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1 | .. SPDX-License-Identifier: CC-BY-2.0-UK | ||
2 | |||
3 | ****************** | ||
4 | Kernel Maintenance | ||
5 | ****************** | ||
6 | |||
7 | Tree Construction | ||
8 | ================= | ||
9 | |||
10 | This section describes construction of the Yocto Project kernel source | ||
11 | repositories as accomplished by the Yocto Project team to create Yocto | ||
12 | Linux kernel repositories. These kernel repositories are found under the | ||
13 | heading "Yocto Linux Kernel" at :yocto_git:`/` and | ||
14 | are shipped as part of a Yocto Project release. The team creates these | ||
15 | repositories by compiling and executing the set of feature descriptions | ||
16 | for every BSP and feature in the product. Those feature descriptions | ||
17 | list all necessary patches, configurations, branches, tags, and feature | ||
18 | divisions found in a Yocto Linux kernel. Thus, the Yocto Project Linux | ||
19 | kernel repository (or tree) and accompanying Metadata in the | ||
20 | ``yocto-kernel-cache`` are built. | ||
21 | |||
22 | The existence of these repositories allow you to access and clone a | ||
23 | particular Yocto Project Linux kernel repository and use it to build | ||
24 | images based on their configurations and features. | ||
25 | |||
26 | You can find the files used to describe all the valid features and BSPs | ||
27 | in the Yocto Project Linux kernel in any clone of the Yocto Project | ||
28 | Linux kernel source repository and ``yocto-kernel-cache`` Git trees. For | ||
29 | example, the following commands clone the Yocto Project baseline Linux | ||
30 | kernel that branches off ``linux.org`` version 4.12 and the | ||
31 | ``yocto-kernel-cache``, which contains stores of kernel Metadata: | ||
32 | :: | ||
33 | |||
34 | $ git clone git://git.yoctoproject.org/linux-yocto-4.12 | ||
35 | $ git clone git://git.yoctoproject.org/linux-kernel-cache | ||
36 | |||
37 | For more information on | ||
38 | how to set up a local Git repository of the Yocto Project Linux kernel | ||
39 | files, see the | ||
40 | ":ref:`kernel-dev/kernel-dev-common:preparing the build host to work on the kernel`" | ||
41 | section. | ||
42 | |||
43 | Once you have cloned the kernel Git repository and the cache of Metadata | ||
44 | on your local machine, you can discover the branches that are available | ||
45 | in the repository using the following Git command: $ git branch -a | ||
46 | Checking out a branch allows you to work with a particular Yocto Linux | ||
47 | kernel. For example, the following commands check out the | ||
48 | "standard/beagleboard" branch of the Yocto Linux kernel repository and | ||
49 | the "yocto-4.12" branch of the ``yocto-kernel-cache`` repository: | ||
50 | :: | ||
51 | |||
52 | $ cd ~/linux-yocto-4.12 | ||
53 | $ git checkout -b my-kernel-4.12 remotes/origin/standard/beagleboard | ||
54 | $ cd ~/linux-kernel-cache | ||
55 | $ git checkout -b my-4.12-metadata remotes/origin/yocto-4.12 | ||
56 | |||
57 | .. note:: | ||
58 | |||
59 | Branches in the | ||
60 | yocto-kernel-cache | ||
61 | repository correspond to Yocto Linux kernel versions (e.g. | ||
62 | "yocto-4.12", "yocto-4.10", "yocto-4.9", and so forth). | ||
63 | |||
64 | Once you have checked out and switched to appropriate branches, you can | ||
65 | see a snapshot of all the kernel source files used to used to build that | ||
66 | particular Yocto Linux kernel for a particular board. | ||
67 | |||
68 | To see the features and configurations for a particular Yocto Linux | ||
69 | kernel, you need to examine the ``yocto-kernel-cache`` Git repository. | ||
70 | As mentioned, branches in the ``yocto-kernel-cache`` repository | ||
71 | correspond to Yocto Linux kernel versions (e.g. ``yocto-4.12``). | ||
72 | Branches contain descriptions in the form of ``.scc`` and ``.cfg`` | ||
73 | files. | ||
74 | |||
75 | You should realize, however, that browsing your local | ||
76 | ``yocto-kernel-cache`` repository for feature descriptions and patches | ||
77 | is not an effective way to determine what is in a particular kernel | ||
78 | branch. Instead, you should use Git directly to discover the changes in | ||
79 | a branch. Using Git is an efficient and flexible way to inspect changes | ||
80 | to the kernel. | ||
81 | |||
82 | .. note:: | ||
83 | |||
84 | Ground up reconstruction of the complete kernel tree is an action | ||
85 | only taken by the Yocto Project team during an active development | ||
86 | cycle. When you create a clone of the kernel Git repository, you are | ||
87 | simply making it efficiently available for building and development. | ||
88 | |||
89 | The following steps describe what happens when the Yocto Project Team | ||
90 | constructs the Yocto Project kernel source Git repository (or tree) | ||
91 | found at :yocto_git:`/` given the introduction of a new | ||
92 | top-level kernel feature or BSP. The following actions effectively | ||
93 | provide the Metadata and create the tree that includes the new feature, | ||
94 | patch, or BSP: | ||
95 | |||
96 | 1. *Pass Feature to the OpenEmbedded Build System:* A top-level kernel | ||
97 | feature is passed to the kernel build subsystem. Normally, this | ||
98 | feature is a BSP for a particular kernel type. | ||
99 | |||
100 | 2. *Locate Feature:* The file that describes the top-level feature is | ||
101 | located by searching these system directories: | ||
102 | |||
103 | - The in-tree kernel-cache directories, which are located in the | ||
104 | :yocto_git:`yocto-kernel-cache </cgit/cgit.cgi/yocto-kernel-cache/tree/bsp>` | ||
105 | repository organized under the "Yocto Linux Kernel" heading in the | ||
106 | :yocto_git:`Yocto Project Source Repositories <>`. | ||
107 | |||
108 | - Areas pointed to by ``SRC_URI`` statements found in kernel recipes | ||
109 | |||
110 | For a typical build, the target of the search is a feature | ||
111 | description in an ``.scc`` file whose name follows this format (e.g. | ||
112 | ``beaglebone-standard.scc`` and ``beaglebone-preempt-rt.scc``): | ||
113 | :: | ||
114 | |||
115 | bsp_root_name-kernel_type.scc | ||
116 | |||
117 | 3. *Expand Feature:* Once located, the feature description is either | ||
118 | expanded into a simple script of actions, or into an existing | ||
119 | equivalent script that is already part of the shipped kernel. | ||
120 | |||
121 | 4. *Append Extra Features:* Extra features are appended to the top-level | ||
122 | feature description. These features can come from the | ||
123 | :term:`KERNEL_FEATURES` | ||
124 | variable in recipes. | ||
125 | |||
126 | 5. *Locate, Expand, and Append Each Feature:* Each extra feature is | ||
127 | located, expanded and appended to the script as described in step | ||
128 | three. | ||
129 | |||
130 | 6. *Execute the Script:* The script is executed to produce files | ||
131 | ``.scc`` and ``.cfg`` files in appropriate directories of the | ||
132 | ``yocto-kernel-cache`` repository. These files are descriptions of | ||
133 | all the branches, tags, patches and configurations that need to be | ||
134 | applied to the base Git repository to completely create the source | ||
135 | (build) branch for the new BSP or feature. | ||
136 | |||
137 | 7. *Clone Base Repository:* The base repository is cloned, and the | ||
138 | actions listed in the ``yocto-kernel-cache`` directories are applied | ||
139 | to the tree. | ||
140 | |||
141 | 8. *Perform Cleanup:* The Git repositories are left with the desired | ||
142 | branches checked out and any required branching, patching and tagging | ||
143 | has been performed. | ||
144 | |||
145 | The kernel tree and cache are ready for developer consumption to be | ||
146 | locally cloned, configured, and built into a Yocto Project kernel | ||
147 | specific to some target hardware. | ||
148 | |||
149 | .. note:: | ||
150 | |||
151 | - The generated ``yocto-kernel-cache`` repository adds to the kernel | ||
152 | as shipped with the Yocto Project release. Any add-ons and | ||
153 | configuration data are applied to the end of an existing branch. | ||
154 | The full repository generation that is found in the official Yocto | ||
155 | Project kernel repositories at :yocto_git:`/` is the | ||
156 | combination of all supported boards and configurations. | ||
157 | |||
158 | - The technique the Yocto Project team uses is flexible and allows | ||
159 | for seamless blending of an immutable history with additional | ||
160 | patches specific to a deployment. Any additions to the kernel | ||
161 | become an integrated part of the branches. | ||
162 | |||
163 | - The full kernel tree that you see on :yocto_git:`/` is | ||
164 | generated through repeating the above steps for all valid BSPs. | ||
165 | The end result is a branched, clean history tree that makes up the | ||
166 | kernel for a given release. You can see the script (``kgit-scc``) | ||
167 | responsible for this in the | ||
168 | :yocto_git:`yocto-kernel-tools </cgit.cgi/yocto-kernel-tools/tree/tools>` | ||
169 | repository. | ||
170 | |||
171 | - The steps used to construct the full kernel tree are the same | ||
172 | steps that BitBake uses when it builds a kernel image. | ||
173 | |||
174 | Build Strategy | ||
175 | ============== | ||
176 | |||
177 | Once you have cloned a Yocto Linux kernel repository and the cache | ||
178 | repository (``yocto-kernel-cache``) onto your development system, you | ||
179 | can consider the compilation phase of kernel development, which is | ||
180 | building a kernel image. Some prerequisites exist that are validated by | ||
181 | the build process before compilation starts: | ||
182 | |||
183 | - The :term:`SRC_URI` points to the | ||
184 | kernel Git repository. | ||
185 | |||
186 | - A BSP build branch with Metadata exists in the ``yocto-kernel-cache`` | ||
187 | repository. The branch is based on the Yocto Linux kernel version and | ||
188 | has configurations and features grouped under the | ||
189 | ``yocto-kernel-cache/bsp`` directory. For example, features and | ||
190 | configurations for the BeagleBone Board assuming a | ||
191 | ``linux-yocto_4.12`` kernel reside in the following area of the | ||
192 | ``yocto-kernel-cache`` repository: yocto-kernel-cache/bsp/beaglebone | ||
193 | |||
194 | .. note:: | ||
195 | |||
196 | In the previous example, the "yocto-4.12" branch is checked out in | ||
197 | the | ||
198 | yocto-kernel-cache | ||
199 | repository. | ||
200 | |||
201 | The OpenEmbedded build system makes sure these conditions exist before | ||
202 | attempting compilation. Other means, however, do exist, such as as | ||
203 | bootstrapping a BSP. | ||
204 | |||
205 | Before building a kernel, the build process verifies the tree and | ||
206 | configures the kernel by processing all of the configuration "fragments" | ||
207 | specified by feature descriptions in the ``.scc`` files. As the features | ||
208 | are compiled, associated kernel configuration fragments are noted and | ||
209 | recorded in the series of directories in their compilation order. The | ||
210 | fragments are migrated, pre-processed and passed to the Linux Kernel | ||
211 | Configuration subsystem (``lkc``) as raw input in the form of a | ||
212 | ``.config`` file. The ``lkc`` uses its own internal dependency | ||
213 | constraints to do the final processing of that information and generates | ||
214 | the final ``.config`` file that is used during compilation. | ||
215 | |||
216 | Using the board's architecture and other relevant values from the | ||
217 | board's template, kernel compilation is started and a kernel image is | ||
218 | produced. | ||
219 | |||
220 | The other thing that you notice once you configure a kernel is that the | ||
221 | build process generates a build tree that is separate from your kernel's | ||
222 | local Git source repository tree. This build tree has a name that uses | ||
223 | the following form, where ``${MACHINE}`` is the metadata name of the | ||
224 | machine (BSP) and "kernel_type" is one of the Yocto Project supported | ||
225 | kernel types (e.g. "standard"): | ||
226 | :: | ||
227 | |||
228 | linux-${MACHINE}-kernel_type-build | ||
229 | |||
230 | The existing support in the ``kernel.org`` tree achieves this default | ||
231 | functionality. | ||
232 | |||
233 | This behavior means that all the generated files for a particular | ||
234 | machine or BSP are now in the build tree directory. The files include | ||
235 | the final ``.config`` file, all the ``.o`` files, the ``.a`` files, and | ||
236 | so forth. Since each machine or BSP has its own separate | ||
237 | :term:`Build Directory` in its own separate | ||
238 | branch of the Git repository, you can easily switch between different | ||
239 | builds. | ||