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。

版权声明：本文为博主原创文章，未经博主允许不得转载。