ref-manual: First draft of the "Image Generation" section
Fixes [YOCTO #2808] Added the first draft of the expanded explanation for generating an image. (From yocto-docs rev: 72e4db25d89cad62e1cd9ee1d638af374ec1bfc0) Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
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</note>
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</para>
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</section>
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<section id='image-generation-dev-environment'>
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<title>Image Generation</title>
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<para>
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Once packages are split and stored in the Package Feeds area,
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the OpenEmbedded build system uses BitBake to generate the
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root filesystem image:
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<imagedata fileref="figures/image-generation.png" align="center" width="6in" depth="7in" />
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</para>
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<para>
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The image generation process consists of several stages and
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depends on many variables.
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The <filename>do_rootfs</filename> uses these key variables
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to help create the list of packages to actually install:
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<itemizedlist>
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<listitem><para><link linkend='var-IMAGE_INSTALL'><filename>IMAGE_INSTALL</filename></link>:
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Lists out the base set of packages to install from
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the Package Feeds area.</para></listitem>
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<listitem><para><link linkend='var-IMAGE_FEATURES'><filename>IMAGE_FEATURES</filename></link>:
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Specifies features to include in the image.
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These features map to additional packages for
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installation.</para></listitem>
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<listitem><para><link linkend='var-PACKAGE_CLASSES'><filename>PACKAGE_CLASSES</filename></link>:
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Specifies the package manager to use and consequently
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helps determine where to locate packages within the
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Package Feeds area.</para></listitem>
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<listitem><para><link linkend='var-PACKAGE_EXCLUDE'><filename>PACKAGE_EXCLUDE</filename></link>:
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Specifies packages that should not be installed.
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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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In addition to the previously mentioned variables, the image
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generation process uses
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<link linkend='var-IMAGE_LINGUAS'><filename>IMAGE_LINGUAS</filename></link>
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to determine what language for which packages are being
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installed.
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</para>
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<para>
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Part of the image generation process includes compressing the
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root filesystem image.
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Compression is accomplished through several optimization
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routines designed to reduce the overall size of the image.
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</para>
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<para>
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The process runs as many post installation scripts as possible.
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Any scripts that cannot be run are run when the system is
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first booted.
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If you are using a read-only root filesystem, all the post
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installation scripts are run during the package installation
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phase since the root filesystem cannot be written into.
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</para>
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<para>
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During the Prelink phase, optimization processes are run across
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the image.
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These processes include <filename>mklibs</filename> and
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<filename>prelink</filename>.
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The <filename>mklibs</filename> optimizes the size of the
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libraries.
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The <filename>prelink</filename> process optimizes the dynamic
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linking of shared libraries to reduce start up time of the
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executables.
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See the
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<link linkend='var-IMAGE_POSTPROCESS_COMMAND'><filename>IMAGE_POSTPROCESS_COMMAND</filename></link>
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and
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<link linkend='var-ROOTFS_POSTPROCESS_COMMAND'><filename>ROOTFS_POSTPROCESS_COMMAND</filename></link>
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variables for additional information.
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</para>
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<para>
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After the root filesystem has been constructed, the image
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generation process turns everything into an image file or
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a set of image files.
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we need to turn into an image file or set of image files.
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The formats used for the root filesystem depend on the
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<link linkend='var-IMAGE_FSTYPES'><filename>IMAGE_FSTYPES</filename></link>
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variable.
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</para>
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<note>
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The entire image generation process is run under Pseudo.
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Running under Pseudo ensures that the files in the root
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filesystem have correct ownership.
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</note>
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</section>
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</section>
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<section id="package-feeds-dev-environment">
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