ANDROID音訊系統散記之四:4.0音訊系統HAL初探
昨天(2011-11-15)釋出了Android4.0的原始碼,今天download下來,開始挺進4.0時代。簡單看了一下,發現音訊系統方面與2.3的有較多地方不同,下面逐一描述。
一、程式碼模組位置
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
AudioFlinger相關程式碼,好像這部分與2.3相差不大,至少介面是相容的。值得注意的是:2.3位於這裡的還有AudioHardwareGeneric、AudioHardwareInterface、A2dpAudioInterface等一系列介面程式碼,現在都移除了。實際上,這些介面變更為legacy(有另外更好的實現方式,但也相容之前的方法),取而代之的是要實現hardware/libhardware/include/hardware/audio.h提供的介面,這是一個較大的變化。兩種Audio Hardware HAL介面定義:
1/ legacy:hardware/libhardware_legacy/include/hardware_legacy/AudioHardwareInterface.h
2/ current: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
上面提及的AudioHardwareGeneric、AudioHardwareInterface、A2dpAudioInterface等都放到libhardware_legacy裡。事實上legacy也要封裝成current中的audio.h,確切的說需要一個聯絡legacy interface和not legacy interface的中間層,這裡的audio_hw_hal.cpp就充當這樣的一個角色了。因此,我們其實也可以把2.3之前的alsa_sound這一套東西也搬過來。
hardware/libhardware/modules/audio/
+-- Android.mk
+-- audio_hw.c
+-- audio_policy.c
這是一個stub(類似於2.3中的AudioHardwareStub),大多數函式只是簡單的返回一個值,並沒有實際操作,只是保證Android能得到一個audio hardware hal例項,從而啟動執行,當然聲音沒有輸出到外設的。在底層音訊驅動或audio hardware hal還沒有實現好的情況下,可以使用這個stub device,先讓Android跑起來。device/samsung/tuna/audio/
+-- Android.mk
+-- audio_hw.c
+-- ril_interface.c
+-- ril_interface.h
這是Samsung Tuna的音訊裝置抽象層,很有參考價值,計劃以後就在它的基礎上進行移植。它呼叫tinyalsa的介面,可見這個方案的底層音訊驅動是alsa。3、tinyalsa
external/tinyalsa/
+-- Android.mk
+-- include
| +-- tinyalsa
| +-- asoundlib.h
+-- mixer.c ##類alsa-lib的control,作用音訊部件開關、音量調節等
+-- pcm.c ##類alsa-lib的pcm,作用音訊pcm資料回放錄製
+-- README
+-- tinycap.c ##類alsa_arecord
+-- tinymix.c ##類alsa_amixer
+-- tinyplay.c ##類alsa_aplay
在2.3時代,Android還隱晦把它放在android2.3.1-gingerbread/device/samsung/crespo/libaudio,現在終於把alsa-lib一腳踢開,小三變正室了,正名tinyalsa。這其實是歷史的必然了,alsa-lib太過複雜繁瑣了,我看得也很不爽;更重要的商業上面的考慮,必須移除被GNU GPL授權證所約束的部份,alsa-lib並不是個例。
注意:上面的hardware/libhardware_legacy/audio/、hardware/libhardware/modules/audio/、device/samsung/tuna/audio/是同層的。之一是legacy audio,用於相容2.2時代的alsa_sound;之二是stub audio介面;之三是Samsung Tuna的音訊抽象層實現。呼叫層次:AudioFlinger -> audio_hw -> tinyalsa。
注意:上面的hardware/libhardware_legacy/audio/、hardware/libhardware/modules/audio/、device/samsung/tuna/audio/是同層的。之一是legacy audio,用於相容2.2時代的alsa_sound;之二是stub audio介面;之三是Samsung Tuna的音訊抽象層實現。呼叫層次:AudioFlinger -> audio_hw -> tinyalsa。
二、Audio Hardware HAL載入
1、AudioFlinger
//載入audio hardware hal
static int load_audio_interface(const char *if_name, const hw_module_t **mod,
audio_hw_device_t **dev)
{
int rc;
//根據classid和if_name找到指定的動態庫並載入,這裡載入的是音訊動態庫,如libaudio.primary.tuna.so
rc = hw_get_module_by_class(AUDIO_HARDWARE_MODULE_ID, if_name, mod);
if (rc)
goto out;
//載入好的動態庫模組必有個open方法,呼叫open方法開啟音訊裝置模組
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;
}
//音訊裝置介面,hw_get_module_by_class需要根據這些字串找到相關的音訊模組庫
static const char *audio_interfaces[] = {
"primary", //主音訊裝置,一般為本機codec
"a2dp", //a2dp裝置,藍芽高保真音訊
"usb", //usb-audio裝置,這個東東我2.3就考慮要實現了,現在終於支援了
};
#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;
//開啟audio_interfaces陣列定義的所有音訊裝置
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是一個Vector,儲存已開啟的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;
}
//對audio hw devices進行一些初始化,如mode、master volume的設定
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;
}
}
}
以上對AudioFlinger進行的分析,主要是通過hw_get_module_by_class()找到模組介面名字if_name相匹配的模組庫,載入,然後audio_hw_device_open()呼叫模組的open方法,完成音訊裝置模組的初始化。
留意AudioFlinger的建構函式只有簡單的私有變數的初始化操作了,把音訊裝置初始化放到onFirstRef(),Android終於改進了這一點,好的設計根本不應該把可能會失敗的操作放到建構函式中。onFirstRef是RefBase類的一個虛擬函式,在構造sp的時候就會被呼叫。因此,在構造sp<AudioFlinger>的時候就會觸發onFirstRef方法,從而完成音訊裝置模組初始化。
2、hw_get_module_by_class
我們接下來看看hw_get_module_by_class,實現在hardware/libhardware/ hardware.c中,它作用載入指定名字的模組庫(.so檔案),這個應該是用於載入所有硬體裝置相關的庫檔案,並不只是音訊裝置。
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);
//這裡我們以音訊庫為例,AudioFlinger呼叫到這個函式時,
//class_id=AUDIO_HARDWARE_MODULE_ID="audio",inst="primary"(或"a2dp"或"usb")
//那麼此時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) {
//通過property_get找到廠家標記如"ro.product.board=tuna",這時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", //如沒有指定的庫檔案,則載入default.so,即stub-device
HAL_LIBRARY_PATH1, name);
if (access(path, R_OK) == 0) break;
}
}
//到這裡,完成一個模組庫的完整路徑名稱,如path="/system/lib/hw/audio.primary.tuna.so"
//如何生成audio.primary.tuna.so?請看相關的Android.mk檔案,其中有定義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); //載入模組庫
}
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;
這個結構體很重要,註釋很詳細。dlsym拿到這個結構體的首地址後,就可以呼叫Modules methods進行裝置模組的初始化了。裝置模組中,都應該按照這個格式初始化好這個結構體,否則dlsym找不到它,也就無法呼叫Modules methods進行初始化了。例如,在audio_hw.c中,它是這樣定義的:
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,經過一番周折,又dlopen又dlsym的,終於進入我們的audio_hw。這部分沒什麼好說的,按照hardware/libhardware/include/hardware/audio.h定義的介面實現就行了。這些介面全扔到一個結構體裡面的,這樣做的好處是:不必用大量的dlsym來獲取各個介面函式的地址,只需找到這個結構體即可,從易用性和可擴充性來說,都是首選方式。
介面定義如下:
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實現
之前提到兩種Audio Hardware HAL介面定義:
1/ legacy:hardware/libhardware_legacy/include/hardware_legacy/AudioHardwareInterface.h
2/ current:hardware/libhardware/include/hardware/audio.h
前者是2.3及之前的音訊裝置介面定義,後者是4.0的介面定義。
1/ legacy:hardware/libhardware_legacy/include/hardware_legacy/AudioHardwareInterface.h
2/ current:hardware/libhardware/include/hardware/audio.h
前者是2.3及之前的音訊裝置介面定義,後者是4.0的介面定義。
為了相容以前的設計,4.0實現一箇中間層:hardware/libhardware_legacy/audio/audio_hw_hal.cpp,結構與其他的audio_hw.c大同小異,差別在於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;
}
......
}
看到那個熟悉的createAudioHardware()沒有?這是以前我提到的Vendor Specific Audio介面,然後新的介面再呼叫ladev->hwif的函式就是了。因此老一套的alsa-lib、alsa-utils和alsa_sound也可以照搬過來,這裡的檔案被編譯成靜態庫的,因此你需要修改alsa_sound裡面的Android.mk檔案,連結這個靜態庫。還有alsa_sound的名稱空間原來是“android”,現在需要改成“android_audio_legacy”。
四、a2dp Audio HAL的實現
external/Bluetooth/bluez/audio/android_audio_hw.c
大致與上面提到的audio_hw.c類似,因為都是基於audio.h定義的介面來實現的。
如果需要編譯這個庫,須在BoardConfig.mk裡定義:
BOARD_HAVE_BLUETOOTH := true
開始還提到現在支援3種audio裝置了,分別是primary、a2dp和usb。目前剩下usb audio hal我沒有找到,不知是否需要自己去實現?其實alsa-driver都支援大部分的usb-audio裝置了,因此上層也可呼叫tinyalsa的介面,就像samsung tuna的audio_hw.c那樣。
五、音質改進???
--to be continued…
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