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1<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
2"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
3
4<chapter id='kernel-concepts'>
5
6<title>Yocto Project Kernel Concepts</title>
7
8<section id='concepts-org'>
9 <title>Introduction</title>
10 <para>
11 This chapter provides conceptual information about the Yocto Project kernel:
12 <itemizedlist>
13 <listitem><para>Kernel Goals</para></listitem>
14 <listitem><para>Yocto Project Kernel Development and Maintenance Overview</para></listitem>
15 <listitem><para>Kernel Architecture</para></listitem>
16 <listitem><para>Kernel Tools</para></listitem>
17 </itemizedlist>
18 </para>
19</section>
20
21 <section id='kernel-goals'>
22 <title>Kernel Goals</title>
23 <para>
24 The complexity of embedded kernel design has increased dramatically.
25 Whether it is managing multiple implementations of a particular feature or tuning and
26 optimizing board specific features, flexibility and maintainability are key concerns.
27 The Yocto Project Linux kernel is presented with the embedded
28 developer's needs in mind and has evolved to assist in these key concerns.
29 For example, prior methods such as applying hundreds of patches to an extracted
30 tarball have been replaced with proven techniques that allow easy inspection,
31 bisection and analysis of changes.
32 Application of these techniques also creates a platform for performing integration and
33 collaboration with the thousands of upstream development projects.
34 </para>
35 <para>
36 With all these considerations in mind, the Yocto Project kernel and development team
37 strives to attain these goals:
38 <itemizedlist>
39 <listitem><para>Allow the end user to leverage community best practices to seamlessly
40 manage the development, build and debug cycles.</para></listitem>
41 <listitem><para>Create a platform for performing integration and collaboration with the
42 thousands of upstream development projects that exist.</para></listitem>
43 <listitem><para>Provide mechanisms that support many different work flows, front-ends and
44 management techniques.</para></listitem>
45 <listitem><para>Deliver the most up-to-date kernel possible while still ensuring that
46 the baseline kernel is the the most stable official release.</para></listitem>
47 <listitem><para>Include major technological features as part of Yocto Project's up-rev
48 strategy.</para></listitem>
49 <listitem><para>Present a git tree, that just like the upstream kernel.org tree, has a
50 clear and continuous history.</para></listitem>
51 <listitem><para>Deliver a key set of supported kernel types, where each type is tailored
52 to a specific use case (i.g. networking, consumer, devices, and so forth).</para></listitem>
53 <listitem><para>Employ a git branching strategy that from a customer's point of view
54 results in a linear path from the baseline kernel.org, through a select group of features and
55 ends with their BSP-specific commits.</para></listitem>
56 </itemizedlist>
57 </para>
58 </section>
59
60 <section id='kernel-big-picture'>
61 <title>Yocto Project Kernel Development and Maintenance Overview</title>
62 <para>
63 Yocto Project kernel, like other kernels, is based off the Linux kernel release
64 from <ulink url='http://www.kernel.org'></ulink>.
65 At the beginning of our major development cycle, we choose our Yocto Project kernel
66 based on factors like release timing, the anticipated release timing of "final" (i.e. non "rc")
67 upstream kernel.org versions, and Yocto Project feature requirements.
68 Typically this will be a kernel that is in the
69 final stages of development by the community (i.e. still in the release
70 candidate or "rc" phase) and not yet a final release.
71 But by being in the final stages of external development, we know that the
72 kernel.org final release will clearly land within the early stages of
73 the Yocto Project development window.
74 </para>
75 <para>
76 This balance allows us to deliver the most up-to-date kernel
77 as possible, while still ensuring that we have a stable official release as
78 our baseline kernel version.
79 </para>
80 <para>
81 The following figure represents the overall place the Yocto Project kernel fills.
82 </para>
83 <para>
84 <imagedata fileref="figures/kernel-big-picture.png" width="6in" depth="6in" align="center" scale="100" />
85 </para>
86 <para>
87 In the figure the ultimate source for the Yocto Project kernel is a released kernel
88 from kernel.org.
89 In addition to a foundational kernel from kernel.org the commercially released
90 Yocto Project kernel contains a mix of important new mainline
91 developments, non-mainline developments, Board Support Package (BSP) developments,
92 and custom features.
93 These additions result in a commercially released Yocto Project kernel that caters
94 to specific embedded designer needs for targeted hardware.
95 </para>
96 <para>
97 Once a Yocto Project kernel is officially released the Yocto Project team goes into
98 their next development cycle, or "uprev" cycle.
99 It is important to note that the most sustainable and stable way
100 to include feature development upstream is through a kernel uprev process.
101 Back-porting of hundreds of individual fixes and minor features from various
102 kernel versions is not sustainable and can easily compromise quality.
103 During the uprev cycle, the Yocto Project team uses an ongoing analysis of
104 kernel development, BSP support, and release timing to select the best
105 possible kernel.org version.
106 The team continually monitors community kernel
107 development to look for significant features of interest.
108 The illustration depicts this by showing the team looking back to kernel.org for new features,
109 BSP features, and significant bug fixes.
110 The team does consider back-porting large features if they have a significant advantage.
111 User or community demand can also trigger a back-port or creation of new
112 functionality in the Yocto Project baseline kernel during the uprev cycle.
113 </para>
114 <para>
115 Generally speaking, every new kernel both adds features and introduces new bugs.
116 These consequences are the basic properties of upstream kernel development and are
117 managed by the Yocto Project team's kernel strategy.
118 It is the Yocto Project team's policy to not back-port minor features to the released kernel.
119 They only consider back-porting significant technological jumps - and, that is done
120 after a complete gap analysis.
121 The reason for this policy is that simply back-porting any small to medium sized change
122 from an evolving kernel can easily create mismatches, incompatibilities and very
123 subtle errors.
124 </para>
125 <para>
126 These policies result in both a stable and a cutting
127 edge kernel that mixes forward ports of existing features and significant and critical
128 new functionality.
129 Forward porting functionality in the Yocto Project kernel can be thought of as a
130 "micro uprev."
131 The many “micro uprevs” produce a kernel version with a mix of
132 important new mainline, non-mainline, BSP developments and feature integrations.
133 This kernel gives insight into new features and allows focused
134 amounts of testing to be done on the kernel, which prevents
135 surprises when selecting the next major uprev.
136 The quality of these cutting edge kernels is evolving and the kernels are used in very special
137 cases for BSP and feature development.
138 </para>
139 </section>
140
141 <section id='kernel-architecture'>
142 <title>Kernel Architecture</title>
143 <para>
144 This section describes the architecture of the Yocto Project kernel and provides information
145 on the mechanisms used to achieve that architecture.
146 </para>
147
148 <section id='architecture-overview'>
149 <title>Overview</title>
150 <para>
151 As mentioned earlier, a key goal of Yocto Project is to present the developer with
152 a kernel that has a clear and continuous history that is visible to the user.
153 The architecture and mechanisms used achieve that goal in a manner similar to the
154 upstream kernel.org.
155
156 </para>
157 <para>
158 You can think of the Yocto Project kernel as consisting of a baseline kernel with
159 added features logically structured on top of the baseline.
160 The features are tagged and organized by way of a branching strategy implemented by the
161 source code manager (SCM) git.
162 The result is that the user has the ability to see the added features and
163 the commits that make up those features.
164 In addition to being able to see added features, the user can also view the history of what
165 made up the baseline kernel as well.
166 </para>
167 <para>
168 The following illustration shows the conceptual Yocto Project kernel.
169 </para>
170 <para>
171 <imagedata fileref="figures/kernel-architecture-overview.png" width="6in" depth="7in" align="center" scale="100" />
172 </para>
173 <para>
174 In the illustration, the "kernel.org Branch Point" marks the specific spot (or release) from
175 which the Yocto Project kernel is created. From this point "up" in the tree features and
176 differences are organized and tagged.
177 </para>
178 <para>
179 The "Yocto Project Baseline Kernel" contains functionality that is common to every kernel
180 type and BSP that is organized further up the tree. Placing these common features in the
181 tree this way means features don't have to be duplicated along individual branches of the
182 structure.
183 </para>
184 <para>
185 From the Yocto Project Baseline Kernel branch points represent specific functionality
186 for individual BSPs as well as real-time kernels.
187 The illustration represents this through three BSP-specific branches and a real-time
188 kernel branch.
189 Each branch represents some unique functionality for the BSP or a real-time kernel.
190 </para>
191 <para>
192 The real-time kernel branch has common features for all real-time kernels and contains
193 more branches for individual BSP-specific real-time kernels.
194 The illustration shows three branches as an example.
195 Each branch points the way to specific, unique features for a respective real-time
196 kernel as they apply to a given BSP.
197 </para>
198 <para>
199 The resulting tree structure presents a clear path of markers (or branches) to the user
200 that for all practical purposes is the kernel needed for any given set of requirements.
201 </para>
202 </section>
203
204 <section id='branching-and-workflow'>
205 <title>Branching Strategy and Workflow</title>
206 <para>
207 The Yocto Project team creates kernel branches at points where functionality is
208 no longer shared and thus, needs to be isolated.
209 For example, board-specific incompatibilities would require different functionality
210 and would require a branch to separate the features.
211 Likewise, for specific kernel features the same branching strategy is used.
212 This branching strategy results in a tree that has features organized to be specific
213 for particular functionality, single kernel types, or a subset of kernel types.
214 This strategy results in not having to store the same feature twice internally in the
215 tree.
216 Rather we store the unique differences required to apply the feature onto the kernel type
217 in question.
218 </para>
219 <para>
220 BSP-specific code additions are handled in a similar manner to kernel-specific additions.
221 Some BSPs only make sense given certain kernel types.
222 So, for these types, we create branches off the end of that kernel type for all
223 of the BSPs that are supported on that kernel type.
224 From the perspective of the tools that create the BSP branch, the BSP is really no
225 different than a feature.
226 Consequently, the same branching strategy applies to BSPs as it does to features.
227 So again, rather than store the BSP twice, only the unique differences for the BSP across
228 the supported multiple kernels are uniquely stored.
229 </para>
230 <para>
231 While this strategy results in a tree with a significant number of branches, it is
232 important to realize that from the customer's point of view, there is a linear
233 path that travels from the baseline kernel.org, through a select group of features and
234 ends with their BSP-specific commits.
235 In other words, the divisions of the kernel are transparent and are not relevant
236 to the developer on a day-to-day basis.
237 From the customer's perspective, this is the "master" branch.
238 They do not need not be aware of the existence of any other branches at all.
239 Of course there is value in the existence of these branches
240 in the tree, should a person decide to explore them.
241 For example, a comparison between two BSPs at either the commit level or at the line-by-line
242 code diff level is now a trivial operation.
243 </para>
244 <para>
245 Working with the kernel as a structured tree follows recognized community best practices.
246 In particular, the kernel as shipped with the product should be
247 considered an 'upstream source' and viewed as a series of
248 historical and documented modifications (commits).
249 These modifications represent the development and stabilization done
250 by the Yocto Project kernel development team.
251 </para>
252 <para>
253 Because commits only change at significant release points in the product life cycle,
254 developers can work on a branch created
255 from the last relevant commit in the shipped Yocto Project kernel.
256 As mentioned previously, the structure is transparent to the user
257 because the kernel tree is left in this state after cloning and building the kernel.
258 </para>
259 </section>
260
261 <section id='source-code-manager-git'>
262 <title>Source Code Manager - git</title>
263 <para>
264 The Source Code Manager (SCM) is git and it is the obvious mechanism for meeting the
265 previously mentioned goals.
266 Not only is it the SCM for kernel.org but git continues to grow in popularity and
267 supports many different work flows, front-ends and management techniques.
268 </para>
269 <note><para>
270 It should be noted that you can use as much, or as little, of what git has to offer
271 as is appropriate to your project.
272 </para></note>
273 </section>
274 </section>
275
276 <section id='kernel-tools'>
277 <title>Kernel Tools</title>
278 <para>
279Since most standard workflows involve moving forward with an existing tree by
280continuing to add and alter the underlying baseline, the tools that manage
281Yocto Project's kernel construction are largely hidden from the developer to
282present a simplified view of the kernel for ease of use.
283</para>
284<para>
285The fundamental properties of the tools that manage and construct the
286kernel are:
287<itemizedlist>
288 <listitem><para>the ability to group patches into named, reusable features</para></listitem>
289 <listitem><para>to allow top down control of included features</para></listitem>
290 <listitem><para>the binding of kernel configuration to kernel patches/features</para></listitem>
291 <listitem><para>the presentation of a seamless git repository that blends Yocto Project value with the kernel.org history and development</para></listitem>
292</itemizedlist>
293</para>
294<!--<para>
295The tools that construct a kernel tree will be discussed later in this
296document. The following tools form the foundation of the Yocto Project
297kernel toolkit:
298<itemizedlist>
299 <listitem><para>git : distributed revision control system created by Linus Torvalds</para></listitem>
300 <listitem><para>guilt: quilt on top of git</para></listitem>
301 <listitem><para>*cfg : kernel configuration management and classification</para></listitem>
302 <listitem><para>kgit*: Yocto Project kernel tree creation and management tools</para></listitem>
303 <listitem><para>scc : series &amp; configuration compiler</para></listitem>
304</itemizedlist>
305</para> -->
306 </section>
307
308
309
310
311
312</chapter>
313<!--
314vim: expandtab tw=80 ts=4
315-->
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