</filename>
	directories are applied to the tree.
	</para></listitem>
	<listitem><para>
	<emphasis>Perform Cleanup:</emphasis>
	The Git repositories are left with the desired branches
	checked out and any required branching, patching and
	tagging has been performed.
	</para></listitem>
	</orderedlist>
	</para>

	<para>
	The kernel tree and cache are ready for developer consumption to
	be locally cloned, configured, and built into a Yocto Project
	kernel specific to some target hardware.
	<note><title>Notes</title>
	<itemizedlist>
	<listitem><para>
	The generated <filename>yocto-kernel-cache</filename>
	repository adds to the kernel as shipped with the Yocto
	Project release.
	Any add-ons and configuration data are applied to the
	end of an existing branch.
	The full repository generation that is found in the
	official Yocto Project kernel repositories at
	<ulink url='&YOCTO_GIT_URL;'>http://git.yoctoproject.org</ulink>
	is the combination of all supported boards and
	configurations.
	</para></listitem>
	<listitem><para>
	The technique the Yocto Project team uses is flexible
	and allows for seamless blending of an immutable
	history with additional patches specific to a
	deployment.
	Any additions to the kernel become an integrated part
	of the branches.
	</para></listitem>
	<listitem><para>
	The full kernel tree that you see on
	<ulink url='&YOCTO_GIT_URL;'></ulink> is
	generated through repeating the above steps for all
	valid BSPs.
	The end result is a branched, clean history tree that
	makes up the kernel for a given release.
	You can see the script (<filename>kgit-scc</filename>)
	responsible for this in the
	<ulink url='&YOCTO_GIT_URL;/cgit.cgi/yocto-kernel-tools/tree/tools'><filename>yocto-kernel-tools</filename></ulink>
	repository.
	</para></listitem>
	<listitem><para>
	The steps used to construct the full kernel tree are
	the same steps that BitBake uses when it builds a
	kernel image.
	</para></listitem>
	</itemizedlist>
	</note>
	</para>
	</section>

	<section id='build-strategy'>
	<title>Build Strategy</title>

	<para>
	Once you have cloned a Yocto Linux kernel repository and the
	cache repository (<filename>yocto-kernel-cache</filename>) onto
	your development system, you can consider the compilation phase
	of kernel development, which is building a kernel image.
	Some prerequisites exist that are validated by the build process
	before compilation starts:
	</para>

	<itemizedlist>
	<listitem><para>
	The
	<ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
	points to the kernel Git repository.
	</para></listitem>
	<listitem><para>
	A BSP build branch with Metadata exists in the
	<filename>yocto-kernel-cache</filename> repository.
	The branch is based on the Yocto Linux kernel version and
	has configurations and features grouped under the
	<filename>yocto-kernel-cache/bsp</filename> directory.
	For example, features and configurations for the
	BeagleBone Board assuming a
	<filename>linux-yocto_4.12</filename> kernel reside in the
	following area of the <filename>yocto-kernel-cache</filename>
	repository:
	<literallayout class='monospaced'>
	yocto-kernel-cache/bsp/beaglebone
	</literallayout>
	<note>
	In the previous example, the "yocto-4.12" branch is
	checked out in the <filename>yocto-kernel-cache</filename>
	repository.
	</note>
	</para></listitem>
	</itemizedlist>

	<para>
	The OpenEmbedded build system makes sure these conditions exist
	before attempting compilation.
	Other means, however, do exist, such as as bootstrapping a BSP.
	</para>

	<para>
	Before building a kernel, the build process verifies the tree
	and configures the kernel by processing all of the
	configuration "fragments" specified by feature descriptions
	in the <filename>.scc</filename> files.
	As the features are compiled, associated kernel configuration
	fragments are noted and recorded in the series of directories
	in their compilation order.
	The fragments are migrated, pre-processed and passed to the
	Linux Kernel Configuration subsystem (<filename>lkc</filename>) as
	raw input in the form of a <filename>.config</filename> file.
	The <filename>lkc</filename> uses its own internal dependency
	constraints to do the final processing of that information and
	generates the final <filename>.config</filename> file that is used
	during compilation.
	</para>

	<para>
	Using the board's architecture and other relevant values from
	the board's template, kernel compilation is started and a kernel
	image is produced.
	</para>

	<para>
	The other thing that you notice once you configure a kernel is that
	the build process generates a build tree that is separate from
	your kernel's local Git source repository tree.
	This build tree has a name that uses the following form, where
	<filename>${MACHINE}</filename> is the metadata name of the
	machine (BSP) and "kernel_type" is one of the Yocto Project
	supported kernel types (e.g. "standard"):
	<literallayout class='monospaced'>
	linux-${MACHINE}-<replaceable>kernel_type</replaceable>-build
	</literallayout>
	</para>

	<para>
	The existing support in the <filename>kernel.org</filename> tree
	achieves this default functionality.
	</para>

	<para>
	This behavior means that all the generated files for a particular
	machine or BSP are now in the build tree directory.
	The files include the final <filename>.config</filename> file,
	all the <filename>.o</filename> files, the <filename>.a</filename>
	files, and so forth.
	Since each machine or BSP has its own separate
	<ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>
	in its own separate branch of the Git repository, you can easily
	switch between different builds.
	</para>
	</section>
	</appendix>
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