.. SPDX-License-Identifier: CC-BY-SA-2.0-UK

************
Introduction
************

Overview
========

Regardless of how you intend to make use of the Yocto Project, chances
are you will work with the Linux kernel. This manual describes how to
set up your build host to support kernel development, introduces the
kernel development process, provides background information on the Yocto
Linux kernel :term:`Metadata`, describes
common tasks you can perform using the kernel tools, shows you how to
use the kernel Metadata needed to work with the kernel inside the Yocto
Project, and provides insight into how the Yocto Project team develops
and maintains Yocto Linux kernel Git repositories and Metadata.

Each Yocto Project release has a set of Yocto Linux kernel recipes,
whose Git repositories you can view in the Yocto
:yocto_git:`Source Repositories <>` under the "Yocto Linux Kernel"
heading. New recipes for the release track the latest Linux kernel
upstream developments from https://www.kernel.org and introduce
newly-supported platforms. Previous recipes in the release are refreshed
and supported for at least one additional Yocto Project release. As they
align, these previous releases are updated to include the latest from
the Long Term Support Initiative (LTSI) project. You can learn more
about Yocto Linux kernels and LTSI in the
":ref:`kernel-dev/concepts-appx:yocto project kernel development and maintenance`" section.

Also included is a Yocto Linux kernel development recipe
(``linux-yocto-dev.bb``) should you want to work with the very latest in
upstream Yocto Linux kernel development and kernel Metadata development.

.. note::

   For more on Yocto Linux kernels, see the
   ":ref:`kernel-dev/concepts-appx:yocto project kernel development and maintenance`"
   section.

The Yocto Project also provides a powerful set of kernel tools for
managing Yocto Linux kernel sources and configuration data. You can use
these tools to make a single configuration change, apply multiple
patches, or work with your own kernel sources.

In particular, the kernel tools allow you to generate configuration
fragments that specify only what you must, and nothing more.
Configuration fragments only need to contain the highest level visible
``CONFIG`` options as presented by the Yocto Linux kernel ``menuconfig``
system. Contrast this against a complete Yocto Linux kernel ``.config``
file, which includes all the automatically selected ``CONFIG`` options.
This efficiency reduces your maintenance effort and allows you to
further separate your configuration in ways that make sense for your
project. A common split separates policy and hardware. For example, all
your kernels might support the ``proc`` and ``sys`` filesystems, but
only specific boards require sound, USB, or specific drivers. Specifying
these configurations individually allows you to aggregate them together
as needed, but maintains them in only one place. Similar logic applies
to separating source changes.

If you do not maintain your own kernel sources and need to make only
minimal changes to the sources, the released recipes provide a vetted
base upon which to layer your changes. Doing so allows you to benefit
from the continual kernel integration and testing performed during
development of the Yocto Project.

If, instead, you have a very specific Linux kernel source tree and are
unable to align with one of the official Yocto Linux kernel recipes,
you have a way to use the Yocto Project Linux kernel tools with your
own kernel sources.

The remainder of this manual provides instructions for completing
specific Linux kernel development tasks. These instructions assume you
are comfortable working with :oe_wiki:`BitBake </Bitbake>` recipes and basic
open-source development tools. Understanding these concepts will
facilitate the process of working with the kernel recipes. If you find
you need some additional background, please be sure to review and
understand the following documentation:

-  :doc:`/brief-yoctoprojectqs/index` document.

-  :doc:`/overview-manual/index`.

-  :ref:`devtool
   workflow <sdk-manual/extensible:using \`\`devtool\`\` in your sdk workflow>`
   as described in the Yocto Project Application Development and the
   Extensible Software Development Kit (eSDK) manual.

-  The ":ref:`dev-manual/common-tasks:understanding and creating layers`"
   section in the Yocto Project Development Tasks Manual.

-  The ":ref:`kernel-dev/intro:kernel modification workflow`" section.

Kernel Modification Workflow
============================

Kernel modification involves changing the Yocto Project kernel, which
could involve changing configuration options as well as adding new
kernel recipes. Configuration changes can be added in the form of
configuration fragments, while recipe modification comes through the
kernel's ``recipes-kernel`` area in a kernel layer you create.

This section presents a high-level overview of the Yocto Project kernel
modification workflow. The illustration and accompanying list provide
general information and references for further information.

.. image:: figures/kernel-dev-flow.png
   :width: 100%

1. *Set up Your Host Development System to Support Development Using the
   Yocto Project*: See the ":doc:`/dev-manual/start`" section in
   the Yocto Project Development Tasks Manual for options on how to get
   a build host ready to use the Yocto Project.

2. *Set Up Your Host Development System for Kernel Development:* It is
   recommended that you use ``devtool`` and an extensible SDK for kernel
   development. Alternatively, you can use traditional kernel
   development methods with the Yocto Project. Either way, there are
   steps you need to take to get the development environment ready.

   Using ``devtool`` and the eSDK requires that you have a clean build
   of the image and that you are set up with the appropriate eSDK. For
   more information, see the
   ":ref:`kernel-dev/common:getting ready to develop using \`\`devtool\`\``"
   section.

   Using traditional kernel development requires that you have the
   kernel source available in an isolated local Git repository. For more
   information, see the
   ":ref:`kernel-dev/common:getting ready for traditional kernel development`"
   section.

3. *Make Changes to the Kernel Source Code if applicable:* Modifying the
   kernel does not always mean directly changing source files. However,
   if you have to do this, you make the changes to the files in the
   eSDK's Build Directory if you are using ``devtool``. For more
   information, see the
   ":ref:`kernel-dev/common:using \`\`devtool\`\` to patch the kernel`"
   section.

   If you are using traditional kernel development, you edit the source
   files in the kernel's local Git repository. For more information, see the
   ":ref:`kernel-dev/common:using traditional kernel development to patch the kernel`"
   section.

4. *Make Kernel Configuration Changes if Applicable:* If your situation
   calls for changing the kernel's configuration, you can use
   :ref:`menuconfig <kernel-dev/common:using \`\`menuconfig\`\`>`,
   which allows you to
   interactively develop and test the configuration changes you are
   making to the kernel. Saving changes you make with ``menuconfig``
   updates the kernel's ``.config`` file.

   .. note::

      Try to resist the temptation to directly edit an existing ``.config``
      file, which is found in the Build Directory among the source code
      used for the build. Doing so, can produce unexpected results when
      the OpenEmbedded build system regenerates the configuration file.

   Once you are satisfied with the configuration changes made using
   ``menuconfig`` and you have saved them, you can directly compare the
   resulting ``.config`` file against an existing original and gather
   those changes into a
   :ref:`configuration fragment file <kernel-dev/common:creating configuration fragments>` to be
   referenced from within the kernel's ``.bbappend`` file.

   Additionally, if you are working in a BSP layer and need to modify
   the BSP's kernel's configuration, you can use ``menuconfig``.

5. *Rebuild the Kernel Image With Your Changes:* Rebuilding the kernel
   image applies your changes. Depending on your target hardware, you
   can verify your changes on actual hardware or perhaps QEMU.

The remainder of this developer's guide covers common tasks typically
used during kernel development, advanced Metadata usage, and Yocto Linux
kernel maintenance concepts.