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-.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
-
-************************
-Advanced Kernel Concepts
-************************
-
-Yocto Project Kernel Development and Maintenance
-================================================
-
-Kernels available through the Yocto Project (Yocto Linux kernels), like
-other kernels, are based off the Linux kernel releases from
-https://www.kernel.org. At the beginning of a major Linux kernel
-development cycle, the Yocto Project team chooses a Linux kernel based
-on factors such as release timing, the anticipated release timing of
-final upstream ``kernel.org`` versions, and Yocto Project feature
-requirements. Typically, the Linux kernel chosen is in the final stages
-of development by the Linux community. In other words, the Linux kernel
-is in the release candidate or "rc" phase and has yet to reach final
-release. But, by being in the final stages of external development, the
-team knows that the ``kernel.org`` final release will clearly be within
-the early stages of the Yocto Project development window.
-
-This balance allows the Yocto Project team to deliver the most
-up-to-date Yocto Linux kernel possible, while still ensuring that the
-team has a stable official release for the baseline Linux kernel
-version.
-
-As implied earlier, the ultimate source for Yocto Linux kernels are
-released kernels from ``kernel.org``. In addition to a foundational
-kernel from ``kernel.org``, the available Yocto Linux kernels contain a
-mix of important new mainline developments, non-mainline developments
-(when no alternative exists), Board Support Package (BSP) developments,
-and custom features. These additions result in a commercially released
-Yocto Project Linux kernel that caters to specific embedded designer
-needs for targeted hardware.
-
-You can find a web interface to the Yocto Linux kernels in the
-:ref:`overview-manual/development-environment:yocto project source repositories`
-at :yocto_git:`/`. If you look at the interface, you will see to
-the left a grouping of Git repositories titled "Yocto Linux Kernel".
-Within this group, you will find several Linux Yocto kernels developed
-and included with Yocto Project releases:
-
--  *linux-yocto-4.1:* The stable Yocto Project kernel to use with
-   the Yocto Project Release 2.0. This kernel is based on the Linux 4.1
-   released kernel.
-
--  *linux-yocto-4.4:* The stable Yocto Project kernel to use with
-   the Yocto Project Release 2.1. This kernel is based on the Linux 4.4
-   released kernel.
-
--  *linux-yocto-4.6:* A temporary kernel that is not tied to any
-   Yocto Project release.
-
--  *linux-yocto-4.8:* The stable yocto Project kernel to use with
-   the Yocto Project Release 2.2.
-
--  *linux-yocto-4.9:* The stable Yocto Project kernel to use with
-   the Yocto Project Release 2.3. This kernel is based on the Linux 4.9
-   released kernel.
-
--  *linux-yocto-4.10:* The default stable Yocto Project kernel to
-   use with the Yocto Project Release 2.3. This kernel is based on the
-   Linux 4.10 released kernel.
-
--  *linux-yocto-4.12:* The default stable Yocto Project kernel to
-   use with the Yocto Project Release 2.4. This kernel is based on the
-   Linux 4.12 released kernel.
-
--  *yocto-kernel-cache:* The ``linux-yocto-cache`` contains patches
-   and configurations for the linux-yocto kernel tree. This repository
-   is useful when working on the linux-yocto kernel. For more
-   information on this "Advanced Kernel Metadata", see the
-   ":doc:`/kernel-dev/advanced`" Chapter.
-
--  *linux-yocto-dev:* A development kernel based on the latest
-   upstream release candidate available.
-
-.. note::
-
-   Long Term Support Initiative (LTSI) for Yocto Linux kernels is as
-   follows:
-
-   -  For Yocto Project releases 1.7, 1.8, and 2.0, the LTSI kernel is
-      ``linux-yocto-3.14``.
-
-   -  For Yocto Project releases 2.1, 2.2, and 2.3, the LTSI kernel is
-      ``linux-yocto-4.1``.
-
-   -  For Yocto Project release 2.4, the LTSI kernel is
-      ``linux-yocto-4.9``
-
-   -  ``linux-yocto-4.4`` is an LTS kernel.
-
-Once a Yocto Linux kernel is officially released, the Yocto Project team
-goes into their next development cycle, or upward revision (uprev)
-cycle, while still continuing maintenance on the released kernel. It is
-important to note that the most sustainable and stable way to include
-feature development upstream is through a kernel uprev process.
-Back-porting hundreds of individual fixes and minor features from
-various kernel versions is not sustainable and can easily compromise
-quality.
-
-During the uprev cycle, the Yocto Project team uses an ongoing analysis
-of Linux kernel development, BSP support, and release timing to select
-the best possible ``kernel.org`` Linux kernel version on which to base
-subsequent Yocto Linux kernel development. The team continually monitors
-Linux community kernel development to look for significant features of
-interest. The team does consider back-porting large features if they
-have a significant advantage. User or community demand can also trigger
-a back-port or creation of new functionality in the Yocto Project
-baseline kernel during the uprev cycle.
-
-Generally speaking, every new Linux kernel both adds features and
-introduces new bugs. These consequences are the basic properties of
-upstream Linux kernel development and are managed by the Yocto Project
-team's Yocto Linux kernel development strategy. It is the Yocto Project
-team's policy to not back-port minor features to the released Yocto
-Linux kernel. They only consider back-porting significant technological
-jumps --- and, that is done after a complete gap analysis. The reason
-for this policy is that back-porting any small to medium sized change
-from an evolving Linux kernel can easily create mismatches,
-incompatibilities and very subtle errors.
-
-The policies described in this section result in both a stable and a
-cutting edge Yocto Linux kernel that mixes forward ports of existing
-Linux kernel features and significant and critical new functionality.
-Forward porting Linux kernel functionality into the Yocto Linux kernels
-available through the Yocto Project can be thought of as a "micro
-uprev". The many "micro uprevs" produce a Yocto Linux kernel version
-with a mix of important new mainline, non-mainline, BSP developments and
-feature integrations. This Yocto Linux kernel gives insight into new
-features and allows focused amounts of testing to be done on the kernel,
-which prevents surprises when selecting the next major uprev. The
-quality of these cutting edge Yocto Linux kernels is evolving and the
-kernels are used in leading edge feature and BSP development.
-
-Yocto Linux Kernel Architecture and Branching Strategies
-========================================================
-
-As mentioned earlier, a key goal of the Yocto Project is to present the
-developer with a kernel that has a clear and continuous history that is
-visible to the user. The architecture and mechanisms, in particular the
-branching strategies, used achieve that goal in a manner similar to
-upstream Linux kernel development in ``kernel.org``.
-
-You can think of a Yocto Linux kernel as consisting of a baseline Linux
-kernel with added features logically structured on top of the baseline.
-The features are tagged and organized by way of a branching strategy
-implemented by the Yocto Project team using the Source Code Manager
-(SCM) Git.
-
-.. note::
-
-   -  Git is the obvious SCM for meeting the Yocto Linux kernel
-      organizational and structural goals described in this section. Not
-      only is Git the SCM for Linux kernel development in ``kernel.org``
-      but, Git continues to grow in popularity and supports many
-      different work flows, front-ends and management techniques.
-
-   -  You can find documentation on Git at https://git-scm.com/doc. You can
-      also get an introduction to Git as it applies to the Yocto Project in the
-      ":ref:`overview-manual/development-environment:git`" section in the Yocto Project
-      Overview and Concepts Manual. The latter reference provides an
-      overview of Git and presents a minimal set of Git commands that
-      allows you to be functional using Git. You can use as much, or as
-      little, of what Git has to offer to accomplish what you need for
-      your project. You do not have to be a "Git Expert" in order to use
-      it with the Yocto Project.
-
-Using Git's tagging and branching features, the Yocto Project team
-creates kernel branches at points where functionality is no longer
-shared and thus, needs to be isolated. For example, board-specific
-incompatibilities would require different functionality and would
-require a branch to separate the features. Likewise, for specific kernel
-features, the same branching strategy is used.
-
-This "tree-like" architecture results in a structure that has features
-organized to be specific for particular functionality, single kernel
-types, or a subset of kernel types. Thus, the user has the ability to
-see the added features and the commits that make up those features. In
-addition to being able to see added features, the user can also view the
-history of what made up the baseline Linux kernel.
-
-Another consequence of this strategy results in not having to store the
-same feature twice internally in the tree. Rather, the kernel team
-stores the unique differences required to apply the feature onto the
-kernel type in question.
-
-.. note::
-
-   The Yocto Project team strives to place features in the tree such
-   that features can be shared by all boards and kernel types where
-   possible. However, during development cycles or when large features
-   are merged, the team cannot always follow this practice. In those
-   cases, the team uses isolated branches to merge features.
-
-BSP-specific code additions are handled in a similar manner to
-kernel-specific additions. Some BSPs only make sense given certain
-kernel types. So, for these types, the team creates branches off the end
-of that kernel type for all of the BSPs that are supported on that
-kernel type. From the perspective of the tools that create the BSP
-branch, the BSP is really no different than a feature. Consequently, the
-same branching strategy applies to BSPs as it does to kernel features.
-So again, rather than store the BSP twice, the team only stores the
-unique differences for the BSP across the supported multiple kernels.
-
-While this strategy can result in a tree with a significant number of
-branches, it is important to realize that from the developer's point of
-view, there is a linear path that travels from the baseline
-``kernel.org``, through a select group of features and ends with their
-BSP-specific commits. In other words, the divisions of the kernel are
-transparent and are not relevant to the developer on a day-to-day basis.
-From the developer's perspective, this path is the development branch.
-The developer does not need to be aware of the existence of
-any other branches at all. Of course, it can make sense to have these
-branches in the tree, should a person decide to explore them. For
-example, a comparison between two BSPs at either the commit level or at
-the line-by-line code ``diff`` level is now a trivial operation.
-
-The following illustration shows the conceptual Yocto Linux kernel.
-
-.. image:: figures/kernel-architecture-overview.png
-   :width: 100%
-
-In the illustration, the "Kernel.org Branch Point" marks the specific
-spot (or Linux kernel release) from which the Yocto Linux kernel is
-created. From this point forward in the tree, features and differences
-are organized and tagged.
-
-The "Yocto Project Baseline Kernel" contains functionality that is
-common to every kernel type and BSP that is organized further along in
-the tree. Placing these common features in the tree this way means
-features do not have to be duplicated along individual branches of the
-tree structure.
-
-From the "Yocto Project Baseline Kernel", branch points represent
-specific functionality for individual Board Support Packages (BSPs) as
-well as real-time kernels. The illustration represents this through
-three BSP-specific branches and a real-time kernel branch. Each branch
-represents some unique functionality for the BSP or for a real-time
-Yocto Linux kernel.
-
-In this example structure, the "Real-time (rt) Kernel" branch has common
-features for all real-time Yocto Linux kernels and contains more
-branches for individual BSP-specific real-time kernels. The illustration
-shows three branches as an example. Each branch points the way to
-specific, unique features for a respective real-time kernel as they
-apply to a given BSP.
-
-The resulting tree structure presents a clear path of markers (or
-branches) to the developer that, for all practical purposes, is the
-Yocto Linux kernel needed for any given set of requirements.
-
-.. note::
-
-   Keep in mind the figure does not take into account all the supported
-   Yocto Linux kernels, but rather shows a single generic kernel just
-   for conceptual purposes. Also keep in mind that this structure
-   represents the
-   :ref:`overview-manual/development-environment:yocto project source repositories`
-   that are either pulled from during the build or established on the
-   host development system prior to the build by either cloning a
-   particular kernel's Git repository or by downloading and unpacking a
-   tarball.
-
-Working with the kernel as a structured tree follows recognized
-community best practices. In particular, the kernel as shipped with the
-product, should be considered an "upstream source" and viewed as a
-series of historical and documented modifications (commits). These
-modifications represent the development and stabilization done by the
-Yocto Project kernel development team.
-
-Because commits only change at significant release points in the product
-life cycle, developers can work on a branch created from the last
-relevant commit in the shipped Yocto Project Linux kernel. As mentioned
-previously, the structure is transparent to the developer because the
-kernel tree is left in this state after cloning and building the kernel.
-
-Kernel Build File Hierarchy
-===========================
-
-Upstream storage of all the available kernel source code is one thing,
-while representing and using the code on your host development system is
-another. Conceptually, you can think of the kernel source repositories
-as all the source files necessary for all the supported Yocto Linux
-kernels. As a developer, you are just interested in the source files for
-the kernel on which you are working. And, furthermore, you need them
-available on your host system.
-
-Kernel source code is available on your host system several different
-ways:
-
--  *Files Accessed While using devtool:* ``devtool``, which is
-   available with the Yocto Project, is the preferred method by which to
-   modify the kernel. See the ":ref:`kernel-dev/intro:kernel modification workflow`" section.
-
--  *Cloned Repository:* If you are working in the kernel all the time,
-   you probably would want to set up your own local Git repository of
-   the Yocto Linux kernel tree. For information on how to clone a Yocto
-   Linux kernel Git repository, see the
-   ":ref:`kernel-dev/common:preparing the build host to work on the kernel`"
-   section.
-
--  *Temporary Source Files from a Build:* If you just need to make some
-   patches to the kernel using a traditional BitBake workflow (i.e. not
-   using the ``devtool``), you can access temporary kernel source files
-   that were extracted and used during a kernel build.
-
-The temporary kernel source files resulting from a build using BitBake
-have a particular hierarchy. When you build the kernel on your
-development system, all files needed for the build are taken from the
-source repositories pointed to by the
-:term:`SRC_URI` variable and gathered
-in a temporary work area where they are subsequently used to create the
-unique kernel. Thus, in a sense, the process constructs a local source
-tree specific to your kernel from which to generate the new kernel
-image.
-
-The following figure shows the temporary file structure created on your
-host system when you build the kernel using BitBake. This
-:term:`Build Directory` contains all the source files used during the build.
-
-.. image:: figures/kernel-overview-2-generic.png
-   :align: center
-   :width: 70%
-
-Again, for additional information on the Yocto Project kernel's
-architecture and its branching strategy, see the
-":ref:`kernel-dev/concepts-appx:yocto linux kernel architecture and branching strategies`"
-section. You can also reference the
-":ref:`kernel-dev/common:using \`\`devtool\`\` to patch the kernel`"
-and
-":ref:`kernel-dev/common:using traditional kernel development to patch the kernel`"
-sections for detailed example that modifies the kernel.
-
-Determining Hardware and Non-Hardware Features for the Kernel Configuration Audit Phase
-=======================================================================================
-
-This section describes part of the kernel configuration audit phase that
-most developers can ignore. For general information on kernel
-configuration including ``menuconfig``, ``defconfig`` files, and
-configuration fragments, see the
-":ref:`kernel-dev/common:configuring the kernel`" section.
-
-During this part of the audit phase, the contents of the final
-``.config`` file are compared against the fragments specified by the
-system. These fragments can be system fragments, distro fragments, or
-user-specified configuration elements. Regardless of their origin, the
-OpenEmbedded build system warns the user if a specific option is not
-included in the final kernel configuration.
-
-By default, in order to not overwhelm the user with configuration
-warnings, the system only reports missing "hardware" options as they
-could result in a boot failure or indicate that important hardware is
-not available.
-
-To determine whether or not a given option is "hardware" or
-"non-hardware", the kernel Metadata in ``yocto-kernel-cache`` contains
-files that classify individual or groups of options as either hardware
-or non-hardware. To better show this, consider a situation where the
-``yocto-kernel-cache`` contains the following files::
-
-   yocto-kernel-cache/features/drm-psb/hardware.cfg
-   yocto-kernel-cache/features/kgdb/hardware.cfg
-   yocto-kernel-cache/ktypes/base/hardware.cfg
-   yocto-kernel-cache/bsp/mti-malta32/hardware.cfg
-   yocto-kernel-cache/bsp/qemu-ppc32/hardware.cfg
-   yocto-kernel-cache/bsp/qemuarma9/hardware.cfg
-   yocto-kernel-cache/bsp/mti-malta64/hardware.cfg
-   yocto-kernel-cache/bsp/arm-versatile-926ejs/hardware.cfg
-   yocto-kernel-cache/bsp/common-pc/hardware.cfg
-   yocto-kernel-cache/bsp/common-pc-64/hardware.cfg
-   yocto-kernel-cache/features/rfkill/non-hardware.cfg
-   yocto-kernel-cache/ktypes/base/non-hardware.cfg
-   yocto-kernel-cache/features/aufs/non-hardware.kcf
-   yocto-kernel-cache/features/ocf/non-hardware.kcf
-   yocto-kernel-cache/ktypes/base/non-hardware.kcf
-   yocto-kernel-cache/ktypes/base/hardware.kcf
-   yocto-kernel-cache/bsp/qemu-ppc32/hardware.kcf
-
-Here are explanations for the various files:
-
--  ``hardware.kcf``: Specifies a list of kernel Kconfig files that
-   contain hardware options only.
-
--  ``non-hardware.kcf``: Specifies a list of kernel Kconfig files that
-   contain non-hardware options only.
-
--  ``hardware.cfg``: Specifies a list of kernel ``CONFIG_`` options that
-   are hardware, regardless of whether or not they are within a Kconfig
-   file specified by a hardware or non-hardware Kconfig file (i.e.
-   ``hardware.kcf`` or ``non-hardware.kcf``).
-
--  ``non-hardware.cfg``: Specifies a list of kernel ``CONFIG_`` options
-   that are not hardware, regardless of whether or not they are within a
-   Kconfig file specified by a hardware or non-hardware Kconfig file
-   (i.e. ``hardware.kcf`` or ``non-hardware.kcf``).
-
-Here is a specific example using the
-``kernel-cache/bsp/mti-malta32/hardware.cfg``::
-
-   CONFIG_SERIAL_8250
-   CONFIG_SERIAL_8250_CONSOLE
-   CONFIG_SERIAL_8250_NR_UARTS
-   CONFIG_SERIAL_8250_PCI
-   CONFIG_SERIAL_CORE
-   CONFIG_SERIAL_CORE_CONSOLE
-   CONFIG_VGA_ARB
-
-The kernel configuration audit automatically detects
-these files (hence the names must be exactly the ones discussed here),
-and uses them as inputs when generating warnings about the final
-``.config`` file.
-
-A user-specified kernel Metadata repository, or recipe space feature,
-can use these same files to classify options that are found within its
-``.cfg`` files as hardware or non-hardware, to prevent the OpenEmbedded
-build system from producing an error or warning when an option is not in
-the final ``.config`` file.