documentation/kernel-manual: Scrub for 1.1
I went through and made sure examples are relevant, wording is correct, large blocks of unused text was removed, and some references included to other YP documents. (From yocto-docs rev: 2231082530dd9cecc234f5f024c4e246afb2968d) Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
This commit is contained in:
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@ -46,12 +46,14 @@
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the baseline kernel is the most stable official release.</para></listitem>
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<listitem><para>Include major technological features as part of Yocto Project's up-rev
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strategy.</para></listitem>
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<listitem><para>Present a Git tree, that just like the upstream kernel.org tree, has a
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clear and continuous history.</para></listitem>
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<listitem><para>Present a kernel Git repository that, similar to the upstream
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<filename>kernel.org</filename> tree,
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has a clear and continuous history.</para></listitem>
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<listitem><para>Deliver a key set of supported kernel types, where each type is tailored
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to a specific use case (i.e. networking, consumer, devices, and so forth).</para></listitem>
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<listitem><para>Employ a Git branching strategy that from a customer's point of view
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results in a linear path from the baseline kernel.org, through a select group of features and
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to a specific use case (e.g. networking, consumer, devices, and so forth).</para></listitem>
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<listitem><para>Employ a Git branching strategy that, from a developer's point of view,
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results in a linear path from the baseline <filename>kernel.org</filename>,
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through a select group of features and
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ends with their BSP-specific commits.</para></listitem>
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</itemizedlist>
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</para>
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@ -60,27 +62,29 @@
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<section id='kernel-big-picture'>
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<title>Yocto Project Kernel Development and Maintenance Overview</title>
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<para>
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Yocto Project kernel, like other kernels, is based off the Linux kernel release
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The Yocto Project kernel, like other kernels, is based off the Linux kernel release
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from <ulink url='http://www.kernel.org'></ulink>.
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At the beginning of our major development cycle, we choose our Yocto Project kernel
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based on factors like release timing, the anticipated release timing of "final" (i.e. non "rc")
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upstream kernel.org versions, and Yocto Project feature requirements.
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Typically this will be a kernel that is in the
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final stages of development by the community (i.e. still in the release
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candidate or "rc" phase) and not yet a final release.
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But by being in the final stages of external development, we know that the
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kernel.org final release will clearly land within the early stages of
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At the beginning of a major development cycle, the Yocto Project team
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chooses its Yocto Project kernel
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based on factors like release timing, the anticipated release timing of final
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upstream <filename>kernel.org</filename> versions, and Yocto Project feature requirements.
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Typically, the kernel chosen is in the
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final stages of development by the community.
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In other words, the kernel is in the release
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candidate or "rc" phase and not yet a final release.
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But, by being in the final stages of external development, the team knows that the
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<filename>kernel.org</filename> final release will clearly be within the early stages of
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the Yocto Project development window.
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</para>
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<para>
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This balance allows us to deliver the most up-to-date kernel
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as possible, while still ensuring that we have a stable official release as
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our baseline kernel version.
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This balance allows the team to deliver the most up-to-date kernel
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as possible, while still ensuring that the team has a stable official release as
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the baseline kernel version.
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</para>
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<para>
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The ultimate source for the Yocto Project kernel is a released kernel
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from kernel.org.
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In addition to a foundational kernel from kernel.org the released
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from <filename>kernel.org</filename>.
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In addition to a foundational kernel from <filename>kernel.org</filename>, the released
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Yocto Project kernel contains a mix of important new mainline
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developments, non-mainline developments (when there is no alternative),
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Board Support Package (BSP) developments,
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@ -88,37 +92,21 @@
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These additions result in a commercially released Yocto Project kernel that caters
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to specific embedded designer needs for targeted hardware.
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</para>
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<!-- <para>
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The following figure represents the overall place the Yocto Project kernel fills.
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</para>
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<para>
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<imagedata fileref="figures/kernel-big-picture.png" width="6in" depth="6in" align="center" scale="100" />
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</para>
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<para>
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In the figure the ultimate source for the Yocto Project kernel is a released kernel
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from kernel.org.
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In addition to a foundational kernel from kernel.org the commercially released
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Yocto Project kernel contains a mix of important new mainline
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developments, non-mainline developments, Board Support Package (BSP) developments,
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and custom features.
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These additions result in a commercially released Yocto Project kernel that caters
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to specific embedded designer needs for targeted hardware.
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</para> -->
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<para>
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Once a Yocto Project kernel is officially released the Yocto Project team goes into
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their next development cycle, or "uprev" cycle while continuing maintenance on the
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Once a Yocto Project kernel is officially released, the Yocto Project team goes into
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their next development cycle, or "uprev" cycle, while still continuing maintenance on the
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released kernel.
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It is important to note that the most sustainable and stable way
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to include feature development upstream is through a kernel uprev process.
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Back-porting of hundreds of individual fixes and minor features from various
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Back-porting hundreds of individual fixes and minor features from various
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kernel versions is not sustainable and can easily compromise quality.
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</para>
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<para>
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During the uprev cycle, the Yocto Project team uses an ongoing analysis of
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kernel development, BSP support, and release timing to select the best
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possible kernel.org version.
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possible <filename>kernel.org</filename> version.
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The team continually monitors community kernel
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development to look for significant features of interest.
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<!-- The illustration depicts this by showing the team looking back to kernel.org for new features,
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BSP features, and significant bug fixes. -->
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The team does consider back-porting large features if they have a significant advantage.
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User or community demand can also trigger a back-port or creation of new
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functionality in the Yocto Project baseline kernel during the uprev cycle.
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It is the Yocto Project team's policy to not back-port minor features to the released kernel.
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They only consider back-porting significant technological jumps - and, that is done
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after a complete gap analysis.
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The reason for this policy is that simply back-porting any small to medium sized change
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The reason for this policy is that back-porting any small to medium sized change
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from an evolving kernel can easily create mismatches, incompatibilities and very
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subtle errors.
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</para>
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@ -163,18 +151,23 @@
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As mentioned earlier, a key goal of Yocto Project is to present the developer with
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a kernel that has a clear and continuous history that is visible to the user.
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The architecture and mechanisms used achieve that goal in a manner similar to the
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upstream kernel.org.
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upstream <filename>kernel.org</filename>.
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</para>
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<para>
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You can think of the Yocto Project kernel as consisting of a baseline kernel with
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added features logically structured on top of the baseline.
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The features are tagged and organized by way of a branching strategy implemented by the
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source code manager (SCM) Git.
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For information on Git as applied to the Yocto Project, see the
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"<ulink url='http://www.yoctoproject.org/docs/1.1/dev-manual/dev-manual.html#git'>Git</ulink>"
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section in <ulink url='http://www.yoctoproject.org/docs/1.1/dev-manual/dev-manual.html'>The
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Yocto Project Development Manual</ulink>.
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</para>
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<para>
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The result is that the user has the ability to see the added features and
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the commits that make up those features.
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In addition to being able to see added features, the user can also view the history of what
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made up the baseline kernel as well.
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made up the baseline kernel.
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</para>
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<para>
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The following illustration shows the conceptual Yocto Project kernel.
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<imagedata fileref="figures/kernel-architecture-overview.png" width="6in" depth="7in" align="center" scale="100" />
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</para>
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<para>
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In the illustration, the "kernel.org Branch Point" marks the specific spot (or release) from
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which the Yocto Project kernel is created. From this point "up" in the tree features and
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differences are organized and tagged.
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In the illustration, the "<filename>kernel.org</filename> Branch Point"
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marks the specific spot (or release) from
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which the Yocto Project kernel is created.
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From this point "up" in the tree, features and differences are organized and tagged.
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</para>
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<para>
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The "Yocto Project Baseline Kernel" contains functionality that is common to every kernel
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type and BSP that is organized further up the tree. Placing these common features in the
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type and BSP that is organized further up the tree.
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Placing these common features in the
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tree this way means features don't have to be duplicated along individual branches of the
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structure.
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</para>
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<para>
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From the Yocto Project Baseline Kernel branch points represent specific functionality
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From the Yocto Project Baseline Kernel, branch points represent specific functionality
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for individual BSPs as well as real-time kernels.
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The illustration represents this through three BSP-specific branches and a real-time
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kernel branch.
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kernel as they apply to a given BSP.
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</para>
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<para>
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The resulting tree structure presents a clear path of markers (or branches) to the user
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that for all practical purposes is the kernel needed for any given set of requirements.
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The resulting tree structure presents a clear path of markers (or branches) to the
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developer that, for all practical purposes, is the kernel needed for any given set
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of requirements.
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</para>
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</section>
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no longer shared and thus, needs to be isolated.
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For example, board-specific incompatibilities would require different functionality
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and would require a branch to separate the features.
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Likewise, for specific kernel features the same branching strategy is used.
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Likewise, for specific kernel features, the same branching strategy is used.
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</para>
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<para>
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This branching strategy results in a tree that has features organized to be specific
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for particular functionality, single kernel types, or a subset of kernel types.
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This strategy results in not having to store the same feature twice internally in the
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tree.
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Rather we store the unique differences required to apply the feature onto the kernel type
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in question.
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</para>
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<note><para>
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This strategy also results in not having to store the same feature twice
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internally in the tree.
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Rather, the kernel team stores the unique differences required to apply the
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feature onto the kernel type in question.
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<note>
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The Yocto Project team strives to place features in the tree such that they can be
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shared by all boards and kernel types where possible.
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However, during development cycles or when large features are merged this practice
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cannot always be followed.
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In those cases isolated branches are used for feature merging.
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</para></note>
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However, during development cycles or when large features are merged,
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the team cannot always follow this practice.
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In those cases, the team uses isolated branches to merge features.
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</note>
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</para>
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<para>
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BSP-specific code additions are handled in a similar manner to kernel-specific additions.
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Some BSPs only make sense given certain kernel types.
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So, for these types, we create branches off the end of that kernel type for all
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So, for these types, the team creates branches off the end of that kernel type for all
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of the BSPs that are supported on that kernel type.
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From the perspective of the tools that create the BSP branch, the BSP is really no
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different than a feature.
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Consequently, the same branching strategy applies to BSPs as it does to features.
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So again, rather than store the BSP twice, only the unique differences for the BSP across
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the supported multiple kernels are uniquely stored.
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So again, rather than store the BSP twice, the team only stores the unique
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differences for the BSP across the supported multiple kernels.
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</para>
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<para>
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While this strategy can result in a tree with a significant number of branches, it is
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important to realize that from the user's point of view, there is a linear
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path that travels from the baseline kernel.org, through a select group of features and
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ends with their BSP-specific commits.
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important to realize that from the developer's point of view, there is a linear
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path that travels from the baseline <filename>kernel.org</filename>, through a select
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group of features and ends with their BSP-specific commits.
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In other words, the divisions of the kernel are transparent and are not relevant
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to the developer on a day-to-day basis.
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From the user's perspective, this is the "master" branch.
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They do not need not be aware of the existence of any other branches at all.
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Of course there is value in the existence of these branches
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From the developer's perspective, this path is the "master" branch.
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The developer does not need not be aware of the existence of any other branches at all.
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Of course, there is value in the existence of these branches
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in the tree, should a person decide to explore them.
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For example, a comparison between two BSPs at either the commit level or at the line-by-line
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code diff level is now a trivial operation.
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code <filename>diff</filename> level is now a trivial operation.
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</para>
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<para>
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Working with the kernel as a structured tree follows recognized community best practices.
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In particular, the kernel as shipped with the product should be
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considered an 'upstream source' and viewed as a series of
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In particular, the kernel as shipped with the product, should be
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considered an "upstream source" and viewed as a series of
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historical and documented modifications (commits).
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These modifications represent the development and stabilization done
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by the Yocto Project kernel development team.
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Because commits only change at significant release points in the product life cycle,
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developers can work on a branch created
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from the last relevant commit in the shipped Yocto Project kernel.
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As mentioned previously, the structure is transparent to the user
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As mentioned previously, the structure is transparent to the developer
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because the kernel tree is left in this state after cloning and building the kernel.
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</para>
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</section>
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<section id='source-code-manager-git'>
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<title>Source Code Manager - Git</title>
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<para>
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The Source Code Manager (SCM) is Git and it is the obvious mechanism for meeting the
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previously mentioned goals.
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Not only is it the SCM for kernel.org but Git continues to grow in popularity and
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supports many different work flows, front-ends and management techniques.
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The Source Code Manager (SCM) is Git.
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This SCM is the obvious mechanism for meeting the previously mentioned goals.
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Not only is it the SCM for <filename>kernel.org</filename> but,
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Git continues to grow in popularity and supports many different work flows,
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front-ends and management techniques.
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</para>
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<para>
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You can find documentation on Git at <ulink url='http://git-scm.com/documentation'></ulink>.
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Also, the Yocto Project Development manual has an introduction to Git and describes a
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minimal set of commands that allow you to be functional with Git.
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You can also get an introduction to Git as it applies to the Yocto Project in the
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"<ulink url='http://www.yoctoproject.org/docs/1.1/dev-manual/dev-manual.html#git'>Git</ulink>"
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section in <ulink url='http://www.yoctoproject.org/docs/1.1/dev-manual/dev-manual.html'>The
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Yocto Project Development Manual</ulink>.
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This section overviews Git and describes a minimal set of commands that allow you to be
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functional using Git.
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<note>
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You can use as much, or as little, of what Git has to offer to accomplish what
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you need for your project.
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You do not have to be a "Git Master" in order to use it with the Yocto Project.
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</note>
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</para>
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<note><para>
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It should be noted that you can use as much, or as little, of what Git has to offer
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as is appropriate to your project.
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</para></note>
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</section>
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</section>
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present a simplified view of the kernel for ease of use.
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</para>
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<para>
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The fundamental properties of the tools that manage and construct the
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Yocto Project kernel are:
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Fundamentally, the kernel tools that manage and construct the
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Yocto Project kernel accomplish the following:
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<itemizedlist>
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<listitem><para>Group patches into named, reusable features.</para></listitem>
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<listitem><para>Allow top down control of included features.</para></listitem>
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<listitem><para>Bind kernel configuration to kernel patches and features.</para></listitem>
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<listitem><para>Allow top-down control of included features.</para></listitem>
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<listitem><para>Bind kernel configurations to kernel patches and features.</para></listitem>
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<listitem><para>Present a seamless Git repository that blends Yocto Project value
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with the kernel.org history and development.</para></listitem>
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with the <filename>kernel.org</filename> history and development.</para></listitem>
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</itemizedlist>
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</para>
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<!--<para>
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WRITER NOTE: Put this in for post 1.1 if possible:
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The tools that construct a kernel tree will be discussed later in this
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document. The following tools form the foundation of the Yocto Project
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kernel toolkit:
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|
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@ -20,17 +20,20 @@
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on its history, organization, benefits, and use.
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The manual consists of two sections:
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<itemizedlist>
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<listitem><para>Concepts - Describes concepts behind the kernel.
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<listitem><para><emphasis>Concepts:</emphasis> Describes concepts behind the kernel.
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You will understand how the kernel is organized and why it is organized in
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the way it is. You will understand the benefits of the kernel's organization
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and the mechanisms used to work with the kernel and how to apply it in your
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design process.</para></listitem>
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<listitem><para>Using the Kernel - Describes best practices and "how-to" information
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that lets you put the kernel to practical use. Some examples are "How to Build a
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Project Specific Tree", "How to Examine Changes in a Branch", and "Saving Kernel
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Modifications."</para></listitem>
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<listitem><para><emphasis>Using the Kernel:</emphasis> Describes best practices
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and "how-to" information
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that lets you put the kernel to practical use.
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Some examples are "How to Build a
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Project Specific Tree", "How to Examine Changes in a Branch", and "How to
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Save Kernel Modifications."</para></listitem>
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</itemizedlist>
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</para>
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<para>
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For more information on the kernel, see the following links:
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<itemizedlist>
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<listitem><para><ulink url='http://userweb.kernel.org/~akpm/stuff/tpp.txt'></ulink></para></listitem>
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<listitem><para><ulink url='http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob_plain;f=Documentation/HOWTO;hb=HEAD'></ulink></para></listitem>
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</itemizedlist>
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<para>
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You can find more information on Yocto Project by visiting the website at
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<ulink url='http://www.yoctoproject.org'></ulink>.
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</para>
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<para>
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For more discussion on the Yocto Project kernel, you can also see the
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"<ulink url='http://www.yoctoproject.org/docs/1.1/dev-manual/dev-manual.html#kernel-overview'>Kernel Overview</ulink>",
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"<ulink url='http://www.yoctoproject.org/docs/1.1/dev-manual/dev-manual.html#kernel-modification-workflow'>Kernel Modification Workflow</ulink>", and
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"<ulink url='http://www.yoctoproject.org/docs/1.1/dev-manual/dev-manual.html#dev-manual-kernel-appendix'>Kernel Modification Example</ulink>" sections all in
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<ulink url='http://www.yoctoproject.org/docs/1.1/dev-manual/dev-manual.html'>The Yocto Project Development Manual</ulink>.
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</para>
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<para>
|
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For general information on the Yocto Project, visit the website at
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<ulink url='http://www.yoctoproject.org'></ulink>.
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</para>
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</section>
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Reference in New Issue