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<chapter id='kernel-dev-common'>
<title>Common Tasks</title>
<para>
This chapter presents several common tasks you perform when you
work with the Yocto Project Linux kernel.
These tasks include preparing a layer, modifying an existing recipe,
iterative development, working with your own sources, and incorporating
out-of-tree modules.
<note>
The examples presented in this chapter work with the Yocto Project
1.2.2 Release and forward.
</note>
</para>
<section id='creating-and-preparing-a-layer'>
<title>Creating and Preparing a Layer</title>
<para>
If you are going to be modifying kernel recipes, it is recommended
that you create and prepare your own layer in which to do your
work.
Your layer contains its own
<ulink url='&YOCTO_DOCS_DEV_URL;#bitbake-term'>BitBake</ulink>
append files
(<filename>.bbappend</filename>) and provides a convenient
mechanism to create your own recipe files
(<filename>.bb</filename>).
For details on how to create and work with layers, see the following
sections in the Yocto Project Development Manual:
<itemizedlist>
<listitem><para>"<ulink url='&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers'>Understanding and Creating Layers</ulink>" for
general information on layers and how to create layers.</para></listitem>
<listitem><para>"<ulink url='&YOCTO_DOCS_DEV_URL;#set-up-your-layer-for-the-build'>Set Up Your Layer for the Build</ulink>" for
specific instructions on setting up a layer for kernel
development.</para></listitem>
</itemizedlist>
</para>
</section>
<section id='modifying-an-existing-recipe'>
<title>Modifying an Existing Recipe</title>
<para>
In many cases, you can customize an existing linux-yocto recipe to
meet the needs of your project.
Each release of the Yocto Project provides a few Linux
kernel recipes from which you can choose.
These are located in the
<ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>
in <filename>meta/recipes-kernel/linux</filename>.
</para>
<para>
Modifying an existing recipe can consist of the following:
<itemizedlist>
<listitem><para>Creating the append file</para></listitem>
<listitem><para>Applying patches</para></listitem>
<listitem><para>Changing the configuration</para></listitem>
</itemizedlist>
</para>
<para>
Before modifying an existing recipe, be sure that you have created
a minimal, custom layer from which you can work.
See the "<link linkend='creating-and-preparing-a-layer'>Creating and Preparing a Layer</link>"
section for some general resources.
You can also see the
"<ulink url='&YOCTO_DOCS_DEV_URL;#set-up-your-layer-for-the-build'>Set Up Your Layer for the Build</ulink>" section
of the Yocto Project Development Manual for a detailed
example.
</para>
<section id='creating-the-append-file'>
<title>Creating the Append File</title>
<para>
You create this file in your custom layer.
You also name it accordingly based on the linux-yocto recipe
you are using.
For example, if you are modifying the
<filename>meta/recipes-kernel/linux/linux-yocto_3.19.bb</filename>
recipe, the append file will typically be located as follows
within your custom layer:
<literallayout class='monospaced'>
<replaceable>your-layer</replaceable>/recipes-kernel/linux/linux-yocto_3.19.bbappend
</literallayout>
The append file should initially extend the
<ulink url='&YOCTO_DOCS_REF_URL;#var-FILESPATH'><filename>FILESPATH</filename></ulink>
search path by prepending the directory that contains your
files to the
<ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>
variable as follows:
<literallayout class='monospaced'>
FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
</literallayout>
The path <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>
expands to "linux-yocto" in the current directory for this
example.
If you add any new files that modify the kernel recipe and you
have extended <filename>FILESPATH</filename> as
described above, you must place the files in your layer in the
following area:
<literallayout class='monospaced'>
<replaceable>your-layer</replaceable>/recipes-kernel/linux/linux-yocto/
</literallayout>
<note>If you are working on a new machine Board Support Package
(BSP), be sure to refer to the
<ulink url='&YOCTO_DOCS_BSP_URL;'>Yocto Project Board Support Package (BSP) Developer's Guide</ulink>.
</note>
</para>
</section>
<section id='applying-patches'>
<title>Applying Patches</title>
<para>
If you have a single patch or a small series of patches
that you want to apply to the Linux kernel source, you
can do so just as you would with any other recipe.
You first copy the patches to the path added to
<ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>
in your <filename>.bbappend</filename> file as described in
the previous section, and then reference them in
<ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
statements.
</para>
<para>
For example, you can apply a three-patch series by adding the
following lines to your linux-yocto
<filename>.bbappend</filename> file in your layer:
<literallayout class='monospaced'>
SRC_URI += "file://0001-first-change.patch"
SRC_URI += "file://0002-second-change.patch"
SRC_URI += "file://0003-third-change.patch"
</literallayout>
The next time you run BitBake to build the Linux kernel,
BitBake detects the change in the recipe and fetches and
applies the patches before building the kernel.
</para>
<para>
For a detailed example showing how to patch the kernel, see the
"<ulink url='&YOCTO_DOCS_DEV_URL;#patching-the-kernel'>Patching the Kernel</ulink>"
section in the Yocto Project Development Manual.
</para>
</section>
<section id='changing-the-configuration'>
<title>Changing the Configuration</title>
<para>
You can make wholesale or incremental changes to the final
<filename>.config</filename> file used for the eventual
Linux kernel configuration by including a
<filename>defconfig</filename> file and by specifying
configuration fragments in the
<ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
to be applied to that file.
</para>
<para>
If you have a complete, working Linux kernel
<filename>.config</filename>
file you want to use for the configuration, as before, copy
that file to the appropriate <filename>${PN}</filename>
directory in your layer's
<filename>recipes-kernel/linux</filename> directory,
and rename the copied file to "defconfig".
Then, add the following lines to the linux-yocto
<filename>.bbappend</filename> file in your layer:
<literallayout class='monospaced'>
FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
SRC_URI += "file://defconfig"
</literallayout>
The <filename>SRC_URI</filename> tells the build system how to
search for the file, while the
<ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>
extends the
<ulink url='&YOCTO_DOCS_REF_URL;#var-FILESPATH'><filename>FILESPATH</filename></ulink>
variable (search directories) to include the
<filename>${PN}</filename> directory you created to hold the
configuration changes.
</para>
<note>
The build system applies the configurations from the
<filename>defconfig</filename> file before applying any
subsequent configuration fragments.
The final kernel configuration is a combination of the
configurations in the <filename>defconfig</filename> file and
any configuration fragments you provide.
You need to realize that if you have any configuration
fragments, the build system applies these on top of and
after applying the existing <filename>defconfig</filename>
file configurations.
</note>
<para>
Generally speaking, the preferred approach is to determine the
incremental change you want to make and add that as a
configuration fragment.
For example, if you want to add support for a basic serial
console, create a file named <filename>8250.cfg</filename> in
the <filename>${PN}</filename> directory with the following
content (without indentation):
<literallayout class='monospaced'>
CONFIG_SERIAL_8250=y
CONFIG_SERIAL_8250_CONSOLE=y
CONFIG_SERIAL_8250_PCI=y
CONFIG_SERIAL_8250_NR_UARTS=4
CONFIG_SERIAL_8250_RUNTIME_UARTS=4
CONFIG_SERIAL_CORE=y
CONFIG_SERIAL_CORE_CONSOLE=y
</literallayout>
Next, include this configuration fragment and extend the
<filename>FILESPATH</filename> variable in your
<filename>.bbappend</filename> file:
<literallayout class='monospaced'>
FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
SRC_URI += "file://8250.cfg"
</literallayout>
The next time you run BitBake to build the Linux kernel, BitBake
detects the change in the recipe and fetches and applies the
new configuration before building the kernel.
</para>
<para>
For a detailed example showing how to configure the kernel,
see the
"<ulink url='&YOCTO_DOCS_DEV_URL;#configuring-the-kernel'>Configuring the Kernel</ulink>"
section in the Yocto Project Development Manual.
</para>
</section>
<section id='using-an-in-tree-defconfig-file'>
<title>Using an "In-Tree"&nbsp;&nbsp;<filename>defconfig</filename> File</title>
<para>
It might be desirable to have kernel configuration fragment
support through a <filename>defconfig</filename> file that
is pulled from the kernel source tree for the configured
machine.
By default, the OpenEmbedded build system looks for
<filename>defconfig</filename> files in the layer used for
Metadata, which is "out-of-tree", and then configures them
using the following:
<literallayout class='monospaced'>
SRC_URI += "file://defconfig"
</literallayout>
If you do not want to maintain copies of
<filename>defconfig</filename> files in your layer but would
rather allow users to use the default configuration from the
kernel tree and still be able to add configuration fragments
to the
<ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
through, for example, append files, you can direct the
OpenEmbedded build system to use a
<filename>defconfig</filename> file that is "in-tree".
</para>
<para>
To specify an "in-tree" <filename>defconfig</filename> file,
edit the recipe that builds your kernel so that it has the
following command form:
<literallayout class='monospaced'>
KBUILD_DEFCONFIG_<ulink url='&YOCTO_DOCS_REF_URL;#var-KMACHINE'>KMACHINE</ulink> ?= <replaceable>defconfig_file</replaceable>
</literallayout>
You need to append the variable with
<filename>KMACHINE</filename> and then supply the path to
your "in-tree" <filename>defconfig</filename> file.
</para>
<para>
Aside from modifying your kernel recipe and providing your own
<filename>defconfig</filename> file, you need to be sure no
files or statements set <filename>SRC_URI</filename> to use a
<filename>defconfig</filename> other than your "in-tree"
file (e.g. a kernel's <filename>linux-</filename><replaceable>machine</replaceable><filename>.inc</filename>
file).
In other words, if the build system detects a statement
that identifies an "out-of-tree"
<filename>defconfig</filename> file, that statement
will override your
<filename>KBUILD_DEFCONFIG</filename> variable.
</para>
<para>
See the
<ulink url='&YOCTO_DOCS_REF_URL;#var-KBUILD_DEFCONFIG'><filename>KBUILD_DEFCONFIG</filename></ulink>
variable description for more information.
</para>
</section>
</section>
<section id='using-an-iterative-development-process'>
<title>Using an Iterative Development Process</title>
<para>
If you do not have existing patches or configuration files,
you can iteratively generate them from within the BitBake build
environment as described within this section.
During an iterative workflow, running a previously completed BitBake
task causes BitBake to invalidate the tasks that follow the
completed task in the build sequence.
Invalidated tasks rebuild the next time you run the build using
BitBake.
</para>
<para>
As you read this section, be sure to substitute the name
of your Linux kernel recipe for the term
"linux-yocto".
</para>
<section id='tip-dirty-string'>
<title>"-dirty" String</title>
<!--
<para>
<emphasis>AR - Darren Hart:</emphasis> This section
originated from the old Yocto Project Kernel Architecture
and Use Manual.
It was decided we need to put it in this section here.
Darren needs to figure out where we want it and what part
of it we want (all, revision???)
</para>
-->
<para>
If kernel images are being built with "-dirty" on the
end of the version string, this simply means that
modifications in the source directory have not been committed.
<literallayout class='monospaced'>
$ git status
</literallayout>
</para>
<para>
You can use the above Git command to report modified,
removed, or added files.
You should commit those changes to the tree regardless of
whether they will be saved, exported, or used.
Once you commit the changes, you need to rebuild the kernel.
</para>
<para>
To force a pickup and commit of all such pending changes,
enter the following:
<literallayout class='monospaced'>
$ git add .
$ git commit -s -a -m "getting rid of -dirty"
</literallayout>
</para>
<para>
Next, rebuild the kernel.
</para>
</section>
<section id='generating-configuration-files'>
<title>Generating Configuration Files</title>
<para>
You can manipulate the <filename>.config</filename> file
used to build a linux-yocto recipe with the
<filename>menuconfig</filename> command as follows:
<literallayout class='monospaced'>
$ bitbake linux-yocto -c menuconfig
</literallayout>
This command starts the Linux kernel configuration tool,
which allows you to prepare a new
<filename>.config</filename> file for the build.
When you exit the tool, be sure to save your changes
at the prompt.
</para>
<para>
The resulting <filename>.config</filename> file is
located in
<filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink><filename>}</filename> under the
<filename>linux-${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGE_ARCH'><filename>PACKAGE_ARCH</filename></ulink><filename>}-${<ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_KERNEL_TYPE'><filename>LINUX_KERNEL_TYPE</filename></ulink>}-build</filename> directory.
You can use the entire <filename>.config</filename> file as the
<filename>defconfig</filename> file as described in the
"<link linkend='changing-the-configuration'>Changing the Configuration</link>" section.
For more information on the <filename>.config</filename> file,
see the
"<ulink url='&YOCTO_DOCS_DEV_URL;#using-menuconfig'>Using <filename>menuconfig</filename></ulink>"
section in the Yocto Project Development Manual.
</para>
<para>
A better method is to create a configuration fragment using the
differences between two configuration files: one previously
created and saved, and one freshly created using the
<filename>menuconfig</filename> tool.
</para>
<para>
To create a configuration fragment using this method, follow
these steps:
<orderedlist>
<listitem><para>Complete a build at least through the kernel
configuration task as follows:
<literallayout class='monospaced'>
$ bitbake linux-yocto -c kernel_configme -f
</literallayout>
This step ensures that you will be creating a
<filename>.config</filename> file from a known state.
Because situations exist where your build state might
become unknown, it is best to run the previous
command prior to starting up
<filename>menuconfig</filename>.
</para></listitem>
<listitem><para>Run the <filename>menuconfig</filename>
command:
<literallayout class='monospaced'>
$ bitbake linux-yocto -c menuconfig
</literallayout></para></listitem>
<listitem><para>Run the <filename>diffconfig</filename>
command to prepare a configuration fragment.
The resulting file <filename>fragment.cfg</filename>
will be placed in the
<filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink><filename>}</filename> directory:
<literallayout class='monospaced'>
$ bitbake linux-yocto -c diffconfig
</literallayout></para></listitem>
</orderedlist>
</para>
<para>
The <filename>diffconfig</filename> command creates a file that is a
list of Linux kernel <filename>CONFIG_</filename> assignments.
See the "<link linkend='changing-the-configuration'>Changing the Configuration</link>"
section for information on how to use the output as a
configuration fragment.
<note>
You can also use this method to create configuration
fragments for a BSP.
See the "<link linkend='bsp-descriptions'>BSP Descriptions</link>"
section for more information.
</note>
</para>
<para>
The kernel tools also provide configuration validation.
You can use these tools to produce warnings for when a
requested configuration does not appear in the final
<filename>.config</filename> file or when you override a
policy configuration in a hardware configuration fragment.
Here is an example with some sample output of the command
that runs these tools:
<literallayout class='monospaced'>
$ bitbake linux-yocto -c kernel_configcheck -f
...
NOTE: validating kernel configuration
This BSP sets 3 invalid/obsolete kernel options.
These config options are not offered anywhere within this kernel.
The full list can be found in your kernel src dir at:
meta/cfg/standard/mybsp/invalid.cfg
This BSP sets 21 kernel options that are possibly non-hardware related.
The full list can be found in your kernel src dir at:
meta/cfg/standard/mybsp/specified_non_hdw.cfg
WARNING: There were 2 hardware options requested that do not
have a corresponding value present in the final ".config" file.
This probably means you are not getting the config you wanted.
The full list can be found in your kernel src dir at:
meta/cfg/standard/mybsp/mismatch.cfg
</literallayout>
</para>
<para>
The output describes the various problems that you can
encounter along with where to find the offending configuration
items.
You can use the information in the logs to adjust your
configuration files and then repeat the
<filename>kernel_configme</filename> and
<filename>kernel_configcheck</filename> commands until
they produce no warnings.
</para>
<para>
For more information on how to use the
<filename>menuconfig</filename> tool, see the
"<ulink url='&YOCTO_DOCS_DEV_URL;#using-menuconfig'>Using <filename>menuconfig</filename></ulink>"
section in the Yocto Project Development Manual.
</para>
</section>
<section id='modifying-source-code'>
<title>Modifying Source Code</title>
<para>
You can experiment with source code changes and create a
simple patch without leaving the BitBake environment.
To get started, be sure to complete a build at
least through the kernel configuration task:
<literallayout class='monospaced'>
$ bitbake linux-yocto -c kernel_configme -f
</literallayout>
Taking this step ensures you have the sources prepared
and the configuration completed.
You can find the sources in the
<filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink><filename>}/linux</filename> directory.
</para>
<para>
You can edit the sources as you would any other Linux source
tree.
However, keep in mind that you will lose changes if you
trigger the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-fetch'><filename>do_fetch</filename></ulink>
task for the recipe.
You can avoid triggering this task by not using BitBake to
run the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-cleanall'><filename>cleanall</filename></ulink>,
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-cleansstate'><filename>cleansstate</filename></ulink>,
or forced
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-fetch'><filename>fetch</filename></ulink>
commands.
Also, do not modify the recipe itself while working
with temporary changes or BitBake might run the
<filename>fetch</filename> command depending on the
changes to the recipe.
</para>
<para>
To test your temporary changes, instruct BitBake to run the
<filename>compile</filename> again.
The <filename>-f</filename> option forces the command to run
even though BitBake might think it has already done so:
<literallayout class='monospaced'>
$ bitbake linux-yocto -c compile -f
</literallayout>
If the compile fails, you can update the sources and repeat
the <filename>compile</filename>.
Once compilation is successful, you can inspect and test
the resulting build (i.e. kernel, modules, and so forth) from
the following build directory:
<literallayout class='monospaced'>
${WORKDIR}/linux-${PACKAGE_ARCH}-${LINUX_KERNEL_TYPE}-build
</literallayout>
Alternatively, you can run the <filename>deploy</filename>
command to place the kernel image in the
<filename>tmp/deploy/images</filename> directory:
<literallayout class='monospaced'>
$ bitbake linux-yocto -c deploy
</literallayout>
And, of course, you can perform the remaining installation and
packaging steps by issuing:
<literallayout class='monospaced'>
$ bitbake linux-yocto
</literallayout>
</para>
<para>
For rapid iterative development, the edit-compile-repeat loop
described in this section is preferable to rebuilding the
entire recipe because the installation and packaging tasks
are very time consuming.
</para>
<para>
Once you are satisfied with your source code modifications,
you can make them permanent by generating patches and
applying them to the
<ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
statement as described in the
"<link linkend='applying-patches'>Applying Patches</link>"
section.
If you are not familiar with generating patches, refer to the
"<ulink url='&YOCTO_DOCS_DEV_URL;#creating-the-patch'>Creating the Patch</ulink>"
section in the Yocto Project Development Manual.
</para>
</section>
</section>
<section id='working-with-your-own-sources'>
<title>Working With Your Own Sources</title>
<para>
If you cannot work with one of the Linux kernel
versions supported by existing linux-yocto recipes, you can
still make use of the Yocto Project Linux kernel tooling by
working with your own sources.
When you use your own sources, you will not be able to
leverage the existing kernel
<ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink> and
stabilization work of the linux-yocto sources.
However, you will be able to manage your own Metadata in the same
format as the linux-yocto sources.
Maintaining format compatibility facilitates converging with
linux-yocto on a future, mutually-supported kernel version.
</para>
<para>
To help you use your own sources, the Yocto Project provides a
linux-yocto custom recipe
(<filename>linux-yocto-custom.bb</filename>) that uses
<filename>kernel.org</filename> sources
and the Yocto Project Linux kernel tools for managing
kernel Metadata.
You can find this recipe in the
<filename>poky</filename> Git repository of the
Yocto Project <ulink url='&YOCTO_GIT_URL;'>Source Repository</ulink>
at:
<literallayout class="monospaced">
poky/meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb
</literallayout>
</para>
<para>
Here are some basic steps you can use to work with your own sources:
<orderedlist>
<listitem><para>Copy the <filename>linux-yocto-custom.bb</filename>
recipe to your layer and give it a meaningful name.
The name should include the version of the Linux kernel you
are using (e.g.
<filename>linux-yocto-myproject_3.19.bb</filename>,
where "3.19" is the base version of the Linux kernel
with which you would be working).</para></listitem>
<listitem><para>In the same directory inside your layer,
create a matching directory
to store your patches and configuration files (e.g.
<filename>linux-yocto-myproject</filename>).
</para></listitem>
<listitem><para>Make sure you have either a
<filename>defconfig</filename> file or configuration
fragment files.
When you use the <filename>linux-yocto-custom.bb</filename>
recipe, you must specify a configuration.
If you do not have a <filename>defconfig</filename> file,
you can run the following:
<literallayout class='monospaced'>
$ make defconfig
</literallayout>
After running the command, copy the resulting
<filename>.config</filename> to the
<filename>files</filename> directory as "defconfig" and
then add it to the
<ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
variable in the recipe.</para>
<para>Running the <filename>make defconfig</filename>
command results in the default configuration for your
architecture as defined by your kernel.
However, no guarantee exists that this configuration is
valid for your use case, or that your board will even boot.
This is particularly true for non-x86 architectures.
To use non-x86 <filename>defconfig</filename> files, you
need to be more specific and find one that matches your
board (i.e. for arm, you look in
<filename>arch/arm/configs</filename> and use the one that
is the best starting point for your board).
</para></listitem>
<listitem><para>Edit the following variables in your recipe
as appropriate for your project:
<itemizedlist>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>:
The <filename>SRC_URI</filename> should specify
a Git repository that uses one of the supported Git
fetcher protocols (i.e. <filename>file</filename>,
<filename>git</filename>, <filename>http</filename>,
and so forth).
The <filename>SRC_URI</filename> variable should
also specify either a <filename>defconfig</filename>
file or some configuration fragment files.
The skeleton recipe provides an example
<filename>SRC_URI</filename> as a syntax reference.
</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_VERSION'><filename>LINUX_VERSION</filename></ulink>:
The Linux kernel version you are using (e.g.
"3.19").</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-LINUX_VERSION_EXTENSION'><filename>LINUX_VERSION_EXTENSION</filename></ulink>:
The Linux kernel <filename>CONFIG_LOCALVERSION</filename>
that is compiled into the resulting kernel and visible
through the <filename>uname</filename> command.
</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-SRCREV'><filename>SRCREV</filename></ulink>:
The commit ID from which you want to build.
</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-PR'><filename>PR</filename></ulink>:
Treat this variable the same as you would in any other
recipe.
Increment the variable to indicate to the OpenEmbedded
build system that the recipe has changed.
</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>:
The default <filename>PV</filename> assignment is
typically adequate.
It combines the <filename>LINUX_VERSION</filename>
with the Source Control Manager (SCM) revision
as derived from the
<ulink url='&YOCTO_DOCS_REF_URL;#var-SRCPV'><filename>SRCPV</filename></ulink>
variable.
The combined results are a string with
the following form:
<literallayout class='monospaced'>
3.19.11+git1+68a635bf8dfb64b02263c1ac80c948647cc76d5f_1+218bd8d2022b9852c60d32f0d770931e3cf343e2
</literallayout>
While lengthy, the extra verbosity in <filename>PV</filename>
helps ensure you are using the exact
sources from which you intend to build.
</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-COMPATIBLE_MACHINE'><filename>COMPATIBLE_MACHINE</filename></ulink>:
A list of the machines supported by your new recipe.
This variable in the example recipe is set
by default to a regular expression that matches
only the empty string, "(^$)".
This default setting triggers an explicit build
failure.
You must change it to match a list of the machines
that your new recipe supports.
For example, to support the <filename>qemux86</filename>
and <filename>qemux86-64</filename> machines, use
the following form:
<literallayout class='monospaced'>
COMPATIBLE_MACHINE = "qemux86|qemux86-64"
</literallayout></para></listitem>
</itemizedlist></para></listitem>
<listitem><para>Provide further customizations to your recipe
as needed just as you would customize an existing
linux-yocto recipe.
See the "<link linkend='modifying-an-existing-recipe'>Modifying
an Existing Recipe</link>" section for information.
</para></listitem>
</orderedlist>
</para>
</section>
<section id='working-with-out-of-tree-modules'>
<title>Working with Out-of-Tree Modules</title>
<para>
This section describes steps to build out-of-tree modules on
your target and describes how to incorporate out-of-tree modules
in the build.
</para>
<section id='building-out-of-tree-modules-on-the-target'>
<title>Building Out-of-Tree Modules on the Target</title>
<para>
While the traditional Yocto Project development model would be
to include kernel modules as part of the normal build
process, you might find it useful to build modules on the
target.
This could be the case if your target system is capable
and powerful enough to handle the necessary compilation.
Before deciding to build on your target, however, you should
consider the benefits of using a proper cross-development
environment from your build host.
</para>
<para>
If you want to be able to build out-of-tree modules on
the target, there are some steps you need to take
on the target that is running your SDK image.
Briefly, the <filename>kernel-dev</filename> package
is installed by default on all
<filename>*.sdk</filename> images and the
<filename>kernel-devsrc</filename> package is installed
on many of the <filename>*.sdk</filename> images.
However, you need to create some scripts prior to
attempting to build the out-of-tree modules on the target
that is running that image.
</para>
<para>
Prior to attempting to build the out-of-tree modules,
you need to be on the target as root and you need to
change to the <filename>/usr/src/kernel</filename> directory.
Next, <filename>make</filename> the scripts:
<literallayout class='monospaced'>
# cd /usr/src/kernel
# make scripts
</literallayout>
Because all SDK image recipes include
<filename>dev-pkgs</filename>, the
<filename>kernel-dev</filename> packages will be installed
as part of the SDK image and the
<filename>kernel-devsrc</filename> packages will be installed
as part of applicable SDK images.
The SDK uses the scripts when building out-of-tree
modules.
Once you have switched to that directory and created the
scripts, you should be able to build your out-of-tree modules
on the target.
</para>
</section>
<section id='incorporating-out-of-tree-modules'>
<title>Incorporating Out-of-Tree Modules</title>
<para>
While it is always preferable to work with sources integrated
into the Linux kernel sources, if you need an external kernel
module, the <filename>hello-mod.bb</filename> recipe is
available as a template from which you can create your
own out-of-tree Linux kernel module recipe.
</para>
<para>
This template recipe is located in the
<filename>poky</filename> Git repository of the
Yocto Project <ulink url='&YOCTO_GIT_URL;'>Source Repository</ulink>
at:
<literallayout class="monospaced">
poky/meta-skeleton/recipes-kernel/hello-mod/hello-mod_0.1.bb
</literallayout>
</para>
<para>
To get started, copy this recipe to your layer and give it a
meaningful name (e.g. <filename>mymodule_1.0.bb</filename>).
In the same directory, create a new directory named
<filename>files</filename> where you can store any source files,
patches, or other files necessary for building
the module that do not come with the sources.
Finally, update the recipe as needed for the module.
Typically, you will need to set the following variables:
<itemizedlist>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-DESCRIPTION'><filename>DESCRIPTION</filename></ulink>
</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-LICENSE'><filename>LICENSE*</filename></ulink>
</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>
</para></listitem>
</itemizedlist>
</para>
<para>
Depending on the build system used by the module sources,
you might need to make some adjustments.
For example, a typical module <filename>Makefile</filename>
looks much like the one provided with the
<filename>hello-mod</filename> template:
<literallayout class='monospaced'>
obj-m := hello.o
SRC := $(shell pwd)
all:
$(MAKE) -C $(KERNEL_SRC) M=$(SRC)
modules_install:
$(MAKE) -C $(KERNEL_SRC) M=$(SRC) modules_install
...
</literallayout>
</para>
<para>
The important point to note here is the
<ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_SRC'><filename>KERNEL_SRC</filename></ulink>
variable.
The
<ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-module'><filename>module</filename></ulink>
class sets this variable and the
<ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_PATH'><filename>KERNEL_PATH</filename></ulink>
variable to
<filename>${<ulink url='&YOCTO_DOCS_REF_URL;#var-STAGING_KERNEL_DIR'><filename>STAGING_KERNEL_DIR</filename></ulink>}</filename>
with the necessary Linux kernel build information to build
modules.
If your module <filename>Makefile</filename> uses a different
variable, you might want to override the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-compile'><filename>do_compile()</filename></ulink>
step, or create a patch to
the <filename>Makefile</filename> to work with the more typical
<filename>KERNEL_SRC</filename> or
<filename>KERNEL_PATH</filename> variables.
</para>
<para>
After you have prepared your recipe, you will likely want to
include the module in your images.
To do this, see the documentation for the following variables in
the Yocto Project Reference Manual and set one of them
appropriately for your machine configuration file:
<itemizedlist>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_ESSENTIAL_EXTRA_RDEPENDS'><filename>MACHINE_ESSENTIAL_EXTRA_RDEPENDS</filename></ulink>
</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS'><filename>MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS</filename></ulink>
</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_EXTRA_RDEPENDS'><filename>MACHINE_EXTRA_RDEPENDS</filename></ulink>
</para></listitem>
<listitem><para><ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_EXTRA_RRECOMMENDS'><filename>MACHINE_EXTRA_RRECOMMENDS</filename></ulink>
</para></listitem>
</itemizedlist>
</para>
<para>
Modules are often not required for boot and can be excluded from
certain build configurations.
The following allows for the most flexibility:
<literallayout class='monospaced'>
MACHINE_EXTRA_RRECOMMENDS += "kernel-module-mymodule"
</literallayout>
The value is derived by appending the module filename without
the <filename>.ko</filename> extension to the string
"kernel-module-".
</para>
<para>
Because the variable is
<ulink url='&YOCTO_DOCS_REF_URL;#var-RRECOMMENDS'><filename>RRECOMMENDS</filename></ulink>
and not a
<ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'><filename>RDEPENDS</filename></ulink>
variable, the build will not fail if this module is not
available to include in the image.
</para>
</section>
</section>
<section id='inspecting-changes-and-commits'>
<title>Inspecting Changes and Commits</title>
<para>
A common question when working with a kernel is:
"What changes have been applied to this tree?"
Rather than using "grep" across directories to see what has
changed, you can use Git to inspect or search the kernel tree.
Using Git is an efficient way to see what has changed in the tree.
</para>
<section id='what-changed-in-a-kernel'>
<title>What Changed in a Kernel?</title>
<para>
Following are a few examples that show how to use Git
commands to examine changes.
These examples are by no means the only way to see changes.
<note>
In the following examples, unless you provide a commit
range, <filename>kernel.org</filename> history is blended
with Yocto Project kernel changes.
You can form ranges by using branch names from the
kernel tree as the upper and lower commit markers with
the Git commands.
You can see the branch names through the web interface
to the Yocto Project source repositories at
<ulink url='http://git.yoctoproject.org/cgit.cgi'></ulink>.
</note>
To see a full range of the changes, use the
<filename>git whatchanged</filename> command and specify a
commit range for the branch
(<replaceable>commit</replaceable><filename>..</filename><replaceable>commit</replaceable>).
</para>
<para>
Here is an example that looks at what has changed in the
<filename>emenlow</filename> branch of the
<filename>linux-yocto-3.19</filename> kernel.
The lower commit range is the commit associated with the
<filename>standard/base</filename> branch, while
the upper commit range is the commit associated with the
<filename>standard/emenlow</filename> branch.
<literallayout class='monospaced'>
$ git whatchanged origin/standard/base..origin/standard/emenlow
</literallayout>
</para>
<para>
To see short, one line summaries of changes use the
<filename>git log</filename> command:
<literallayout class='monospaced'>
$ git log --oneline origin/standard/base..origin/standard/emenlow
</literallayout>
</para>
<para>
Use this command to see code differences for the changes:
<literallayout class='monospaced'>
$ git diff origin/standard/base..origin/standard/emenlow
</literallayout>
</para>
<para>
Use this command to see the commit log messages and the
text differences:
<literallayout class='monospaced'>
$ git show origin/standard/base..origin/standard/emenlow
</literallayout>
</para>
<para>
Use this command to create individual patches for
each change.
Here is an example that that creates patch files for each
commit and places them in your <filename>Documents</filename>
directory:
<literallayout class='monospaced'>
$ git format-patch -o $HOME/Documents origin/standard/base..origin/standard/emenlow
</literallayout>
</para>
</section>
<section id='showing-a-particular-feature-or-branch-change'>
<title>Showing a Particular Feature or Branch Change</title>
<para>
Tags in the Yocto Project kernel tree divide changes for
significant features or branches.
The <filename>git show</filename>&nbsp;<replaceable>tag</replaceable>
command shows changes based on a tag.
Here is an example that shows <filename>systemtap</filename>
changes:
<literallayout class='monospaced'>
$ git show systemtap
</literallayout>
You can use the
<filename>git branch --contains</filename>&nbsp;<replaceable>tag</replaceable>
command to show the branches that contain a particular feature.
This command shows the branches that contain the
<filename>systemtap</filename> feature:
<literallayout class='monospaced'>
$ git branch --contains systemtap
</literallayout>
</para>
</section>
</section>
</chapter>
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