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Complete Yocto mirror with license table for TQMa6UL (2038-compliance)

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.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
*******************************************************
Working with Advanced Metadata (``yocto-kernel-cache``)
*******************************************************
Overview
========
In addition to supporting configuration fragments and patches, the Yocto
Project kernel tools also support rich
:term:`Metadata` that you can use to define
complex policies and Board Support Package (BSP) support. The purpose of
the Metadata and the tools that manage it is to help you manage the
complexity of the configuration and sources used to support multiple
BSPs and Linux kernel types.
Kernel Metadata exists in many places. One area in the
:ref:`overview-manual/development-environment:yocto project source repositories`
is the ``yocto-kernel-cache`` Git repository. You can find this repository
grouped under the "Yocto Linux Kernel" heading in the
:yocto_git:`Yocto Project Source Repositories <>`.
Kernel development tools ("kern-tools") are also available in the Yocto Project
Source Repositories under the "Yocto Linux Kernel" heading in the
``yocto-kernel-tools`` Git repository. The recipe that builds these
tools is ``meta/recipes-kernel/kern-tools/kern-tools-native_git.bb`` in
the :term:`Source Directory` (e.g.
``poky``).
Using Kernel Metadata in a Recipe
=================================
As mentioned in the introduction, the Yocto Project contains kernel
Metadata, which is located in the ``yocto-kernel-cache`` Git repository.
This Metadata defines Board Support Packages (BSPs) that correspond to
definitions in linux-yocto recipes for corresponding BSPs. A BSP
consists of an aggregation of kernel policy and enabled
hardware-specific features. The BSP can be influenced from within the
linux-yocto recipe.
.. note::
A Linux kernel recipe that contains kernel Metadata (e.g. inherits
from the ``linux-yocto.inc`` file) is said to be a "linux-yocto style" recipe.
Every linux-yocto style recipe must define the
:term:`KMACHINE` variable. This
variable is typically set to the same value as the :term:`MACHINE` variable,
which is used by :term:`BitBake`.
However, in some cases, the variable might instead refer to the
underlying platform of the :term:`MACHINE`.
Multiple BSPs can reuse the same :term:`KMACHINE` name if they are built
using the same BSP description. Multiple Corei7-based BSPs could share
the same "intel-corei7-64" value for :term:`KMACHINE`. It is important to
realize that :term:`KMACHINE` is just for kernel mapping, while :term:`MACHINE`
is the machine type within a BSP Layer. Even with this distinction,
however, these two variables can hold the same value. See the
":ref:`kernel-dev/advanced:bsp descriptions`" section for more information.
Every linux-yocto style recipe must also indicate the Linux kernel
source repository branch used to build the Linux kernel. The
:term:`KBRANCH` variable must be set
to indicate the branch.
.. note::
You can use the :term:`KBRANCH` value to define an alternate branch typically
with a machine override as shown here from the ``meta-yocto-bsp`` layer::
KBRANCH:beaglebone-yocto = "standard/beaglebone"
The linux-yocto style recipes can optionally define the following
variables:
- :term:`KERNEL_FEATURES`
- :term:`LINUX_KERNEL_TYPE`
:term:`LINUX_KERNEL_TYPE`
defines the kernel type to be used in assembling the configuration. If
you do not specify a :term:`LINUX_KERNEL_TYPE`, it defaults to "standard".
Together with :term:`KMACHINE`, :term:`LINUX_KERNEL_TYPE` defines the search
arguments used by the kernel tools to find the appropriate description
within the kernel Metadata with which to build out the sources and
configuration. The linux-yocto recipes define "standard", "tiny", and
"preempt-rt" kernel types. See the ":ref:`kernel-dev/advanced:kernel types`"
section for more information on kernel types.
During the build, the kern-tools search for the BSP description file
that most closely matches the :term:`KMACHINE` and :term:`LINUX_KERNEL_TYPE`
variables passed in from the recipe. The tools use the first BSP
description they find that matches both variables. If the tools cannot find
a match, they issue a warning.
The tools first search for the :term:`KMACHINE` and then for the
:term:`LINUX_KERNEL_TYPE`. If the tools cannot find a partial match, they
will use the sources from the :term:`KBRANCH` and any configuration
specified in the :term:`SRC_URI`.
You can use the
:term:`KERNEL_FEATURES`
variable to include features (configuration fragments, patches, or both)
that are not already included by the :term:`KMACHINE` and
:term:`LINUX_KERNEL_TYPE` variable combination. For example, to include a
feature specified as "features/netfilter/netfilter.scc", specify::
KERNEL_FEATURES += "features/netfilter/netfilter.scc"
To include a
feature called "cfg/sound.scc" just for the ``qemux86`` machine,
specify::
KERNEL_FEATURES:append:qemux86 = " cfg/sound.scc"
The value of
the entries in :term:`KERNEL_FEATURES` are dependent on their location
within the kernel Metadata itself. The examples here are taken from the
``yocto-kernel-cache`` repository. Each branch of this repository
contains "features" and "cfg" subdirectories at the top-level. For more
information, see the ":ref:`kernel-dev/advanced:kernel metadata syntax`"
section.
Kernel Metadata Syntax
======================
The kernel Metadata consists of three primary types of files: ``scc``
[1]_ description files, configuration fragments, and patches. The
``scc`` files define variables and include or otherwise reference any of
the three file types. The description files are used to aggregate all
types of kernel Metadata into what ultimately describes the sources and
the configuration required to build a Linux kernel tailored to a
specific machine.
The ``scc`` description files are used to define two fundamental types
of kernel Metadata:
- Features
- Board Support Packages (BSPs)
Features aggregate sources in the form of patches and configuration
fragments into a modular reusable unit. You can use features to
implement conceptually separate kernel Metadata descriptions such as
pure configuration fragments, simple patches, complex features, and
kernel types. :ref:`kernel-dev/advanced:kernel types` define general kernel
features and policy to be reused in the BSPs.
BSPs define hardware-specific features and aggregate them with kernel
types to form the final description of what will be assembled and built.
While the kernel Metadata syntax does not enforce any logical separation
of configuration fragments, patches, features or kernel types, best
practices dictate a logical separation of these types of Metadata. The
following Metadata file hierarchy is recommended::
base/
bsp/
cfg/
features/
ktypes/
patches/
The ``bsp`` directory contains the :ref:`kernel-dev/advanced:bsp descriptions`.
The remaining directories all contain "features". Separating ``bsp`` from the
rest of the structure aids conceptualizing intended usage.
Use these guidelines to help place your ``scc`` description files within
the structure:
- If your file contains only configuration fragments, place the file in
the ``cfg`` directory.
- If your file contains only source-code fixes, place the file in the
``patches`` directory.
- If your file encapsulates a major feature, often combining sources
and configurations, place the file in ``features`` directory.
- If your file aggregates non-hardware configuration and patches in
order to define a base kernel policy or major kernel type to be
reused across multiple BSPs, place the file in ``ktypes`` directory.
These distinctions can easily become blurred --- especially as out-of-tree
features slowly merge upstream over time. Also, remember that how the
description files are placed is a purely logical organization and has no
impact on the functionality of the kernel Metadata. There is no impact
because all of ``cfg``, ``features``, ``patches``, and ``ktypes``,
contain "features" as far as the kernel tools are concerned.
Paths used in kernel Metadata files are relative to base, which is
either
:term:`FILESEXTRAPATHS` if
you are creating Metadata in
:ref:`recipe-space <kernel-dev/advanced:recipe-space metadata>`,
or the top level of
:yocto_git:`yocto-kernel-cache </yocto-kernel-cache/tree/>`
if you are creating
:ref:`kernel-dev/advanced:metadata outside the recipe-space`.
.. [1]
``scc`` stands for Series Configuration Control, but the naming has
less significance in the current implementation of the tooling than
it had in the past. Consider ``scc`` files to be description files.
Configuration
-------------
The simplest unit of kernel Metadata is the configuration-only feature.
This feature consists of one or more Linux kernel configuration
parameters in a configuration fragment file (``.cfg``) and a ``.scc``
file that describes the fragment.
As an example, consider the Symmetric Multi-Processing (SMP) fragment
used with the ``linux-yocto-4.12`` kernel as defined outside of the
recipe space (i.e. ``yocto-kernel-cache``). This Metadata consists of
two files: ``smp.scc`` and ``smp.cfg``. You can find these files in the
``cfg`` directory of the ``yocto-4.12`` branch in the
``yocto-kernel-cache`` Git repository::
cfg/smp.scc:
define KFEATURE_DESCRIPTION "Enable SMP for 32 bit builds"
define KFEATURE_COMPATIBILITY all
kconf hardware smp.cfg
cfg/smp.cfg:
CONFIG_SMP=y
CONFIG_SCHED_SMT=y
# Increase default NR_CPUS from 8 to 64 so that platform with
# more than 8 processors can be all activated at boot time
CONFIG_NR_CPUS=64
# The following is needed when setting NR_CPUS to something
# greater than 8 on x86 architectures, it should be automatically
# disregarded by Kconfig when using a different arch
CONFIG_X86_BIGSMP=y
You can find general information on configuration
fragment files in the ":ref:`kernel-dev/common:creating configuration fragments`" section.
Within the ``smp.scc`` file, the
:term:`KFEATURE_DESCRIPTION`
statement provides a short description of the fragment. Higher level
kernel tools use this description.
Also within the ``smp.scc`` file, the ``kconf`` command includes the
actual configuration fragment in an ``.scc`` file, and the "hardware"
keyword identifies the fragment as being hardware enabling, as opposed
to general policy, which would use the "non-hardware" keyword. The
distinction is made for the benefit of the configuration validation
tools, which warn you if a hardware fragment overrides a policy set by a
non-hardware fragment.
.. note::
The description file can include multiple ``kconf`` statements, one per
fragment.
As described in the
":ref:`kernel-dev/common:validating configuration`" section, you can
use the following BitBake command to audit your configuration::
$ bitbake linux-yocto -c kernel_configcheck -f
Patches
-------
Patch descriptions are very similar to configuration fragment
descriptions, which are described in the previous section. However,
instead of a ``.cfg`` file, these descriptions work with source patches
(i.e. ``.patch`` files).
A typical patch includes a description file and the patch itself. As an
example, consider the build patches used with the ``linux-yocto-4.12``
kernel as defined outside of the recipe space (i.e.
``yocto-kernel-cache``). This Metadata consists of several files:
``build.scc`` and a set of ``*.patch`` files. You can find these files
in the ``patches/build`` directory of the ``yocto-4.12`` branch in the
``yocto-kernel-cache`` Git repository.
The following listings show the ``build.scc`` file and part of the
``modpost-mask-trivial-warnings.patch`` file::
patches/build/build.scc:
patch arm-serialize-build-targets.patch
patch powerpc-serialize-image-targets.patch
patch kbuild-exclude-meta-directory-from-distclean-processi.patch
# applied by kgit
# patch kbuild-add-meta-files-to-the-ignore-li.patch
patch modpost-mask-trivial-warnings.patch
patch menuconfig-check-lxdiaglog.sh-Allow-specification-of.patch
patches/build/modpost-mask-trivial-warnings.patch:
From bd48931bc142bdd104668f3a062a1f22600aae61 Mon Sep 17 00:00:00 2001
From: Paul Gortmaker <paul.gortmaker@windriver.com>
Date: Sun, 25 Jan 2009 17:58:09 -0500
Subject: [PATCH] modpost: mask trivial warnings
Newer HOSTCC will complain about various stdio fcns because
.
.
.
char *dump_write = NULL, *files_source = NULL;
int opt;
--
2.10.1
generated by cgit v0.10.2 at 2017-09-28 15:23:23 (GMT)
The description file can
include multiple patch statements where each statement handles a single
patch. In the example ``build.scc`` file, there are five patch statements
for the five patches in the directory.
You can create a typical ``.patch`` file using ``diff -Nurp`` or
``git format-patch`` commands. For information on how to create patches,
see 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.
Features
--------
Features are complex kernel Metadata types that consist of configuration
fragments, patches, and possibly other feature description files. As an
example, consider the following generic listing::
features/myfeature.scc
define KFEATURE_DESCRIPTION "Enable myfeature"
patch 0001-myfeature-core.patch
patch 0002-myfeature-interface.patch
include cfg/myfeature_dependency.scc
kconf non-hardware myfeature.cfg
This example shows how the ``patch`` and ``kconf`` commands are used as well
as how an additional feature description file is included with the
``include`` command.
Typically, features are less granular than configuration fragments and
are more likely than configuration fragments and patches to be the types
of things you want to specify in the :term:`KERNEL_FEATURES` variable of the
Linux kernel recipe. See the
":ref:`kernel-dev/advanced:using kernel metadata in a recipe`" section earlier
in the manual.
Kernel Types
------------
A kernel type defines a high-level kernel policy by aggregating non-hardware
configuration fragments with patches you want to use when building a Linux
kernel of a specific type (e.g. a real-time kernel). Syntactically, kernel
types are no different than features as described in the
":ref:`kernel-dev/advanced:features`" section. The :term:`LINUX_KERNEL_TYPE`
variable in the kernel recipe selects the kernel type. For example, in the
``linux-yocto_4.12.bb`` kernel recipe found in ``poky/meta/recipes-kernel/linux``, a
:ref:`require <bitbake-user-manual/bitbake-user-manual-metadata:\`\`require\`\` directive>`
directive includes the ``poky/meta/recipes-kernel/linux/linux-yocto.inc`` file,
which has the following statement that defines the default kernel type::
LINUX_KERNEL_TYPE ??= "standard"
Another example would be the real-time kernel (i.e.
``linux-yocto-rt_4.12.bb``). This kernel recipe directly sets the kernel
type as follows::
LINUX_KERNEL_TYPE = "preempt-rt"
.. note::
You can find kernel recipes in the ``meta/recipes-kernel/linux`` directory
of the :ref:`overview-manual/development-environment:yocto project source repositories`
(e.g. ``poky/meta/recipes-kernel/linux/linux-yocto_4.12.bb``). See the
":ref:`kernel-dev/advanced:using kernel metadata in a recipe`"
section for more information.
Three kernel types ("standard", "tiny", and "preempt-rt") are supported
for Linux Yocto kernels:
- "standard": Includes the generic Linux kernel policy of the Yocto
Project linux-yocto kernel recipes. This policy includes, among other
things, which file systems, networking options, core kernel features,
and debugging and tracing options are supported.
- "preempt-rt": Applies the ``PREEMPT_RT`` patches and the
configuration options required to build a real-time Linux kernel.
This kernel type inherits from the "standard" kernel type.
- "tiny": Defines a bare minimum configuration meant to serve as a base
for very small Linux kernels. The "tiny" kernel type is independent
from the "standard" configuration. Although the "tiny" kernel type
does not currently include any source changes, it might in the
future.
For any given kernel type, the Metadata is defined by the ``.scc`` (e.g.
``standard.scc``). Here is a partial listing for the ``standard.scc``
file, which is found in the ``ktypes/standard`` directory of the
``yocto-kernel-cache`` Git repository::
# Include this kernel type fragment to get the standard features and
# configuration values.
# Note: if only the features are desired, but not the configuration
# then this should be included as:
# include ktypes/standard/standard.scc nocfg
# if no chained configuration is desired, include it as:
# include ktypes/standard/standard.scc nocfg inherit
include ktypes/base/base.scc
branch standard
kconf non-hardware standard.cfg
include features/kgdb/kgdb.scc
.
.
.
include cfg/net/ip6_nf.scc
include cfg/net/bridge.scc
include cfg/systemd.scc
include features/rfkill/rfkill.scc
As with any ``.scc`` file, a kernel type definition can aggregate other
``.scc`` files with ``include`` commands. These definitions can also
directly pull in configuration fragments and patches with the ``kconf``
and ``patch`` commands, respectively.
.. note::
It is not strictly necessary to create a kernel type ``.scc``
file. The Board Support Package (BSP) file can implicitly define the
kernel type using a ``define`` :term:`KTYPE` ``myktype`` line. See the
":ref:`kernel-dev/advanced:bsp descriptions`" section for more
information.
BSP Descriptions
----------------
BSP descriptions (i.e. ``*.scc`` files) combine kernel types with
hardware-specific features. The hardware-specific Metadata is typically
defined independently in the BSP layer, and then aggregated with each
supported kernel type.
.. note::
For BSPs supported by the Yocto Project, the BSP description files
are located in the ``bsp`` directory of the ``yocto-kernel-cache``
repository organized under the "Yocto Linux Kernel" heading in the
:yocto_git:`Yocto Project Source Repositories <>`.
This section overviews the BSP description structure, the aggregation
concepts, and presents a detailed example using a BSP supported by the
Yocto Project (i.e. BeagleBone Board). For complete information on BSP
layer file hierarchy, see the :doc:`/bsp-guide/index`.
Description Overview
~~~~~~~~~~~~~~~~~~~~
For simplicity, consider the following root BSP layer description files
for the BeagleBone board. These files employ both a structure and naming
convention for consistency. The naming convention for the file is as
follows::
bsp_root_name-kernel_type.scc
Here are some example root layer
BSP filenames for the BeagleBone Board BSP, which is supported by the
Yocto Project::
beaglebone-standard.scc
beaglebone-preempt-rt.scc
Each file uses the root name (i.e "beaglebone") BSP name followed by the
kernel type.
Examine the ``beaglebone-standard.scc`` file::
define KMACHINE beaglebone
define KTYPE standard
define KARCH arm
include ktypes/standard/standard.scc
branch beaglebone
include beaglebone.scc
# default policy for standard kernels
include features/latencytop/latencytop.scc
include features/profiling/profiling.scc
Every top-level BSP description file
should define the :term:`KMACHINE`,
:term:`KTYPE`, and
:term:`KARCH` variables. These
variables allow the OpenEmbedded build system to identify the
description as meeting the criteria set by the recipe being built. This
example supports the "beaglebone" machine for the "standard" kernel and
the "arm" architecture.
Be aware that there is no hard link between the :term:`KTYPE` variable and a kernel
type description file. Thus, if you do not have the
kernel type defined in your kernel Metadata as it is here, you only need
to ensure that the
:term:`LINUX_KERNEL_TYPE`
variable in the kernel recipe and the :term:`KTYPE` variable in the BSP
description file match.
To separate your kernel policy from your hardware configuration, you
include a kernel type (``ktype``), such as "standard". In the previous
example, this is done using the following::
include ktypes/standard/standard.scc
This file aggregates all the configuration
fragments, patches, and features that make up your standard kernel
policy. See the ":ref:`kernel-dev/advanced:kernel types`" section for more
information.
To aggregate common configurations and features specific to the kernel
for `mybsp`, use the following::
include mybsp.scc
You can see that in the BeagleBone example with the following::
include beaglebone.scc
For information on how to break a complete ``.config`` file into the various
configuration fragments, see the ":ref:`kernel-dev/common:creating configuration fragments`" section.
Finally, if you have any configurations specific to the hardware that
are not in a ``*.scc`` file, you can include them as follows::
kconf hardware mybsp-extra.cfg
The BeagleBone example does not include these
types of configurations. However, the Malta 32-bit board does
("mti-malta32"). Here is the ``mti-malta32-le-standard.scc`` file::
define KMACHINE mti-malta32-le
define KMACHINE qemumipsel
define KTYPE standard
define KARCH mips
include ktypes/standard/standard.scc
branch mti-malta32
include mti-malta32.scc
kconf hardware mti-malta32-le.cfg
Example
~~~~~~~
Many real-world examples are more complex. Like any other ``.scc`` file,
BSP descriptions can aggregate features. Consider the Minnow BSP
definition given the ``linux-yocto-4.4`` branch of the
``yocto-kernel-cache`` (i.e. ``yocto-kernel-cache/bsp/minnow/minnow.scc``)::
include cfg/x86.scc
include features/eg20t/eg20t.scc
include cfg/dmaengine.scc
include features/power/intel.scc
include cfg/efi.scc
include features/usb/ehci-hcd.scc
include features/usb/ohci-hcd.scc
include features/usb/usb-gadgets.scc
include features/usb/touchscreen-composite.scc
include cfg/timer/hpet.scc
include features/leds/leds.scc
include features/spi/spidev.scc
include features/i2c/i2cdev.scc
include features/mei/mei-txe.scc
# Earlyprintk and port debug requires 8250
kconf hardware cfg/8250.cfg
kconf hardware minnow.cfg
kconf hardware minnow-dev.cfg
.. note::
Although the Minnow Board BSP is unused, the Metadata remains and is
being used here just as an example.
The ``minnow.scc`` description file includes a hardware configuration
fragment (``minnow.cfg``) specific to the Minnow BSP as well as several
more general configuration fragments and features enabling hardware
found on the machine. This ``minnow.scc`` description file is then
included in each of the three "minnow" description files for the
supported kernel types (i.e. "standard", "preempt-rt", and "tiny").
Consider the "minnow" description for the "standard" kernel type (i.e.
``minnow-standard.scc``)::
define KMACHINE minnow
define KTYPE standard
define KARCH i386
include ktypes/standard
include minnow.scc
# Extra minnow configs above the minimal defined in minnow.scc
include cfg/efi-ext.scc
include features/media/media-all.scc
include features/sound/snd_hda_intel.scc
# The following should really be in standard.scc
# USB live-image support
include cfg/usb-mass-storage.scc
include cfg/boot-live.scc
# Basic profiling
include features/latencytop/latencytop.scc
include features/profiling/profiling.scc
# Requested drivers that don't have an existing scc
kconf hardware minnow-drivers-extra.cfg
The ``include`` command midway through the file includes the ``minnow.scc`` description
that defines all enabled hardware for the BSP that is common to all
kernel types. Using this command significantly reduces duplication.
Now consider the "minnow" description for the "tiny" kernel type (i.e.
``minnow-tiny.scc``)::
define KMACHINE minnow
define KTYPE tiny
define KARCH i386
include ktypes/tiny
include minnow.scc
As you might expect,
the "tiny" description includes quite a bit less. In fact, it includes
only the minimal policy defined by the "tiny" kernel type and the
hardware-specific configuration required for booting the machine along
with the most basic functionality of the system as defined in the base
"minnow" description file.
Notice again the three critical variables:
:term:`KMACHINE`,
:term:`KTYPE`, and
:term:`KARCH`. Of these variables, only
:term:`KTYPE` has changed to specify the "tiny" kernel type.
Kernel Metadata Location
========================
Kernel Metadata always exists outside of the kernel tree either defined
in a kernel recipe (recipe-space) or outside of the recipe. Where you
choose to define the Metadata depends on what you want to do and how you
intend to work. Regardless of where you define the kernel Metadata, the
syntax used applies equally.
If you are unfamiliar with the Linux kernel and only wish to apply a
configuration and possibly a couple of patches provided to you by
others, the recipe-space method is recommended. This method is also a
good approach if you are working with Linux kernel sources you do not
control or if you just do not want to maintain a Linux kernel Git
repository on your own. For partial information on how you can define
kernel Metadata in the recipe-space, see the
":ref:`kernel-dev/common:modifying an existing recipe`" section.
Conversely, if you are actively developing a kernel and are already
maintaining a Linux kernel Git repository of your own, you might find it
more convenient to work with kernel Metadata kept outside the
recipe-space. Working with Metadata in this area can make iterative
development of the Linux kernel more efficient outside of the BitBake
environment.
Recipe-Space Metadata
---------------------
When stored in recipe-space, the kernel Metadata files reside in a
directory hierarchy below :term:`FILESEXTRAPATHS`. For
a linux-yocto recipe or for a Linux kernel recipe derived by copying
:oe_git:`meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb
</openembedded-core/tree/meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb>`
into your layer and modifying it, :term:`FILESEXTRAPATHS` is typically set to
``${``\ :term:`THISDIR`\ ``}/${``\ :term:`PN`\ ``}``.
See the ":ref:`kernel-dev/common:modifying an existing recipe`"
section for more information.
Here is an example that shows a trivial tree of kernel Metadata stored
in recipe-space within a BSP layer::
meta-my_bsp_layer/
`-- recipes-kernel
`-- linux
`-- linux-yocto
|-- bsp-standard.scc
|-- bsp.cfg
`-- standard.cfg
When the Metadata is stored in recipe-space, you must take steps to
ensure BitBake has the necessary information to decide what files to
fetch and when they need to be fetched again. It is only necessary to
specify the ``.scc`` files on the
:term:`SRC_URI`. BitBake parses them
and fetches any files referenced in the ``.scc`` files by the
``include``, ``patch``, or ``kconf`` commands. Because of this, it is
necessary to bump the recipe :term:`PR`
value when changing the content of files not explicitly listed in the
:term:`SRC_URI`.
If the BSP description is in recipe space, you cannot simply list the
``*.scc`` in the :term:`SRC_URI` statement. You need to use the following
form from your kernel append file::
SRC_URI:append:myplatform = " \
file://myplatform;type=kmeta;destsuffix=myplatform \
"
Metadata Outside the Recipe-Space
---------------------------------
When stored outside of the recipe-space, the kernel Metadata files
reside in a separate repository. The OpenEmbedded build system adds the
Metadata to the build as a "type=kmeta" repository through the
:term:`SRC_URI` variable. As an
example, consider the following :term:`SRC_URI` statement from the
``linux-yocto_5.15.bb`` kernel recipe::
SRC_URI = "git://git.yoctoproject.org/linux-yocto.git;name=machine;branch=${KBRANCH};protocol=https \
git://git.yoctoproject.org/yocto-kernel-cache;type=kmeta;name=meta;branch=yocto-5.15;destsuffix=${KMETA};protocol=https"
``${KMETA}``, in this context, is simply used to name the directory into
which the Git fetcher places the Metadata. This behavior is no different
than any multi-repository :term:`SRC_URI` statement used in a recipe (e.g.
see the previous section).
You can keep kernel Metadata in a "kernel-cache", which is a directory
containing configuration fragments. As with any Metadata kept outside
the recipe-space, you simply need to use the :term:`SRC_URI` statement with
the "type=kmeta" attribute. Doing so makes the kernel Metadata available
during the configuration phase.
If you modify the Metadata, you must not forget to update the :term:`SRCREV`
statements in the kernel's recipe. In particular, you need to update the
``SRCREV_meta`` variable to match the commit in the ``KMETA`` branch you
wish to use. Changing the data in these branches and not updating the
:term:`SRCREV` statements to match will cause the build to fetch an older
commit.
Organizing Your Source
======================
Many recipes based on the ``linux-yocto-custom.bb`` recipe use Linux
kernel sources that have only a single branch. This type of
repository structure is fine for linear development supporting a single
machine and architecture. However, if you work with multiple boards and
architectures, a kernel source repository with multiple branches is more
efficient. For example, suppose you need a series of patches for one
board to boot. Sometimes, these patches are works-in-progress or
fundamentally wrong, yet they are still necessary for specific boards.
In these situations, you most likely do not want to include these
patches in every kernel you build (i.e. have the patches as part of the
default branch). It is situations like these that give rise to
multiple branches used within a Linux kernel sources Git repository.
Here are repository organization strategies maximizing source reuse,
removing redundancy, and logically ordering your changes. This section
presents strategies for the following cases:
- Encapsulating patches in a feature description and only including the
patches in the BSP descriptions of the applicable boards.
- Creating a machine branch in your kernel source repository and
applying the patches on that branch only.
- Creating a feature branch in your kernel source repository and
merging that branch into your BSP when needed.
The approach you take is entirely up to you and depends on what works
best for your development model.
Encapsulating Patches
---------------------
If you are reusing patches from an external tree and are not working on
the patches, you might find the encapsulated feature to be appropriate.
Given this scenario, you do not need to create any branches in the
source repository. Rather, you just take the static patches you need and
encapsulate them within a feature description. Once you have the feature
description, you simply include that into the BSP description as
described in the ":ref:`kernel-dev/advanced:bsp descriptions`" section.
You can find information on how to create patches and BSP descriptions
in the ":ref:`kernel-dev/advanced:patches`" and
":ref:`kernel-dev/advanced:bsp descriptions`" sections.
Machine Branches
----------------
When you have multiple machines and architectures to support, or you are
actively working on board support, it is more efficient to create
branches in the repository based on individual machines. Having machine
branches allows common source to remain in the development branch with any
features specific to a machine stored in the appropriate machine branch.
This organization method frees you from continually reintegrating your
patches into a feature.
Once you have a new branch, you can set up your kernel Metadata to use
the branch a couple different ways. In the recipe, you can specify the
new branch as the :term:`KBRANCH` to use for the board as follows::
KBRANCH = "mynewbranch"
Another method is to use the ``branch`` command in the BSP
description::
mybsp.scc:
define KMACHINE mybsp
define KTYPE standard
define KARCH i386
include standard.scc
branch mynewbranch
include mybsp-hw.scc
If you find yourself with numerous branches, you might consider using a
hierarchical branching system similar to what the Yocto Linux Kernel Git
repositories use::
common/kernel_type/machine
If you had two kernel types, "standard" and "small" for instance, three
machines, and common as ``mydir``, the branches in your Git repository
might look like this::
mydir/base
mydir/standard/base
mydir/standard/machine_a
mydir/standard/machine_b
mydir/standard/machine_c
mydir/small/base
mydir/small/machine_a
This organization can help clarify the branch relationships. In this
case, ``mydir/standard/machine_a`` includes everything in ``mydir/base``
and ``mydir/standard/base``. The "standard" and "small" branches add
sources specific to those kernel types that for whatever reason are not
appropriate for the other branches.
.. note::
The "base" branches are an artifact of the way Git manages its data
internally on the filesystem: Git will not allow you to use
``mydir/standard`` and ``mydir/standard/machine_a`` because it would have to
create a file and a directory named "standard".
Feature Branches
----------------
When you are actively developing new features, it can be more efficient
to work with that feature as a branch, rather than as a set of patches
that have to be regularly updated. The Yocto Project Linux kernel tools
provide for this with the ``git merge`` command.
To merge a feature branch into a BSP, insert the ``git merge`` command
after any ``branch`` commands::
mybsp.scc:
define KMACHINE mybsp
define KTYPE standard
define KARCH i386
include standard.scc
branch mynewbranch
git merge myfeature
include mybsp-hw.scc
SCC Description File Reference
==============================
This section provides a brief reference for the commands you can use
within an SCC description file (``.scc``):
- ``branch [ref]``: Creates a new branch relative to the current branch
(typically ``${KTYPE}``) using the currently checked-out branch, or
"ref" if specified.
- ``define``: Defines variables, such as
:term:`KMACHINE`,
:term:`KTYPE`,
:term:`KARCH`, and
:term:`KFEATURE_DESCRIPTION`.
- ``include SCC_FILE``: Includes an SCC file in the current file. The
file is parsed as if you had inserted it inline.
- ``kconf [hardware|non-hardware] CFG_FILE``: Queues a configuration
fragment for merging into the final Linux ``.config`` file.
- ``git merge GIT_BRANCH``: Merges the feature branch into the current
branch.
- ``patch PATCH_FILE``: Applies the patch to the current Git branch.

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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.

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.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
**********************
Kernel Development FAQ
**********************
Common Questions and Solutions
==============================
Here are some solutions for common questions.
How do I use my own Linux kernel ``.config`` file?
--------------------------------------------------
Refer to the
":ref:`kernel-dev/common:changing the configuration`"
section for information.
How do I create configuration fragments?
----------------------------------------
A: Refer to the
":ref:`kernel-dev/common:creating configuration fragments`"
section for information.
How do I use my own Linux kernel sources?
-----------------------------------------
Refer to the
":ref:`kernel-dev/common:working with your own sources`"
section for information.
How do I install/not-install the kernel image on the root filesystem?
---------------------------------------------------------------------
The kernel image (e.g. ``vmlinuz``) is provided by the
``kernel-image`` package. Image recipes depend on ``kernel-base``. To
specify whether or not the kernel image is installed in the generated
root filesystem, override ``RRECOMMENDS:${KERNEL_PACKAGE_NAME}-base`` to include or not
include "kernel-image". See the
":ref:`dev-manual/layers:appending other layers metadata with your layer`"
section in the
Yocto Project Development Tasks Manual for information on how to use an
append file to override metadata.
How do I install a specific kernel module?
------------------------------------------
Linux kernel modules are packaged individually. To ensure a
specific kernel module is included in an image, include it in the
appropriate machine :term:`RRECOMMENDS` variable.
These other variables are useful for installing specific modules:
- :term:`MACHINE_ESSENTIAL_EXTRA_RDEPENDS`
- :term:`MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS`
- :term:`MACHINE_EXTRA_RDEPENDS`
- :term:`MACHINE_EXTRA_RRECOMMENDS`
For example, set the following in the ``qemux86.conf`` file to include
the ``ab123`` kernel modules with images built for the ``qemux86``
machine::
MACHINE_EXTRA_RRECOMMENDS += "kernel-module-ab123"
For more information, see the
":ref:`kernel-dev/common:incorporating out-of-tree modules`" section.
How do I change the Linux kernel command line?
----------------------------------------------
The Linux kernel command line is
typically specified in the machine config using the :term:`APPEND` variable.
For example, you can add some helpful debug information doing the
following::
APPEND += "printk.time=y initcall_debug debug"

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.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
=============================================
Yocto Project Linux Kernel Development Manual
=============================================
|
.. toctree::
:caption: Table of Contents
:numbered:
intro
common
advanced
concepts-appx
maint-appx
faq
.. include:: /boilerplate.rst

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.. 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/layers: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%
#. *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.
#. *Set Up Your Host Development System for Kernel Development:* It is
recommended that you use ``devtool`` 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`` requires that you have a clean build
of the image. 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.
#. *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
Yocto's :term:`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.
#. *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 :term:`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``.
#. *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.

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.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
******************
Kernel Maintenance
******************
Tree Construction
=================
This section describes construction of the Yocto Project kernel source
repositories as accomplished by the Yocto Project team to create Yocto
Linux kernel repositories. These kernel repositories are found under the
heading "Yocto Linux Kernel" at :yocto_git:`/` and
are shipped as part of a Yocto Project release. The team creates these
repositories by compiling and executing the set of feature descriptions
for every BSP and feature in the product. Those feature descriptions
list all necessary patches, configurations, branches, tags, and feature
divisions found in a Yocto Linux kernel. Thus, the Yocto Project Linux
kernel repository (or tree) and accompanying Metadata in the
``yocto-kernel-cache`` are built.
The existence of these repositories allow you to access and clone a
particular Yocto Project Linux kernel repository and use it to build
images based on their configurations and features.
You can find the files used to describe all the valid features and BSPs
in the Yocto Project Linux kernel in any clone of the Yocto Project
Linux kernel source repository and ``yocto-kernel-cache`` Git trees. For
example, the following commands clone the Yocto Project baseline Linux
kernel that branches off ``linux.org`` version 4.12 and the
``yocto-kernel-cache``, which contains stores of kernel Metadata::
$ git clone git://git.yoctoproject.org/linux-yocto-4.12
$ git clone git://git.yoctoproject.org/linux-kernel-cache
For more information on
how to set up a local Git repository of the Yocto Project Linux kernel
files, see the
":ref:`kernel-dev/common:preparing the build host to work on the kernel`"
section.
Once you have cloned the kernel Git repository and the cache of Metadata
on your local machine, you can discover the branches that are available
in the repository using the following Git command::
$ git branch -a
Checking out a branch allows you to work with a particular Yocto Linux
kernel. For example, the following commands check out the
"standard/beagleboard" branch of the Yocto Linux kernel repository and
the "yocto-4.12" branch of the ``yocto-kernel-cache`` repository::
$ cd ~/linux-yocto-4.12
$ git checkout -b my-kernel-4.12 remotes/origin/standard/beagleboard
$ cd ~/linux-kernel-cache
$ git checkout -b my-4.12-metadata remotes/origin/yocto-4.12
.. note::
Branches in the ``yocto-kernel-cache`` repository correspond to Yocto Linux
kernel versions (e.g. "yocto-4.12", "yocto-4.10", "yocto-4.9", and so forth).
Once you have checked out and switched to appropriate branches, you can
see a snapshot of all the kernel source files used to build that
particular Yocto Linux kernel for a particular board.
To see the features and configurations for a particular Yocto Linux
kernel, you need to examine the ``yocto-kernel-cache`` Git repository.
As mentioned, branches in the ``yocto-kernel-cache`` repository
correspond to Yocto Linux kernel versions (e.g. ``yocto-4.12``).
Branches contain descriptions in the form of ``.scc`` and ``.cfg``
files.
You should realize, however, that browsing your local
``yocto-kernel-cache`` repository for feature descriptions and patches
is not an effective way to determine what is in a particular kernel
branch. Instead, you should use Git directly to discover the changes in
a branch. Using Git is an efficient and flexible way to inspect changes
to the kernel.
.. note::
Ground up reconstruction of the complete kernel tree is an action
only taken by the Yocto Project team during an active development
cycle. When you create a clone of the kernel Git repository, you are
simply making it efficiently available for building and development.
The following steps describe what happens when the Yocto Project Team
constructs the Yocto Project kernel source Git repository (or tree)
found at :yocto_git:`/` given the introduction of a new
top-level kernel feature or BSP. The following actions effectively
provide the Metadata and create the tree that includes the new feature,
patch, or BSP:
#. *Pass Feature to the OpenEmbedded Build System:* A top-level kernel
feature is passed to the kernel build subsystem. Normally, this
feature is a BSP for a particular kernel type.
#. *Locate Feature:* The file that describes the top-level feature is
located by searching these system directories:
- The in-tree kernel-cache directories, which are located in the
:yocto_git:`yocto-kernel-cache </yocto-kernel-cache/tree/bsp>`
repository organized under the "Yocto Linux Kernel" heading in the
:yocto_git:`Yocto Project Source Repositories <>`.
- Areas pointed to by :term:`SRC_URI` statements found in kernel recipes.
For a typical build, the target of the search is a feature
description in an ``.scc`` file whose name follows this format (e.g.
``beaglebone-standard.scc`` and ``beaglebone-preempt-rt.scc``)::
bsp_root_name-kernel_type.scc
#. *Expand Feature:* Once located, the feature description is either
expanded into a simple script of actions, or into an existing
equivalent script that is already part of the shipped kernel.
#. *Append Extra Features:* Extra features are appended to the top-level
feature description. These features can come from the
:term:`KERNEL_FEATURES`
variable in recipes.
#. *Locate, Expand, and Append Each Feature:* Each extra feature is
located, expanded and appended to the script as described in step
three.
#. *Execute the Script:* The script is executed to produce files
``.scc`` and ``.cfg`` files in appropriate directories of the
``yocto-kernel-cache`` repository. These files are descriptions of
all the branches, tags, patches and configurations that need to be
applied to the base Git repository to completely create the source
(build) branch for the new BSP or feature.
#. *Clone Base Repository:* The base repository is cloned, and the
actions listed in the ``yocto-kernel-cache`` directories are applied
to the tree.
#. *Perform Cleanup:* The Git repositories are left with the desired
branches checked out and any required branching, patching and tagging
has been performed.
The kernel tree and cache are ready for developer consumption to be
locally cloned, configured, and built into a Yocto Project kernel
specific to some target hardware.
.. note::
- The generated ``yocto-kernel-cache`` repository adds to the kernel
as shipped with the Yocto Project release. Any add-ons and
configuration data are applied to the end of an existing branch.
The full repository generation that is found in the official Yocto
Project kernel repositories at :yocto_git:`/` is the
combination of all supported boards and configurations.
- The technique the Yocto Project team uses is flexible and allows
for seamless blending of an immutable history with additional
patches specific to a deployment. Any additions to the kernel
become an integrated part of the branches.
- The full kernel tree that you see on :yocto_git:`/` is
generated through repeating the above steps for all valid BSPs.
The end result is a branched, clean history tree that makes up the
kernel for a given release. You can see the script (``kgit-scc``)
responsible for this in the
:yocto_git:`yocto-kernel-tools </yocto-kernel-tools/tree/tools>`
repository.
- The steps used to construct the full kernel tree are the same
steps that BitBake uses when it builds a kernel image.
Build Strategy
==============
Once you have cloned a Yocto Linux kernel repository and the cache
repository (``yocto-kernel-cache``) onto your development system, you
can consider the compilation phase of kernel development, which is
building a kernel image. Some prerequisites are validated by
the build process before compilation starts:
- The :term:`SRC_URI` points to the
kernel Git repository.
- A BSP build branch with Metadata exists in the ``yocto-kernel-cache``
repository. The branch is based on the Yocto Linux kernel version and
has configurations and features grouped under the
``yocto-kernel-cache/bsp`` directory. For example, features and
configurations for the BeagleBone Board assuming a
``linux-yocto_4.12`` kernel reside in the following area of the
``yocto-kernel-cache`` repository: yocto-kernel-cache/bsp/beaglebone
.. note::
In the previous example, the "yocto-4.12" branch is checked out in
the ``yocto-kernel-cache`` repository.
The OpenEmbedded build system makes sure these conditions are satisfied before
attempting compilation. Other means, however, do exist, such as
bootstrapping a BSP.
Before building a kernel, the build process verifies the tree and
configures the kernel by processing all of the configuration "fragments"
specified by feature descriptions in the ``.scc`` files. As the features
are compiled, associated kernel configuration fragments are noted and
recorded in the series of directories in their compilation order. The
fragments are migrated, pre-processed and passed to the Linux Kernel
Configuration subsystem (``lkc``) as raw input in the form of a
``.config`` file. The ``lkc`` uses its own internal dependency
constraints to do the final processing of that information and generates
the final ``.config`` file that is used during compilation.
Using the board's architecture and other relevant values from the
board's template, kernel compilation is started and a kernel image is
produced.
The other thing that you notice once you configure a kernel is that the
build process generates a build tree that is separate from your kernel's
local Git source repository tree. This build tree has a name that uses
the following form, where ``${MACHINE}`` is the metadata name of the
machine (BSP) and "kernel_type" is one of the Yocto Project supported
kernel types (e.g. "standard")::
linux-${MACHINE}-kernel_type-build
The existing support in the ``kernel.org`` tree achieves this default
functionality.
This behavior means that all the generated files for a particular
machine or BSP are now in the build tree directory. The files include
the final ``.config`` file, all the ``.o`` files, the ``.a`` files, and
so forth. Since each machine or BSP has its own separate
:term:`Build Directory` in its own separate branch of the Git repository,
you can easily switch between different builds.