1.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
2
3************
4Introduction
5************
6
7Overview
8========
9
10Regardless of how you intend to make use of the Yocto Project, chances
11are you will work with the Linux kernel. This manual describes how to
12set up your build host to support kernel development, introduces the
13kernel development process, provides background information on the Yocto
14Linux kernel :term:`Metadata`, describes
15common tasks you can perform using the kernel tools, shows you how to
16use the kernel Metadata needed to work with the kernel inside the Yocto
17Project, and provides insight into how the Yocto Project team develops
18and maintains Yocto Linux kernel Git repositories and Metadata.
19
20Each Yocto Project release has a set of Yocto Linux kernel recipes,
21whose Git repositories you can view in the Yocto
22:yocto_git:`Source Repositories <>` under the "Yocto Linux Kernel"
23heading. New recipes for the release track the latest Linux kernel
24upstream developments from https://www.kernel.org and introduce
25newly-supported platforms. Previous recipes in the release are refreshed
26and supported for at least one additional Yocto Project release. As they
27align, these previous releases are updated to include the latest from
28the Long Term Support Initiative (LTSI) project. You can learn more
29about Yocto Linux kernels and LTSI in the
30":ref:`kernel-dev/concepts-appx:yocto project kernel development and maintenance`" section.
31
32Also included is a Yocto Linux kernel development recipe
33(``linux-yocto-dev.bb``) should you want to work with the very latest in
34upstream Yocto Linux kernel development and kernel Metadata development.
35
36.. note::
37
38   For more on Yocto Linux kernels, see the
39   ":ref:`kernel-dev/concepts-appx:yocto project kernel development and maintenance`"
40   section.
41
42The Yocto Project also provides a powerful set of kernel tools for
43managing Yocto Linux kernel sources and configuration data. You can use
44these tools to make a single configuration change, apply multiple
45patches, or work with your own kernel sources.
46
47In particular, the kernel tools allow you to generate configuration
48fragments that specify only what you must, and nothing more.
49Configuration fragments only need to contain the highest level visible
50``CONFIG`` options as presented by the Yocto Linux kernel ``menuconfig``
51system. Contrast this against a complete Yocto Linux kernel ``.config``
52file, which includes all the automatically selected ``CONFIG`` options.
53This efficiency reduces your maintenance effort and allows you to
54further separate your configuration in ways that make sense for your
55project. A common split separates policy and hardware. For example, all
56your kernels might support the ``proc`` and ``sys`` filesystems, but
57only specific boards require sound, USB, or specific drivers. Specifying
58these configurations individually allows you to aggregate them together
59as needed, but maintains them in only one place. Similar logic applies
60to separating source changes.
61
62If you do not maintain your own kernel sources and need to make only
63minimal changes to the sources, the released recipes provide a vetted
64base upon which to layer your changes. Doing so allows you to benefit
65from the continual kernel integration and testing performed during
66development of the Yocto Project.
67
68If, instead, you have a very specific Linux kernel source tree and are
69unable to align with one of the official Yocto Linux kernel recipes,
70you have a way to use the Yocto Project Linux kernel tools with your
71own kernel sources.
72
73The remainder of this manual provides instructions for completing
74specific Linux kernel development tasks. These instructions assume you
75are comfortable working with :oe_wiki:`BitBake </Bitbake>` recipes and basic
76open-source development tools. Understanding these concepts will
77facilitate the process of working with the kernel recipes. If you find
78you need some additional background, please be sure to review and
79understand the following documentation:
80
81-  :doc:`/brief-yoctoprojectqs/index` document.
82
83-  :doc:`/overview-manual/index`.
84
85-  :ref:`devtool
86   workflow <sdk-manual/extensible:using \`\`devtool\`\` in your sdk workflow>`
87   as described in the Yocto Project Application Development and the
88   Extensible Software Development Kit (eSDK) manual.
89
90-  The ":ref:`dev-manual/layers:understanding and creating layers`"
91   section in the Yocto Project Development Tasks Manual.
92
93-  The ":ref:`kernel-dev/intro:kernel modification workflow`" section.
94
95Kernel Modification Workflow
96============================
97
98Kernel modification involves changing the Yocto Project kernel, which
99could involve changing configuration options as well as adding new
100kernel recipes. Configuration changes can be added in the form of
101configuration fragments, while recipe modification comes through the
102kernel's ``recipes-kernel`` area in a kernel layer you create.
103
104This section presents a high-level overview of the Yocto Project kernel
105modification workflow. The illustration and accompanying list provide
106general information and references for further information.
107
108.. image:: figures/kernel-dev-flow.png
109   :width: 100%
110
111#. *Set up Your Host Development System to Support Development Using the
112   Yocto Project*: See the ":doc:`/dev-manual/start`" section in
113   the Yocto Project Development Tasks Manual for options on how to get
114   a build host ready to use the Yocto Project.
115
116#. *Set Up Your Host Development System for Kernel Development:* It is
117   recommended that you use ``devtool`` for kernel
118   development. Alternatively, you can use traditional kernel
119   development methods with the Yocto Project. Either way, there are
120   steps you need to take to get the development environment ready.
121
122   Using ``devtool`` requires that you have a clean build
123   of the image. For
124   more information, see the
125   ":ref:`kernel-dev/common:getting ready to develop using \`\`devtool\`\``"
126   section.
127
128   Using traditional kernel development requires that you have the
129   kernel source available in an isolated local Git repository. For more
130   information, see the
131   ":ref:`kernel-dev/common:getting ready for traditional kernel development`"
132   section.
133
134#. *Make Changes to the Kernel Source Code if applicable:* Modifying the
135   kernel does not always mean directly changing source files. However,
136   if you have to do this, you make the changes to the files in the
137   Yocto's :term:`Build Directory` if you are using ``devtool``. For more
138   information, see the
139   ":ref:`kernel-dev/common:using \`\`devtool\`\` to patch the kernel`"
140   section.
141
142   If you are using traditional kernel development, you edit the source
143   files in the kernel's local Git repository. For more information, see the
144   ":ref:`kernel-dev/common:using traditional kernel development to patch the kernel`"
145   section.
146
147#. *Make Kernel Configuration Changes if Applicable:* If your situation
148   calls for changing the kernel's configuration, you can use
149   :ref:`menuconfig <kernel-dev/common:using \`\`menuconfig\`\`>`,
150   which allows you to
151   interactively develop and test the configuration changes you are
152   making to the kernel. Saving changes you make with ``menuconfig``
153   updates the kernel's ``.config`` file.
154
155   .. note::
156
157      Try to resist the temptation to directly edit an existing ``.config``
158      file, which is found in the :term:`Build Directory` among the source code
159      used for the build. Doing so, can produce unexpected results when
160      the OpenEmbedded build system regenerates the configuration file.
161
162   Once you are satisfied with the configuration changes made using
163   ``menuconfig`` and you have saved them, you can directly compare the
164   resulting ``.config`` file against an existing original and gather
165   those changes into a
166   :ref:`configuration fragment file <kernel-dev/common:creating configuration fragments>` to be
167   referenced from within the kernel's ``.bbappend`` file.
168
169   Additionally, if you are working in a BSP layer and need to modify
170   the BSP's kernel's configuration, you can use ``menuconfig``.
171
172#. *Rebuild the Kernel Image With Your Changes:* Rebuilding the kernel
173   image applies your changes. Depending on your target hardware, you
174   can verify your changes on actual hardware or perhaps QEMU.
175
176The remainder of this developer's guide covers common tasks typically
177used during kernel development, advanced Metadata usage, and Yocto Linux
178kernel maintenance concepts.
179