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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>
This commit is contained in:
Scott Rifenbark 2013-01-28 11:29:51 -06:00 committed by Richard Purdie
parent 36b5f97e26
commit a24cb73f39
3 changed files with 166 additions and 1 deletions
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3
documentation/Makefile
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@ -289,7 +289,8 @@ XSLTOPTS = --stringparam html.stylesheet kernel-dev-style.css \
--stringparam section.label.includes.component.label 1 \
--xinclude
ALLPREQ = html pdf tarball
TARFILES = kernel-dev.html kernel-dev.pdf kernel-dev-style.css figures/kernel-dev-title.png
TARFILES = kernel-dev.html kernel-dev.pdf kernel-dev-style.css figures/kernel-dev-title.png \
figures/kernel-architecture-overview.png
MANUALS = $(DOC)/$(DOC).html $(DOC)/$(DOC).pdf
FIGURES = figures
STYLESHEET = $(DOC)/*.css

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documentation/kernel-dev/kernel-dev-concepts-appx.xml
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@ -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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