353 lines
17 KiB
XML
353 lines
17 KiB
XML
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
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[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >
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<appendix id='kernel-dev-maint-appx'>
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<title>Kernel Maintenance</title>
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<section id='tree-construction'>
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<title>Tree Construction</title>
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<para>
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This section describes construction of the Yocto Project kernel
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source repositories as accomplished by the Yocto Project team to
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create Yocto Linux kernel repositories.
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These kernel repositories are found under the heading "Yocto Linux
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Kernel" at
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<ulink url='&YOCTO_GIT_URL;/cgit.cgi'>&YOCTO_GIT_URL;/cgit.cgi</ulink>
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and are shipped as part of a Yocto Project release.
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The team creates these repositories by compiling and executing the
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set of feature descriptions for every BSP and feature in the
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product.
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Those feature descriptions list all necessary patches,
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configurations, branches, tags, and feature divisions found in a
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Yocto Linux kernel.
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Thus, the Yocto Project Linux kernel repository (or tree) and
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accompanying Metadata in the
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<filename>yocto-kernel-cache</filename> are built.
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</para>
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<para>
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The existence of these repositories allow you to access and clone a
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particular Yocto Project Linux kernel repository and use it to
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build images based on their configurations and features.
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</para>
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<para>
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You can find the files used to describe all the valid features and
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BSPs in the Yocto Project Linux kernel in any clone of the Yocto
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Project Linux kernel source repository and
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<filename>yocto-kernel-cache</filename> Git trees.
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For example, the following commands clone the Yocto Project
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baseline Linux kernel that branches off
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<filename>linux.org</filename> version 4.12 and the
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<filename>yocto-kernel-cache</filename>, which contains stores of
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kernel Metadata:
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<literallayout class='monospaced'>
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$ git clone git://git.yoctoproject.org/linux-yocto-4.12
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$ git clone git://git.yoctoproject.org/linux-kernel-cache
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</literallayout>
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For more information on how to set up a local Git repository of
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the Yocto Project Linux kernel files, see the
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"<link linkend='preparing-the-build-host-to-work-on-the-kernel'>Preparing the Build Host to Work on the Kernel</link>"
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section.
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</para>
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<para>
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Once you have cloned the kernel Git repository and the
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cache of Metadata on your local machine, you can discover the
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branches that are available in the repository using the following
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Git command:
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<literallayout class='monospaced'>
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$ git branch -a
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</literallayout>
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Checking out a branch allows you to work with a particular
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Yocto Linux kernel.
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For example, the following commands check out the
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"standard/beagleboard" branch of the Yocto Linux kernel repository
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and the "yocto-4.12" branch of the
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<filename>yocto-kernel-cache</filename> repository:
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<literallayout class='monospaced'>
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$ cd ~/linux-yocto-4.12
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$ git checkout -b my-kernel-4.12 remotes/origin/standard/beagleboard
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$ cd ~/linux-kernel-cache
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$ git checkout -b my-4.12-metadata remotes/origin/yocto-4.12
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</literallayout>
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<note>
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Branches in the <filename>yocto-kernel-cache</filename>
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repository correspond to Yocto Linux kernel versions
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(e.g. "yocto-4.12", "yocto-4.10", "yocto-4.9", and so forth).
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</note>
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Once you have checked out and switched to appropriate branches,
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you can see a snapshot of all the kernel source files used to
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used to build that particular Yocto Linux kernel for a
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particular board.
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</para>
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<para>
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To see the features and configurations for a particular Yocto
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Linux kernel, you need to examine the
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<filename>yocto-kernel-cache</filename> Git repository.
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As mentioned, branches in the
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<filename>yocto-kernel-cache</filename> repository correspond to
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Yocto Linux kernel versions (e.g. <filename>yocto-4.12</filename>).
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Branches contain descriptions in the form of
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<filename>.scc</filename> and <filename>.cfg</filename> files.
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</para>
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<para>
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You should realize, however, that browsing your local
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<filename>yocto-kernel-cache</filename> repository for feature
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descriptions and patches is not an effective way to determine what
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is in a particular kernel branch.
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Instead, you should use Git directly to discover the changes in
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a branch.
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Using Git is an efficient and flexible way to inspect changes to
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the kernel.
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<note>
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Ground up reconstruction of the complete kernel tree is an
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action only taken by the Yocto Project team during an active
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development cycle.
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When you create a clone of the kernel Git repository, you are
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simply making it efficiently available for building and
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development.
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</note>
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</para>
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<para>
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The following steps describe what happens when the Yocto Project
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Team constructs the Yocto Project kernel source Git repository
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(or tree) found at
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<ulink url='&YOCTO_GIT_URL;/cgit.cgi'></ulink> given the
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introduction of a new top-level kernel feature or BSP.
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These are the actions that effectively provide the Metadata
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and create the tree that includes the new feature, patch or BSP:
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<orderedlist>
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<listitem><para>
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<emphasis>Pass Feature to Build Subsystem:</emphasis>
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A top-level kernel feature is passed to the kernel build
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subsystem.
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Normally, this feature is a BSP for a particular kernel
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type.
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</para></listitem>
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<listitem><para>
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<emphasis>Locate Feature:</emphasis>
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The file that describes the top-level feature is located
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by searching these system directories:
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<itemizedlist>
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<listitem><para>
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The in-tree kernel-cache directories, which are
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located in the
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<filename>yocto-kernel-cache</filename>
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repository
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</para></listitem>
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<listitem><para>
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Areas pointed to by <filename>SRC_URI</filename>
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statements found in kernel recipes
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</para></listitem>
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</itemizedlist>
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For a typical build, the target of the search is a
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feature description in an <filename>.scc</filename> file
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whose name follows this format:
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<literallayout class='monospaced'>
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<replaceable>bsp_name</replaceable>-<replaceable>kernel_type</replaceable>.scc
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</literallayout>
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</para></listitem>
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<listitem><para>
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<emphasis>Expand Feature:</emphasis>
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Once located, the feature description is either expanded
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into a simple script of actions, or into an existing
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equivalent script that is already part of the shipped
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kernel.
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</para></listitem>
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<listitem><para>
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<emphasis>Append Extra Features:</emphasis>
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Extra features are appended to the top-level feature
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description.
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These features can come from the
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<ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES'><filename>KERNEL_FEATURES</filename></ulink>
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variable in recipes.
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</para></listitem>
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<listitem><para>
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<emphasis>Locate, Expand, and Append Each Feature:</emphasis>
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Each extra feature is located, expanded and appended to
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the script as described in step three.
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</para></listitem>
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<listitem><para>
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<emphasis>Execute the Script:</emphasis>
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The script is executed to produce files
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<filename>.scc</filename> and <filename>.cfg</filename>
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files in appropriate directories of the
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<filename>yocto-kernel-cache</filename> repository.
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These files are descriptions of all the branches, tags,
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patches and configurations that need to be applied to the
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base Git repository to completely create the
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source (build) branch for the new BSP or feature.
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</para></listitem>
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<listitem><para>
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<emphasis>Clone Base Repository:</emphasis>
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The base repository is cloned, and the actions
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listed in the <filename>yocto-kernel-cache</filename>
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directories are applied to the tree.
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</para></listitem>
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<listitem><para>
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<emphasis>Perform Cleanup:</emphasis>
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The Git repositories are left with the desired branches
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checked out and any required branching, patching and
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tagging has been performed.
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</para></listitem>
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</orderedlist>
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</para>
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<para>
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The kernel tree and cache are ready for developer consumption to
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be locally cloned, configured, and built into a Yocto Project
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kernel specific to some target hardware.
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<note><title>Notes</title>
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<itemizedlist>
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<listitem><para>
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The generated <filename>yocto-kernel-cache</filename>
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repository adds to the kernel as shipped with the Yocto
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Project release.
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Any add-ons and configuration data are applied to the
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end of an existing branch.
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The full repository generation that is found in the
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official Yocto Project kernel repositories at
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<ulink url='&YOCTO_GIT_URL;/cgit.cgi'>http://git.yoctoproject.org/cgit.cgi</ulink>
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is the combination of all supported boards and
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configurations.
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</para></listitem>
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<listitem><para>
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The technique the Yocto Project team uses is flexible
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and allows for seamless blending of an immutable
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history with additional patches specific to a
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deployment.
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Any additions to the kernel become an integrated part
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of the branches.
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</para></listitem>
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<listitem><para>
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The full kernel tree that you see on
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<ulink url='&YOCTO_GIT_URL;/cgit.cgi'></ulink> is
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generated through repeating the above steps for all
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valid BSPs.
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The end result is a branched, clean history tree that
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makes up the kernel for a given release.
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You can see the script (<filename>kgit-scc</filename>)
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responsible for this in the
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<ulink url='&YOCTO_GIT_URL;/cgit.cgi/yocto-kernel-tools/tree/tools'><filename>yocto-kernel-tools</filename></ulink>
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repository.
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</para></listitem>
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<listitem><para>
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The steps used to construct the full kernel tree are
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the same steps that BitBake uses when it builds a
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kernel image.
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</para></listitem>
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</itemizedlist>
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</note>
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</para>
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</section>
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<section id='build-strategy'>
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<title>Build Strategy</title>
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<para>
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Once you have cloned a Yocto Linux kernel repository and the
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cache repository (<filename>yocto-kernel-cache</filename>) onto
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your development system, you can consider the compilation phase
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of kernel development, which is building a kernel image.
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Some prerequisites exist that are validated by the build process
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before compilation starts:
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</para>
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<itemizedlist>
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<listitem><para>
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The
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<ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
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points to the kernel Git repository.
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</para></listitem>
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<listitem><para>
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A BSP build branch with Metadata exists in the
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<filename>yocto-kernel-cache</filename> repository.
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The branch is based on the Yocto Linux kernel version and
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has configurations and features grouped under the
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<filename>yocto-kernel-cache/bsp</filename> directory.
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For example, features and configurations for the
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BeagleBone Board assuming a
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<filename>linux-yocto_4.12</filename> kernel reside in the
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following area of the <filename>yocto-kernel-cache</filename>
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repository:
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<literallayout class='monospaced'>
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yocto-kernel-cache/bsp/beaglebone
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</literallayout>
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<note>
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In the previous example, the "yocto-4.12" branch is
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checked out in the <filename>yocto-kernel-cache</filename>
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repository.
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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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The OpenEmbedded build system makes sure these conditions exist
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before attempting compilation.
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Other means, however, do exist, such as as bootstrapping a BSP.
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</para>
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<para>
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Before building a kernel, the build process verifies the tree
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and configures the kernel by processing all of the
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configuration "fragments" specified by feature descriptions
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in the <filename>.scc</filename> files.
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As the features are compiled, associated kernel configuration
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fragments are noted and recorded in the series of directories
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in their compilation order.
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The fragments are migrated, pre-processed and passed to the
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Linux Kernel Configuration subsystem (<filename>lkc</filename>) as
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raw input in the form of a <filename>.config</filename> file.
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The <filename>lkc</filename> uses its own internal dependency
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constraints to do the final processing of that information and
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generates the final <filename>.config</filename> file that is used
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during compilation.
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</para>
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<para>
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Using the board's architecture and other relevant values from
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the board's template, kernel compilation is started and a kernel
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image is produced.
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</para>
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<para>
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The other thing that you notice once you configure a kernel is that
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the build process generates a build tree that is separate from
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your kernel's local Git source repository tree.
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This build tree has a name that uses the following form, where
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<filename>${MACHINE}</filename> is the metadata name of the
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machine (BSP) and "kernel_type" is one of the Yocto Project
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supported kernel types (e.g. "standard"):
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<literallayout class='monospaced'>
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linux-${MACHINE}-<replaceable>kernel_type</replaceable>-build
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</literallayout>
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</para>
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<para>
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The existing support in the <filename>kernel.org</filename> tree
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achieves this default functionality.
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</para>
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<para>
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This behavior means that all the generated files for a particular
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machine or BSP are now in the build tree directory.
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The files include the final <filename>.config</filename> file,
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all the <filename>.o</filename> files, the <filename>.a</filename>
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files, and so forth.
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Since each machine or BSP has its own separate
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<ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>
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in its own separate branch of the Git repository, you can easily
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switch between different builds.
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</para>
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</section>
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</appendix>
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<!--
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vim: expandtab tw=80 ts=4
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-->
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