hw_get_module() 会调用hw_get_module_by_class函数，首先通过property_get 函数获取系统属性"ro.hardware"的值，如果找到，则通过hw_module_exists 函数去查找.so 文件存不存在。如果存在，直接加载；不存在，就再继续查找variant_keys 数组中，其他的系统属性值存不存在。如果存在，直接加载，不存在，则最后去加载.default.so文件。如果此文件也不存在，那么硬件抽象层模块加载失败了。

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;/* * 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();    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;}/* Check that the id matches */if (strcmp(id, hmi->id) != 0) {    ALOGE("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 {    ALOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",            id, path, *pHmi, handle);}*pHmi = hmi;return status;}

  前面已经说过的，硬件抽象层模块，最后其实被打包成一个动态链接库.so 文件。上面的Load函数中，通过dlopen 函数加载此.so文件到内存中，然后调用dlsym函数来获取HAL_MODULE_INFO_SYM_AS_STR这个符号。这个符号里面包含硬件抽象层模块的所有信息。根据前面硬件抽象层模块的编写规范, HAL_MODULE_INFO_SYM_AS_STR 的值被定义为"HMI"，且每一个硬件抽象层模块都必须包含一个名称为"HMI"的符号，且这个符号的第一个成员变量的类型必须定义成hw_module_t。因此，可以将HMI
强转成一个hw_module_t 的结构体指针。后边会调用strcmp 函数来验证加载得到的硬件抽象层模块ID跟所要求加载的ID 是否一致。如果一致就说明成功了，最好会把handle 保存在结构体指针hmi的成员变量dso中，返还给调用者。

  以上就是硬件抽象模块的加载过程

##四.Android 6.0版本Fingerprint Hal层分析 ##

1. 在上文中提到，在系统启动过程中，Fingerprintd 会调用hw_get_module 函数来获取来加载指纹硬件抽象层模块文件.so文件。

2. fingerprintd加载指纹模块成功之后，会得到了相应的fingerprint_module_t，之后就会去调用它的open函数,在这里实现fingerprint.h 定义的接口。然后赋值给device，fingerprintd 就能够用这个device 操纵hal的指纹模块了。一般也在这个地方，进行一些初始化操作。

   static int fingerprint_open(const hw_module_t* module, const char __unused id,
   hw_device_t* device)
   {
   if (device == NULL) {
   ALOGE(“NULL device on open”);
   return -EINVAL;
   }

    fingerprint_device_t *dev = malloc(sizeof(fingerprint_device_t)); memset(dev, 0, sizeof(fingerprint_device_t)); dev->common.tag = HARDWARE_DEVICE_TAG; dev->common.version = FINGERPRINT_MODULE_API_VERSION_2_0; dev->common.module = (struct hw_module_t*) module; dev->common.close = fingerprint_close; dev->pre_enroll = fingerprint_pre_enroll; dev->enroll = fingerprint_enroll; dev->get_authenticator_id = fingerprint_get_auth_id; dev->cancel = fingerprint_cancel; dev->remove = fingerprint_remove; dev->set_active_group = fingerprint_set_active_group; dev->authenticate = fingerprint_authenticate; dev->set_notify = set_notify_callback; dev->notify = NULL; *device = (hw_device_t*) dev; return 0;

   }

   fingerprint_module_t HAL_MODULE_INFO_SYM = {
   .common = {
   .tag = HARDWARE_MODULE_TAG,
   .module_api_version = FINGERPRINT_MODULE_API_VERSION_2_0,
   .hal_api_version = HARDWARE_HAL_API_VERSION,
   .id = FINGERPRINT_HARDWARE_MODULE_ID,
   .name = “Demo Fingerprint HAL”,
   .author = “The Android Open Source Project”,
   .methods = &fingerprint_module_methods,
   },
   };

在硬件抽象层的加载过程中，会对id 进行验证，这个id比较重要，不能轻易修改。

####hardware/libhardware/include/hardware/fingerprint.h####

/* Callback function type */typedef void (*fingerprint_notify_t)(const fingerprint_msg_t *msg);/* Synchronous operation */typedef struct fingerprint_device {/** * Common methods of the fingerprint device. This *must* be the first member * of fingerprint_device as users of this structure will cast a hw_device_t * to fingerprint_device pointer in contexts where it's known * the hw_device_t references a fingerprint_device.*/struct hw_device_t common;/* * Client provided callback function to receive notifications. * Do not set by hand, use the function above instead. */fingerprint_notify_t notify;/* * Set notification callback: * Registers a user function that would receive notifications from the HAL * The call will block if the HAL state machine is in busy state until HAL * leaves the busy state. * * Function return: 0 if callback function is successfuly registered *                  or a negative number in case of error, generally from the errno.h set. */int (*set_notify)(struct fingerprint_device *dev, fingerprint_notify_t notify);/* * Fingerprint pre-enroll enroll request: * Generates a unique token to upper layers to indicate the start of an enrollment transaction. * This token will be wrapped by security for verification and passed to enroll() for * verification before enrollment will be allowed. This is to ensure adding a new fingerprint * template was preceded by some kind of credential confirmation (e.g. device password). * * Function return: 0 if function failed *                  otherwise, a uint64_t of token */uint64_t (*pre_enroll)(struct fingerprint_device *dev);/* * Fingerprint enroll request: * Switches the HAL state machine to collect and store a new fingerprint * template. Switches back as soon as enroll is complete * (fingerprint_msg.type == FINGERPRINT_TEMPLATE_ENROLLING && *  fingerprint_msg.data.enroll.samples_remaining == 0) * or after timeout_sec seconds. * The fingerprint template will be assigned to the group gid. User has a choice * to supply the gid or set it to 0 in which case a unique group id will be generated. * * Function return: 0 if enrollment process can be successfully started *                  or a negative number in case of error, generally from the errno.h set. *                  A notify() function may be called indicating the error condition. */int (*enroll)(struct fingerprint_device *dev, const hw_auth_token_t *hat,                uint32_t gid, uint32_t timeout_sec);/* * Finishes the enroll operation and invalidates the pre_enroll() generated challenge. * This will be called at the end of a multi-finger enrollment session to indicate * that no more fingers will be added. * * Function return: 0 if the request is accepted *                  or a negative number in case of error, generally from the errno.h set. */int (*post_enroll)(struct fingerprint_device *dev);/* * get_authenticator_id: * Returns a token associated with the current fingerprint set. This value will * change whenever a new fingerprint is enrolled, thus creating a new fingerprint * set. * * Function return: current authenticator id or 0 if function failed. */uint64_t (*get_authenticator_id)(struct fingerprint_device *dev);/* * Cancel pending enroll or authenticate, sending FINGERPRINT_ERROR_CANCELED * to all running clients. Switches the HAL state machine back to the idle state. * Unlike enroll_done() doesn't invalidate the pre_enroll() challenge. * * Function return: 0 if cancel request is accepted *                  or a negative number in case of error, generally from the errno.h set. */int (*cancel)(struct fingerprint_device *dev);/* * Enumerate all the fingerprint templates found in the directory set by * set_active_group() * This is a synchronous call. The function takes: * - A pointer to an array of fingerprint_finger_id_t. * - The size of the array provided, in fingerprint_finger_id_t elements. * Max_size is a bi-directional parameter and returns the actual number * of elements copied to the caller supplied array. * In the absence of errors the function returns the total number of templates * in the user directory. * If the caller has no good guess on the size of the array he should call this * function witn *max_size == 0 and use the return value for the array allocation. * The caller of this function has a complete list of the templates when *max_size * is the same as the function return. * * Function return: Total number of fingerprint templates in the current storage directory. *                  or a negative number in case of error, generally from the errno.h set. */int (*enumerate)(struct fingerprint_device *dev, fingerprint_finger_id_t *results,    uint32_t *max_size);/* * Fingerprint remove request: * Deletes a fingerprint template. * Works only within a path set by set_active_group(). * notify() will be called with details on the template deleted. * fingerprint_msg.type == FINGERPRINT_TEMPLATE_REMOVED and * fingerprint_msg.data.removed.id indicating the template id removed. * * Function return: 0 if fingerprint template(s) can be successfully deleted *                  or a negative number in case of error, generally from the errno.h set. */int (*remove)(struct fingerprint_device *dev, uint32_t gid, uint32_t fid);/* * Restricts the HAL operation to a set of fingerprints belonging to a * group provided. * The caller must provide a path to a storage location within the user's * data directory. ** Function return: 0 on success *                  or a negative number in case of error, generally from the errno.h set. */int (*set_active_group)(struct fingerprint_device *dev, uint32_t gid,                        const char *store_path);/* * Authenticates an operation identifed by operation_id * * Function return: 0 on success *                  or a negative number in case of error, generally from the errno.h set. */int (*authenticate)(struct fingerprint_device *dev, uint64_t operation_id, uint32_t gid);//Add by zhangbin@meitu.com for MEITU_TAG_FINGERPRINT: update fpc_tac sw18.2/* *  Set fingerprint model work mode, Just like DeepSleep :)*/int (*set_work_mode)(struct fingerprint_device *device, int mode);/* *  Just Use in FactoryMode, unify factory test API*/int (*factory_test)(struct fingerprint_device *device, int argc, void* argv);/* Reserved for backward binary compatibility *///void *reserved[4];void *reserved[2];//Add end} fingerprint_device_t;

我们需要在fingerprint.c/cpp 里面去实现这个fingerprint.h 里面的函数，即可。

• enroll: 切换HAL状态机以启动指纹模板的收集和存储。 一旦注册完成或超时，HAL状态机将返回到空闲状态
• pre_enroll:生成一个唯一的令牌来指示指纹注册的开始。 向注册功能提供token，以确保先前的身份验证，例如使用密码。 一旦设备凭证被确认，令牌就被包装并进行HMAC，以防止篡改。 在注册期间必须检查token，以验证token是否仍然有效。
• pos_enroll: 完成注册操作并使pre_enroll（）生成的challenge无效。 这将在多手指登录会话结束时调用，以指示不再添加手指
• get_authenticator_id:返回与当前指纹集关联的token
• cancel: 取消任何待处理的注册或验证操作。 HAL状态机返回到空闲状态。
• enumerate:同步调用枚举所有已知的指纹模板
• remove: 删除单个的指纹模板
• set_active_group: 将HAL操作限制为属于指定组（由组标识符或GID标识）的一组指纹。
• authenticate: 验证指纹相关操作（由操作ID标识）
• set_notify - 注册将从HAL获取通知的用户功能。 如果HAL状态机处于忙状态，则该功能被阻塞，直到HAL离开忙碌状态

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