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documentation/dev-manual/dev-manual-cases.xml: partial kernel case 

(From yocto-docs rev: 798652fa1544dd141516ce9fb085f63d7a5da4cf)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
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Scott Rifenbark 2011-07-20 06:06:05 -07:00 committed by Richard Purdie
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<section id='modifying-a-kernel'>
<title>Modifying a Kernel</title>
<para>
Kernel modification involves changing or adding configurations to an existing kernel, or
adding recipes to the kernel that are needed to support specific hardware features.
The process is similar to creating a Board Support Package (BSP) except that it does not
involve a BSP layer.
</para>
<para>
This section presents a brief overview of the kernel structure and then provides a simple
example that shows how to modify the kernel.
</para>
<section id='yocto-project-kernel-overview'>
<title>Yocto Project Kernel Overview</title>
<para>
When one thinks of the source files for a kernel they usually think of a fixed structure
of files that contain kernel patches.
The Yocto Project, however, employs mechanisims that in a sense result in a kernel source
generator.
</para>
<para>
The Yocto Project uses the source code management (SCM) tool Git to manage and track Yocto
Project files.
Git employs branching strategies that effectively produce a tree-like structure whose
branches represent diversions from more general code.
For example, suppose two kernels are basically identical with the exception of a couple
different features in each.
In the Yocto Project kernel Git repository a main branch can contain the common or shared
parts of the kernel source and two branches that diverge from that common branch can
each contain the features specific to the respective kernel.
The result is a tree whose "leaves" represent the end of a specific path that yields
a set of kernel source files necessary for a specific piece of hardware and its features.
</para>
<para>
A big advantage to this scheme is the sharing of common features by keeping them in
"larger" branches that are further up the tree.
This practice eliminates redundant storage of similar features shared among kernels.
</para>
<para>
When you build the kernel on your development system all files needed for the build
are taken from the local tree (the Git repository) and gathered in a temporary work area
where they are subsequently used to create the unique kernel.
Thus, in a sense, the process uses a local, all-inclusive source tree to generate
(or choose) the specific kernel source files you need for the build - a source
generator if you will.
</para>
<para>
For a complete discussion of the Yocto Project kernel's architcture and its branching strategy,
see the <ulink url='http://www.yoctoproject.org/docs/1.1/kernel-manual/kernel-manual.html'>
The Yocto Project Kernel Architecture and Use Manual</ulink>.
</para>
<para>
You can find a web interface to the Yocto Project source repository at
<ulink url='http://git.yoctoproject.org/'></ulink>.
Within the interface you will see groups of related source code, each of which can
be cloned using Git to result in a working Git repository on your local system.
The Yocto Project supports four types of kernels in its source repositories at
<ulink url='http://git.yoctoproject.org/'></ulink>:
<itemizedlist>
<listitem><para><emphasis><filename>linux-yocto-2.6.34</filename></emphasis> - The
stable Linux Yocto kernel that is based on the Linux 2.6.34 release.</para></listitem>
<listitem><para><emphasis><filename>linux-yocto-2.6.37</filename></emphasis> - The current
Linux Yocto kernel that is based on the Linux 2.6.37 release.</para></listitem>
<listitem><para><emphasis><filename>linux-yocto-dev</filename></emphasis> - A development
kernel based on the Linux 2.6.39-rc1 release.</para></listitem>
<listitem><para><emphasis><filename>linux-2.6</filename></emphasis> - A kernel based on
minimal Linux mainline tracking.
[WRITER'S NOTE: I don't know which Git repository the user needs to clone to get this
repository on their development system.]</para></listitem>
</itemizedlist>
</para>
</section>
<section id='modifying-a-kernel'>
<title>Modifying a Kernel</title>
<para>
The remainder of this section presents a simple example that modifies a kernel.
For the purpose of this example we are going to base our new kernel on the current
Linux Yocto 2.6.37 release.
<para>
[WRITER'S NOTE: This section is a second example that focuses on just modifying the kernel.
I don't have any information on this yet.