dev-manual: Created new "Plugins" section in the wic section.
Add a new section discussing plugins, taken directly from the corresponding wic help section. (From yocto-docs rev: c1b4c378a496413f2dde8ad2f043a537cba24b6e) Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
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Tom Zanussi
Richard Purdie
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commit
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</section>
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</section>
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<section id='openembedded-kickstart-plugins'>
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<title>Plugins</title>
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<para>
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Plugins allow <filename>wic</filename> functionality to
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be extended and specialized by users.
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This section documents the plugin interface, which is
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currently restricted to source plugins.
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</para>
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<para>
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Source plugins provide a mechanism to customize
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various aspects of the image generation process in
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<filename>wic</filename>, mainly the contents of
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partitions.
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The plugins provide a mechanism for mapping values
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specified in <filename>.wks</filename> files using the
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<filename>‐‐source</filename> keyword to a
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particular plugin implementation that populates a
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corresponding partition.
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</para>
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<para>
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A source plugin is created as a subclass of
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<filename>SourcePlugin</filename>.
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The plugin file containing it is added to
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<filename>scripts/lib/mic/plugins/source/</filename> to
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make the plugin implementation available to the
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<filename>wic</filename> implementation.
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For more information, see
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<filename>scripts/lib/mic/pluginbase.py</filename>.
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</para>
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<para>
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Source plugins can also be implemented and added by
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external layers.
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As such, any plugins found in a
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<filename>scripts/lib/mic/plugins/source/</filename>
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directory in an external layer are also made
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available.
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</para>
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<para>
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When the <filename>wic</filename> implementation needs
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to invoke a partition-specific implementation, it looks
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for the plugin that has the same name as the
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<filename>‐‐source</filename> parameter given to
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that partition.
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For example, if the partition is set up as follows:
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<literallayout class='monospaced'>
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part /boot --source bootimg-pcbios ...
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</literallayout>
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The methods defined as class members of the plugin
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having the matching <filename>bootimg-pcbios.name</filename>
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class member are used.
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</para>
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<para>
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To be more concrete, here is the plugin definition that
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matches a
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<filename>'‐‐source bootimg-pcbios'</filename> usage,
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along with an example
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method called by the <filename>wic</filename> implementation
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when it needs to invoke an implementation-specific
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partition-preparation function:
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<literallayout class='monospaced'>
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class BootimgPcbiosPlugin(SourcePlugin):
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name = 'bootimg-pcbios'
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@classmethod
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def do_prepare_partition(self, part, ...)
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</literallayout>
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If the subclass itself does not implement a function, a
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default version in a superclass is located and
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used, which is why all plugins must be derived from
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<filename>SourcePlugin</filename>.
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</para>
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<para>
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The <filename>SourcePlugin</filename> class defines the
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following methods, which is the current set of methods
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that can be implemented or overridden by
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<filename>‐‐source</filename> plugins.
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Any methods not implemented by a
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<filename>SourcePlugin</filename> subclass inherit the
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implementations present in the
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<filename>SourcePlugin</filename> class.
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For more information, see the
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<filename>SourcePlugin</filename> source for details:
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</para>
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<para>
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<itemizedlist>
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<listitem><para><emphasis><filename>do_prepare_partition()</filename>:</emphasis>
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Called to do the actual content population for a
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partition.
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In other words, the method prepares the final
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partition image that is incorporated into the
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disk image.
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</para></listitem>
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<listitem><para><emphasis><filename>do_configure_partition()</filename>:</emphasis>
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Called before
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<filename>do_prepare_partition()</filename>.
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This method is typically used to create custom
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configuration files for a partition (e.g. syslinux or
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grub configuration files).
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</para></listitem>
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<listitem><para><emphasis><filename>do_install_disk()</filename>:</emphasis>
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Called after all partitions have been prepared and
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assembled into a disk image.
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This method provides a hook to allow finalization of a
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disk image, (e.g. writing an MBR).
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</para></listitem>
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<listitem><para><emphasis><filename>do_stage_partition()</filename>:</emphasis>
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Special content-staging hook called before
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<filename>do_prepare_partition()</filename>.
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This method is normally empty.</para>
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<para>Typically, a partition just uses the passed-in
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parameters (e.g. the unmodified value of
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<filename>bootimg_dir</filename>).
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However, in some cases things might need to be
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more tailored.
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As an example, certain files might additionally
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need to be taken from
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<filename>bootimg_dir + /boot</filename>.
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This hook allows those files to be staged in a
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customized fashion.
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<note>
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<filename>get_bitbake_var()</filename>
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allows you to access non-standard variables
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that you might want to use for this.
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</note>
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</para></listitem>
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</itemizedlist>
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</para>
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<para>
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This scheme is extensible.
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Adding more hooks is a simple matter of adding more
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plugin methods to <filename>SourcePlugin</filename> and
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derived classes.
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The code that then needs to call the plugin methods uses
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<filename>plugin.get_source_plugin_methods()</filename>
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to find the method or methods needed by the call.
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Location is accomplished by filling up a dict with keys
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containing the method names of interest.
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On success, these will be filled in with the actual
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methods.
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Please see the <filename>wic</filename>
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implementation for examples and details.
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</para>
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</section>
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<section id='openembedded-kickstart-wks-reference'>
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<title>OpenEmbedded Kickstart (.wks) Reference</title>
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