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<html devsite>
<head>
<title>Android Common Kernels</title>
<meta name="project_path" value="/_project.yaml" />
<meta name="book_path" value="/_book.yaml" />
</head>
<body>
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<p>The
<a href="https://android.googlesource.com/kernel/common/" class="external">AOSP
common kernels</a> are downstream of Long Term Supported (LTS) kernels and
include patches of interest to the Android community that have not been merged
into LTS. These patches can include:</p>
<ul>
<li>Features tailored for Android needs (e.g. interactive <code>cpufreq</code>
governor).</li>
<li>Features rejected by upstream due to implementation concerns (e.g. MTP/PTP,
paranoid networking).</li>
<li>Features ready for Android devices but still under development upstream
(e.g. Energy Aware Scheduling/EAS).</li>
<li>Vendor/OEM features that are useful for others (e.g. <code>sdcardfs</code>).
</li>
</ul>
<h2 id="list-of-kernels">List of common kernels</h2>
<p>To view a list of Android common kernels, refer to
<a href="https://android.googlesource.com/kernel/common/" class=external>https://android.googlesource.com/kernel/common/</a>
(shown below).</p>
<p><img src="../images/android-diffs.png"></p>
<p class="img-caption"><strong>Figure 1.</strong> List of Android common
kernels</p>
<h3 id="differences-lts">Differences from LTS</h3>
<p>When compared to LTS (4.14.0), the Android common kernel has 355 changes,
32266 insertions, and 1546 deletions (as of February 2018).</p>
<p><img src="../images/kernel_lts_diff.png"></p>
<p class="img-caption"><strong>Figure 2.</strong> Android-specific code over
time</p>
<p>The largest features include:</p>
<ul>
<li>19.8% Energy Aware Scheduling (kernel/sched)</li>
<li>13.8% Networking (net/netfilter)</li>
<li>13.5% Sdcardfs (fs/sdcardfs)</li>
<li>9.4% USB (drivers/usb)</li>
<li>7.2% SoC (arch/arm64, arch/x86)</li>
<li>6.2% f2fs (fs/f2fs -- backports from upstream)</li>
<li>6.1% Input (drivers/input/misc)</li>
<li>5.4% FIQ Debugger (drivers/staging/android/fiq_debugger)</li>
<li>3.6% Goldfish Emulator (drivers/platform/goldfish)</li>
<li>3.4% Verity (drivers/md)</li>
<li>11.6% Other</li>
</ul>
<h2 id="requirements">Requirements</h2>
<p>All AOSP common kernels must provide the following:</p>
<ul>
<li>Method for downstream partners to get timely updates that include all
LTS patches.</li>
<li>Mechanism to guarantee that new feature development does not interfere with
merging from AOSP common (even for previous Android releases).</li>
<li>Method for downstream partners to easily identify security patches that are
part of an <a href="/security/bulletin/">Android Security Bulletin (ASB)</a>.
This satisfies carriers who require a full requalification if OEMs attempt to
include patches beyond those listed in the bulletin.</li>
</ul>
<p>In addition, regular testing must be performed on AOSP common kernels and
branches must be tagged when passing.</p>
<h3 id="lts-merges">LTS merges</h3>
<p>To ensure downstream partners can get timely updates that include all LTS
patches, android-<var>X</var>.<var>Y</var> gets regular merges from LTS and is
validated via automated VTS, CTS, and build/boot tests.</p>
<h3 id="android-dessert-branches">Android-dessert branches</h3>
<p>To guarantee that new feature development does not interfere with merging
from the AOSP common kernel (even for previous Android releases),
android-<var>X</var>.<var>Y</var>-<var>androidRel</var> is cloned from
android-<var>X</var>.<var>Y</var> prior to the initial dessert release, gets regular
merges from LTS, and is tested against the associated Android release. For
example, the android-4.4-n branch gets merges from the LTS 4.4.y branch. </p>
<h3 id="android-release-branches">Android-release branches</h3>
<p>To ensure downstream partners can easily identify security patches that are
part of an ASB,
android-<var>X</var>.<var>Y</var>-<var>androidRel</var>-<var>type</var> is
cloned from android-<var>X</var>.<var>Y</var>-<var>androidRel</var> at the time
of the Android release and gets only the patches listed in the bulletin.</p>
<p>After the patches associated with a bulletin are confirmed to be merged
into a release branch, the branch is tagged with the ASB level. For example, the
tag <strong>ASB-2017-10-05</strong> indicates the release branch contains
patches from the Android Security Bulletin for October 5th, 2017. Parent
branches contain those security patches, so if the android-4.4-o-release branch
is tagged with <strong>ASB-2017-10-01</strong>, android-4.4-o and android-4.4
are also up-to-date with that bulletin. Example:</p>
<ul>
<li>Before releasing Android N MR1, <strong>android-4.4-n-mr1</strong> is cloned
from <strong>android-4.4-n</strong>.</li>
<li>Only patches listed in ASBs are merged, allowing OEMs (who have strict
requirements from carriers to avoid full qualification on security updates) to
find the patches listed in the bulletin.</li>
<li><strong>android-4.4-n-mr2</strong> will be
<strong>android-4.4-n-mr1</strong> plus LTS patches that were merged between the
releases.</li>
<li>Each month when the ASB is released publicly, the release branches are
updated with any patches cited in the bulletin that are upstream
(device-specific patches cited in the bulletin are not applied to the common
kernels).</li>
</ul>
<h3 id="regular-testing">Regular testing</h3>
<p>Regular testing is performed on all on AOSP common kernels and test results
are available to the public. Specifically:</p>
<ul>
<li>After LTS updates or other patches are merged, VTS and a subset of CTS
is run and results are made available at
<a href="https://qa-reports.linaro.org/lkft" class="external">https://qa-reports.linaro.org/lkft</a>.
</li>
<li>To continually test for build/boot breaks in a variety of architectures and
builds, <code>kernelci</code> is run and results are made available at
<a href="https://kernelci.org/job/android/" class="external">https://kernelci.org/job/android</a>.
</li>
</ul>
<h3 id="branch-hierarchy">Branch hierarchy (android-4.4)</h3>
<p>The branch hierarchy for the android-4.4 kernel uses the following structure:
</p>
<p><img src="../images/kernel_branch_hierarchy_44.png"></p>
<p class="img-caption"><strong>Figure 3.</strong> Branch hierarchy for the
android-4.4 kernel.</p>
<h2 id="guidelines">Guidelines</h2>
<p>Android implementations should use the following kernel guidelines:</p>
<ul>
<li>Use the new AOSP common kernels as upstream merge sources.<ul>
<li>To get patches from LTS, merge from android-<var>X</var>.<var>Y</var>.<ul>
<li>Merge regularly during development phase.</li>
<li>When updating device to a new Android release, merge either from the
android-<var>X</var>.<var>Y</var> branch or the release branch for the target
release (e.g. for an update to Nougat MR2, merge from the android-4.4-n-mr2
branch).</li>
</ul>
<li>When constrained by the carrier for a security release, merge from release
branches for security updates.</li>
</ul>
<li>Send fixes upstream to mainline, LTS, or AOSP common.</li>
</ul>
</body>
</html>
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