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@@ -83,6 +83,170 @@
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feature and BSP development.
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</para>
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</section>
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+
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+ <section id='kernel-architecture'>
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+ <title>Kernel Architecture</title>
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+ <para>
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+ This section describes the architecture of the kernels available through the
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+ Yocto Project and provides information
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+ on the mechanisms used to achieve that architecture.
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+ </para>
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+
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+ <section id='architecture-overview'>
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+ <title>Overview</title>
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+ <para>
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+ As mentioned earlier, a key goal of the Yocto Project is to present the
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+ developer with
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+ a kernel that has a clear and continuous history that is visible to the user.
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+ The architecture and mechanisms used achieve that goal in a manner similar to the
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+ upstream <filename>kernel.org</filename>.
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+ </para>
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+ <para>
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+ You can think of a Yocto Project kernel as consisting of a baseline Linux kernel with
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+ added features logically structured on top of the baseline.
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+ The features are tagged and organized by way of a branching strategy implemented by the
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+ source code manager (SCM) Git.
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+ For information on Git as applied to the Yocto Project, see the
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+ "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>" section in the
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+ Yocto Project Development Manual.
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+ </para>
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+ <para>
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+ The result is that the user has the ability to see the added features and
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+ the commits that make up those features.
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+ In addition to being able to see added features, the user can also view the history of what
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+ made up the baseline kernel.
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+ </para>
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+ <para>
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+ The following illustration shows the conceptual Yocto Project kernel.
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+ </para>
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+ <para>
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+ <imagedata fileref="figures/kernel-architecture-overview.png" width="6in" depth="7in" align="center" scale="100" />
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+ </para>
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+ <para>
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+ In the illustration, the "Kernel.org Branch Point"
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+ marks the specific spot (or release) from
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+ which the Yocto Project kernel is created.
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+ From this point "up" in the tree, features and differences are organized and tagged.
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+ </para>
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+ <para>
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+ The "Yocto Project Baseline Kernel" contains functionality that is common to every kernel
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+ type and BSP that is organized further up the tree.
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+ Placing these common features in the
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+ tree this way means features don't have to be duplicated along individual branches of the
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+ structure.
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+ </para>
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+ <para>
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+ From the Yocto Project Baseline Kernel, branch points represent specific functionality
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+ for individual BSPs as well as real-time kernels.
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+ The illustration represents this through three BSP-specific branches and a real-time
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+ kernel branch.
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+ Each branch represents some unique functionality for the BSP or a real-time kernel.
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+ </para>
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+ <para>
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+ In this example structure, the real-time kernel branch has common features for all
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+ real-time kernels and contains
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+ more branches for individual BSP-specific real-time kernels.
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+ The illustration shows three branches as an example.
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+ Each branch points the way to specific, unique features for a respective real-time
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+ kernel as they apply to a given BSP.
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+ </para>
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+ <para>
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+ The resulting tree structure presents a clear path of markers (or branches) to the
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+ developer that, for all practical purposes, is the kernel needed for any given set
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+ of requirements.
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+ </para>
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+ </section>
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+
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+ <section id='branching-and-workflow'>
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+ <title>Branching Strategy and Workflow</title>
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+ <para>
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+ The Yocto Project team creates kernel branches at points where functionality is
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+ no longer shared and thus, needs to be isolated.
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+ For example, board-specific incompatibilities would require different functionality
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+ and would require a branch to separate the features.
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+ Likewise, for specific kernel features, the same branching strategy is used.
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+ </para>
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+ <para>
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+ This branching strategy results in a tree that has features organized to be specific
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+ for particular functionality, single kernel types, or a subset of kernel types.
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+ This strategy also results in not having to store the same feature twice
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+ internally in the tree.
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+ Rather, the kernel team stores the unique differences required to apply the
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+ feature onto the kernel type in question.
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+ <note>
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+ The Yocto Project team strives to place features in the tree such that they can be
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+ shared by all boards and kernel types where possible.
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+ However, during development cycles or when large features are merged,
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+ the team cannot always follow this practice.
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+ In those cases, the team uses isolated branches to merge features.
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+ </note>
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+ </para>
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+ <para>
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+ BSP-specific code additions are handled in a similar manner to kernel-specific additions.
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+ Some BSPs only make sense given certain kernel types.
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+ So, for these types, the team creates branches off the end of that kernel type for all
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+ of the BSPs that are supported on that kernel type.
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+ From the perspective of the tools that create the BSP branch, the BSP is really no
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+ different than a feature.
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+ Consequently, the same branching strategy applies to BSPs as it does to features.
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+ So again, rather than store the BSP twice, the team only stores the unique
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+ differences for the BSP across the supported multiple kernels.
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+ </para>
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+ <para>
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+ While this strategy can result in a tree with a significant number of branches, it is
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+ important to realize that from the developer's point of view, there is a linear
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+ path that travels from the baseline <filename>kernel.org</filename>, through a select
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+ group of features and ends with their BSP-specific commits.
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+ In other words, the divisions of the kernel are transparent and are not relevant
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+ to the developer on a day-to-day basis.
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+ From the developer's perspective, this path is the "master" branch.
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+ The developer does not need to be aware of the existence of any other branches at all.
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+ Of course, there is value in the existence of these branches
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+ in the tree, should a person decide to explore them.
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+ For example, a comparison between two BSPs at either the commit level or at the line-by-line
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+ code <filename>diff</filename> level is now a trivial operation.
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+ </para>
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+ <para>
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+ Working with the kernel as a structured tree follows recognized community best practices.
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+ In particular, the kernel as shipped with the product, should be
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+ considered an "upstream source" and viewed as a series of
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+ historical and documented modifications (commits).
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+ These modifications represent the development and stabilization done
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+ by the Yocto Project kernel development team.
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+ </para>
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+ <para>
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+ Because commits only change at significant release points in the product life cycle,
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+ developers can work on a branch created
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+ from the last relevant commit in the shipped Yocto Project kernel.
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+ As mentioned previously, the structure is transparent to the developer
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+ because the kernel tree is left in this state after cloning and building the kernel.
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+ </para>
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+ </section>
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+
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+ <section id='source-code-manager-git'>
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+ <title>Source Code Manager - Git</title>
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+ <para>
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+ The Source Code Manager (SCM) is Git.
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+ This SCM is the obvious mechanism for meeting the previously mentioned goals.
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+ Not only is it the SCM for <filename>kernel.org</filename> but,
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+ Git continues to grow in popularity and supports many different work flows,
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+ front-ends and management techniques.
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+ </para>
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+ <para>
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+ You can find documentation on Git at <ulink url='http://git-scm.com/documentation'></ulink>.
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+ You can also get an introduction to Git as it applies to the Yocto Project in the
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+ "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>"
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+ section in the Yocto Project Development Manual.
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+ These referenced sections overview Git and describe a minimal set of
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+ commands that allows you to be functional using Git.
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+ <note>
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+ You can use as much, or as little, of what Git has to offer to accomplish what
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+ you need for your project.
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+ You do not have to be a "Git Master" in order to use it with the Yocto Project.
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+ </note>
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+ </para>
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+ </section>
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+ </section>
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</appendix>
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<!--
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vim: expandtab tw=80 ts=4
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Scott Rifenbark
