en/devices/architecture/hal.html - platform/docs/source.android.com - Git at Google

 <html devsite>
   <head>
     <title>Hardware Abstraction Layer (HAL)</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 HAL defines a standard interface for hardware vendors to implement,
 which enables Android to be agnostic about lower-level driver implementations.
 Using a HAL allows you to implement functionality without affecting or modifying
 the higher level system. HAL implementations are packaged into modules and
 loaded by the Android system at the appropriate time.</p>

 <img src="../images/ape_fwk_hal.png">

 <p class="img-caption"><strong>Figure 1.</strong> HAL components</p>

 <p>You must implement the corresponding HAL (and driver) for the specific
 hardware your product provides. HAL implementations are typically built into
 shared library modules (<code>.so</code> files), but as Android does not mandate
 a standard interaction between a HAL implementation and device drivers, you can
 do what is best for your situation. However, to enable the Android system to
 correctly interact with your hardware, you <strong>must</strong> abide by the
 contract defined in each hardware-specific HAL interface.</p>

 <p>To guarantee that HALs have a predictable structure, each hardware-specific
 HAL interface has properties defined in
 <code>hardware/libhardware/include/hardware/hardware.h</code>. This interface
 allows the Android system to load correct versions of your HAL modules in a
 consistent way. A HAL interface consists of two components: modules and devices.
 </p>

 <h2 id="hal-module">HAL modules</h2>
 <p>A module represents your packaged HAL implementation, which is stored as a
 shared library (<code>.so file</code>). 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 metadata such as the version, name, and author of the module. Android
 uses this metadata to find and load the HAL module correctly.</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 initiate
 communication with the hardware for which the HAL is serving as an abstraction.
 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 class="devsite-click-to-copy">
 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>. Example from the Nexus 9 audio HAL:</p>

 <pre class="devsite-click-to-copy">
 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 = "NVIDIA Tegra Audio HAL",
         .author = "The Android Open Source Project",
         .methods = &hal_module_methods,
     },
 };
 </pre>

 <h2 id="hal-device">HAL devices</h2>
 <p>A device abstracts the 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.</p>

 <p>A device is represented by the <code>hw_device_t</code> struct. Similar to a
 module, each type of device defines a 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 class="devsite-click-to-copy">
 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. For details,
 see the <a href="/reference/hal/">HAL reference documentation</a> as well as
 the individual instructions for each HAL.</p>

 <h2 id="hal-building">Building 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 specific format so they can be found and loaded properly. The naming
 scheme varies slightly from module to module, but follows the general pattern
 of: <code>&lt;module_type&gt;.&lt;device_name&gt;</code>.
 </p>

 <p>For details on setting up the build for each HAL, see the HAL-specific
 documentation through the Porting section of this website.</p>

   </body>
 </html>
	<html devsite>
	<head>
	<title>Hardware Abstraction Layer (HAL)</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 HAL defines a standard interface for hardware vendors to implement,
	which enables Android to be agnostic about lower-level driver implementations.
	Using a HAL allows you to implement functionality without affecting or modifying
	the higher level system. HAL implementations are packaged into modules and
	loaded by the Android system at the appropriate time.</p>

	<img src="../images/ape_fwk_hal.png">

	<p class="img-caption"><strong>Figure 1.</strong> HAL components</p>

	<p>You must implement the corresponding HAL (and driver) for the specific
	hardware your product provides. HAL implementations are typically built into
	shared library modules (<code>.so</code> files), but as Android does not mandate
	a standard interaction between a HAL implementation and device drivers, you can
	do what is best for your situation. However, to enable the Android system to
	correctly interact with your hardware, you <strong>must</strong> abide by the
	contract defined in each hardware-specific HAL interface.</p>

	<p>To guarantee that HALs have a predictable structure, each hardware-specific
	HAL interface has properties defined in
	<code>hardware/libhardware/include/hardware/hardware.h</code>. This interface
	allows the Android system to load correct versions of your HAL modules in a
	consistent way. A HAL interface consists of two components: modules and devices.
	</p>

	<h2 id="hal-module">HAL modules</h2>
	<p>A module represents your packaged HAL implementation, which is stored as a
	shared library (<code>.so file</code>). 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 metadata such as the version, name, and author of the module. Android
	uses this metadata to find and load the HAL module correctly.</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 initiate
	communication with the hardware for which the HAL is serving as an abstraction.
	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 class="devsite-click-to-copy">
	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>. Example from the Nexus 9 audio HAL:</p>

	<pre class="devsite-click-to-copy">
	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 = "NVIDIA Tegra Audio HAL",
	.author = "The Android Open Source Project",
	.methods = &hal_module_methods,
	},
	};
	</pre>

	<h2 id="hal-device">HAL devices</h2>
	<p>A device abstracts the 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.</p>

	<p>A device is represented by the <code>hw_device_t</code> struct. Similar to a
	module, each type of device defines a 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 class="devsite-click-to-copy">
	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. For details,
	see the <a href="/reference/hal/">HAL reference documentation</a> as well as
	the individual instructions for each HAL.</p>

	<h2 id="hal-building">Building 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 specific format so they can be found and loaded properly. The naming
	scheme varies slightly from module to module, but follows the general pattern
	of: <code><module_type>.<device_name></code>.
	</p>

	<p>For details on setting up the build for each HAL, see the HAL-specific
	documentation through the Porting section of this website.</p>

	</body>
	</html>