kernel-dev: Added "Kernel Architecture" section.

Moved the "Kernel Architecture" section from the YP Kernel
Architecture and Use Manual to this manual.  The section
included the kernel-architecture-overview.png figure.  So,
I added that PNG file to the "figures" folder.  Finally, I
had to also add the PNG file to the Makefile tarfile list
for kernel-dev.  Note that because the figure was part of
the old YP Kernel Architecture and Use Manual, I did not have
to add the figure to the mega-manual tarfile list.

(From yocto-docs rev: fbc5508ce162ea7915fd5dce74338b6a5bfd7ce1)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 164 insertions, 0 deletions

diff --git a/documentation/kernel-dev/kernel-dev-concepts-appx.xml b/documentation/kernel-dev/kernel-dev-concepts-appx.xml
index d78d2dc86c..732c0c310a 100644
--- a/documentation/kernel-dev/kernel-dev-concepts-appx.xml
+++ b/documentation/kernel-dev/kernel-dev-concepts-appx.xml
@@ -83,6 +83,170 @@
             feature and BSP development.
         </para>
     </section>
+
+    <section id='kernel-architecture'>
+        <title>Kernel Architecture</title>
+        <para>
+            This section describes the architecture of the kernels available through the
+            Yocto Project 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 the 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 <filename>kernel.org</filename>.
+            </para>
+            <para>
+                You can think of a Yocto Project kernel as consisting of a baseline Linux 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.
+                For information on Git as applied to the Yocto Project, see the
+                "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>" section in the
+                Yocto Project Development Manual.
+            </para>
+            <para>
+                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.
+            </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>
+                In this example structure, 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
+                developer 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.
+            </para>
+            <para>
+                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 also results in not having to store the same feature twice
+                internally in the tree.
+                Rather, the kernel team stores the unique differences required to apply the
+                feature onto the kernel type in question.
+                <note>
+                    The Yocto Project team strives to place features in the tree such that they can be
+                    shared by all boards and kernel types where possible.
+                    However, during development cycles or when large features are merged,
+                    the team cannot always follow this practice.
+                    In those cases, the team uses isolated branches to merge features.
+                </note>
+            </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, the team creates 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, the team only stores the unique
+                differences for the BSP across the supported multiple kernels.
+            </para>
+            <para>
+                While this strategy can result in a tree with a significant number of branches, it is
+                important to realize that from the developer's point of view, there is a linear
+                path that travels from the baseline <filename>kernel.org</filename>, 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 developer's perspective, this path is the "master" branch.
+                The developer does not need to 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 <filename>diff</filename> 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 developer
+                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.
+                This SCM is the obvious mechanism for meeting the previously mentioned goals.
+                Not only is it the SCM for <filename>kernel.org</filename> but,
+                Git continues to grow in popularity and supports many different work flows,
+                front-ends and management techniques.
+            </para>
+            <para>
+                You can find documentation on Git at <ulink url='http://git-scm.com/documentation'></ulink>.
+                You can also get an introduction to Git as it applies to the Yocto Project in the
+                "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>"
+                section in the Yocto Project Development Manual.
+                These referenced sections overview Git and describe a minimal set of
+                commands that allows you to be functional using Git.
+                <note>
+                    You can use as much, or as little, of what Git has to offer to accomplish what
+                    you need for your project.
+                    You do not have to be a "Git Master" in order to use it with the Yocto Project.
+                </note>
+            </para>
+        </section>
+    </section>
 </appendix>
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