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
AudioFlinger相关代码,好像这部分与2.3相差不大,至少接口是兼容的。值得注意的是:2.3位于这里的还有AudioHardwareGeneric、AudioHardwareInterface、A2dpAudioInterface等一系列接口代码,现在都移除了。实际上,这些接口变更为legacy(有另外更好的实现方式,但也兼容之前的方法),取而代之的是要实现hardware/libhardware/include/hardware/audio.h提供的接口,这是一个较大的变化。

两种Audio Hardware HAL接口定义:
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里。
事实上legacy也要封装成current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
这是一个stub(类似于2.3中的AudioHardwareStub),大多数函数只是简单的返回一个值,并没有实际操作,只是保证Android能得到一个audio hardware hal实例,从而启动运行,当然声音没有输出到外设的。在底层音频驱动或audio hardware hal还没有实现好的情况下,可以使用这个stub device,先让Android跑起来。

device/samsung/tuna/audio/
+-- Android.mk
+-- audio_hw.c
+-- ril_interface.c
+-- ril_interface.h
这是Samsung Tuna的音频设备抽象层,很有参考价值,计划以后就在它的基础上进行移植。它调用tinyalsa的接口,可见这个方案的底层音频驱动是alsa。

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
在2.3时代,Android还隐晦把它放在android2.3.1-gingerbread/device/samsung/crespo/libaudio,现在终于把alsa-lib一脚踢开,小三变正室了,正名tinyalsa。
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。

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;
}

load()函数不详细分析了,它通过dlopen加载库文件,然后dlsym找到hal_module_info的首地址。我们先看看hal_module_info的定义:
/**
 * 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;
这个结构体很重要,注释很详细。dlsym拿到这个结构体的首地址后,就可以调用Modules methods进行设备模块的初始化了。设备模块中,都应该按照这个格式初始化好这个结构体,否则dlsym找不到它,也就无法调用Modules methods进行初始化了。

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;

注:这是比较标准的C接口设计方法了,但是个人感觉还是用C++比较好,直观易读。2.3之前都是用C++实现这些接口设计的,到了4.0,不知道为何采纳用C?不会理由是做底层的不懂C++吧?!

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/ current: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;
    }

    ......
}
看到那个熟悉的createAudioHardware()没有?这是以前我提到的Vendor Specific Audio接口,然后新的接口再调用ladev->hwif的函数就是了。
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


4.0的a2dp audio hal放到bluez里实现了,我找了好一会才找到:
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。

五、音质改进???


可使用audio echo cancel和更好的resampler(SRC)???

--to be continued…

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