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<chapter id='kernel-dev-advanced'>
<title>Working with Advanced Metadata</title>
<section id='kernel-dev-advanced-overview'>
<title>Overview</title>
<para>
In addition to supporting configuration fragments and patches, the
Yocto Project kernel tools also support rich
<ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink> 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, known as
the kern-tools (<filename>kern-tools-native_git.bb</filename>), is
to help you manage the complexity of the configuration and sources
used to support multiple BSPs and Linux kernel types.
</para>
</section>
<section id='using-kernel-metadata-in-a-recipe'>
<title>Using Kernel Metadata in a Recipe</title>
<para>
The kernel sources in the Yocto Project contain kernel Metadata, which
is located in the <filename>meta</filename> branches of the kernel
source Git repositories.
This Metadata defines Board Support Packages (BSPs) that
correspond to definitions in linux-yocto recipes for the same 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>
Linux kernel source that contains kernel Metadata is said to be
"linux-yocto style" kernel source.
A Linux kernel recipe that inherits from the
<filename>linux-yocto.inc</filename> include file is said to be a
"linux-yocto style" recipe.
</note>
</para>
<para>
Every linux-yocto style recipe must define the
<ulink url='&YOCTO_DOCS_REF_URL;#var-KMACHINE'><filename>KMACHINE</filename></ulink>
variable.
This variable is typically set to the same value as the
<ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>
variable, which is used by
<ulink url='&YOCTO_DOCS_DEV_URL;#bitbake-term'>BitBake</ulink>.
However, in some cases, the variable might instead refer to the
underlying platform of the <filename>MACHINE</filename>.
</para>
<para>
Multiple BSPs can reuse the same <filename>KMACHINE</filename>
name if they are built using the same BSP description.
The "ep108-zynqmp" and "qemuzynqmp" BSP combination
in the <filename>meta-xilinx</filename>
layer is a good example of two BSPs using the same
<filename>KMACHINE</filename> value (i.e. "zynqmp").
See the <link linkend='bsp-descriptions'>BSP Descriptions</link> section
for more information.
</para>
<para>
Every linux-yocto style recipe must also indicate the Linux kernel
source repository branch used to build the Linux kernel.
The <ulink url='&YOCTO_DOCS_REF_URL;#var-KBRANCH'><filename>KBRANCH</filename></ulink>
variable must be set to indicate the branch.
<note>
You can use the <filename>KBRANCH</filename> value to define an
alternate branch typically with a machine override as shown here
from the <filename>meta-emenlow</filename> layer:
<literallayout class='monospaced'>
KBRANCH_emenlow-noemgd = "standard/base"
</literallayout>
</note>
</para>
<para>
The linux-yocto style recipes can optionally define the following
variables:
<literallayout class='monospaced'>
<ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'>KERNEL_FEATURES</ulink>
<ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_KERNEL_TYPE'>LINUX_KERNEL_TYPE</ulink>
</literallayout>
</para>
<para>
<filename>LINUX_KERNEL_TYPE</filename> defines the kernel type to be
used in assembling the configuration.
If you do not specify a <filename>LINUX_KERNEL_TYPE</filename>,
it defaults to "standard".
Together with
<ulink url='&YOCTO_DOCS_REF_URL;#var-KMACHINE'><filename>KMACHINE</filename></ulink>,
<filename>LINUX_KERNEL_TYPE</filename> 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 "<link linkend='kernel-types'>Kernel Types</link>" section
for more information on kernel types.
</para>
<para>
During the build, the kern-tools search for the BSP description
file that most closely matches the <filename>KMACHINE</filename>
and <filename>LINUX_KERNEL_TYPE</filename> variables passed in from the
recipe.
The tools use the first BSP description it finds that match
both variables.
If the tools cannot find a match, they issue a warning such as
the following:
<literallayout class='monospaced'>
WARNING: Can't find any BSP hardware or required configuration fragments.
WARNING: Looked at meta/cfg/broken/emenlow-broken/hdw_frags.txt and
meta/cfg/broken/emenlow-broken/required_frags.txt in directory:
meta/cfg/broken/emenlow-broken
</literallayout>
In this example, <filename>KMACHINE</filename> was set to "emenlow-broken"
and <filename>LINUX_KERNEL_TYPE</filename> was set to "broken".
</para>
<para>
The tools first search for the <filename>KMACHINE</filename> and
then for the <filename>LINUX_KERNEL_TYPE</filename>.
If the tools cannot find a partial match, they will use the
sources from the <filename>KBRANCH</filename> and any configuration
specified in the
<ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>.
</para>
<para>
You can use the <filename>KERNEL_FEATURES</filename> variable
to include features (configuration fragments, patches, or both) that
are not already included by the <filename>KMACHINE</filename> and
<filename>LINUX_KERNEL_TYPE</filename> variable combination.
For example, to include a feature specified as
"features/netfilter/netfilter.scc",
specify:
<literallayout class='monospaced'>
KERNEL_FEATURES += "features/netfilter/netfilter.scc"
</literallayout>
To include a feature called "cfg/sound.scc" just for the
<filename>qemux86</filename> machine, specify:
<literallayout class='monospaced'>
KERNEL_FEATURES_append_qemux86 = " cfg/sound.scc"
</literallayout>
The value of the entries in <filename>KERNEL_FEATURES</filename>
are dependent on their location within the kernel Metadata itself.
The examples here are taken from the <filename>meta</filename>
branch of the <filename>linux-yocto-3.19</filename> repository.
Within that branch, "features" and "cfg" are subdirectories of the
<filename>meta/cfg/kernel-cache</filename> directory.
For more information, see the
"<link linkend='kernel-metadata-syntax'>Kernel Metadata Syntax</link>" section.
<note>
The processing of the these variables has evolved some between the
0.9 and 1.3 releases of the Yocto Project and associated
kern-tools sources.
The descriptions in this section are accurate for 1.3 and later
releases of the Yocto Project.
</note>
</para>
</section>
<section id='kernel-metadata-location'>
<title>Kernel Metadata Location</title>
<para>
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.
</para>
<para>
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
"<link linkend='modifying-an-existing-recipe'>Modifying an Existing Recipe</link>"
section.
</para>
<para>
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.
</para>
<section id='recipe-space-metadata'>
<title>Recipe-Space Metadata</title>
<para>
When stored in recipe-space, the kernel Metadata files reside in a
directory hierarchy below
<ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>.
For a linux-yocto recipe or for a Linux kernel recipe derived
by copying and modifying
<filename>oe-core/meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb</filename>
to a recipe in your layer, <filename>FILESEXTRAPATHS</filename>
is typically set to
<filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-THISDIR'><filename>THISDIR</filename></ulink><filename>}/${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink><filename>}</filename>.
See the "<link linkend='modifying-an-existing-recipe'>Modifying an Existing Recipe</link>"
section for more information.
</para>
<para>
Here is an example that shows a trivial tree of kernel Metadata
stored in recipe-space within a BSP layer:
<literallayout class='monospaced'>
meta-<replaceable>my_bsp_layer</replaceable>/
`-- recipes-kernel
`-- linux
`-- linux-yocto
|-- bsp-standard.scc
|-- bsp.cfg
`-- standard.cfg
</literallayout>
</para>
<para>
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 <filename>.scc</filename>
files on the
<ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>.
BitBake parses them and fetches any files referenced in the
<filename>.scc</filename> files by the <filename>include</filename>,
<filename>patch</filename>, or <filename>kconf</filename> commands.
Because of this, it is necessary to bump the recipe
<ulink url='&YOCTO_DOCS_REF_URL;#var-PR'><filename>PR</filename></ulink>
value when changing the content of files not explicitly listed
in the <filename>SRC_URI</filename>.
</para>
</section>
<section id='metadata-outside-the-recipe-space'>
<title>Metadata Outside the Recipe-Space</title>
<para>
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 "ktype=meta" repository through the
<ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
variable.
As an example, consider the following <filename>SRC_URI</filename>
statement from the <filename>linux-yocto_4.4.bb</filename>
kernel recipe:
<literallayout class='monospaced'>
SRC_URI = "git://git.yoctoproject.org/linux-yocto-4.4.git;name=machine;branch=${KBRANCH}; \
git://git.yoctoproject.org/yocto-kernel-cache;type=kmeta;name=meta;branch=yocto-4.4;destsuffix=${KMETA}"
</literallayout>
<filename>${KMETA}</filename>, 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
<filename>SRC_URI</filename> statement used in a recipe.
</para>
<para>
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 <filename>SRC_URI</filename> statement with the
"type=kmeta" attribute.
Doing so makes the kernel Metadata available during the
configuration phase.
</para>
<!--
<para>
Following is an example that shows how a trivial tree of Metadata
is stored in a custom Linux kernel Git repository:
<literallayout class='monospaced'>
meta/
`‐‐ cfg
`‐‐ kernel-cache
|‐‐ bsp-standard.scc
|‐‐ bsp.cfg
`‐‐ standard.cfg
</literallayout>
</para>
<para>
To use a branch different from where the sources reside,
specify the branch in the <filename>KMETA</filename> variable
in your Linux kernel recipe.
Here is an example:
<literallayout class='monospaced'>
KMETA = "meta"
</literallayout>
To use the same branch as the sources, set
<filename>KMETA</filename> to an empty string:
<literallayout class='monospaced'>
KMETA = ""
</literallayout>
If you are working with your own sources and want to create an
orphan <filename>meta</filename> branch, use these commands
from within your Linux kernel Git repository:
<literallayout class='monospaced'>
$ git checkout ‐‐orphan meta
$ git rm -rf .
$ git commit ‐‐allow-empty -m "Create orphan meta branch"
</literallayout>
</para>
-->
<para>
If you modify the Metadata, you must not forget to update the
<ulink url='&YOCTO_DOCS_REF_URL;#var-SRCREV'><filename>SRCREV</filename></ulink>
statements in the kernel's recipe.
In particular, you need to update the
<filename>SRCREV_meta</filename> variable to match the commit in
the <filename>KMETA</filename> branch you wish to use.
Changing the data in these branches and not updating the
<filename>SRCREV</filename> statements to match will cause the
build to fetch an older commit.
</para>
</section>
</section>
<section id='kernel-metadata-syntax'>
<title>Kernel Metadata Syntax</title>
<para>
The kernel Metadata consists of three primary types of files:
<filename>scc</filename>
<footnote>
<para>
<filename>scc</filename> 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 <filename>scc</filename> files to be description files.
</para>
</footnote>
description files, configuration fragments, and patches.
The <filename>scc</filename> 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.
</para>
<para>
The <filename>scc</filename> description files are used to define two
fundamental types of kernel Metadata:
<itemizedlist>
<listitem><para>Features</para></listitem>
<listitem><para>Board Support Packages (BSPs)</para></listitem>
</itemizedlist>
</para>
<para>
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.
<link linkend='kernel-types'>Kernel types</link> define general
kernel features and policy to be reused in the BSPs.
</para>
<para>
BSPs define hardware-specific features and aggregate them with kernel
types to form the final description of what will be assembled and built.
</para>
<para>
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:
<literallayout class='monospaced'>
<replaceable>base</replaceable>/
bsp/
cfg/
features/
ktypes/
patches/
</literallayout>
</para>
<para>
The <filename>bsp</filename> directory contains the
<link linkend='bsp-descriptions'>BSP descriptions</link>.
The remaining directories all contain "features".
Separating <filename>bsp</filename> from the rest of the structure
aids conceptualizing intended usage.
</para>
<para>
Use these guidelines to help place your <filename>scc</filename>
description files within the structure:
<itemizedlist>
<listitem><para>If your file contains
only configuration fragments, place the file in the
<filename>cfg</filename> directory.</para></listitem>
<listitem><para>If your file contains
only source-code fixes, place the file in the
<filename>patches</filename> directory.</para></listitem>
<listitem><para>If your file encapsulates
a major feature, often combining sources and configurations,
place the file in <filename>features</filename> directory.
</para></listitem>
<listitem><para>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 <filename>ktypes</filename>
directory.
</para></listitem>
</itemizedlist>
</para>
<para>
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 <filename>cfg</filename>,
<filename>features</filename>, <filename>patches</filename>, and
<filename>ktypes</filename>, contain "features" as far as the kernel
tools are concerned.
</para>
<para>
Paths used in kernel Metadata files are relative to
<filename><base></filename>, which is either
<ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>
if you are creating Metadata in
<link linkend='recipe-space-metadata'>recipe-space</link>,
or <filename>meta/cfg/kernel-cache/</filename> if you are creating
<link linkend='metadata-outside-the-recipe-space'>Metadata outside of the recipe-space</link>.
</para>
<section id='configuration'>
<title>Configuration</title>
<para>
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
(<filename>.cfg</filename>) and a <filename>.scc</filename> file
that describes the fragment.
</para>
<para>
The Symmetric Multi-Processing (SMP) fragment included in the
<filename>linux-yocto-3.19</filename> Git repository
consists of the following two files:
<literallayout class='monospaced'>
cfg/smp.scc:
define KFEATURE_DESCRIPTION "Enable SMP"
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
</literallayout>
You can find information on configuration fragment files in the
"<ulink url='&YOCTO_DOCS_DEV_URL;#creating-config-fragments'>Creating Configuration Fragments</ulink>"
section of the Yocto Project Development Manual and in
the "<link linkend='generating-configuration-files'>Generating Configuration Files</link>"
section earlier in this manual.
</para>
<para>
<ulink url='&YOCTO_DOCS_REF_URL;#var-KFEATURE_DESCRIPTION'><filename>KFEATURE_DESCRIPTION</filename></ulink>
provides a short description of the fragment.
Higher level kernel tools use this description.
</para>
<para>
The <filename>kconf</filename> command is used to include the
actual configuration fragment in an <filename>.scc</filename>
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
<filename>kconf</filename> statements, one per fragment.
</note>
</para>
<para>
As described in the
"<link linkend='generating-configuration-files'>Generating Configuration Files</link>"
section, you can use the following BitBake command to audit your
configuration:
<literallayout class='monospaced'>
$ bitbake linux-yocto -c kernel_configcheck -f
</literallayout>
</para>
</section>
<section id='patches'>
<title>Patches</title>
<para>
Patch descriptions are very similar to configuration fragment
descriptions, which are described in the previous section.
However, instead of a <filename>.cfg</filename> file, these
descriptions work with source patches.
</para>
<para>
A typical patch includes a description file and the patch itself:
<literallayout class='monospaced'>
patches/mypatch.scc:
patch mypatch.patch
patches/mypatch.patch:
<replaceable>typical-patch</replaceable>
</literallayout>
You can create the typical <filename>.patch</filename>
file using <filename>diff -Nurp</filename> or
<filename>git format-patch</filename>.
</para>
<para>
The description file can include multiple patch statements,
one per patch.
</para>
</section>
<section id='features'>
<title>Features</title>
<para>
Features are complex kernel Metadata types that consist
of configuration fragments (<filename>kconf</filename>), patches
(<filename>patch</filename>), and possibly other feature
description files (<filename>include</filename>).
</para>
<para>
Here is an example that shows a feature description file:
<literallayout class='monospaced'>
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
</literallayout>
This example shows how the <filename>patch</filename> and
<filename>kconf</filename> commands are used as well as
how an additional feature description file is included.
</para>
<para>
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 <filename>KERNEL_FEATURES</filename> variable of the
Linux kernel recipe.
See the "<link linkend='using-kernel-metadata-in-a-recipe'>Using Kernel Metadata in a Recipe</link>"
section earlier in the manual.
</para>
</section>
<section id='kernel-types'>
<title>Kernel Types</title>
<para>
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 kernels of a
specific type.
Syntactically, kernel types are no different than features
as described in the "<link linkend='features'>Features</link>"
section.
The <filename>LINUX_KERNEL_TYPE</filename> variable in the kernel
recipe selects the kernel type.
See the "<link linkend='using-kernel-metadata-in-a-recipe'>Using Kernel Metadata in a Recipe</link>"
section for more information.
</para>
<para>
As an example, the <filename>linux-yocto-3.19</filename>
tree defines three kernel types: "standard",
"tiny", and "preempt-rt":
<itemizedlist>
<listitem><para>"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.
</para></listitem>
<listitem><para>"preempt-rt":
Applies the <filename>PREEMPT_RT</filename>
patches and the configuration options required to
build a real-time Linux kernel.
This kernel type inherits from the "standard" kernel type.
</para></listitem>
<listitem><para>"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.
</para></listitem>
</itemizedlist>
</para>
<para>
The "standard" kernel type is defined by
<filename>standard.scc</filename>:
<literallayout class='monospaced'>
# Include this kernel type fragment to get the standard features and
# configuration values.
# Include all standard features
include standard-nocfg.scc
kconf non-hardware standard.cfg
# individual cfg block section
include cfg/fs/devtmpfs.scc
include cfg/fs/debugfs.scc
include cfg/fs/btrfs.scc
include cfg/fs/ext2.scc
include cfg/fs/ext3.scc
include cfg/fs/ext4.scc
include cfg/net/ipv6.scc
include cfg/net/ip_nf.scc
include cfg/net/ip6_nf.scc
include cfg/net/bridge.scc
</literallayout>
</para>
<para>
As with any <filename>.scc</filename> file, a
kernel type definition can aggregate other
<filename>.scc</filename> files with
<filename>include</filename> commands.
These definitions can also directly pull in
configuration fragments and patches with the
<filename>kconf</filename> and <filename>patch</filename>
commands, respectively.
</para>
<note>
It is not strictly necessary to create a kernel type
<filename>.scc</filename> file.
The Board Support Package (BSP) file can implicitly define
the kernel type using a <filename>define
<ulink url='&YOCTO_DOCS_REF_URL;#var-KTYPE'>KTYPE</ulink> myktype</filename>
line.
See the "<link linkend='bsp-descriptions'>BSP Descriptions</link>"
section for more information.
</note>
</section>
<section id='bsp-descriptions'>
<title>BSP Descriptions</title>
<para>
BSP descriptions combine kernel types with hardware-specific
features.
The hardware-specific portion is typically defined
independently, and then aggregated with each supported kernel
type.
Consider this simple BSP description that supports the
<replaceable>mybsp</replaceable> machine:
<literallayout class='monospaced'>
<replaceable>mybsp</replaceable>.scc:
define KMACHINE <replaceable>mybsp</replaceable>
define KTYPE standard
define KARCH i386
kconf <replaceable>mybsp</replaceable>.cfg
</literallayout>
Every BSP description should define the
<ulink url='&YOCTO_DOCS_REF_URL;#var-KMACHINE'><filename>KMACHINE</filename></ulink>,
<ulink url='&YOCTO_DOCS_REF_URL;#var-KTYPE'><filename>KTYPE</filename></ulink>,
and <ulink url='&YOCTO_DOCS_REF_URL;#var-KARCH'><filename>KARCH</filename></ulink>
variables.
These variables allow the OpenEmbedded build system to identify
the description as meeting the criteria set by the recipe being
built.
This simple example supports the "mybsp" machine for the "standard"
kernel and the "i386" architecture.
</para>
<para>
Be aware that a hard link between the
<filename>KTYPE</filename> variable and a kernel type
description file does not exist.
Thus, if you do not have kernel types defined in your kernel
Metadata, you only need to ensure that the kernel recipe's
<ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_KERNEL_TYPE'><filename>LINUX_KERNEL_TYPE</filename></ulink>
variable and the <filename>KTYPE</filename> variable in the
BSP description file match.
<note>
Future versions of the tooling make the specification of
<filename>KTYPE</filename> in the BSP optional.
</note>
</para>
<para>
If you did want to separate your kernel policy from your
hardware configuration, you could do so by specifying a kernel
type, such as "standard" and including that description file
in the BSP description file.
See the "<link linkend='kernel-types'>Kernel Types</link>" section
for more information.
</para>
<para>
You might also have multiple hardware configurations that you
aggregate into a single hardware description file that you
could include in the BSP description file, rather than referencing
a single <filename>.cfg</filename> file.
Consider the following:
<literallayout class='monospaced'>
<replaceable>mybsp</replaceable>.scc:
define KMACHINE mybsp
define KTYPE standard
define KARCH i386
include standard.scc
include <replaceable>mybsp</replaceable>-hw.scc
</literallayout>
</para>
<para>
In the above example, <filename>standard.scc</filename>
aggregates all the configuration fragments, patches, and
features that make up your standard kernel policy whereas
<replaceable>mybsp</replaceable><filename>-hw.scc</filename>
aggregates all those necessary
to support the hardware available on the
<replaceable>mybsp</replaceable> machine.
For information on how to break a complete
<filename>.config</filename> file into the various
configuration fragments, see the
"<link linkend='generating-configuration-files'>Generating Configuration Files</link>"
section.
</para>
<para>
Many real-world examples are more complex.
Like any other <filename>.scc</filename> file, BSP
descriptions can aggregate features.
Consider the Minnow BSP definition from the
<filename>linux-yocto-3.19</filename>
Git repository:
<literallayout class='monospaced'>
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 cfg/timer/rtc.scc
include features/leds/leds.scc
include features/spi/spidev.scc
include features/i2c/i2cdev.scc
# Earlyprintk and port debug requires 8250
kconf hardware cfg/8250.cfg
kconf hardware minnow.cfg
kconf hardware minnow-dev.cfg
</literallayout>
</para>
<para>
The <filename>minnow.scc</filename> description file includes
a hardware configuration fragment
(<filename>minnow.cfg</filename>) specific to the Minnow
BSP as well as several more general configuration
fragments and features enabling hardware found on the
machine.
This 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:
<literallayout class='monospaced'>
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
</literallayout>
The <filename>include</filename> command midway through the file
includes the <filename>minnow.scc</filename> description that
defines all hardware enablements for the BSP that is common to all
kernel types.
Using this command significantly reduces duplication.
</para>
<para>
Now consider the "minnow" description for the "tiny" kernel type:
<literallayout class='monospaced'>
minnow-tiny.scc:
define KMACHINE minnow
define KTYPE tiny
define KARCH i386
include ktypes/tiny
include minnow.scc
</literallayout>
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.
</para>
<para>
Notice again the three critical variables:
<filename>KMACHINE</filename>, <filename>KTYPE</filename>,
and <filename>KARCH</filename>.
Of these variables, only the <filename>KTYPE</filename> has changed.
It is now set to "tiny".
</para>
</section>
</section>
<section id='organizing-your-source'>
<title>Organizing Your Source</title>
<para>
Many recipes based on the <filename>linux-yocto-custom.bb</filename>
recipe use Linux kernel sources that have only a single
branch - "master".
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 lone "master" branch).
It is situations like these that give rise to multiple branches used
within a Linux kernel sources Git repository.
</para>
<para>
Repository organization strategies exist that maximize source reuse,
remove redundancy, and logically order your changes.
This section presents strategies for the following cases:
<itemizedlist>
<listitem><para>Encapsulating patches in a feature description
and only including the patches in the BSP descriptions of
the applicable boards.</para></listitem>
<listitem><para>Creating a machine branch in your
kernel source repository and applying the patches on that
branch only.</para></listitem>
<listitem><para>Creating a feature branch in your
kernel source repository and merging that branch into your
BSP when needed.</para></listitem>
</itemizedlist>
</para>
<para>
The approach you take is entirely up to you
and depends on what works best for your development model.
</para>
<section id='encapsulating-patches'>
<title>Encapsulating Patches</title>
<para>
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
"<link linkend='bsp-descriptions'>BSP Descriptions</link>"
section.
</para>
<para>
You can find information on how to create patches and BSP
descriptions in the "<link linkend='patches'>Patches</link>" and
"<link linkend='bsp-descriptions'>BSP Descriptions</link>"
sections.
</para>
</section>
<section id='machine-branches'>
<title>Machine Branches</title>
<para>
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
"master" 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.
</para>
<para>
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
<filename>KBRANCH</filename> to use for the board as
follows:
<literallayout class='monospaced'>
KBRANCH = "mynewbranch"
</literallayout>
Another method is to use the <filename>branch</filename> command
in the BSP description:
<literallayout class='monospaced'>
mybsp.scc:
define KMACHINE mybsp
define KTYPE standard
define KARCH i386
include standard.scc
branch mynewbranch
include mybsp-hw.scc
</literallayout>
</para>
<para>
If you find
yourself with numerous branches, you might consider using a
hierarchical branching system similar to what the linux-yocto Linux
kernel repositories use:
<literallayout class='monospaced'>
<replaceable>common</replaceable>/<replaceable>kernel_type</replaceable>/<replaceable>machine</replaceable>
</literallayout>
</para>
<para>
If you had two kernel types, "standard" and "small" for
instance, three machines, and <replaceable>common</replaceable>
as <filename>mydir</filename>, the branches in your
Git repository might look like this:
<literallayout class='monospaced'>
mydir/base
mydir/standard/base
mydir/standard/machine_a
mydir/standard/machine_b
mydir/standard/machine_c
mydir/small/base
mydir/small/machine_a
</literallayout>
</para>
<para>
This organization can help clarify the branch relationships.
In this case, <filename>mydir/standard/machine_a</filename>
includes everything in <filename>mydir/base</filename> and
<filename>mydir/standard/base</filename>.
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 <filename>mydir/standard</filename> and
<filename>mydir/standard/machine_a</filename> because it
would have to create a file and a directory named "standard".
</note>
</para>
</section>
<section id='feature-branches'>
<title>Feature Branches</title>
<para>
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 <filename>git merge</filename> command.
</para>
<para>
To merge a feature branch into a BSP, insert the
<filename>git merge</filename> command after any
<filename>branch</filename> commands:
<literallayout class='monospaced'>
mybsp.scc:
define KMACHINE mybsp
define KTYPE standard
define KARCH i386
include standard.scc
branch mynewbranch
git merge myfeature
include mybsp-hw.scc
</literallayout>
</para>
</section>
</section>
<section id='scc-reference'>
<title>SCC Description File Reference</title>
<para>
This section provides a brief reference for the commands you can use
within an SCC description file (<filename>.scc</filename>):
<itemizedlist>
<listitem><para><filename>branch [ref]</filename>:
Creates a new branch relative to the current branch
(typically <filename>${KTYPE}</filename>) using
the currently checked-out branch, or "ref" if specified.
</para></listitem>
<listitem><para><filename>define</filename>:
Defines variables, such as <filename>KMACHINE</filename>,
<filename>KTYPE</filename>, <filename>KARCH</filename>,
and <filename>KFEATURE_DESCRIPTION</filename>.</para></listitem>
<listitem><para><filename>include SCC_FILE</filename>:
Includes an SCC file in the current file.
The file is parsed as if you had inserted it inline.
</para></listitem>
<listitem><para><filename>kconf [hardware|non-hardware] CFG_FILE</filename>:
Queues a configuration fragment for merging into the final
Linux <filename>.config</filename> file.</para></listitem>
<listitem><para><filename>git merge GIT_BRANCH</filename>:
Merges the feature branch into the current branch.
</para></listitem>
<listitem><para><filename>patch PATCH_FILE</filename>:
Applies the patch to the current Git branch.</para></listitem>
</itemizedlist>
</para>
</section>
</chapter>
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