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