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

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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