src/devices/porting.jd

page.title=Porting
@jd:body

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    <h2>In this document</h2>
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  <p>Android provides you with the freedom to implement your own device specifications
  and the drivers to support them. The hardware abstraction layer (HAL) gives you a
  standard way to create software hooks in between the Android
  platform stack and your hardware. In addition, the Android operating system 
  is open-sourced to help you through your device's bringup.</p>

  <p>To ensure that your devices maintain a high level of quality and offers a consistent
  experience for your users, they must must also
  pass the tests in the compatibility test suite (CTS). CTS ensures that anyone
  building a device meets a quality standard that ensures apps run reliabaly well
  and gives users a good experience. For more information, see the 
  <a href="{@docRoot}compatibility/index.html">Compatibility</a> section.</p>

 <h2>Android Low-Level System Architecture</h2>

<p>Before you begin porting Android to your hardware, it is important to have an
understanding of how Android works at a high level. Because your drivers and HAL code interact
with many layers of Android code, this understanding can help you find
your way through the many layers of code that are available to you through the AOSP
(Android Open Source Project) source tree. The following diagram shows a system
level view of how Android works:
</p>

<img src="images/system-architecture.png">

<p class="img-caption"><strong>Figure 1.</strong> Android System Architecture</p>

  <h4>Application framework</h4>
  <p>This is the level that most application developers concern themselves with. You should be
    aware of the APIs available to developers as many of them map 1:1 to the underlying HAL
    interfaces and can provide information as to how to implement your driver.
  </p>

  <h4>Binder IPC</h4>
  <p>
  The Binder Inter-Process Communication mechanism allows the application framework to
  cross process boundaries and call into the Android system services code. This basically allows
  high level framework APIs to interact with Android's system services. At the application framework level, all
  of this communication is hidden from the developer and things appear to "just work." 
  </p>

  <h4>System services</h4>
  <p>Most of the functionality exposed through the application framework APIs  must
    communicate with some sort of system service to access the underlying hardware. Services
    are divided into modular components with focused functionality 
    such as the Window Manager, Search Service, or Notification Manager. System services are grouped
    into two buckets: system and media. The system services include things such as the Window or
    Notification Manager. The media services include all the services involved in playing and
    recording media.
  </p>
  
<h4>Hardware abstraction layer (HAL)</h4>
<p>The HAL serves as a standard interface that allows the Android system to call into the device
  driver layer while being agnostic about the lower-level implementations of your drivers and hardware.
  You must implement the corresponding HAL (and driver) for the particular piece of hardware that your product
  provides. Android does not mandate a standard interaction between your HAL implementation and your device drivers, so
  you have free reign to do what is best for your situation. However, you must abide by the contract
  defined in each hardware-specific HAL interface for the Android system to be able
  to correctly interact with your hardware. HAL implementations are typically built into
  shared library modules (<code>.so</code> files).
</p>
<h4>Linux Kernel</h4>
<p>For the most part, developing your device drivers is the same as developing a typical Linux device driver.
  Android uses a specialized version of the Linux kernel with a few special additions such as
  wakelocks, a memory management system that is more agressive in preserving memory,
  the Binder IPC driver, and other features that are important for a mobile embedded platform like Android.
  These additions have less to do with driver development than with the system's functionality. The PDK
  does not provide kernel sources, so you must provide your own. You can use any version of the kernel that
  you want as long as it supports the required features, such as the binder driver. However, we recommend
  using the latest version of the Android kernel. For the latest Android kernel, see
  <a href="{@docRoot}source/building-kernels.html" >Building Kernels</a>.
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