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      1 page.title=Porting Android to Devices
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     19 <div id="qv-wrapper">
     20   <div id="qv">
     21     <h2>In this document</h2>
     22     <ol id="auto-toc">
     23     </ol>
     24   </div>
     25 </div>
     26 
     27   <p>Android provides you with the freedom to implement your own device specifications
     28   and the drivers to support them. The hardware abstraction layer (HAL) gives you a
     29   standard way to create software hooks in between the Android
     30   platform stack and your hardware. In addition, the Android operating system
     31   is open-sourced to help you through your device's bringup.</p>
     32 
     33   <p>To ensure that your devices maintain a high level of quality and offers a consistent
     34   experience for your users, they must must also
     35   pass the tests in the compatibility test suite (CTS). CTS ensures that anyone
     36   building a device meets a quality standard that ensures apps run reliabaly well
     37   and gives users a good experience. For more information, see the
     38   <a href="{@docRoot}compatibility/index.html">Compatibility</a> section.</p>
     39 
     40  <h2>Android Low-Level System Architecture</h2>
     41 
     42 <p>Before you begin porting Android to your hardware, it is important to have an
     43 understanding of how Android works at a high level. Because your drivers and HAL code interact
     44 with many layers of Android code, this understanding can help you find
     45 your way through the many layers of code that are available to you through the AOSP
     46 (Android Open Source Project) source tree. The following diagram shows a system
     47 level view of how Android works:
     48 </p>
     49 
     50 <img src="images/system-architecture.png">
     51 
     52 <p class="img-caption"><strong>Figure 1.</strong> Android System Architecture</p>
     53 
     54   <h4>Application framework</h4>
     55   <p>This is the level that most application developers concern themselves with. You should be
     56     aware of the APIs available to developers as many of them map 1:1 to the underlying HAL
     57     interfaces and can provide information as to how to implement your driver.
     58   </p>
     59 
     60   <h4>Binder IPC</h4>
     61   <p>
     62   The Binder Inter-Process Communication mechanism allows the application framework to
     63   cross process boundaries and call into the Android system services code. This basically allows
     64   high level framework APIs to interact with Android's system services. At the application framework level, all
     65   of this communication is hidden from the developer and things appear to "just work."
     66   </p>
     67 
     68   <h4>System services</h4>
     69   <p>Most of the functionality exposed through the application framework APIs  must
     70     communicate with some sort of system service to access the underlying hardware. Services
     71     are divided into modular components with focused functionality
     72     such as the Window Manager, Search Service, or Notification Manager. System services are grouped
     73     into two buckets: system and media. The system services include things such as the Window or
     74     Notification Manager. The media services include all the services involved in playing and
     75     recording media.
     76   </p>
     77 
     78 <h4>Hardware abstraction layer (HAL)</h4>
     79 <p>The HAL serves as a standard interface that allows the Android system to call into the device
     80   driver layer while being agnostic about the lower-level implementations of your drivers and hardware.
     81   You must implement the corresponding HAL (and driver) for the particular piece of hardware that your product
     82   provides. Android does not mandate a standard interaction between your HAL implementation and your device drivers, so
     83   you have free reign to do what is best for your situation. However, you must abide by the contract
     84   defined in each hardware-specific HAL interface for the Android system to be able
     85   to correctly interact with your hardware. HAL implementations are typically built into
     86   shared library modules (<code>.so</code> files).
     87 </p>
     88 <h4>Linux Kernel</h4>
     89 <p>For the most part, developing your device drivers is the same as developing a typical Linux device driver.
     90   Android uses a specialized version of the Linux kernel with a few special additions such as
     91   wakelocks, a memory management system that is more agressive in preserving memory,
     92   the Binder IPC driver, and other features that are important for a mobile embedded platform like Android.
     93   These additions have less to do with driver development than with the system's functionality. 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
     94   using the latest version of the Android kernel. For the latest Android kernel, see
     95   <a href="{@docRoot}source/building-kernels.html" >Building Kernels</a>.
     96 </p>