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1<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
3[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >
5<chapter id='kernel-how-to'>
7<title>Working with the Yocto Project Kernel</title>
10<section id='actions-org'>
11 <title>Introduction</title>
12 <para>
13 This chapter describes how to accomplish tasks involving a kernel's tree structure.
14 The information is designed to help the developer that wants to modify the Yocto
15 Project kernel and contribute changes upstream to the Yocto Project.
16 The information covers the following:
17 <itemizedlist>
18 <listitem><para>Tree construction</para></listitem>
19 <listitem><para>Build strategies</para></listitem>
20 <listitem><para>Workflow examples</para></listitem>
21 </itemizedlist>
22 </para>
25 <section id='tree-construction'>
26 <title>Tree Construction</title>
27 <para>
28 This section describes construction of the Yocto Project kernel source repositories
29 as accomplished by the Yocto Project team to create kernel repositories.
30 These kernel repositories are found under the heading "Yocto Linux Kernel" at
31 <ulink url='&YOCTO_GIT_URL;/cgit.cgi'>&YOCTO_GIT_URL;/cgit.cgi</ulink>
32 and can be shipped as part of a Yocto Project release.
33 The team creates these repositories by
34 compiling and executing the set of feature descriptions for every BSP/feature
35 in the product.
36 Those feature descriptions list all necessary patches,
37 configuration, branching, tagging and feature divisions found in a kernel.
38 Thus, the Yocto Project kernel repository (or tree) is built.
39 </para>
40 <para>
41 The existence of this tree allows you to access and clone a particular
42 Yocto Project kernel repository and use it to build images based on their configurations
43 and features.
44 </para>
45 <para>
46 You can find the files used to describe all the valid features and BSPs
47 in the Yocto Project kernel in any clone of the Yocto Project kernel source repository
48 Git tree.
49 For example, the following command clones the Yocto Project baseline kernel that
50 branched off of <filename></filename> version 3.4:
51 <literallayout class='monospaced'>
52 $ git clone git://
53 </literallayout>
54 For another example of how to set up a local Git repository of the Yocto Project
55 kernel files, see the
56 "<ulink url='&YOCTO_DOCS_DEV_URL;#local-kernel-files'>Yocto Project Kernel</ulink>" bulleted
57 item in the Yocto Project Development Manual.
58 </para>
59 <para>
60 Once you have cloned the kernel Git repository on your local machine, you can
61 switch to the <filename>meta</filename> branch within the repository.
62 Here is an example that assumes the local Git repository for the kernel is in
63 a top-level directory named <filename>linux-yocto-3.4</filename>:
64 <literallayout class='monospaced'>
65 $ cd ~/linux-yocto-3.4
66 $ git checkout -b meta origin/meta
67 </literallayout>
68 Once you have checked out and switched to the <filename>meta</filename> branch,
69 you can see a snapshot of all the kernel configuration and feature descriptions that are
70 used to build that particular kernel repository.
71 These descriptions are in the form of <filename>.scc</filename> files.
72 </para>
73 <para>
74 You should realize, however, that browsing your local kernel repository
75 for feature descriptions and patches is not an effective way to determine what is in a
76 particular kernel branch.
77 Instead, you should use Git directly to discover the changes in a branch.
78 Using Git is an efficient and flexible way to inspect changes to the kernel.
79 For examples showing how to use Git to inspect kernel commits, see the following sections
80 in this chapter.
81 <note>
82 Ground up reconstruction of the complete kernel tree is an action only taken by the
83 Yocto Project team during an active development cycle.
84 When you create a clone of the kernel Git repository, you are simply making it
85 efficiently available for building and development.
86 </note>
87 </para>
88 <para>
89 The following steps describe what happens when the Yocto Project Team constructs
90 the Yocto Project kernel source Git repository (or tree) found at
91 <ulink url='&YOCTO_GIT_URL;/cgit.cgi'></ulink> given the
92 introduction of a new top-level kernel feature or BSP.
93 These are the actions that effectively create the tree
94 that includes the new feature, patch or BSP:
95 <orderedlist>
96 <listitem><para>A top-level kernel feature is passed to the kernel build subsystem.
97 Normally, this feature is a BSP for a particular kernel type.</para></listitem>
98 <listitem><para>The file that describes the top-level feature is located by searching
99 these system directories:
100 <itemizedlist>
101 <listitem><para>The in-tree kernel-cache directories, which are located
102 in <filename>meta/cfg/kernel-cache</filename></para></listitem>
103 <listitem><para>Areas pointed to by <filename>SRC_URI</filename> statements
104 found in recipes</para></listitem>
105 </itemizedlist>
106 For a typical build, the target of the search is a
107 feature description in an <filename>.scc</filename> file
108 whose name follows this format:
109 <literallayout class='monospaced'>
110 &lt;bsp_name&gt;-&lt;kernel_type&gt;.scc
111 </literallayout>
112 </para></listitem>
113 <listitem><para>Once located, the feature description is either compiled into a simple script
114 of actions, or into an existing equivalent script that is already part of the
115 shipped kernel.</para></listitem>
116 <listitem><para>Extra features are appended to the top-level feature description.
117 These features can come from the
118 <ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'><filename>KERNEL_FEATURES</filename></ulink>
119 variable in recipes.</para></listitem>
120 <listitem><para>Each extra feature is located, compiled and appended to the script
121 as described in step three.</para></listitem>
122 <listitem><para>The script is executed to produce a series of <filename>meta-*</filename>
123 directories.
124 These directories are descriptions of all the branches, tags, patches and configurations that
125 need to be applied to the base Git repository to completely create the
126 source (build) branch for the new BSP or feature.</para></listitem>
127 <listitem><para>The base repository is cloned, and the actions
128 listed in the <filename>meta-*</filename> directories are applied to the
129 tree.</para></listitem>
130 <listitem><para>The Git repository is left with the desired branch checked out and any
131 required branching, patching and tagging has been performed.</para></listitem>
132 </orderedlist>
133 </para>
134 <para>
135 The kernel tree is now ready for developer consumption to be locally cloned,
136 configured, and built into a Yocto Project kernel specific to some target hardware.
137 <note><para>The generated <filename>meta-*</filename> directories add to the kernel
138 as shipped with the Yocto Project release.
139 Any add-ons and configuration data are applied to the end of an existing branch.
140 The full repository generation that is found in the
141 official Yocto Project kernel repositories at
142 <ulink url='&YOCTO_GIT_URL;/cgit.cgi'></ulink>
143 is the combination of all supported boards and configurations.</para>
144 <para>The technique the Yocto Project team uses is flexible and allows for seamless
145 blending of an immutable history with additional patches specific to a
146 deployment.
147 Any additions to the kernel become an integrated part of the branches.</para>
148 </note>
149 </para>
150 </section>
152 <section id='build-strategy'>
153 <title>Build Strategy</title>
154 <para>
155 Once a local Git repository of the Yocto Project kernel exists on a development system,
156 you can consider the compilation phase of kernel development - building a kernel image.
157 Some prerequisites exist that are validated by the build process before compilation
158 starts:
159 </para>
161 <itemizedlist>
162 <listitem><para>The
163 <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink> points
164 to the kernel Git repository.</para></listitem>
165 <listitem><para>A BSP build branch exists.
166 This branch has the following form:
167 <literallayout class='monospaced'>
168 &lt;kernel_type&gt;/&lt;bsp_name&gt;
169 </literallayout></para></listitem>
170 </itemizedlist>
172 <para>
173 The OpenEmbedded build system makes sure these conditions exist before attempting compilation.
174 Other means, however, do exist, such as as bootstrapping a BSP, see
175 the "<link linkend='workflow-examples'>Workflow Examples</link>".
176 </para>
178 <para>
179 Before building a kernel, the build process verifies the tree
180 and configures the kernel by processing all of the
181 configuration "fragments" specified by feature descriptions in the <filename>.scc</filename>
182 files.
183 As the features are compiled, associated kernel configuration fragments are noted
184 and recorded in the <filename>meta-*</filename> series of directories in their compilation order.
185 The fragments are migrated, pre-processed and passed to the Linux Kernel
186 Configuration subsystem (<filename>lkc</filename>) as raw input in the form
187 of a <filename>.config</filename> file.
188 The <filename>lkc</filename> uses its own internal dependency constraints to do the final
189 processing of that information and generates the final <filename>.config</filename> file
190 that is used during compilation.
191 </para>
193 <para>
194 Using the board's architecture and other relevant values from the board's template,
195 kernel compilation is started and a kernel image is produced.
196 </para>
198 <para>
199 The other thing that you notice once you configure a kernel is that
200 the build process generates a build tree that is separate from your kernel's local Git
201 source repository tree.
202 This build tree has a name that uses the following form, where
203 <filename>${MACHINE}</filename> is the metadata name of the machine (BSP) and "kernel_type" is one
204 of the Yocto Project supported kernel types (e.g. "standard"):
205 <literallayout class='monospaced'>
206 linux-${MACHINE}-&lt;kernel_type&gt;-build
207 </literallayout>
208 </para>
210 <para>
211 The existing support in the <filename></filename> tree achieves this
212 default functionality.
213 </para>
215 <para>
216 This behavior means that all the generated files for a particular machine or BSP are now in
217 the build tree directory.
218 The files include the final <filename>.config</filename> file, all the <filename>.o</filename>
219 files, the <filename>.a</filename> files, and so forth.
220 Since each machine or BSP has its own separate build directory in its own separate branch
221 of the Git repository, you can easily switch between different builds.
222 </para>
223 </section>
225 <section id='workflow-examples'>
226 <title>Workflow Examples</title>
228 <para>
229 As previously noted, the Yocto Project kernel has built-in Git integration.
230 However, these utilities are not the only way to work with the kernel repository.
231 The Yocto Project has not made changes to Git or to other tools that
232 would invalidate alternate workflows.
233 Additionally, the way the kernel repository is constructed results in using
234 only core Git functionality, thus allowing any number of tools or front ends to use the
235 resulting tree.
236 </para>
238 <para>
239 This section contains several workflow examples.
240 Many of the examples use Git commands.
241 You can find Git documentation at
242 <ulink url=''></ulink>.
243 You can find a simple overview of using Git with the Yocto Project in the
244 "<ulink url='&YOCTO_DOCS_DEV_URL;#git'>Git</ulink>"
245 section of the Yocto Project Development Manual.
246 </para>
248 <section id='change-inspection-kernel-changes-commits'>
249 <title>Change Inspection: Changes/Commits</title>
251 <para>
252 A common question when working with a kernel is:
253 "What changes have been applied to this tree?"
254 </para>
256 <para>
257 In projects that have a collection of directories that
258 contain patches to the kernel, it is possible to inspect or "grep" the contents
259 of the directories to get a general feel for the changes.
260 This sort of patch inspection is not an efficient way to determine what has been
261 done to the kernel.
262 The reason it is inefficient is because there are many optional patches that are
263 selected based on the kernel type and the feature description.
264 Additionally, patches could exist in directories that are not included in the search.
265 </para>
267 <para>
268 A more efficient way to determine what has changed in the branch is to use
269 Git and inspect or search the kernel tree.
270 This method gives you a full view of not only the source code modifications,
271 but also provides the reasons for the changes.
272 </para>
274 <section id='what-changed-in-a-kernel'>
275 <title>What Changed in a Kernel?</title>
277 <para>
278 Following are a few examples that show how to use Git commands to examine changes.
279 Because Git repositories in the Yocto Project do not break existing Git
280 functionality, and because there exists many permutations of these types of
281 Git commands, many methods exist by which you can discover changes.
282 <note>
283 In the following examples, unless you provide a commit range,
284 <filename></filename> history is blended with Yocto Project
285 kernel changes.
286 You can form ranges by using branch names from the kernel tree as the
287 upper and lower commit markers with the Git commands.
288 You can see the branch names through the web interface to the
289 Yocto Project source repositories at
290 <ulink url=''></ulink>.
291 For example, the branch names for the <filename>linux-yocto-3.4</filename>
292 kernel repository can be seen at
293 <ulink url=''></ulink>.
294 </note>
295 To see a full range of the changes, use the
296 <filename>git whatchanged</filename> command and specify a commit range
297 for the branch (<filename>&lt;commit&gt;..&lt;commit&gt;</filename>).
298 </para>
300 <para>
301 Here is an example that looks at what has changed in the
302 <filename>emenlow</filename> branch of the
303 <filename>linux-yocto-3.4</filename> kernel.
304 The lower commit range is the commit associated with the
305 <filename>standard/base</filename> branch, while
306 the upper commit range is the commit associated with the
307 <filename>standard/emenlow</filename> branch.
308 <literallayout class='monospaced'>
309 $ git whatchanged origin/standard/base..origin/standard/emenlow
310 </literallayout>
311 </para>
313 <para>
314 To see a summary of changes use the <filename>git log</filename> command.
315 Here is an example using the same branches:
316 <literallayout class='monospaced'>
317 $ git log --oneline origin/standard/base..origin/standard/emenlow
318 </literallayout>
319 The <filename>git log</filename> output might be more useful than
320 the <filename>git whatchanged</filename> as you get
321 a short, one-line summary of each change and not the entire commit.
322 </para>
324 <para>
325 If you want to see code differences associated with all the changes, use
326 the <filename>git diff</filename> command.
327 Here is an example:
328 <literallayout class='monospaced'>
329 $ git diff origin/standard/base..origin/standard/emenlow
330 </literallayout>
331 </para>
333 <para>
334 You can see the commit log messages and the text differences using the
335 <filename>git show</filename> command:
336 Here is an example:
337 <literallayout class='monospaced'>
338 $ git show origin/standard/base..origin/standard/emenlow
339 </literallayout>
340 </para>
342 <para>
343 You can create individual patches for each change by using the
344 <filename>git format-patch</filename> command.
345 Here is an example that that creates patch files for each commit and
346 places them in your <filename>Documents</filename> directory:
347 <literallayout class='monospaced'>
348 $ git format-patch -o $HOME/Documents origin/standard/base..origin/standard/emenlow
349 </literallayout>
350 </para>
351 </section>
353 <section id='show-a-particular-feature-or-branch-change'>
354 <title>Show a Particular Feature or Branch Change</title>
356 <para>
357 Developers use tags in the Yocto Project kernel tree to divide changes for significant
358 features or branches.
359 Once you know a particular tag, you can use Git commands
360 to show changes associated with the tag and find the branches that contain
361 the feature.
362 <note>
363 Because BSP branch, <filename></filename>, and feature tags are all
364 present, there could be many tags.
365 </note>
366 The <filename>git show &lt;tag&gt;</filename> command shows changes that are tagged by
367 a feature.
368 Here is an example that shows changes tagged by the <filename>systemtap</filename>
369 feature:
370 <literallayout class='monospaced'>
371 $ git show systemtap
372 </literallayout>
373 You can use the <filename>git branch --contains &lt;tag&gt;</filename> command
374 to show the branches that contain a particular feature.
375 This command shows the branches that contain the <filename>systemtap</filename>
376 feature:
377 <literallayout class='monospaced'>
378 $ git branch --contains systemtap
379 </literallayout>
380 </para>
382 <para>
383 You can use many other comparisons to isolate BSP and kernel changes.
384 For example, you can compare against <filename></filename> tags
385 such as the <filename>v3.4</filename> tag.
386 </para>
387 </section>
388 </section>
390 <section id='development-saving-kernel-modifications'>
391 <title>Development: Saving Kernel Modifications</title>
393 <para>
394 Another common operation is to build a BSP supplied by the Yocto Project, make some
395 changes, rebuild, and then test.
396 Those local changes often need to be exported, shared or otherwise maintained.
397 </para>
399 <para>
400 Since the Yocto Project kernel source tree is backed by Git, this activity is
401 much easier as compared to with previous releases.
402 Because Git tracks file modifications, additions and deletions, it is easy
403 to modify the code and later realize that you need to save the changes.
404 It is also easy to determine what has changed.
405 This method also provides many tools to commit, undo and export those modifications.
406 </para>
408 <para>
409 This section and its sub-sections, describe general application of Git's
410 <filename>push</filename> and <filename>pull</filename> commands, which are used to
411 get your changes upstream or source your code from an upstream repository.
412 The Yocto Project provides scripts that help you work in a collaborative development
413 environment.
414 For information on these scripts, see the
415 "<ulink url='&YOCTO_DOCS_DEV_URL;#pushing-a-change-upstream'>Using Scripts to Push a Change
416 Upstream and Request a Pull</ulink>" and
417 "<ulink url='&YOCTO_DOCS_DEV_URL;#submitting-a-patch'>Using Email to Submit a Patch</ulink>"
418 sections in the Yocto Project Development Manual.
419 </para>
421 <para>
422 There are many ways to save kernel modifications.
423 The technique employed
424 depends on the destination for the patches:
426 <itemizedlist>
427 <listitem><para>Bulk storage</para></listitem>
428 <listitem><para>Internal sharing either through patches or by using Git</para></listitem>
429 <listitem><para>External submissions</para></listitem>
430 <listitem><para>Exporting for integration into another Source Code
431 Manager (SCM)</para></listitem>
432 </itemizedlist>
433 </para>
435 <para>
436 Because of the following list of issues, the destination of the patches also influences
437 the method for gathering them:
439 <itemizedlist>
440 <listitem><para>Bisectability</para></listitem>
441 <listitem><para>Commit headers</para></listitem>
442 <listitem><para>Division of subsystems for separate submission or review</para></listitem>
443 </itemizedlist>
444 </para>
446 <section id='bulk-export'>
447 <title>Bulk Export</title>
449 <para>
450 This section describes how you can "bulk" export changes that have not
451 been separated or divided.
452 This situation works well when you are simply storing patches outside of the kernel
453 source repository, either permanently or temporarily, and you are not committing
454 incremental changes during development.
455 <note>
456 This technique is not appropriate for full integration of upstream submission
457 because changes are not properly divided and do not provide an avenue for per-change
458 commit messages.
459 Therefore, this example assumes that changes have not been committed incrementally
460 during development and that you simply must gather and export them.
461 </note>
462 <literallayout class='monospaced'>
463 # bulk export of ALL modifications without separation or division
464 # of the changes
466 $ git add .
467 $ git commit -s -a -m &lt;msg&gt;
468 or
469 $ git commit -s -a # and interact with $EDITOR
470 </literallayout>
471 </para>
473 <para>
474 The previous operations capture all the local changes in the project source
475 tree in a single Git commit.
476 And, that commit is also stored in the project's source tree.
477 </para>
479 <para>
480 Once the changes are exported, you can restore them manually using a template
481 or through integration with the <filename>default_kernel</filename>.
482 </para>
484 </section>
486 <section id='incremental-planned-sharing'>
487 <title>Incremental/Planned Sharing</title>
489 <para>
490 This section describes how to save modifications when you are making incremental
491 commits or practicing planned sharing.
492 The examples in this section assume that you have incrementally committed
493 changes to the tree during development and now need to export them.
494 The sections that follow
495 describe how you can export your changes internally through either patches or by
496 using Git commands.
497 </para>
499 <para>
500 During development, the following commands are of interest.
501 For full Git documentation, refer to the Git documentation at
502 <ulink url=''></ulink>.
504 <literallayout class='monospaced'>
505 # edit a file
506 $ vi &lt;path&gt;/file
507 # stage the change
508 $ git add &lt;path&gt;/file
509 # commit the change
510 $ git commit -s
511 # remove a file
512 $ git rm &lt;path&gt;/file
513 # commit the change
514 $ git commit -s
516 ... etc.
517 </literallayout>
518 </para>
520 <para>
521 Distributed development with Git is possible when you use a universally
522 agreed-upon unique commit identifier (set by the creator of the commit) that maps to a
523 specific change set with a specific parent.
524 This identifier is created for you when
525 you create a commit, and is re-created when you amend, alter or re-apply
526 a commit.
527 As an individual in isolation, this is of no interest.
528 However, if you
529 intend to share your tree with normal Git <filename>push</filename> and
530 <filename>pull</filename> operations for
531 distributed development, you should consider the ramifications of changing a
532 commit that you have already shared with others.
533 </para>
535 <para>
536 Assuming that the changes have not been pushed upstream, or pulled into
537 another repository, you can update both the commit content and commit messages
538 associated with development by using the following commands:
540 <literallayout class='monospaced'>
541 $ Git add &lt;path&gt;/file
542 $ Git commit --amend
543 $ Git rebase or Git rebase -i
544 </literallayout>
545 </para>
547 <para>
548 Again, assuming that the changes have not been pushed upstream, and that
549 no pending works-in-progress exist (use <filename>git status</filename> to check), then
550 you can revert (undo) commits by using the following commands:
552 <literallayout class='monospaced'>
553 # remove the commit, update working tree and remove all
554 # traces of the change
555 $ git reset --hard HEAD^
556 # remove the commit, but leave the files changed and staged for re-commit
557 $ git reset --soft HEAD^
558 # remove the commit, leave file change, but not staged for commit
559 $ git reset --mixed HEAD^
560 </literallayout>
561 </para>
563 <para>
564 You can create branches, "cherry-pick" changes, or perform any number of Git
565 operations until the commits are in good order for pushing upstream
566 or for pull requests.
567 After a <filename>push</filename> or <filename>pull</filename> command,
568 commits are normally considered
569 "permanent" and you should not modify them.
570 If the commits need to be changed, you can incrementally do so with new commits.
571 These practices follow standard Git workflow and the <filename></filename> best
572 practices, which is recommended.
573 <note>
574 It is recommended to tag or branch before adding changes to a Yocto Project
575 BSP or before creating a new one.
576 The reason for this recommendation is because the branch or tag provides a
577 reference point to facilitate locating and exporting local changes.
578 </note>
579 </para>
581 <section id='export-internally-via-patches'>
582 <title>Exporting Changes Internally by Using Patches</title>
584 <para>
585 This section describes how you can extract committed changes from a working directory
586 by exporting them as patches.
587 Once the changes have been extracted, you can use the patches for upstream submission,
588 place them in a Yocto Project template for automatic kernel patching,
589 or apply them in many other common uses.
590 </para>
592 <para>
593 This example shows how to create a directory with sequentially numbered patches.
594 Once the directory is created, you can apply it to a repository using the
595 <filename>git am</filename> command to reproduce the original commit and all
596 the related information such as author, date, commit log, and so forth.
597 <note>
598 The new commit identifiers (ID) will be generated upon re-application.
599 This action reflects that the commit is now applied to an underlying commit
600 with a different ID.
601 </note>
602 <literallayout class='monospaced'>
603 # &lt;first-commit&gt; can be a tag if one was created before development
604 # began. It can also be the parent branch if a branch was created
605 # before development began.
607 $ git format-patch -o &lt;dir&gt; &lt;first commit&gt;..&lt;last commit&gt;
608 </literallayout>
609 </para>
611 <para>
612 In other words:
613 <literallayout class='monospaced'>
614 # Identify commits of interest.
616 # If the tree was tagged before development
617 $ git format-patch -o &lt;save dir&gt; &lt;tag&gt;
619 # If no tags are available
620 $ git format-patch -o &lt;save dir&gt; HEAD^ # last commit
621 $ git format-patch -o &lt;save dir&gt; HEAD^^ # last 2 commits
622 $ git whatchanged # identify last commit
623 $ git format-patch -o &lt;save dir&gt; &lt;commit id&gt;
624 $ git format-patch -o &lt;save dir&gt; &lt;rev-list&gt;
625 </literallayout>
626 </para>
627 </section>
629 <section id='export-internally-via-git'>
630 <title>Exporting Changes Internally by Using Git</title>
632 <para>
633 This section describes how you can export changes from a working directory
634 by pushing the changes into a master repository or by making a pull request.
635 Once you have pushed the changes to the master repository, you can then
636 pull those same changes into a new kernel build at a later time.
637 </para>
639 <para>
640 Use this command form to push the changes:
641 <literallayout class='monospaced'>
642 $ git push ssh://&lt;master_server&gt;/&lt;path_to_repo&gt;
643 &lt;local_branch&gt;:&lt;remote_branch&gt;
644 </literallayout>
645 </para>
647 <para>
648 For example, the following command pushes the changes from your local branch
649 <filename>yocto/standard/common-pc/base</filename> to the remote branch with the same name
650 in the master repository <filename>//</filename>.
651 <literallayout class='monospaced'>
652 $ git push ssh:// \
653 yocto/standard/common-pc/base:yocto/standard/common-pc/base
654 </literallayout>
655 </para>
657 <para>
658 A pull request entails using the <filename>git request-pull</filename> command to compose
659 an email to the
660 maintainer requesting that a branch be pulled into the master repository, see
661 <ulink url=''></ulink> for an example.
662 <note>
663 Other commands such as <filename>git stash</filename> or branching can also be used to save
664 changes, but are not covered in this document.
665 </note>
666 </para>
667 </section>
668 </section>
670 <section id='export-for-external-upstream-submission'>
671 <title>Exporting Changes for External (Upstream) Submission</title>
673 <para>
674 This section describes how to export changes for external upstream submission.
675 If the patch series is large or the maintainer prefers to pull
676 changes, you can submit these changes by using a pull request.
677 However, it is common to send patches as an email series.
678 This method allows easy review and integration of the changes.
679 <note>
680 Before sending patches for review be sure you understand the
681 community standards for submitting and documenting changes and follow their best practices.
682 For example, kernel patches should follow standards such as:
683 <itemizedlist>
684 <listitem><para>
685 <ulink url=''></ulink></para></listitem>
686 <listitem><para>Documentation/SubmittingPatches (in any linux
687 kernel source tree)</para></listitem>
688 </itemizedlist>
689 </note>
690 </para>
692 <para>
693 The messages used to commit changes are a large part of these standards.
694 Consequently, be sure that the headers for each commit have the required information.
695 For information on how to follow the Yocto Project commit message standards, see the
696 "<ulink url='&YOCTO_DOCS_DEV_URL;#how-to-submit-a-change'>How to Submit a
697 Change</ulink>" section in the Yocto Project Development Manual.
698 </para>
700 <para>
701 If the initial commits were not properly documented or do not meet those standards,
702 you can re-base by using the <filename>git rebase -i</filename> command to
703 manipulate the commits and
704 get them into the required format.
705 Other techniques such as branching and cherry-picking commits are also viable options.
706 </para>
708 <para>
709 Once you complete the commits, you can generate the email that sends the patches
710 to the maintainer(s) or lists that review and integrate changes.
711 The command <filename>git send-email</filename> is commonly used to ensure
712 that patches are properly
713 formatted for easy application and avoid mailer-induced patch damage.
714 </para>
716 <para>
717 The following is an example of dumping patches for external submission:
718 <literallayout class='monospaced'>
719 # dump the last 4 commits
720 $ git format-patch --thread -n -o ~/rr/ HEAD^^^^
721 $ git send-email --compose --subject '[RFC 0/N] &lt;patch series summary&gt;' \
722 --to --to \
723 --cc ~/rr
724 # the editor is invoked for the 0/N patch, and when complete the entire
725 # series is sent via email for review
726 </literallayout>
727 </para>
728 </section>
730 <section id='export-for-import-into-other-scm'>
731 <title>Exporting Changes for Import into Another SCM</title>
733 <para>
734 When you want to export changes for import into another
735 Source Code Manager (SCM), you can use any of the previously discussed
736 techniques.
737 However, if the patches are manually applied to a secondary tree and then
738 that tree is checked into the SCM, you can lose change information such as
739 commit logs.
740 This process is not recommended.
741 </para>
743 <para>
744 Many SCMs can directly import Git commits, or can translate Git patches so that
745 information is not lost.
746 Those facilities are SCM-dependent and you should use them whenever possible.
747 </para>
748 </section>
749 </section>
751 <section id='scm-working-with-the-yocto-project-kernel-in-another-scm'>
752 <title>Working with the Yocto Project Kernel in Another SCM</title>
754 <para>
755 This section describes kernel development in an SCM other than Git,
756 which is not the same as exporting changes to another SCM described earlier.
757 For this scenario, you use the OpenEmbedded build system to
758 develop the kernel in a different SCM.
759 The following must be true for you to accomplish this:
760 <itemizedlist>
761 <listitem><para>The delivered Yocto Project kernel must be exported into the second
762 SCM.</para></listitem>
763 <listitem><para>Development must be exported from that secondary SCM into a
764 format that can be used by the OpenEmbedded build system.</para></listitem>
765 </itemizedlist>
766 </para>
768 <section id='exporting-delivered-kernel-to-scm'>
769 <title>Exporting the Delivered Kernel to the SCM</title>
771 <para>
772 Depending on the SCM, it might be possible to export the entire Yocto Project
773 kernel Git repository, branches and all, into a new environment.
774 This method is preferred because it has the most flexibility and potential to maintain
775 the meta data associated with each commit.
776 </para>
778 <para>
779 When a direct import mechanism is not available, it is still possible to
780 export a branch (or series of branches) and check them into a new repository.
781 </para>
783 <para>
784 The following commands illustrate some of the steps you could use to
785 import the <filename>yocto/standard/common-pc/base</filename>
786 kernel into a secondary SCM:
787 <literallayout class='monospaced'>
788 $ git checkout yocto/standard/common-pc/base
789 $ cd .. ; echo linux/.git &gt; .cvsignore
790 $ cvs import -m "initial import" linux MY_COMPANY start
791 </literallayout>
792 </para>
794 <para>
795 You could now relocate the CVS repository and use it in a centralized manner.
796 </para>
798 <para>
799 The following commands illustrate how you can condense and merge two BSPs into a
800 second SCM:
801 <literallayout class='monospaced'>
802 $ git checkout yocto/standard/common-pc/base
803 $ git merge yocto/standard/common-pc-64/base
804 # resolve any conflicts and commit them
805 $ cd .. ; echo linux/.git &gt; .cvsignore
806 $ cvs import -m "initial import" linux MY_COMPANY start
807 </literallayout>
808 </para>
809 </section>
811 <section id='importing-changes-for-build'>
812 <title>Importing Changes for the Build</title>
814 <para>
815 Once development has reached a suitable point in the second development
816 environment, you need to export the changes as patches.
817 To export them, place the changes in a recipe and
818 automatically apply them to the kernel during patching.
819 </para>
820 </section>
821 </section>
823 <section id='bsp-creating'>
824 <title>Creating a BSP Based on an Existing Similar BSP</title>
826 <para>
827 This section overviews the process of creating a BSP based on an
828 existing similar BSP.
829 The information is introductory in nature and does not provide step-by-step examples.
830 For detailed information on how to create a new BSP, see
831 the "<ulink url='&YOCTO_DOCS_BSP_URL;#creating-a-new-bsp-layer-using-the-yocto-bsp-script'>Creating a New BSP Layer Using the yocto-bsp Script</ulink>" section in the
832 Yocto Project Board Support Package (BSP) Developer's Guide, or see the
833 <ulink url='&YOCTO_WIKI_URL;/wiki/Transcript:_creating_one_generic_Atom_BSP_from_another'>Transcript:_creating_one_generic_Atom_BSP_from_another</ulink>
834 wiki page.
835 </para>
837 <para>
838 The basic steps you need to follow are:
839 <orderedlist>
840 <listitem><para><emphasis>Make sure you have set up a local Source Directory:</emphasis>
841 You must create a local
842 <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>
843 by either creating a Git repository (recommended) or
844 extracting a Yocto Project release tarball.</para></listitem>
845 <listitem><para><emphasis>Choose an existing BSP available with the Yocto Project:</emphasis>
846 Try to map your board features as closely to the features of a BSP that is
847 already supported and exists in the Yocto Project.
848 Starting with something as close as possible to your board makes developing
849 your BSP easier.
850 You can find all the BSPs that are supported and ship with the Yocto Project
851 on the Yocto Project's Download page at
852 <ulink url='&YOCTO_HOME_URL;/download'></ulink>.</para></listitem>
853 <listitem><para><emphasis>Be sure you have the Base BSP:</emphasis>
854 You need to either have a local Git repository of the base BSP set up or
855 have downloaded and extracted the files from a release BSP tarball.
856 Either method gives you access to the BSP source files.</para></listitem>
857 <listitem><para><emphasis>Make a copy of the existing BSP, thus isolating your new
858 BSP work:</emphasis>
859 Copying the existing BSP file structure gives you a new area in which to work.</para></listitem>
860 <listitem><para><emphasis>Make configuration and recipe changes to your new BSP:</emphasis>
861 Configuration changes involve the files in the BSP's <filename>conf</filename>
862 directory.
863 Changes include creating a machine-specific configuration file and editing the
864 <filename>layer.conf</filename> file.
865 The configuration changes identify the kernel you will be using.
866 Recipe changes include removing, modifying, or adding new recipe files that
867 instruct the build process on what features to include in the image.</para></listitem>
868 <listitem><para><emphasis>Prepare for the build:</emphasis>
869 Before you actually initiate the build, you need to set up the build environment
870 by sourcing the environment initialization script.
871 After setting up the environment, you need to make some build configuration
872 changes to the <filename>local.conf</filename> and <filename>bblayers.conf</filename>
873 files.</para></listitem>
874 <listitem><para><emphasis>Build the image:</emphasis>
875 The OpenEmbedded build system uses BitBake to create the image.
876 You need to decide on the type of image you are going to build (e.g. minimal, base,
877 core, sato, and so forth) and then start the build using the <filename>bitbake</filename>
878 command.</para></listitem>
879 </orderedlist>
880 </para>
881 </section>
883 <section id='tip-dirty-string'>
884 <title>"-dirty" String</title>
886 <para>
887 If kernel images are being built with "-dirty" on the end of the version
888 string, this simply means that modifications in the source
889 directory have not been committed.
890 <literallayout class='monospaced'>
891 $ git status
892 </literallayout>
893 </para>
895 <para>
896 You can use the above Git command to report modified, removed, or added files.
897 You should commit those changes to the tree regardless of whether they will be saved,
898 exported, or used.
899 Once you commit the changes you need to rebuild the kernel.
900 </para>
902 <para>
903 To brute force pickup and commit all such pending changes, enter the following:
904 <literallayout class='monospaced'>
905 $ git add .
906 $ git commit -s -a -m "getting rid of -dirty"
907 </literallayout>
908 </para>
910 <para>
911 Next, rebuild the kernel.
912 </para>
913 </section>
914 </section>
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