android4.2.2---1. hal层与framework层的调用

1.HAL层
首先在hal层定义结构体sensors_module_t
#define SENSORS_HARDWARE_MODULE_ID "sensors"

struct sensors_module_t HAL_MODULE_INFO_SYM = {
common:
    {
tag:
        HARDWARE_MODULE_TAG,
        version_major: 1,
        version_minor: 0,
id:
        SENSORS_HARDWARE_MODULE_ID,   //每一个hal层都有一个唯一的id
name: "MEMSIC Sensors Module"
        ,
author: "MEMSIC Inc."
        ,
methods:
        &__module_methods,
dso:    NULL,
        reserved: {0},
    },
get_sensors_list:
    __get_sensors_list
};

在__module_methods中定义了open方法

static struct hw_module_methods_t __module_methods = {
open:
    __module_methods_open
};

static int __module_methods_open(const struct hw_module_t *module,
                                 const char *id, struct hw_device_t **device)
{
    struct sensors_poll_context_t *dev;
    dev = (struct sensors_poll_context_t *)malloc(sizeof(*dev));
    //初始化dev
    dev->device.common.tag = HARDWARE_DEVICE_TAG;
    dev->device.common.version = 0;
    dev->device.common.module = (struct hw_module_t *)module;
    dev->device.common.close = __common_close;
    dev->device.activate = __control_activate;
    dev->device.setDelay = __control_set_delay;
    dev->device.poll = __data_poll;
    //赋值给device
    //这儿有个问题: common是一个 struct hw_device_t(小) common;
    //一会在framework中调用的需要的是 sensor_poll_t(大)
    *device = &dev->device.common;
    return res;
}

2. framework层
framework/native/services/sensorservice/SensorDevice.cpp

SensorDevice::SensorDevice()
    : mSensorDevice(0),
       mSensorModule(0)
{
    //fw层会根据唯一的id=SENSORS_HARDWARE_MODULE_ID来找到hal层的so,然后调用dlopen加载到内存中
    //同时定义为 struct sensors_module_t* 的mSensorModule会指向找到的so中的sensor_module_t结构体
    //这样就可以利用hal层定义的函数了
    hw_get_module(SENSORS_HARDWARE_MODULE_ID, (hw_module_t const**)&mSensorModule);
    //struct sensors_poll_device_t* mSensorDevice;
    sensors_open(&mSensorModule->common, &mSensorDevice);
    mSensorDevice->activate(mSensorDevice, list[i].handle, 0);
}

static inline int sensors_open(const struct hw_module_t* module,
        struct sensors_poll_device_t** device) {
    return module->methods->open(module,
            SENSORS_HARDWARE_POLL, (struct hw_device_t**)device);
}

sensors_poll_device_t
                        --> hw_device_t
                        --> 加一些函数指针
在hal层的open中:
       * device  =   & dev - > device . common ;
在framework层的sensor_open中
    module->methods->open(..., (struct hw_device_t**)device);
为什么这儿是将sensors_poll_device_t转为了hw_device_t,而不是直接传sensor_poll_device_t呢?
答: 我感觉这儿是为了屏蔽,即让hal层屏蔽fw层的无关代码. 
      当然也有可能是为了接口的统一.