src/devices/hal.jd

page.title=The Hardware Abstraction Layer
@jd:body

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    <h2>In this document</h2>
    <ol id="auto-toc">
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<p>
  The hardware abstraction layer (HAL) defines a standard interface for hardware vendors to
  implement and allows Android to be agnostic about lower-level driver
  implementations. The HAL allows you to implement functionality
  without affecting or modifying the higher level system. HAL implementations
  are packaged into modules (<code>.so</code>) file and loaded by the Android
  system at the appropriate time.
<h2 id="structure">
  Standard HAL structure
</h2>
<p>
  Each hardware-specific HAL interface has properties that are common to all HAL interfaces. These
  properties are defined in <code>hardware/libhardware/include/hardware/hardware.h</code> and
  guarantees that HALs have a predictable structure.
  This interface allows the Android system to load the correct versions of your
  HAL modules in a consistent way. There are two general components
  that a HAL interface consists of: a module and a device.
</p>
<p>
  A module represents your packaged HAL implementation, which is stored as a shared library (<code>.so file</code>). It contains
  metadata such as the version, name, and author of the module, which helps Android find and load it correctly. The
  <code>hardware/libhardware/include/hardware/hardware.h</code> header file defines a
  struct, <code>hw_module_t</code>, that represents a module and contains information such as
  the module version, author, and name.</p>

  <p>In addition, the <code>hw_module_t</code> struct contains
  a pointer to another struct, <code>hw_module_methods_t</code>, that contains a pointer to
  an "open" function for the module. This open function is used to initate communication with
  the hardware that the HAL is serving as an abstraction for. Each hardware-specific HAL usually 
  extends the generic <code>hw_module_t</code> struct with additional information
  for that specific piece of hardware. For example in the camera HAL, the <code>camera_module_t</code> struct
  contains a <code>hw_module_t</code> struct along with other camera-specific function pointers:
</p>

<pre>
typedef struct camera_module {
    hw_module_t common;
    int (*get_number_of_cameras)(void);
    int (*get_camera_info)(int camera_id, struct camera_info *info);
} camera_module_t;
</pre>

<p>When you implement a HAL and create the module struct, you must name it
  <code>HAL_MODULE_INFO_SYM</code>. For instance, here is an example from the Galaxy Nexus audio HAL:</p>
<pre>
struct audio_module HAL_MODULE_INFO_SYM = {
    .common = {
        .tag = HARDWARE_MODULE_TAG,
        .module_api_version = AUDIO_MODULE_API_VERSION_0_1,
        .hal_api_version = HARDWARE_HAL_API_VERSION,
        .id = AUDIO_HARDWARE_MODULE_ID,
        .name = "Tuna audio HW HAL",
        .author = "The Android Open Source Project",
        .methods = &hal_module_methods,
    },
};
</pre>
<p>
  A device abstracts the actual hardware of your product. For example, an audio module can contain
  a primary audio device, a USB audio device, or a Bluetooth A2DP audio device. A device
  is represented by the <code>hw_device_t</code> struct. Like a module, each type of device
  defines a more-detailed version of the generic <code>hw_device_t</code> that contains
  function pointers for specific features of the hardware. For example, the
  <code>audio_hw_device_t</code> struct type contains function pointers to audio device operations:
</p>

<pre>
struct audio_hw_device {
    struct hw_device_t common;

    /**
     * used by audio flinger to enumerate what devices are supported by
     * each audio_hw_device implementation.
     *
     * Return value is a bitmask of 1 or more values of audio_devices_t
     */
    uint32_t (*get_supported_devices)(const struct audio_hw_device *dev);
  ...
};
typedef struct audio_hw_device audio_hw_device_t;
</pre>

<p>
  In addition to these standard properties, each hardware-specific HAL interface can define more of its
  own features and requirements. See the <a href="{@docRoot}guide/reference/files.html">HAL reference documentation</a>
  as well as the individual instructions for each HAL for more information on how to implement a specific interface.
</p>

<h2 id="modules">HAL modules</h2>
<p>HAL implementations are built into modules (<code>.so</code>) files and are dynamically linked by Android when appropriate.
  You can build your modules by creating <code>Android.mk</code> files for each of your HAL implementations
  and pointing to your source files. In general, your shared libraries must be named in a certain format, so that
  they can be found and loaded properly. The naming scheme varies slightly from module to module, but they follow
  the general pattern of: <code>&lt;module_type&gt;.&lt;device_name&gt;</code>.</p>

  <p>For more information about setting up the build for each HAL, see its respective documentation.</p>

</p>