ANDROID音频系统散记之四：4.0音频系统HAL初探
昨天（2011-11-15）发布了Android4.0的源码，今天download下来，开始挺进4.0时代。简单看了一下，发现音频系统方面与2.3的有较多地方不同，下面逐一描述。
一、代码模块位置
1、AudioFlinger
- frameworks/base/services/audioflinger/
- +-- Android.mk
- +-- AudioBufferProvider.h
- +-- AudioFlinger.cpp
- +-- AudioFlinger.h
- +-- AudioMixer.cpp
- +-- AudioMixer.h
- +-- AudioPolicyService.cpp
- +-- AudioPolicyService.h
- +-- AudioResampler.cpp
- +-- AudioResamplerCubic.cpp
- +-- AudioResamplerCubic.h
- +-- AudioResampler.h
- +-- AudioResamplerSinc.cpp
- +-- AudioResamplerSinc.h
两种Audio Hardware HAL接口定义：
1/ legacy：hardware/libhardware_legacy/include/hardware_legacy /AudioHardwareInterface.h
2/ 非legacy：hardware/libhardware/include/hardware/audio.h
2、audio_hw
- hardware/libhardware_legacy/audio/
- +-- A2dpAudioInterface.cpp
- +-- A2dpAudioInterface.h
- +-- Android.mk
- +-- AudioDumpInterface.cpp
- +-- AudioDumpInterface.h
- +-- AudioHardwareGeneric.cpp
- +-- AudioHardwareGeneric.h
- +-- AudioHardwareInterface.cpp
- +-- AudioHardwareStub.cpp
- +-- AudioHardwareStub.h
- +-- audio_hw_hal.cpp
- +-- AudioPolicyCompatClient.cpp
- +-- AudioPolicyCompatClient.h
- +-- audio_policy_hal.cpp
- +-- AudioPolicyManagerBase.cpp
- +-- AudioPolicyManagerDefault.cpp
- +-- AudioPolicyManagerDefault.h
事实上legacy也要封装成非legacy中的audio.h，确切的说需要一个联系legacy interface和not legacy interface的中间层，这里的audio_hw_hal.cpp就充当这样的一个角色了。因此，我们其实也可以把2.3之前的alsa_sound 这一套东西也搬过来。
- hardware/libhardware/modules/audio/
- +-- Android.mk
- +-- audio_hw.c
- +-- audio_policy.c
- device/samsung/tuna/audio/
- +-- Android.mk
- +-- audio_hw.c
- +-- ril_interface.c
- +-- ril_interface.h
3、tinyalsa
- external/tinyalsa/
- +-- Android.mk
- +-- include
- | +-- tinyalsa
- | +-- asoundlib.h
- +-- mixer.c ##类alsa-lib的control，作用音频部件开关、音量调节等
- +-- pcm.c ##类alsa-lib的pcm，作用音频pcm数据回放录制
- +-- README
- +-- tinycap.c ## 类alsa_arecord
- +-- tinymix.c ##类 alsa_amixer
- +-- tinyplay.c ##类 alsa_aplay
这其实是历史的必然了，alsa-lib太过复杂繁琐了，我看得也很不爽；更重要的商业上面的考虑，必须移除被GNU GPL授权证所约束的部份，alsa-lib并不是个例。
注意：上面的hardware/libhardware_legacy/audio/、hardware/libhardware/modules /audio/、device/samsung/tuna/audio/是同层的。之一是legacy audio，用于兼容2.2时代的alsa_sound；之二是stub audio接口；之三是Samsung Tuna的音频抽象层实现。调用层次：AudioFlinger -> audio_hw -> tinyalsa。
注意：上面的hardware/libhardware_legacy/audio/、hardware/libhardware/modules /audio/、device/samsung/tuna/audio/是同层的。之一是legacy audio，用于兼容2.2时代的alsa_sound；之二是stub audio接口；之三是Samsung Tuna的音频抽象层实现。调用层次：AudioFlinger -> audio_hw -> tinyalsa。
二、Audio Hardware HAL加载
1、AudioFlinger
- // 加载audio hardware hal
- static int load_audio_interface(const char *if_name, const hw_module_t **mod,
- audio_hw_device_t **dev)
- {
- int rc;
- //根据classid和if_name找到指定的动态库并加载，这里加载的是音频动态库，如libaudio.primary.tuna.so
- rc = hw_get_module_by_class(AUDIO_HARDWARE_MODULE_ID, if_name, mod);
- if (rc)
- goto out;
- //加载好的动态库模块必有个open方法，调用open方法打开音频设备模块
- rc = audio_hw_device_open(*mod, dev);
- LOGE_IF(rc, "couldn't open audio hw device in %s.%s (%s)",
- AUDIO_HARDWARE_MODULE_ID, if_name, strerror(-rc));
- if (rc)
- goto out;
- return 0;
- out:
- *mod = NULL;
- *dev = NULL;
- return rc;
- }
- //音频设备接口，hw_get_module_by_class需要根据这些字符串找到相关的音频模块库
- static const char *audio_interfaces[] = {
- "primary", //主音频设备，一般为本机codec
- "a2dp", //a2dp设备，蓝牙高保真音频
- "usb", //usb-audio设备，这个东东我2.3就考虑要实现了，现在终于支持了
- };
- #define ARRAY_SIZE(x) (sizeof((x))/sizeof(((x)[0])))
- // ----------------------------------------------------------------------------
- AudioFlinger::AudioFlinger()
- : BnAudioFlinger(),
- mPrimaryHardwareDev(0), mMasterVolume(1.0f), mMasterMute(false), mNextUniqueId(1),
- mBtNrecIsOff(false)
- {
- }
- void AudioFlinger::onFirstRef()
- {
- int rc = 0;
- Mutex::Autolock _l(mLock);
- /* TODO: move all this work into an Init() function */
- mHardwareStatus = AUDIO_HW_IDLE;
- //打开audio_interfaces数组定义的所有音频设备
- for (size_t i = 0; i < ARRAY_SIZE(audio_interfaces); i++) {
- const hw_module_t *mod;
- audio_hw_device_t *dev;
- rc = load_audio_interface(audio_interfaces[i], &mod, &dev);
- if (rc)
- continue;
- LOGI("Loaded %s audio interface from %s (%s)", audio_interfaces[i],
- mod->name, mod->id);
- mAudioHwDevs.push(dev); //mAudioHwDevs 是一个Vector，存储已打开的audio hw devices
- if (!mPrimaryHardwareDev) {
- mPrimaryHardwareDev = dev;
- LOGI("Using '%s' (%s.%s) as the primary audio interface",
- mod->name, mod->id, audio_interfaces[i]);
- }
- }
- mHardwareStatus = AUDIO_HW_INIT;
- if (!mPrimaryHardwareDev || mAudioHwDevs.size() == 0) {
- LOGE("Primary audio interface not found");
- return;
- }
- //对audio hw devices进行一些初始化，如 mode、master volume的设置
- for (size_t i = 0; i < mAudioHwDevs.size(); i++) {
- audio_hw_device_t *dev = mAudioHwDevs[i];
- mHardwareStatus = AUDIO_HW_INIT;
- rc = dev->init_check(dev);
- if (rc == 0) {
- AutoMutex lock(mHardwareLock);
- mMode = AUDIO_MODE_NORMAL;
- mHardwareStatus = AUDIO_HW_SET_MODE;
- dev->set_mode(dev, mMode);
- mHardwareStatus = AUDIO_HW_SET_MASTER_VOLUME;
- dev->set_master_volume(dev, 1.0f);
- mHardwareStatus = AUDIO_HW_IDLE;
- }
- }
- }
以上对AudioFlinger进行的分析，主要是通过hw_get_module_by_class()找到模块接口名字if_name相匹配的模块库，加载，然后audio_hw_device_open()调用模块的open方法，完成音频设备模块的初始化。
留意AudioFlinger的构造函数只有简单的私有变量的初始化操作了，把音频设备初始化放到onFirstRef()，Android终于改进了这一点，好的设计根本不应该把可能会失败的操作放到构造函数中。onFirstRef是RefBase类的一个虚函数，在构造sp的时候就会被调用。因此，在构造sp<AudioFlinger>的时候就会触发onFirstRef方法，从而完成音频设备模块初始化。
2、hw_get_module_by_class
我们接下来看看hw_get_module_by_class，实现在hardware/libhardware/ hardware.c中，它作用加载指定名字的模块库（.so文件），这个应该是用于加载所有硬件设备相关的库文件，并不只是音频设备。
- int hw_get_module_by_class(const char *class_id, const char *inst,
- const struct hw_module_t **module)
- {
- int status;
- int i;
- const struct hw_module_t *hmi = NULL;
- char prop[PATH_MAX];
- char path[PATH_MAX];
- char name[PATH_MAX];
- if (inst)
- snprintf(name, PATH_MAX, "%s.%s", class_id, inst);
- else
- strlcpy(name, class_id, PATH_MAX);
- //这里我们以音频库为例，AudioFlinger调用到这个函数时，
- //class_id=AUDIO_HARDWARE_MODULE_ID="audio"，inst="primary" (或"a2dp"或"usb")
- //那么此时name="audio.primary"
- /*
- * Here we rely on the fact that calling dlopen multiple times on
- * the same .so will simply increment a refcount (and not load
- * a new copy of the library).
- * We also assume that dlopen() is thread-safe.
- */
- /* Loop through the configuration variants looking for a module */
- for (i=0 ; i<HAL_VARIANT_KEYS_COUNT+1 ; i++) {
- if (i < HAL_VARIANT_KEYS_COUNT) {
- //通过property_get找到厂家标记如"ro.product.board=tuna"，这时prop="tuna"
- if (property_get(variant_keys[i], prop, NULL) == 0) {
- continue;
- }
- snprintf(path, sizeof(path), "%s/%s.%s.so",
- HAL_LIBRARY_PATH2, name, prop); //#define HAL_LIBRARY_PATH2 "/vendor/lib/hw"
- if (access(path, R_OK) == 0) break;
- snprintf(path, sizeof(path), "%s/%s.%s.so",
- HAL_LIBRARY_PATH1, name, prop); //#define HAL_LIBRARY_PATH1 "/system/lib/hw"
- if (access(path, R_OK) == 0) break;
- } else {
- snprintf(path, sizeof(path), "%s/%s.default.so", //如没有指定的库文件，则加载default.so，即stub-device
- HAL_LIBRARY_PATH1, name);
- if (access(path, R_OK) == 0) break;
- }
- }
- //到这里，完成一个模块库的完整路径名称，如path="/system/lib/hw /audio.primary.tuna.so"
- //如何生成audio.primary.tuna.so？请看相关的Android.mk文件，其中有定义 LOCAL_MODULE := audio.primary.tuna
- status = -ENOENT;
- if (i < HAL_VARIANT_KEYS_COUNT+1) {
- /* load the module, if this fails, we're doomed, and we should not try
- * to load a different variant. */
- status = load(class_id, path, module); //加载模块库
- }
- return status;
- }
- /**
- * Every hardware module must have a data structure named HAL_MODULE_INFO_SYM
- * and the fields of this data structure must begin with hw_module_t
- * followed by module specific information.
- */
- typedef struct hw_module_t {
- /** tag must be initialized to HARDWARE_MODULE_TAG */
- uint32_t tag;
- /** major version number for the module */
- uint16_t version_major;
- /** minor version number of the module */
- uint16_t version_minor;
- /** Identifier of module */
- const char *id;
- /** Name of this module */
- const char *name;
- /** Author/owner/implementor of the module */
- const char *author;
- /** Modules methods */
- struct hw_module_methods_t* methods;
- /** module's dso */
- void* dso;
- /** padding to 128 bytes, reserved for future use */
- uint32_t reserved[32-7];
- } hw_module_t;
- typedef struct hw_module_methods_t {
- /** Open a specific device */
- int (*open)(const struct hw_module_t* module, const char* id,
- struct hw_device_t** device);
- } hw_module_methods_t;
例如，在audio_hw.c中，它是这样定义的：
- static struct hw_module_methods_t hal_module_methods = {
- .open = adev_open,
- };
- struct audio_module HAL_MODULE_INFO_SYM = {
- .common = {
- .tag = HARDWARE_MODULE_TAG,
- .version_major = 1,
- .version_minor = 0,
- .id = AUDIO_HARDWARE_MODULE_ID,
- .name = "Tuna audio HW HAL",
- .author = "The Android Open Source Project",
- .methods = &hal_module_methods,
- },
- };
3、audio_hw
好了，经过一番周折，又dlopen又dlsym的，终于进入我们的audio_hw。这部分没什么好说的，按照 hardware/libhardware/include/hardware/audio.h定义的接口实现就行了。这些接口全扔到一个结构体里面的，这样做的好处是：不必用大量的dlsym来获取各个接口函数的地址，只需找到这个结构体即可，从易用性和可扩充性来说，都是首选方式。
接口定义如下：
- 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);
- /**
- * check to see if the audio hardware interface has been initialized.
- * returns 0 on success, -ENODEV on failure.
- */
- int (*init_check)(const struct audio_hw_device *dev);
- /** set the audio volume of a voice call. Range is between 0.0 and 1.0 */
- int (*set_voice_volume)(struct audio_hw_device *dev, float volume);
- /**
- * set the audio volume for all audio activities other than voice call.
- * Range between 0.0 and 1.0. If any value other than 0 is returned,
- * the software mixer will emulate this capability.
- */
- int (*set_master_volume)(struct audio_hw_device *dev, float volume);
- /**
- * setMode is called when the audio mode changes. AUDIO_MODE_NORMAL mode
- * is for standard audio playback, AUDIO_MODE_RINGTONE when a ringtone is
- * playing, and AUDIO_MODE_IN_CALL when a call is in progress.
- */
- int (*set_mode)(struct audio_hw_device *dev, int mode);
- /* mic mute */
- int (*set_mic_mute)(struct audio_hw_device *dev, bool state);
- int (*get_mic_mute)(const struct audio_hw_device *dev, bool *state);
- /* set/get global audio parameters */
- int (*set_parameters)(struct audio_hw_device *dev, const char *kv_pairs);
- /*
- * Returns a pointer to a heap allocated string. The caller is responsible
- * for freeing the memory for it.
- */
- char * (*get_parameters)(const struct audio_hw_device *dev,
- const char *keys);
- /* Returns audio input buffer size according to parameters passed or
- * 0 if one of the parameters is not supported
- */
- size_t (*get_input_buffer_size)(const struct audio_hw_device *dev,
- uint32_t sample_rate, int format,
- int channel_count);
- /** This method creates and opens the audio hardware output stream */
- int (*open_output_stream)(struct audio_hw_device *dev, uint32_t devices,
- int *format, uint32_t *channels,
- uint32_t *sample_rate,
- struct audio_stream_out **out);
- void (*close_output_stream)(struct audio_hw_device *dev,
- struct audio_stream_out* out);
- /** This method creates and opens the audio hardware input stream */
- int (*open_input_stream)(struct audio_hw_device *dev, uint32_t devices,
- int *format, uint32_t *channels,
- uint32_t *sample_rate,
- audio_in_acoustics_t acoustics,
- struct audio_stream_in **stream_in);
- void (*close_input_stream)(struct audio_hw_device *dev,
- struct audio_stream_in *in);
- /** This method dumps the state of the audio hardware */
- int (*dump)(const struct audio_hw_device *dev, int fd);
- };
- typedef struct audio_hw_device audio_hw_device_t;
三、Audio Hardware HAL的legacy实现
之前提到两种Audio Hardware HAL接口定义：
1/ legacy：hardware/libhardware_legacy/include/hardware_legacy /AudioHardwareInterface.h
2/ 非legacy：hardware/libhardware/include/hardware/audio.h
前者是2.3及之前的音频设备接口定义，后者是4.0的接口定义。
1/ legacy：hardware/libhardware_legacy/include/hardware_legacy /AudioHardwareInterface.h
2/ 非legacy：hardware/libhardware/include/hardware/audio.h
前者是2.3及之前的音频设备接口定义，后者是4.0的接口定义。
为了兼容以前的设计，4.0实现一个中间层：hardware/libhardware_legacy/audio/audio_hw_hal.cpp，结构与其他的audio_hw.c大同小异，差别在于open方法：
- static int legacy_adev_open(const hw_module_t* module, const char* name,
- hw_device_t** device)
- {
- ......
- ladev->hwif = createAudioHardware();
- if (!ladev->hwif) {
- ret = -EIO;
- goto err_create_audio_hw;
- }
- ......
- }
因此老一套的alsa-lib、alsa-utils和alsa_sound也可以照搬过来，这里的文件被编译成静态库的，因此你需要修改 alsa_sound里面的Android.mk文件，链接这个静态库。还有alsa_sound的命名空间原来是“android”，现在需要改成 “android_audio_legacy”。
四、a2dp Audio HAL的实现
external/Bluetooth/bluez/audio/android_audio_hw.c
大致与上面提到的audio_hw.c类似，因为都是基于audio.h定义的接口来实现的。
如果需要编译这个库，须在BoardConfig.mk里定义：
BOARD_HAVE_BLUETOOTH := true
开始还提到现在支持3种audio设备了，分别是primary、a2dp和usb。目前剩下usb audio hal我没有找到，不知是否需要自己去实现？其实alsa-driver都支持大部分的usb-audio设备了，因此上层也可调用tinyalsa的接口，就像samsung tuna的audio_hw.c那样。