Android AudioFlinger加载HAL层流程

一、前提

Audio HAL层最终以.so的方式为Android所用，那这个.so的库如何被AudioFlinger所使用？

二、Audio Hardware HAL加载

（1）AudioFlinger

AudioFlinger加载HAL层：

static int load_audio_interface(const char *if_name, const hw_module_t **mod,  
                                audio_hw_device_t **dev)  
{  
    int rc;  

    /* 这里加载的是音频动态库，如audio.primary.msm8916.so,如何加载会独立体现 */
    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;  
}  

audio_interface:

/* 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",              //指本机中的codec  
    "a2dp",                 //a2dp设备，蓝牙高保真音频  
    "usb",                  //usb-audio设备 
};  

AudioFlinger::onFirstRef：

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;  
        }  
    }  
} 

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，加载之后audio_hw_device_open()调用模块的open方法，完成音频设备模块的初始化。

hw_get_module_by_class：

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）。

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=msm8916"，这时prop="msm8916" */
            if (property_get(variant_keys[i], prop, NULL) == 0) {  
                continue;  
            }  
            /* #define HAL_LIBRARY_PATH2 "/vendor/lib/hw" */
            snprintf(path, sizeof(path), "%s/%s.%s.so",  
                     HAL_LIBRARY_PATH2, name, prop);  
            if (access(path, R_OK) == 0) break;  

            /* #define HAL_LIBRARY_PATH1 "/system/lib/hw" */
            snprintf(path, sizeof(path), "%s/%s.%s.so",  
                     HAL_LIBRARY_PATH1, name, prop);   
            if (access(path, R_OK) == 0) break;  
        } else {  
            /* 如没有指定的库文件，则加载default.so */
            snprintf(path, sizeof(path), "%s/%s.default.so",  
                     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.msm8916.so"  */

    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. */  
         //加载模块库:见下面
        status = load(class_id, path, module);   
    }  

    return status;  
} 

load(class_id, path, module)：

static int load(const char *id,
        const char *path,
        const struct hw_module_t **pHmi)
{
    int status;
    void *handle;
    struct hw_module_t *hmi;

    /*
     * load the symbols resolving undefined symbols before
     * dlopen returns. Since RTLD_GLOBAL is not or‘d in with
     * RTLD_NOW the external symbols will not be global
     */
    handle = dlopen(path, RTLD_NOW);
    if (handle == NULL) {
        char const *err_str = dlerror();
        LOGE("load: module=%s\n%s", path, err_str?err_str:"unknown");
        status = -EINVAL;
        goto done;
    }

    /* Get the address of the struct hal_module_info. */
    const char *sym = HAL_MODULE_INFO_SYM_AS_STR;
    hmi = (struct hw_module_t *)dlsym(handle, sym);
        if (hmi == NULL) {
        LOGE("load: couldn‘t find symbol %s", sym);
        status = -EINVAL;
        goto done;
    }

    /* Check that the id matches */
    if (strcmp(id, hmi->id) != 0) {
        LOGE("load: id=%s != hmi->id=%s", id, hmi->id);
        status = -EINVAL;
        goto done;
    }

    hmi->dso = handle;

    /* success */
    status = 0;

    done:
    if (status != 0) {
        hmi = NULL;
        if (handle != NULL) {
            dlclose(handle);
            handle = NULL;
        }
    } else {
        LOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",
                id, path, *pHmi, handle);
    }

    *pHmi = hmi;

    return status;
}

在打开的.so（audio.primary.msm8916.so）中查找HMI符号的地址,并保存在hmi中。至此.so中的hw_module_t已经被成功获取。从而可以根据它获取HAL层相关接口。
1)HAL通过hw_get_module函数获取hw_module_t
2)HAL通过hw_module_t->methods->open获取hw_device_t指针,并在此open函数中初始化audio_hw_device_t结构中的函数。
3)三个重要的数据结构:
a) struct hw_device_t: 表示硬件设备,存储了各种硬件设备的公共属性和方法
b)struct hw_module_t: 可用hw_get_module进行加载的module
c)struct hw_module_methods_t: 用于定义操作设备的方法,其中只定义了一个打开设备的方法open.

hw_module_t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;  

在load（…）中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。

audio_hw.c中hw_module_methods_t 的实例化：

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,  
    },  
};  

audio_module 是我们Audio HAL必须要实现的。

audio_hw_device 接口
接口按照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。

audio_hw_device 接口如下：

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;  

在HAL层adev_open初始化中要对audio_hw_device 进行赋值初始化，HAL层的重头戏其实就是对这些函数进行实例化。

HAL层之后就会调用Tinyalsa，接着就是Audio Driver了。总体顺序：AudioFlinger->Audio HAL->Tinyalsa->Audio Driver。