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documentation/dev-manual/dev-manual-common-tasks.xml: new section 

Created a new section called "Configuring the Kernel."  This is the
how-to counterpart for the concepts part created in the kernel
manual.  The section consists of three sub-sections:
"Using menuconfig", "Creating Config Fragments", and
"Fine-tuning the Kernel Configuration File."  Primary source for
information on this section was Bruce Ashfield.  He has this section
for review.

(From yocto-docs rev: 3ac718bc69b4914c41a0bb94134c085964ee71a7)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
This commit is contained in:
Scott Rifenbark 2012-02-14 10:12:32 -06:00 committed by Richard Purdie
parent d8b27b6662
commit ea70ac509b
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@ -1036,6 +1036,171 @@ so that there are some definite steps on how to do this. I need more detail her
</section>
</section>
<section id='configuring-the-kernel'>
<title>Configuring the Kernel</title>
<para>
Configuring the Linux Yocto kernel consists of making sure the <filename>.config</filename>
file has all the right information in it for the image you are building.
You can use the <filename>menuconfig</filename> tool and configuration fragments to
make sure your <filename>.config</filename> file is just how you need it.
This section describes how to use <filename>menuconfig</filename>, create and use
configuration fragments, and how to interatively tweak your <filename>.config</filename>
file to create the leanest kernel configuration file possible.
</para>
<para>
For concepts on kernel configuration, see the
"<ulink url='http://www.yoctoproject.org/docs/latest/kernel-manual/kernel-manual.html#kernel-configuration'>Kernel Configuration</ulink>"
section in the Yocto Project Kernel Architecture and Use Manual.
</para>
<section id='using-menuconfig'>
<title>Using&nbsp;&nbsp;<filename>menuconfig</filename></title>
<para>
The easiest way to define kernel configurations is to set them through the
<filename>menuconfig</filename> tool.
For general information on <filename>menuconfig</filename>, see
<ulink url='http://en.wikipedia.org/wiki/Menuconfig'></ulink>.
</para>
<para>
To use the <filename>menuconfig</filename> tool in the Yocto Project development
environment, you must build the tool using BitBake.
The following commands build and invoke <filename>menuconfig</filename> assuming the
Yocto Project files top-level directory is <filename>~/poky</filename>:
<literallayout class='monospaced'>
$ cd ~/poky
$ source oe-init-build-env
$ bitbake linux-yocto -c menuconfig
</literallayout>
Once <filename>menuconfig</filename> comes up, its standard interface allows you to
examine and configure all the kernel configuration parameters.
Once you have made your changes, simply exit the tool and save your changes to
create an updated version of the <filename>.config</filename> configuration file.
</para>
<para>
For an example that shows how to change the <filename>SMP_CONFIG</filename> parameter
using <filename>menuconfig</filename>, see the
"<link linkend='changing-the-config-smp-configuration-using-menuconfig'>Changing
the <filename>CONFIG_SMP</filename> Configuration Using <filename>menuconfig</filename></link>"
section.
</para>
</section>
<section id='creating-config-fragments'>
<title>Creating Config Fragments</title>
<para>
Configuration fragments are simply kernel options that appear in a file.
Syntactically, the configuration statement is identical to what would appear
in the <filename>.config</filename>.
For example, issuing the following from the shell would create a config fragment
file named <filename>my_smp.cfg</filename> that enables multi-processor support
within the kernel:
<literallayout class='monospaced'>
$ echo "CONFIG_SMP=y" >> my_smp.cfg
</literallayout>
</para>
<para>
Where do you put your configuration files?
You can place these configuration files in the same area to which the
<filename>SRC_URI</filename> points.
The Yocto Project build process will pick up the configuration and add it to the
kernel's configuration.
For example, assume you add the following to your
<filename>linux-yocto_3.0.bbappend</filename> file:
<literallayout class='monospaced'>
file://my_smp.cfg
</literallayout>
You would put the config fragment file <filename>my_smp.cfg</filename> in your
layer right beneath the directory containing the
<filename>linux-yocto_3.0.bbappend</filename> file and the build system
will pick up and apply the fragment.
</para>
</section>
<section id='fine-tuning-the-kernel-configuration-file'>
<title>Fine-tuning the Kernel Configuration File</title>
<para>
You can make sure the <filename>.config</filename> is as lean or efficient as
possible by reading the output of the kernel configuration fragment audit,
noting any issues, making changes to correct the issues, and then repeating.
</para>
<para>
As part of the Linux Yocto kernel build process, the
<filename>kernel_configcheck</filename> task runs.
This task validates the kernel configuration by checking the final
<filename>.config</filename> file against the input files.
During the check, the task produces warning messages for the following
issues:
<itemizedlist>
<listitem><para>Requested options that did not make the final
<filename>.config</filename> file.</para></listitem>
<listitem><para>Configuration items that appear twice in the same
configuration fragment.</para></listitem>
<listitem><para>Configuration items tagged as 'required' were overridden.
</para></listitem>
<listitem><para>A board overrides a non-board specific option.</para></listitem>
<listitem><para>Listed options not valid for the kernel being processed.
In other words, the option does not appear anywhere.</para></listitem>
</itemizedlist>
<note>
The <filename>kernel_configcheck</filename> task can also optionally report
if an option is overridden during processing.
</note>
</para>
<para>
For each output warning, a message points to the file
that contains a list of the options and a pointer to the config
fragment that defines them.
Collectively, the files are the key to streamlining the configiguration.
</para>
<para>
To streamline the configuration, do the following:
<orderedlist>
<listitem><para>Start with a full configuration that you know
works - it builds and boots successfully.
This configuration file will be your baseline.</para></listitem>
<listitem><para>Separately run the <filename>configme</filename> and
<filename>kernel_configcheck</filename> tasks.</para></listitem>
<listitem><para>Take the resulting list of files from the
<filename>kernel_configcheck</filename> task warnings and do the following:
<itemizedlist>
<listitem><para>Drop values that are redefined in the fragment but do not
change the final <filename>.config</filename> file.</para></listitem>
<listitem><para>Analyze and potentially drop values from the
<filename>.config</filename> file that override required
configurations.</para></listitem>
<listitem><para>Analyze and potentially remove non-board specific options.
</para></listitem>
<listitem><para>Remove repeated and invalid options.</para></listitem>
</itemizedlist></para></listitem>
<listitem><para>After you have worked through the output of the kernel configuration
audit, you can re-run the <filename>configme</filename>
and <filename>kernel_configcheck</filename> tasks to see the results of your
changes.
If you have more issues, you can deal with them as described in the
previous step.</para></listitem>
</orderedlist>
</para>
<para>
Iteratively working through steps two through four eventually yields
a minimal, streamlined configuration file.
Once you have the best <filename>.config</filename>, you can build the Linux
Yocto kernel.
</para>
</section>
</section>
<section id="usingpoky-configuring-DISTRO_PN_ALIAS">
<title>Handling a Package Name Alias</title>
<para>