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diff --git a/import-layers/yocto-poky/documentation/kernel-dev/kernel-dev-concepts-appx.xml b/import-layers/yocto-poky/documentation/kernel-dev/kernel-dev-concepts-appx.xml new file mode 100644 index 000000000..ac91749cd --- /dev/null +++ b/import-layers/yocto-poky/documentation/kernel-dev/kernel-dev-concepts-appx.xml @@ -0,0 +1,253 @@ +<!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> + + <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 do not 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> +<!-- +vim: expandtab tw=80 ts=4 +--> |
