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kernel-dev: Added new appendix for kernel concepts. 

(From yocto-docs rev: e25465c6d177a27d3dee742ebc958ae30f968ffa)

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 2013-01-28 11:18:25 -06:00 committed by Richard Purdie
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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"
[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >
<appendix id='kernel-dev-concepts-appx'>
<title>Advanced Kernel Concepts</title>
<section id='kernel-big-picture'>
<title>Yocto Project Kernel Development and Maintenance</title>
<para>
Kernels available through the Yocto Project, like other kernels, are based off the Linux
kernel releases from <ulink url='http://www.kernel.org'></ulink>.
At the beginning of a major development cycle, the Yocto Project team
chooses its kernel based on factors such as release timing, the anticipated release
timing of final upstream <filename>kernel.org</filename> versions, and Yocto Project
feature requirements.
Typically, the kernel chosen is in the
final stages of development by the community.
In other words, the kernel is in the release
candidate or "rc" phase and not yet a final release.
But, by being in the final stages of external development, the team knows that the
<filename>kernel.org</filename> final release will clearly be within the early stages of
the Yocto Project development window.
</para>
<para>
This balance allows the team to deliver the most up-to-date kernel
possible, while still ensuring that the team has a stable official release for
the baseline Linux kernel version.
</para>
<para>
The ultimate source for kernels available through the Yocto Project are released kernels
from <filename>kernel.org</filename>.
In addition to a foundational kernel from <filename>kernel.org</filename>, the
kernels available contain a mix of important new mainline
developments, non-mainline developments (when there is no alternative),
Board Support Package (BSP) developments,
and custom features.
These additions result in a commercially released Yocto Project Linux kernel that caters
to specific embedded designer needs for targeted hardware.
</para>
<para>
Once a kernel is officially released, the Yocto Project team goes into
their next development cycle, or upward revision (uprev) cycle, while still
continuing maintenance on the released kernel.
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 hundreds of individual fixes and minor features from various
kernel versions is not sustainable and can easily compromise quality.
</para>
<para>
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 <filename>kernel.org</filename> version.
The team continually monitors community kernel
development to look for significant features of interest.
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 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 kernels available through 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 leading edge
feature and BSP development.
</para>
</section>
</appendix>
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