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kernel-dev, dev-manual: Moved Yocto Linux kernel overview

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Fixes [YOCTO #11630]

I took the Yocto Linux kernel overview list of kernels out of the
dev-manual and moved them to the "Yocto Project Kernel Development
and Maintenance" section in Appendix A of the kernel-dev manual.
This is a much more logical area to introduce this type of Yocto
Linux kernel information.

(From yocto-docs rev: 0a5154cb4c23697f2b8e2341c8e1dc07362443d2)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
This commit is contained in:
Scott Rifenbark
2017-06-29 09:32:57 -07:00
committed by Richard Purdie
parent 7d2d54683c
commit bdeb6db2fb
2 changed files with 150 additions and 126 deletions
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207
documentation/kernel-dev/kernel-dev-concepts-appx.xml
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@@ -7,80 +7,169 @@
<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
Kernels available through the Yocto Project (Linux Yocto kernels),
like other kernels, are based off the Linux kernel releases from
<ulink url='http://www.kernel.org'></ulink>.
At the beginning of a major Linux kernel development cycle, the
Yocto Project team chooses a Linux 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.
Typically, the Linux kernel chosen is in the final stages of
development by the Linux community.
In other words, the Linux kernel is in the release candidate
or "rc" phase and has yet to reach 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.
This balance allows the Yocto Project team to deliver the most
up-to-date Linux Yocto 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.
As implied earlier, the ultimate source for Linux Yocto kernels
are released kernels from <filename>kernel.org</filename>.
In addition to a foundational kernel from
<filename>kernel.org</filename>, the available Yocto Linux kernels
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.
You can find a web interface to the Yocto Linux kernels in the
<ulink url='&YOCTO_DOCS_REF_URL;#source-repositories'>Source Repositories</ulink>
at
<ulink url='&YOCTO_GIT_URL;'></ulink>.
If you look at the interface, you will see to the left a
grouping of Git repositories titled "Yocto Linux Kernel".
Within this group, you will find several Linux Yocto kernels
developed and included with Yocto Project releases:
<itemizedlist>
<listitem><para>
<emphasis><filename>linux-yocto-4.1</filename>:</emphasis>
The stable Yocto Project kernel to use with the Yocto
Project Release 2.0.
This kernel is based on the Linux 4.1 released kernel.
</para></listitem>
<listitem><para>
<emphasis><filename>linux-yocto-4.4</filename>:</emphasis>
The stable Yocto Project kernel to use with the Yocto
Project Release 2.1.
This kernel is based on the Linux 4.4 released kernel.
</para></listitem>
<listitem><para>
<emphasis><filename>linux-yocto-4.9</filename>:</emphasis>
The stable Yocto Project kernel to use with the Yocto
Project Release 2.3.
This kernel is based on the Linux 4.9 released kernel.
</para></listitem>
<listitem><para>
<emphasis><filename>linux-yocto-4.10</filename>:</emphasis>
The default stable Yocto Project kernel to use with the
Yocto Project Release 2.3.
This kernel is based on the Linux 4.10 released kernel.
</para></listitem>
<listitem><para>
<emphasis><filename>linux-yocto-dev</filename>:</emphasis>
A development kernel based on the latest upstream release
candidate available.
</para></listitem>
</itemizedlist>
<note><title>Notes</title>
Long Term Support Initiative (LTSI) for Yocto Linux
kernels is as follows:
<itemizedlist>
<listitem><para>
For Yocto Project releases 1.7, 1.8, and 2.0,
the LTSI kernel is
<filename>linux-yocto-3.14</filename>.
</para></listitem>
<listitem><para>
For Yocto Project releases 2.1, 2.2, and 2.3,
the LTSI kernel is <filename>linux-yocto-4.1</filename>.
</para></listitem>
<listitem><para>
<filename>linux-yocto-4.1</filename>,
<filename>linux-yocto-4.4</filename>, and
<filename>linux-yocto-4.9</filename> are all LTS
kernels.
</para></listitem>
</itemizedlist>
</note>
</para>
<para>
Once a Yocto Linux 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.
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.
During the uprev cycle, the Yocto Project team uses an ongoing
analysis of Linux kernel development, BSP support, and release
timing to select the best possible <filename>kernel.org</filename>
Linux kernel version on which to base subsequent Yocto Linux
kernel development.
The team continually monitors Linux 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.
Generally speaking, every new Linux kernel both adds features and
introduces new bugs.
These consequences are the basic properties of upstream
Linux kernel development and are managed by the Yocto Project
team's Linux Yocto kernel development strategy.
It is the Yocto Project team's policy to not back-port minor
features to the released Linux Yocto kernel.
They only consider back-porting significant technological
jumps &dash; 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 Linux 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.
The policies described in this section result in both a stable
and a cutting edge Linux Yocto kernel that mixes forward ports of
existing Linux kernel features and significant and critical new
functionality.
Forward porting Linux kernel functionality into the Linux Yocto
kernels available through the Yocto Project can be thought of as
a "micro uprev."
The many “micro uprevs” produce a Linux Yocto kernel version with
a mix of important new mainline, non-mainline, BSP developments
and feature integrations.
This Yocto Linux 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 Linux Yocto kernels is evolving
and the kernels are used in leading edge feature and BSP
development.
</para>
</section>