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+<!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; ] >
+
+<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 kernel repositories.
+ These kernel repositories are found under the heading "Yocto Linux Kernel" at
+ <ulink url='&YOCTO_GIT_URL;/cgit.cgi'>&YOCTO_GIT_URL;/cgit.cgi</ulink>
+ and can be 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,
+ configuration, branching, tagging and feature divisions found in a kernel.
+ Thus, the Yocto Project kernel repository (or tree) is built.
+ </para>
+ <para>
+ The existence of this tree allows you to access and clone a particular
+ Yocto Project 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 kernel in any clone of the Yocto Project kernel source repository
+ Git tree.
+ For example, the following command clones the Yocto Project baseline kernel that
+ branched off of <filename>linux.org</filename> version 3.19:
+ <literallayout class='monospaced'>
+ $ git clone git://git.yoctoproject.org/linux-yocto-3.19
+ </literallayout>
+ For another example of how to set up a local Git repository of the Yocto Project
+ kernel files, see the
+ "<ulink url='&YOCTO_DOCS_DEV_URL;#local-kernel-files'>Yocto Project Kernel</ulink>" bulleted
+ item in the Yocto Project Development Manual.
+ </para>
+ <para>
+ Once you have cloned the kernel Git repository on your local machine, you can
+ switch to the <filename>meta</filename> branch within the repository.
+ Here is an example that assumes the local Git repository for the kernel is in
+ a top-level directory named <filename>linux-yocto-3.19</filename>:
+ <literallayout class='monospaced'>
+ $ cd linux-yocto-3.19
+ $ git checkout -b meta origin/meta
+ </literallayout>
+ Once you have checked out and switched to the <filename>meta</filename> branch,
+ you can see a snapshot of all the kernel configuration and feature descriptions that are
+ used to build that particular kernel repository.
+ These descriptions are in the form of <filename>.scc</filename> files.
+ </para>
+ <para>
+ You should realize, however, that browsing your local kernel 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;/cgit.cgi'></ulink> given the
+ introduction of a new top-level kernel feature or BSP.
+ These are the actions that effectively create the tree
+ that includes the new feature, patch or BSP:
+ <orderedlist>
+ <listitem><para>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>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 <filename>meta/cfg/kernel-cache</filename></para></listitem>
+ <listitem><para>Areas pointed to by <filename>SRC_URI</filename> statements
+ found in 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:
+ <literallayout class='monospaced'>
+ <replaceable>bsp_name</replaceable>-<replaceable>kernel_type</replaceable>.scc
+ </literallayout>
+ </para></listitem>
+ <listitem><para>Once located, the feature description is either compiled into a simple script
+ of actions, or into an existing equivalent script that is already part of the
+ shipped kernel.</para></listitem>
+ <listitem><para>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>Each extra feature is located, compiled and appended to the script
+ as described in step three.</para></listitem>
+ <listitem><para>The script is executed to produce a series of <filename>meta-*</filename>
+ directories.
+ These directories 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>The base repository is cloned, and the actions
+ listed in the <filename>meta-*</filename> directories are applied to the
+ tree.</para></listitem>
+ <listitem><para>The Git repository is left with the desired branch checked out and any
+ required branching, patching and tagging has been performed.</para></listitem>
+ </orderedlist>
+ </para>
+ <para>
+ The kernel tree is now ready for developer consumption to be locally cloned,
+ configured, and built into a Yocto Project kernel specific to some target hardware.
+ <note><para>The generated <filename>meta-*</filename> directories add 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;/cgit.cgi'>http://git.yoctoproject.org/cgit.cgi</ulink>
+ is the combination of all supported boards and configurations.</para>
+ <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>
+ </note>
+ </para>
+ </section>
+
+ <section id='build-strategy'>
+ <title>Build Strategy</title>
+
+<!--
+ <para>
+ <emphasis>AR - Darren Hart:</emphasis> Some parts of this section
+ need to be in the
+ "<link linkend='using-an-iterative-development-process'>Using an Iterative Development Process</link>"
+ section.
+ Darren needs to figure out which parts and identify them.
+ </para>
+-->
+
+ <para>
+ Once a local Git repository of the Yocto Project kernel exists on a development system,
+ you can consider the compilation phase of kernel development - 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 exists.
+ This branch has the following form:
+ <literallayout class='monospaced'>
+ <replaceable>kernel_type</replaceable>/<replaceable>bsp_name</replaceable>
+ </literallayout></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 <filename>meta-*</filename> 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_DEV_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
+-->