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+<html devsite>
+  <head>
+    <title>Device Tree Overlays</title>
+    <meta name="project_path" value="/_project.yaml" />
+    <meta name="book_path" value="/_book.yaml" />
+  </head>
+  <body>
+  <!--
+      Copyright 2017 The Android Open Source Project
+
+      Licensed under the Apache License, Version 2.0 (the "License");
+      you may not use this file except in compliance with the License.
+      You may obtain a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS,
+      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+      See the License for the specific language governing permissions and
+      limitations under the License.
+  -->
+
+
+<p>A device tree (DT) is a data structure of named nodes and properties that
+describe non-discoverable hardware. Operating systems, such as the Linux kernel
+used in Android, use DTs to support a wide range of hardware configurations used
+by Android-powered devices. Hardware vendors supply their own DT source files,
+which Linux then compiles into the Device Tree Blob (DTB) file used by the
+bootloader.</p>
+
+A <a href="https://lkml.org/lkml/2012/11/5/615" class="external">device tree
+overlay</a> (DTO) enables a central DTB to be overlaid on the device tree. A
+bootloader using DTO can maintain the system-on-chip (SoC) DT and dynamically
+overlay a device-specific DT, adding nodes to the tree and making changes to
+properties in the existing tree.</p>
+
+<p>This page details a typical bootloader workflow for loading a DT and provides
+a list of common DT terms. Other pages in this section describe how to
+<a href="/devices/architecture/dto/implement.html">implement bootloader support
+for DTO</a>, how to
+<a href="/devices/architecture/dto/compile.html">compile</a>, verify, and
+<a href="/devices/architecture/dto/optimize.html">optimize your DTO
+implementation</a>, and how to
+<a href="/devices/architecture/dto/multiple.html">use multiple DTs</a>. You can
+also get details on <a href="/devices/architecture/dto/syntax.html">DTO
+syntax</a> and recommended
+<a href="/devices/architecture/dto/partition.html">DTO/DTBO partition
+formatting</a>.</p>
+
+<h2 id=load-dt>Loading a device tree</h2>
+<p>Loading a device tree in bootloader involves building, partitioning, and
+running.</p>
+
+<figure id="treble_dto_bootloader">
+<img src="../images/treble_dto_bootloader.png">
+<figcaption><strong>Figure 1.</strong> Typical implementation for loading device
+tree in bootloader.</figcaption>
+</figure>
+
+<ol>
+<li>To build:
+<ul>
+<li>Use the device tree compiler (<code>dtc</code>) to compile device tree
+source (<code>.dts</code>) into a device tree blob (<code>.dtb</code>),
+formatted as a flattened device tree.</li>
+<li>Flash the <code>.dtb</code> file into a bootloader runtime-accessible
+location (detailed below).</li>
+</ul>
+</li>
+<li>To partition, determine a bootloader runtime-accessible and trusted location
+in flash memory to put <code>.dtb</code>. Example locations:
+
+<div style="width:75%">
+<div class="attempt-left">
+<table><tr><th style="text-align: center;">Boot Partition</th></tr></table>
+<figure id="treble_dto_partition_1">
+<img src="../images/treble_dto_partition_1.png" style="display:block; margin:0 auto">
+<figcaption>
+<strong>Figure 2.</strong> Put <code>.dtb</code> in boot partition by appending
+to <code>image.gz</code> and passing as "<code>kernel</code>" to
+<code>mkbootimg</code>.
+</figcaption>
+</figure>
+</div>
+<div class="attempt-right">
+<table><tr><th style="text-align: center;">Unique Partition</th></tr></table>
+<figure id="treble_dto_partition_2">
+<img src="../images/treble_dto_partition_2.png" style="display:block; margin:0 auto">
+<figcaption>
+<strong>Figure 3.</strong> Put <code>.dtb</code> in an unique partition (e.g.
+<code>dtb</code> partition).
+</figcaption>
+</figure>
+</div>
+</div>
+</li>
+
+<li style="clear:both">To run:
+<ul>
+<li>Load <code>.dtb</code> from storage into memory.</li>
+<li>Start kernel given the memory address of the loaded DT.</li>
+</ul>
+</li>
+</ol>
+
+<h2 id=terms>Terminology</h2>
+<p>This section uses the following device tree terms:</p>
+<table>
+<tbody>
+<tr>
+<th>DT</th>
+<td>Device Tree</td>
+</tr>
+<tr>
+<th>DTB</th>
+<td>Device Tree Blob</td>
+</tr>
+<tr>
+<th>DTBO</th>
+<td>Device Tree Blob for Overlay</td>
+</tr>
+<tr>
+<th>DTC</th>
+<td>Device Tree Compiler</td>
+</tr>
+<tr>
+<th>DTO</th>
+<td>Device Tree Overlay</td>
+</tr>
+<tr>
+<th>DTS</th>
+<td>Device Tree Source</td>
+</tr>
+<tr>
+<th>FDT</th>
+<td>Flattened Device Tree, a binary format contained in a <code>.dtb</code> blob
+file</td>
+</tr>
+</tbody>
+</table>
+
+  </body>
+</html>