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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[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >
4
5<chapter id='kernel-concepts'>
6
7<title>Yocto Project Kernel Concepts</title>
8
9<section id='concepts-org'>
10 <title>Introduction</title>
11 <para>
12 This chapter provides conceptual information about the kernel:
13 <itemizedlist>
14 <listitem><para>Kernel Goals</para></listitem>
15 <listitem><para>Kernel Development and Maintenance Overview</para></listitem>
16 <listitem><para>Kernel Architecture</para></listitem>
17 <listitem><para>Kernel Tools</para></listitem>
18 </itemizedlist>
19 </para>
20</section>
21
22 <section id='kernel-goals'>
23 <title>Kernel Goals</title>
24 <para>
25 The complexity of embedded kernel design has increased dramatically.
26 Whether it is managing multiple implementations of a particular feature or tuning and
27 optimizing board specific features, both flexibility and maintainability are key concerns.
28 The Linux kernels available through the Yocto Project are presented with the embedded
29 developer's needs in mind and have evolved to assist in these key concerns.
30 For example, prior methods such as applying hundreds of patches to an extracted
31 tarball have been replaced with proven techniques that allow easy inspection,
32 bisection and analysis of changes.
33 Application of these techniques also creates a platform for performing integration and
34 collaboration with the thousands of upstream development projects.
35 </para>
36 <para>
37 With all these considerations in mind, the Yocto Project's kernel and development team
38 strives to attain these goals:
39 <itemizedlist>
40 <listitem><para>Allow the end user to leverage community best practices to seamlessly
41 manage the development, build and debug cycles.</para></listitem>
42 <listitem><para>Create a platform for performing integration and collaboration with the
43 thousands of upstream development projects that exist.</para></listitem>
44 <listitem><para>Provide mechanisms that support many different work flows, front-ends and
45 management techniques.</para></listitem>
46 <listitem><para>Deliver the most up-to-date kernel possible while still ensuring that
47 the baseline kernel is the most stable official release.</para></listitem>
48 <listitem><para>Include major technological features as part of the Yocto Project's
49 upward revision strategy.</para></listitem>
50 <listitem><para>Present a kernel Git repository that, similar to the upstream
51 <filename>kernel.org</filename> tree,
52 has a clear and continuous history.</para></listitem>
53 <listitem><para>Deliver a key set of supported kernel types, where each type is tailored
54 to meet a specific use (e.g. networking, consumer, devices, and so forth).</para></listitem>
55 <listitem><para>Employ a Git branching strategy that, from a developer's point of view,
56 results in a linear path from the baseline <filename>kernel.org</filename>,
57 through a select group of features and
58 ends with their BSP-specific commits.</para></listitem>
59 </itemizedlist>
60 </para>
61 </section>
62
63 <section id='kernel-big-picture'>
64 <title>Yocto Project Kernel Development and Maintenance Overview</title>
65 <para>
66 Kernels available through the Yocto Project, like other kernels, are based off the Linux
67 kernel releases from <ulink url='http://www.kernel.org'></ulink>.
68 At the beginning of a major development cycle, the Yocto Project team
69 chooses its kernel based on factors such as release timing, the anticipated release
70 timing of final upstream <filename>kernel.org</filename> versions, and Yocto Project
71 feature requirements.
72 Typically, the kernel chosen is in the
73 final stages of development by the community.
74 In other words, the kernel is in the release
75 candidate or "rc" phase and not yet a final release.
76 But, by being in the final stages of external development, the team knows that the
77 <filename>kernel.org</filename> final release will clearly be within the early stages of
78 the Yocto Project development window.
79 </para>
80 <para>
81 This balance allows the team to deliver the most up-to-date kernel
82 possible, while still ensuring that the team has a stable official release for
83 the baseline Linux kernel version.
84 </para>
85 <para>
86 The ultimate source for kernels available through the Yocto Project are released kernels
87 from <filename>kernel.org</filename>.
88 In addition to a foundational kernel from <filename>kernel.org</filename>, the
89 kernels available contain a mix of important new mainline
90 developments, non-mainline developments (when there is no alternative),
91 Board Support Package (BSP) developments,
92 and custom features.
93 These additions result in a commercially released Yocto Project Linux kernel that caters
94 to specific embedded designer needs for targeted hardware.
95 </para>
96 <para>
97 Once a kernel is officially released, the Yocto Project team goes into
98 their next development cycle, or upward revision (uprev) cycle, while still
99 continuing maintenance on the released kernel.
100 It is important to note that the most sustainable and stable way
101 to include feature development upstream is through a kernel uprev process.
102 Back-porting hundreds of individual fixes and minor features from various
103 kernel versions is not sustainable and can easily compromise quality.
104 </para>
105 <para>
106 During the uprev cycle, the Yocto Project team uses an ongoing analysis of
107 kernel development, BSP support, and release timing to select the best
108 possible <filename>kernel.org</filename> version.
109 The team continually monitors community kernel
110 development to look for significant features of interest.
111 The team does consider back-porting large features if they have a significant advantage.
112 User or community demand can also trigger a back-port or creation of new
113 functionality in the Yocto Project baseline kernel during the uprev cycle.
114 </para>
115 <para>
116 Generally speaking, every new kernel both adds features and introduces new bugs.
117 These consequences are the basic properties of upstream kernel development and are
118 managed by the Yocto Project team's kernel strategy.
119 It is the Yocto Project team's policy to not back-port minor features to the released kernel.
120 They only consider back-porting significant technological jumps - and, that is done
121 after a complete gap analysis.
122 The reason for this policy is that back-porting any small to medium sized change
123 from an evolving kernel can easily create mismatches, incompatibilities and very
124 subtle errors.
125 </para>
126 <para>
127 These policies result in both a stable and a cutting
128 edge kernel that mixes forward ports of existing features and significant and critical
129 new functionality.
130 Forward porting functionality in the kernels available through the Yocto Project kernel
131 can be thought of as a "micro uprev."
132 The many “micro uprevs” produce a kernel version with a mix of
133 important new mainline, non-mainline, BSP developments and feature integrations.
134 This kernel gives insight into new features and allows focused
135 amounts of testing to be done on the kernel, which prevents
136 surprises when selecting the next major uprev.
137 The quality of these cutting edge kernels is evolving and the kernels are used in leading edge
138 feature and BSP development.
139 </para>
140 </section>
141
142 <section id='kernel-architecture'>
143 <title>Kernel Architecture</title>
144 <para>
145 This section describes the architecture of the kernels available through the
146 Yocto Project and provides information
147 on the mechanisms used to achieve that architecture.
148 </para>
149
150 <section id='architecture-overview'>
151 <title>Overview</title>
152 <para>
153 As mentioned earlier, a key goal of the Yocto Project is to present the
154 developer with
155 a kernel that has a clear and continuous history that is visible to the user.
156 The architecture and mechanisms used achieve that goal in a manner similar to the
157 upstream <filename>kernel.org</filename>.
158 </para>
159 <para>
160 You can think of a Yocto Project kernel as consisting of a baseline Linux kernel with
161 added features logically structured on top of the baseline.
162 The features are tagged and organized by way of a branching strategy implemented by the
163 source code manager (SCM) Git.
164 For information on Git as applied to the Yocto Project, see the
165 "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>" section in the
166 Yocto Project Development Manual.
167 </para>
168 <para>
169 The result is that the user has the ability to see the added features and
170 the commits that make up those features.
171 In addition to being able to see added features, the user can also view the history of what
172 made up the baseline kernel.
173 </para>
174 <para>
175 The following illustration shows the conceptual Yocto Project kernel.
176 </para>
177 <para>
178 <imagedata fileref="figures/kernel-architecture-overview.png" width="6in" depth="7in" align="center" scale="100" />
179 </para>
180 <para>
181 In the illustration, the "Kernel.org Branch Point"
182 marks the specific spot (or release) from
183 which the Yocto Project kernel is created.
184 From this point "up" in the tree, features and differences are organized and tagged.
185 </para>
186 <para>
187 The "Yocto Project Baseline Kernel" contains functionality that is common to every kernel
188 type and BSP that is organized further up the tree.
189 Placing these common features in the
190 tree this way means features don't have to be duplicated along individual branches of the
191 structure.
192 </para>
193 <para>
194 From the Yocto Project Baseline Kernel, branch points represent specific functionality
195 for individual BSPs as well as real-time kernels.
196 The illustration represents this through three BSP-specific branches and a real-time
197 kernel branch.
198 Each branch represents some unique functionality for the BSP or a real-time kernel.
199 </para>
200 <para>
201 In this example structure, the real-time kernel branch has common features for all
202 real-time kernels and contains
203 more branches for individual BSP-specific real-time kernels.
204 The illustration shows three branches as an example.
205 Each branch points the way to specific, unique features for a respective real-time
206 kernel as they apply to a given BSP.
207 </para>
208 <para>
209 The resulting tree structure presents a clear path of markers (or branches) to the
210 developer that, for all practical purposes, is the kernel needed for any given set
211 of requirements.
212 </para>
213 </section>
214
215 <section id='branching-and-workflow'>
216 <title>Branching Strategy and Workflow</title>
217 <para>
218 The Yocto Project team creates kernel branches at points where functionality is
219 no longer shared and thus, needs to be isolated.
220 For example, board-specific incompatibilities would require different functionality
221 and would require a branch to separate the features.
222 Likewise, for specific kernel features, the same branching strategy is used.
223 </para>
224 <para>
225 This branching strategy results in a tree that has features organized to be specific
226 for particular functionality, single kernel types, or a subset of kernel types.
227 This strategy also results in not having to store the same feature twice
228 internally in the tree.
229 Rather, the kernel team stores the unique differences required to apply the
230 feature onto the kernel type in question.
231 <note>
232 The Yocto Project team strives to place features in the tree such that they can be
233 shared by all boards and kernel types where possible.
234 However, during development cycles or when large features are merged,
235 the team cannot always follow this practice.
236 In those cases, the team uses isolated branches to merge features.
237 </note>
238 </para>
239 <para>
240 BSP-specific code additions are handled in a similar manner to kernel-specific additions.
241 Some BSPs only make sense given certain kernel types.
242 So, for these types, the team creates branches off the end of that kernel type for all
243 of the BSPs that are supported on that kernel type.
244 From the perspective of the tools that create the BSP branch, the BSP is really no
245 different than a feature.
246 Consequently, the same branching strategy applies to BSPs as it does to features.
247 So again, rather than store the BSP twice, the team only stores the unique
248 differences for the BSP across the supported multiple kernels.
249 </para>
250 <para>
251 While this strategy can result in a tree with a significant number of branches, it is
252 important to realize that from the developer's point of view, there is a linear
253 path that travels from the baseline <filename>kernel.org</filename>, through a select
254 group of features and ends with their BSP-specific commits.
255 In other words, the divisions of the kernel are transparent and are not relevant
256 to the developer on a day-to-day basis.
257 From the developer's perspective, this path is the "master" branch.
258 The developer does not need to be aware of the existence of any other branches at all.
259 Of course, there is value in the existence of these branches
260 in the tree, should a person decide to explore them.
261 For example, a comparison between two BSPs at either the commit level or at the line-by-line
262 code <filename>diff</filename> level is now a trivial operation.
263 </para>
264 <para>
265 Working with the kernel as a structured tree follows recognized community best practices.
266 In particular, the kernel as shipped with the product, should be
267 considered an "upstream source" and viewed as a series of
268 historical and documented modifications (commits).
269 These modifications represent the development and stabilization done
270 by the Yocto Project kernel development team.
271 </para>
272 <para>
273 Because commits only change at significant release points in the product life cycle,
274 developers can work on a branch created
275 from the last relevant commit in the shipped Yocto Project kernel.
276 As mentioned previously, the structure is transparent to the developer
277 because the kernel tree is left in this state after cloning and building the kernel.
278 </para>
279 </section>
280
281 <section id='source-code-manager-git'>
282 <title>Source Code Manager - Git</title>
283 <para>
284 The Source Code Manager (SCM) is Git.
285 This SCM is the obvious mechanism for meeting the previously mentioned goals.
286 Not only is it the SCM for <filename>kernel.org</filename> but,
287 Git continues to grow in popularity and supports many different work flows,
288 front-ends and management techniques.
289 </para>
290 <para>
291 You can find documentation on Git at <ulink url='http://git-scm.com/documentation'></ulink>.
292 You can also get an introduction to Git as it applies to the Yocto Project in the
293 "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>"
294 section in the Yocto Project Development Manual.
295 These referenced sections overview Git and describe a minimal set of
296 commands that allows you to be functional using Git.
297 <note>
298 You can use as much, or as little, of what Git has to offer to accomplish what
299 you need for your project.
300 You do not have to be a "Git Master" in order to use it with the Yocto Project.
301 </note>
302 </para>
303 </section>
304 </section>
305
306 <section id='kernel-configuration'>
307 <title>Kernel Configuration</title>
308 <para>
309 Kernel configuration, along with kernel features, defines how a kernel
310 image is built for the Yocto Project.
311 Through configuration settings, you can customize a Yocto Project kernel to be
312 specific to particular hardware.
313 For example, you can specify sound support or networking support.
314 This section describes basic concepts behind Kernel configuration within the
315 Yocto Project and references you to other areas for specific configuration
316 applications.
317 </para>
318
319 <para>
320 Conceptually, configuration of a Yocto Project kernel occurs similarly to that needed for any
321 Linux kernel.
322 The build process for a Yocto Project kernel uses a <filename>.config</filename> file, which
323 is created through the Linux Kernel Configuration (LKC) tool.
324 You can directly set various configurations in the
325 <filename>.config</filename> file by using the <filename>menuconfig</filename>
326 tool as built by BitBake.
327 You can also define configurations in the file by using configuration fragments.
328 <note>
329 It is not recommended that you edit the <filename>.config</filename> file directly.
330 </note>
331 Here are some brief descriptions of the ways you can affect the
332 <filename>.config</filename> file:
333 <itemizedlist>
334 <listitem><para><emphasis>The <filename>menuconfig</filename> Tool:</emphasis>
335 One of many front-ends that allows you to define kernel configurations.
336 Some others are <filename>make config</filename>,
337 <filename>make nconfig</filename>, and <filename>make gconfig</filename>.
338 In the Yocto Project environment, you must use BitBake to build the
339 <filename>menuconfig</filename> tool before you can use it to define
340 configurations:
341 <literallayout class='monospaced'>
342 $ bitbake linux-yocto -c menuconfig
343 </literallayout>
344 After the tool is built, you can interact with it normally.
345 You can see how <filename>menuconfig</filename> is used to change a simple
346 kernel configuration in the
347 "<ulink url='&YOCTO_DOCS_DEV_URL;#configuring-the-kernel'>Configuring the Kernel</ulink>"
348 section of the Yocto Project Development Manual.
349 For general information on <filename>menuconfig</filename>, see
350 <ulink url='http://en.wikipedia.org/wiki/Menuconfig'></ulink>.
351 </para></listitem>
352 <listitem><para><emphasis>Configuration Fragments:</emphasis> A file with a
353 list of kernel options just as they would appear syntactically in the
354 <filename>.config</filename> file.
355 Configuration fragments are typically logical groupings and are assembled
356 by the OpenEmbedded build system to produce input used by the LKC
357 that ultimately generates the <filename>.config</filename> file.</para>
358 <para>The
359 <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'>KERNEL_FEATURES</ulink></filename>
360 variable can be used to list configuration fragments.
361 For further discussion on applying configuration fragments, see the
362 "<ulink url='&YOCTO_DOCS_BSP_URL;#bsp-filelayout-kernel'>Linux Kernel Configuration</ulink>"
363 section in the Yocto Project Board Support Package (BSP) Guide.
364 </para></listitem>
365 </itemizedlist>
366 </para>
367 </section>
368
369 <section id='kernel-tools'>
370 <title>Kernel Tools</title>
371 <para>
372 Since most standard workflows involve moving forward with an existing tree by
373 continuing to add and alter the underlying baseline, the tools that manage
374 the Yocto Project's kernel construction are largely hidden from the developer to
375 present a simplified view of the kernel for ease of use.
376 </para>
377 <para>
378 Fundamentally, the kernel tools that manage and construct the
379 Yocto Project kernel accomplish the following:
380 <itemizedlist>
381 <listitem><para>Group patches into named, reusable features.</para></listitem>
382 <listitem><para>Allow top-down control of included features.</para></listitem>
383 <listitem><para>Bind kernel configurations to kernel patches and features.</para></listitem>
384 <listitem><para>Present a seamless Git repository that blends Yocto Project value
385 with the <filename>kernel.org</filename> history and development.</para></listitem>
386 </itemizedlist>
387 </para>
388 </section>
389</chapter>
390<!--
391vim: expandtab tw=80 ts=4
392-->
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