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<html devsite="">
<head>
<title>Dynamically Available HALs</title>
<meta name="project_path" value="/_project.yaml">
<meta name="book_path" value="/_book.yaml">
</head>
{% include "_versions.html" %}
<body>
<!--
Copyright 2018 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.
-->
Android {{ androidPVersionNumber }} supports the dynamic shutdown of Android hardware
subsystems when they are not in use or not needed. For example, when a user
is not using Wi-Fi, the Wi-Fi subsystems should not be taking up memory,
power, or other system resources. In earlier versions of Android, HALs/drivers
were kept open on Android devices for the entire duration an Android
phone was booted.
<p>Implementing dynamic shutdown involves wiring up data flows and executing
dynamic processes as detailed in the following sections.</p>
<h2 id="changes-HAL-definitions">Changes to HAL definitions</h2>
<p>Dynamic shutdown requires information on which processes serve what HAL
interfaces (this information may also be useful later in other contexts) as
well as not starting processes on boot and not restarting them (until
requested again) when they exit.</p>
<pre class="prettyprint"># some init.rc script associated with the HAL
service vendor.some-service-name /vendor/bin/hw/some-binary-service
# init language extension, provides information of what service is served
# if multiple interfaces are served, they can be specified one on each line
interface android.hardware.light@2.0::ILight default
# restarted if hwservicemanager dies
# would also cause the hal to start early during boot if oneshot wasn't set
class hal
# will not be restarted if it exits until it is requested to be restarted
oneshot
# will only be started when requested
disabled
# ... other properties</pre>
<h2 id="changes-init-and-hwservicemanager">Changes to init and hwservicemanager</h2>
<p>Dynamic shutdown also requires the <code>hwservicemanager</code> to tell
<code>init</code> to start requested services. In Android {{ androidPVersionNumber }},
<code>init</code> includes three additional control messages (e.g.
<code>ctl.start</code>): <code>ctl.interface_start</code>,
<code>ctl.interface_stop</code>, and <code>ctl.interface_restart</code>.
These messages can be used to signal <code>init</code> to bring up and down
specific hardware interfaces. When a service is requested and it is not
registered, <code>hwservicemanager</code> will request the service to be
started.</p>
<h2 id="determining-HAL-exit">Determining HAL exit</h2>
<p>Dynamic shutdown requires multiple policies for deciding when to start a
HAL and when to shutdown a HAL. If a HAL decides to exit for any reason, it
will automatically be restarted when it is needed again using the information
provided in the HAL definition and the infrastructure provided by changes to
<code>init</code> and <code>hwservicemanager</code>. This could involve a
couple of different strategies, including:</p>
<ul>
<li>A HAL could choose to call exit on itself if someone calls a close or
similar API on it. This behavior must be specified in the corresponding HAL
interface.</li>
<li>HALs can shut down when their task is completed (documented in the HAL
file).</li>
</ul>
</body>
</html>
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