Revert "poky: subtree update:b23aa6b753..ad30a6d470"
This reverts commit af5e4ef732faedf66c6dc1756432e9de2ac72988.
This commit introduced openbmc/openbmc#3720 and no solution has been
forthcoming. Revert until we can get to the bottom of this.
Change-Id: I2fb0d81eb26cf3dadb2f2abdd1a1bb7a95eaf03c
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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; ] >
+<!--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>
+<!--
+vim: expandtab tw=80 ts=4
+-->