Audio Subsystem

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本文介绍了Android音频系统的两个核心组件：AudioFlinger和AudioHardwareInterface，并详细阐述了如何通过继承和修改AudioHardwareInterface来实现对不同音频堆栈的支持。

ntroduction

Android provides two native layers that handle audio software:

Audio Flinger: the audio software implementation that provides the minimum required audio functions (as illustrated in the diagram below).
AudioHardwareInterface: the hardware abstraction layer that hides driver-specific audio implementations from the Android platform.

Solid elements represent Android blocks and dashed elements represent partner-specific proprietary blocks.

Porting Android to other Audio Stacks

Porting Android to other audio stacks (OSS, ALSA, proprietary user-space audio libraries, etc.) requires inheriting from and modifying AudioHardwareInterface to support the driver-specific implementation.

AudioHardwareInterface Abstract Class

AudioHardwareInterface (//device/servers/audio/flinger) contains several pure virtual functions that the audio driver class being ported needs to implement.

Modifying AudioHardwareInterface

Once the audio driver that inherits AudioHardwareInterface is ready, modify the static function AudioHardwareInterface::create() in order to link/load the driver in Android.

Assume the manufacturer audio driver inherits from AudioHarddwareInterface and that it is compiled into a native shared library (libaudio.so). In this case, use dlopen to load the library.

You can find an example of a similar implementation in//device/libs/media/mediaplayer.cpp. (Note that the example below uses libpv.so because this is a real code snippet from mediaplayer.cpp. If your native shared library is calledlibaudio.so, replace libpv.so with your libaudio.so.)

// load PV library and create PV player
mLibHandle = dlopen("libpv.so", RTLD_NOW);
if (!mLibHandle) {
   LOGE("dlopen failed on libpv.so/n");
   return UNKNOWN_ERROR;
}
createPlayer_f createPlayer = reinterpret_cast(dlsym(mLibHandle, "createPlayer"));
if (!createPlayer) {
   LOGE("dlsym failed on createPlayer in libpv.so/n");
   return UNKNOWN_ERROR;
}

Load the libraries with a call from AudioHardwareInterface::create(), as illustrated below (full code found in //device/servers/audioflinger).

if (property_get("ro.kernel.qemu", value, 0)) {
   LOGD("Running in emulation - using generic audio driver");
   hw = new AudioHardwareGeneric();
}
else {
   // Insert calling of dynamic loading of driver here...
}
if (hw->initCheck() != NO_ERROR) {
   LOGW("Using stubbed audio hardware. No sound will be produced.");
   delete hw;
   hw = new AudioHardwareStub();
}