一、CameraModule的作用

	我理解的CameraModule的主要作用就是承上启下的作用，在Android 开源规则下hal层各个厂商可以根据知识产权将自
己的hal层实现不对外公开，这个CameraModule就是负责加载厂商hal camera so，然后调用相关方法的。

二、加载camera hal so

	在 CameraProvider 中持有 CameraModule的对象，在CameraProvider::initialize() 中 加载camera的so 
	-> hw_get_module(CAMERA_HARDWARE_MODULE_ID,(const hw_module_t **)&rawModule); 并且 new CameraModule

bool CameraProvider::initialize() {
    camera_module_t *rawModule;
    int err = hw_get_module(CAMERA_HARDWARE_MODULE_ID,
            (const hw_module_t **)&rawModule);
    if (err < 0) {
        ALOGE("Could not load camera HAL module: %d (%s)", err, strerror(-err));
        return true;
    }

    mModule = new CameraModule(rawModule);
    ........
  }

我们看看怎么加载的camera so

// CAMERA_HARDWARE_MODULE_ID = camera, hw_module_t 是结构体 camera_module
int hw_get_module(const char *id, const struct hw_module_t **module)
{
	// 在 hw_get_module_by_class里面主要会调用 load() 方法来加载 hal camera so
    return hw_get_module_by_class(id, NULL, module);
}


typedef struct camera_module {
    hw_module_t common;

    int (*get_number_of_cameras)(void);

    int (*get_camera_info)(int camera_id, struct camera_info *info);

    int (*set_callbacks)(const camera_module_callbacks_t *callbacks);

    void (*get_vendor_tag_ops)(vendor_tag_ops_t* ops);

    int (*open_legacy)(const struct hw_module_t* module, const char* id,
            uint32_t halVersion, struct hw_device_t** device);

    int (*set_torch_mode)(const char* camera_id, bool enabled);
    
    int (*init)();
    
    void* reserved[5];
} camera_module_t;


int hw_get_module_by_class(const char *class_id, const char *inst,
                           const struct hw_module_t **module)
{
	...........
    return load(class_id, path, module);
}

static int load(const char *id,
        const char *path,
        const struct hw_module_t **pHmi)
{
    int status = -EINVAL;
    void *handle = NULL;
    struct hw_module_t *hmi = NULL;
    // 注意这边开了__ANDROID_VNDK__就会去走vendor下面找这个so
#ifdef __ANDROID_VNDK__
    const bool try_system = false;
#else
    const bool try_system = true;
#endif

    /*
     * 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
     */
    if (try_system &&
        strncmp(path, HAL_LIBRARY_PATH1, strlen(HAL_LIBRARY_PATH1)) == 0) {
        /* If the library is in system partition, no need to check
         * sphal namespace. Open it with dlopen.
         */
        handle = dlopen(path, RTLD_NOW);
    } else {
    	// 这个path就是so的加载路径 
        handle = android_load_sphal_library(path, RTLD_NOW);
    }
    if (handle == NULL) {
        char const *err_str = dlerror();
        ALOGE("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) {
        ALOGE("load: couldn't find symbol %s", sym);
        status = -EINVAL;
        goto done;
    }
    ............
}