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<title>Yocto Project Kernel Concepts</title>
This chapter provides conceptual information about the Yocto Project kernel:
<listitem><para>Yocto Project Kernel Development and Maintenance Overview</para></listitem>
The complexity of embedded kernel design has increased dramatically.
Whether it is managing multiple implementations of a particular feature or tuning and
optimizing board specific features, flexibility and maintainability are key concerns.
The Yocto Project Linux kernel is presented with the embedded
developer's needs in mind and has evolved to assist in these key concerns.
For example, prior methods such as applying hundreds of patches to an extracted
tarball have been replaced with proven techniques that allow easy inspection,
bisection and analysis of changes.
Application of these techniques also creates a platform for performing integration and
collaboration with the thousands of upstream development projects.
With all these considerations in mind, the Yocto Project kernel and development team
strives to attain these goals:
<listitem><para>Allow the end user to leverage community best practices to seamlessly
manage the development, build and debug cycles.</para></listitem>
<listitem><para>Create a platform for performing integration and collaboration with the
thousands of upstream development projects that exist.</para></listitem>
<listitem><para>Provide mechanisms that support many different work flows, front-ends and
<listitem><para>Deliver the most up-to-date kernel possible while still ensuring that
the baseline kernel is the most stable official release.</para></listitem>
<listitem><para>Include major technological features as part of Yocto Project's
upward revision strategy.</para></listitem>
<listitem><para>Present a kernel Git repository that, similar to the upstream
has a clear and continuous history.</para></listitem>
<listitem><para>Deliver a key set of supported kernel types, where each type is tailored
to meet a specific use (e.g. networking, consumer, devices, and so forth).</para></listitem>
<listitem><para>Employ a Git branching strategy that, from a developer's point of view,
results in a linear path from the baseline <filename>kernel.org</filename>,
through a select group of features and
ends with their BSP-specific commits.</para></listitem>
<title>Yocto Project Kernel Development and Maintenance Overview</title>
The Yocto Project kernel, like other kernels, is based off the Linux kernel release
from <ulink url='http://www.kernel.org'></ulink>.
At the beginning of a major development cycle, the Yocto Project team
chooses its Yocto Project kernel
based on factors like 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.
This balance allows the team to deliver the most up-to-date kernel
as possible, while still ensuring that the team has a stable official release for
the baseline kernel version.
The ultimate source for the Yocto Project kernel is a released kernel
In addition to a foundational kernel from <filename>kernel.org</filename>, the released
Yocto Project kernel contains 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 kernel that caters
to specific embedded designer needs for targeted hardware.
Once a Yocto Project 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.
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.
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
These policies result in both a stable and a cutting
edge kernel that mixes forward ports of existing features and significant and critical
Forward porting functionality in the Yocto Project kernel can be thought of as a
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.
This section describes the architecture of the Yocto Project kernel and provides information
on the mechanisms used to achieve that architecture.
As mentioned earlier, a key goal of the Yocto Project is to present the
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
You can think of the Yocto Project kernel as consisting of a baseline 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
section in <ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html'>The
Yocto Project Development Manual</ulink>.
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.
The following illustration shows the conceptual Yocto Project kernel.
<imagedata fileref="figures/kernel-architecture-overview.png" width="6in" depth="7in" align="center" scale="100" />
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.
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
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
Each branch represents some unique functionality for the BSP or a real-time kernel.
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.
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
<title>Branching Strategy and Workflow</title>
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.
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.
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.
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.
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 not 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.
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.
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.
<title>Source Code Manager - Git</title>
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.
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
section in <ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html'>The
Yocto Project Development Manual</ulink>.
These referenced sections overview Git and describe a minimal set of
commands that allow you to be functional using Git.
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.
Kernel configuration, along with kernel features, defines how a Linux Yocto
kernel image is built.
Through configuration settings, you can customize a Linux Yocto kernel to be
specific to particular hardware.
For example, you can specify sound support or networking support.
This section describes basic concepts behind Kernel configuration within the
Yocto Project and references you to other areas for specific configuration
Conceptually, Linux Yocto kernel configuration occurs similarly to that needed for any
The Linux Yocto kernel build process uses a <filename>.config</filename>, which
is created through the Linux Kernel Coinfiguration (LKC) tool.
You can directly set various configurations in the
<filename>.config</filename> file by using the <filename>menuconfig</filename>
tool as built by BitBake.
You can also affect the configurations in the file by using configuration fragments.
It is not recommended that you edit the <filename>.config</filename> file directly.
Here is are some brief descriptions of the ways you can affect the
<listitem><para><emphasis>The <filename>menuconfig</filename> Tool:</emphasis>
One of many front-ends that allows you to define kernel configurations.
Some others are <filename>make config</filename>,
<filename>make nconfig</filename>, and <filename>make gconfig</filename>.
In the Yocto Project environment, you must use BitBake to build the
<filename>menuconfig</filename> tool before you can use it to define
$ bitbake linux-yocto -c menuconfig
After the tool is built, you can interact with it normally.
You can see how <filename>menuconfig</filename> is used to change a simple
kernel configuration in the
"<ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#changing-the-config-smp-configuration-using-menuconfig'>Changing the <filename>CONFIG_SMP</filename> Configuration Using <filename>menuconfig</filename></ulink>"
section of The Yocto Project Development Manual.
For general information on <filename>menuconfig</filename>, see
<listitem><para><emphasis>Configuration Fragments:</emphasis> A file with a
list of kernel options just as they would appear syntactically in the
Configuration fragments are typically logical groupings and are assembled
by the Yocto Project build system to produce input used by the LKC
that ultimately generates the <filename>.config</filename> file.</para>
variable can be used to list configuration fragments.
For further discussion on applying configuration fragments, see the
"<ulink url='http://www.yoctoproject.org/docs/latest/bsp-guide/bsp-guide.html#linux-kernel-configuration'>Linux Kernel Configuration</ulink>"
section in the Yocto Project Board Support Package (BSP) Guide.
Since most standard workflows involve moving forward with an existing tree by
continuing to add and alter the underlying baseline, the tools that manage
the Yocto Project's kernel construction are largely hidden from the developer to
present a simplified view of the kernel for ease of use.
Fundamentally, the kernel tools that manage and construct the
Yocto Project kernel accomplish the following:
<listitem><para>Group patches into named, reusable features.</para></listitem>
<listitem><para>Allow top-down control of included features.</para></listitem>
<listitem><para>Bind kernel configurations to kernel patches and features.</para></listitem>
<listitem><para>Present a seamless Git repository that blends Yocto Project value
with the <filename>kernel.org</filename> history and development.</para></listitem>
WRITER NOTE: Put this in for post 1.1 if possible:
The tools that construct a kernel tree will be discussed later in this
document. The following tools form the foundation of the Yocto Project
<listitem><para>git : distributed revision control system created by Linus Torvalds</para></listitem>
<listitem><para>guilt: quilt on top of git</para></listitem>
<listitem><para>*cfg : kernel configuration management and classification</para></listitem>
<listitem><para>kgit*: Yocto Project kernel tree creation and management tools</para></listitem>
<listitem><para>scc : series & configuration compiler</para></listitem>
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