documentation/kernel-manual: Added these files so the book could have chapters.

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>

1 files changed, 315 insertions, 0 deletions

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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">
+
+<chapter id='kernel-concepts'>
+
+<title>Yocto Project Kernel Concepts</title>
+
+<section id='concepts-org'>
+    <title>Introduction</title>
+    <para>
+        This chapter provides conceptual information about the Yocto Project kernel:
+        <itemizedlist>
+            <listitem><para>Kernel Goals</para></listitem>
+            <listitem><para>Yocto Project Kernel Development and Maintenance Overview</para></listitem>
+            <listitem><para>Kernel Architecture</para></listitem>
+            <listitem><para>Kernel Tools</para></listitem>
+        </itemizedlist>
+    </para>
+</section>
+
+    <section id='kernel-goals'>
+        <title>Kernel Goals</title>
+        <para>
+            The complexity of embedded kernel design has increased dramatically. 
+            Whether it is managing multiple implementations of a particular feature or tuning and
+            optimizing board specific features, flexibility and maintainability are key concerns. 
+            The Yocto Project Linux kernel is presented with the embedded
+            developer's needs in mind and has evolved to assist in these key concerns. 
+            For example, prior methods such as applying hundreds of patches to an extracted
+            tarball have been replaced with proven techniques that allow easy inspection,
+            bisection and analysis of changes. 
+            Application of these techniques also creates a platform for performing integration and 
+            collaboration with the thousands of upstream development projects.
+        </para>
+        <para>
+            With all these considerations in mind, the Yocto Project kernel and development team
+            strives to attain these goals:
+        <itemizedlist>
+            <listitem><para>Allow the end user to leverage community best practices to seamlessly 
+            manage the development, build and debug cycles.</para></listitem>
+            <listitem><para>Create a platform for performing integration and collaboration with the 
+            thousands of upstream development projects that exist.</para></listitem>
+            <listitem><para>Provide mechanisms that support many different work flows, front-ends and 
+            management techniques.</para></listitem>
+            <listitem><para>Deliver the most up-to-date kernel possible while still ensuring that 
+            the baseline kernel is the the most stable official release.</para></listitem>
+            <listitem><para>Include major technological features as part of Yocto Project's up-rev 
+            strategy.</para></listitem>
+            <listitem><para>Present a git tree, that just like the upstream kernel.org tree, has a 
+            clear and continuous history.</para></listitem>
+            <listitem><para>Deliver a key set of supported kernel types, where each type is tailored 
+            to a specific use case (i.g. networking, consumer, devices, and so forth).</para></listitem>
+            <listitem><para>Employ a git branching strategy that from a customer's point of view
+            results in a linear path from the baseline kernel.org, through a select group of features and
+            ends with their BSP-specific commits.</para></listitem>
+        </itemizedlist>
+        </para>
+    </section>
+
+    <section id='kernel-big-picture'>
+        <title>Yocto Project Kernel Development and Maintenance Overview</title>
+        <para>
+            Yocto Project kernel, like other kernels, is based off the Linux kernel release
+            from <ulink url='http://www.kernel.org'></ulink>.  
+            At the beginning of our major development cycle, we choose our Yocto Project kernel 
+            based on factors like release timing, the anticipated release timing of "final" (i.e. non "rc")
+            upstream kernel.org versions, and Yocto Project feature requirements.
+            Typically this will be a kernel that is in the
+            final stages of development by the community (i.e. still in the release
+            candidate or "rc" phase) and not yet a final release. 
+            But by being in the final stages of external development, we know that the 
+            kernel.org final release will clearly land within the early stages of 
+            the Yocto Project development window.
+        </para>
+        <para>
+            This balance allows us to deliver the most up-to-date kernel
+            as possible, while still ensuring that we have a stable official release as
+            our baseline kernel version.
+        </para>
+        <para>
+            The following figure represents the overall place the Yocto Project kernel fills.
+        </para>
+        <para>
+        <imagedata fileref="figures/kernel-big-picture.png" width="6in" depth="6in" align="center" scale="100" />
+        </para>
+        <para>
+            In the figure the ultimate source for the Yocto Project kernel is a released kernel 
+            from kernel.org.
+            In addition to a foundational kernel from kernel.org the commercially released 
+            Yocto Project kernel contains a mix of important new mainline
+            developments, non-mainline developments, Board Support Package (BSP) developments,
+            and custom features.
+            These additions result in a commercially released Yocto Project kernel that caters 
+            to specific embedded designer needs for targeted hardware. 
+        </para>
+        <para>
+            Once a Yocto Project kernel is officially released the Yocto Project team goes into 
+            their next development cycle, or "uprev" cycle.
+            It is important to note that the most sustainable and stable way
+            to include feature development upstream is through a kernel uprev process.
+            Back-porting of hundreds of individual fixes and minor features from various
+            kernel versions is not sustainable and can easily compromise quality. 
+            During the uprev cycle, the Yocto Project team uses an ongoing analysis of
+            kernel development, BSP support, and release timing to select the best
+            possible kernel.org version.
+            The team continually monitors community kernel
+            development to look for significant features of interest.
+            The illustration depicts this by showing the team looking back to kernel.org for new features, 
+            BSP features, and significant bug fixes.
+            The team does consider back-porting large features if they have a significant advantage. 
+            User or community demand can also trigger a back-port or creation of new
+            functionality in the Yocto Project baseline kernel during the uprev cycle. 
+        </para>
+        <para>
+            Generally speaking, every new kernel both adds features and introduces new bugs.
+            These consequences are the basic properties of upstream kernel development and are
+            managed by the Yocto Project team's kernel strategy. 
+            It is the Yocto Project team's policy to not back-port minor features to the released kernel. 
+            They only consider back-porting significant technological jumps - and, that is done 
+            after a complete gap analysis. 
+            The reason for this policy is that simply back-porting any small to medium sized change 
+            from an evolving kernel can easily create mismatches, incompatibilities and very 
+            subtle errors.
+        </para>
+        <para>
+            These policies result in both a stable and a cutting
+            edge kernel that mixes forward ports of existing features and significant and critical 
+            new functionality. 
+            Forward porting functionality in the Yocto Project kernel can be thought of as a
+            "micro uprev."
+            The many “micro uprevs” produce a kernel version with a mix of 
+            important new mainline, non-mainline, BSP developments and feature integrations. 
+            This kernel gives insight into new features and allows focused
+            amounts of testing to be done on the kernel, which prevents
+            surprises when selecting the next major uprev. 
+            The quality of these cutting edge kernels is evolving and the kernels are used in very special 
+            cases for BSP and feature development.
+        </para>
+    </section>
+
+    <section id='kernel-architecture'>
+        <title>Kernel Architecture</title>
+        <para>
+            This section describes the architecture of the Yocto Project kernel and provides information
+            on the mechanisms used to achieve that architecture.
+        </para>
+        
+        <section id='architecture-overview'>
+            <title>Overview</title>
+            <para>
+                As mentioned earlier, a key goal of Yocto Project is to present the developer with 
+                a kernel that has a clear and continuous history that is visible to the user. 
+                The architecture and mechanisms used achieve that goal in a manner similar to the 
+                upstream kernel.org.
+                
+            </para>
+            <para>
+                You can think of the Yocto Project kernel as consisting of a baseline kernel with
+                added features logically structured on top of the baseline.
+                The features are tagged and organized by way of a branching strategy implemented by the 
+                source code manager (SCM) git. 
+                The result is that the user has the ability to see the added features and 
+                the commits that make up those features.
+                In addition to being able to see added features, the user can also view the history of what 
+                made up the baseline kernel as well.
+            </para>
+            <para>
+                The following illustration shows the conceptual Yocto Project kernel.
+            </para>
+            <para>
+                <imagedata fileref="figures/kernel-architecture-overview.png" width="6in" depth="7in" align="center" scale="100" />
+            </para>
+            <para>
+                In the illustration, the "kernel.org Branch Point" marks the specific spot (or release) from 
+                which the Yocto Project kernel is created.  From this point "up" in the tree features and 
+                differences are organized and tagged.
+            </para>
+            <para>
+                The "Yocto Project Baseline Kernel" contains functionality that is common to every kernel
+                type and BSP that is organized further up the tree.  Placing these common features in the 
+                tree this way means features don't have to be duplicated along individual branches of the 
+                structure.
+            </para>
+            <para>
+                From the Yocto Project Baseline Kernel branch points represent specific functionality
+                for individual BSPs as well as real-time kernels.
+                The illustration represents this through three BSP-specific branches and a real-time 
+                kernel branch.  
+                Each branch represents some unique functionality for the BSP or a real-time kernel.
+            </para>
+            <para>
+                The real-time kernel branch has common features for all real-time kernels and contains
+                more branches for individual BSP-specific real-time kernels.  
+                The illustration shows three branches as an example. 
+                Each branch points the way to specific, unique features for a respective real-time
+                kernel as they apply to a given BSP.
+            </para>
+            <para>
+                The resulting tree structure presents a clear path of markers (or branches) to the user
+                that for all practical purposes is the kernel needed for any given set of requirements.
+            </para>
+        </section>
+ 
+        <section id='branching-and-workflow'>
+            <title>Branching Strategy and Workflow</title>
+            <para>
+                The Yocto Project team creates kernel branches at points where functionality is 
+                no longer shared and thus, needs to be isolated.
+                For example, board-specific incompatibilities would require different functionality
+                and would require a branch to separate the features. 
+                Likewise, for specific kernel features the same branching strategy is used.
+                This branching strategy results in a tree that has features organized to be specific 
+                for particular functionality, single kernel types, or a subset of kernel types.  
+                This strategy results in not having to store the same feature twice internally in the 
+                tree.
+                Rather we store the unique differences required to apply the feature onto the kernel type 
+                in question.
+            </para>
+            <para>
+                BSP-specific code additions are handled in a similar manner to kernel-specific additions. 
+                Some BSPs only make sense given certain kernel types.
+                So, for these types, we create branches off the end of that kernel type for all 
+                of the BSPs that are supported on that kernel type.  
+                From the perspective of the tools that create the BSP branch, the BSP is really no 
+                different than a feature.
+                Consequently, the same branching strategy applies to BSPs as it does to features.
+                So again, rather than store the BSP twice, only the unique differences for the BSP across
+                the supported multiple kernels are uniquely stored.
+            </para>
+            <para>
+                While this strategy results in a tree with a significant number of branches, it is
+                important to realize that from the customer's point of view, there is a linear
+                path that travels from the baseline kernel.org, through a select group of features and
+                ends with their BSP-specific commits.
+                In other words, the divisions of the kernel are transparent and are not relevant 
+                to the developer on a day-to-day basis.  
+                From the customer's perspective, this is the "master" branch.
+                They do not need not be aware of the existence of any other branches at all.  
+                Of course there is value in the existence of these branches
+                in the tree, should a person decide to explore them. 
+                For example, a comparison between two BSPs at either the commit level or at the line-by-line 
+                code diff level is now a trivial operation.
+            </para>
+            <para>
+                Working with the kernel as a structured tree follows recognized community best practices. 
+                In particular, the kernel as shipped with the product should be
+                considered an 'upstream source' and viewed as a series of
+                historical and documented modifications (commits). 
+                These modifications represent the development and stabilization done
+                by the Yocto Project kernel development team.
+            </para>
+            <para>
+                Because commits only change at significant release points in the product life cycle,
+                developers can work on a branch created
+                from the last relevant commit in the shipped Yocto Project kernel. 
+                As mentioned previously, the structure is transparent to the user
+                because the kernel tree is left in this state after cloning and building the kernel.
+            </para>
+        </section>
+     
+        <section id='source-code-manager-git'>
+            <title>Source Code Manager - git</title>
+            <para>
+                The Source Code Manager (SCM) is git and it is the obvious mechanism for meeting the 
+                previously mentioned goals.  
+                Not only is it the SCM for kernel.org but git continues to grow in popularity and
+                supports many different work flows, front-ends and management techniques.
+            </para>
+            <note><para> 
+                It should be noted that you can use as much, or as little, of what git has to offer 
+                as is appropriate to your project.
+            </para></note>
+        </section>
+    </section>
+
+    <section id='kernel-tools'>
+        <title>Kernel Tools</title>
+        <para>
+Since most standard workflows involve moving forward with an existing tree by
+continuing to add and alter the underlying baseline, the tools that manage
+Yocto Project's kernel construction are largely hidden from the developer to
+present a simplified view of the kernel for ease of use.
+</para>
+<para>
+The fundamental properties of the tools that manage and construct the
+kernel are:
+<itemizedlist>
+    <listitem><para>the ability to group patches into named, reusable features</para></listitem>
+    <listitem><para>to allow top down control of included features</para></listitem>
+    <listitem><para>the binding of kernel configuration to kernel patches/features</para></listitem>
+    <listitem><para>the presentation of a seamless git repository that blends Yocto Project value with the kernel.org history and development</para></listitem>
+</itemizedlist>
+</para>
+<!--<para>
+The tools that construct a kernel tree will be discussed later in this 
+document. The following tools form the foundation of the Yocto Project 
+kernel toolkit:
+<itemizedlist>
+    <listitem><para>git  : distributed revision control system created by Linus Torvalds</para></listitem>
+    <listitem><para>guilt: quilt on top of git</para></listitem>
+    <listitem><para>*cfg : kernel configuration management and classification</para></listitem>
+    <listitem><para>kgit*: Yocto Project kernel tree creation and management tools</para></listitem>
+    <listitem><para>scc  : series &amp; configuration compiler</para></listitem>
+</itemizedlist>
+</para> -->
+    </section> 
+
+
+
+
+
+</chapter>
+<!--
+vim: expandtab tw=80 ts=4
+-->