书写hall传感器android hal层代码

以下是一个简单的Hall传感器在Android HAL层的代码示例：

#include <hardware/hardware.h>
#include <hardware/sensors.h>

#include <fcntl.h>
#include <errno.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define SENSOR_NAME "hall-sensor"
#define SENSOR_VENDOR "ACME"
#define SENSOR_VERSION 1
#define SENSOR_HANDLE 0

struct hall_sensor_context_t {
    struct sensors_poll_device_t device;
    sensors_event_t sensor_event;
    int fd;
};

static int hall_sensor_open(const struct hw_module_t* module, const char* name,
        struct hw_device_t** device);

static int hall_sensor_close(struct hw_device_t* device);

static int hall_sensor_activate(struct sensors_poll_device_t *dev, 
        int handle, int enabled);

static int hall_sensor_set_delay(struct sensors_poll_device_t *dev, 
        int handle, int64_t ns);

static int hall_sensor_poll(struct sensors_poll_device_t *dev, 
        sensors_event_t* data, int count);

static struct hw_module_methods_t hall_sensor_module_methods = {
    .open = hall_sensor_open
};

struct sensors_poll_device_t HAL_MODULE_INFO_SYM = {
    .common = {
        .tag = HARDWARE_DEVICE_TAG,
        .version = 0,
        .module = &HAL_MODULE_INFO_SYM.common,
        .close = hall_sensor_close,
    },
    .poll = hall_sensor_poll,
    .activate = hall_sensor_activate,
    .setDelay = hall_sensor_set_delay,
};

static int hall_sensor_open(const struct hw_module_t* module, const char* name,
        struct hw_device_t** device) {

    if (strcmp(name, SENSORS_POLL_DEVICE_NAME)) {
        return -EINVAL;
    }

    struct hall_sensor_context_t* hall_dev = (struct hall_sensor_context_t*) malloc(sizeof(struct hall_sensor_context_t));
    memset(hall_dev, 0, sizeof(*hall_dev));

    hall_dev->device.common.tag = HARDWARE_DEVICE_TAG;
    hall_dev->device.common.version = 0;
    hall_dev->device.common.module = (struct hw_module_t*) module;
    hall_dev->device.common.close = hall_sensor_close;
    hall_dev->device.poll = hall_sensor_poll;
    hall_dev->device.activate = hall_sensor_activate;
    hall_dev->device.setDelay = hall_sensor_set_delay;

    hall_dev->sensor_event.version = sizeof(sensors_event_t);
    hall_dev->sensor_event.sensor = SENSOR_HANDLE;
    hall_dev->sensor_event.type = SENSOR_TYPE_MAGNETIC_FIELD;
    hall_dev->sensor_event.data[0] = 0.0f;
    hall_dev->sensor_event.data[1] = 0.0f;
    hall_dev->sensor_event.data[2] = 0.0f;

    hall_dev->fd = open("/dev/hall-sensor", O_RDONLY);
    if (hall_dev->fd < 0) {
        ALOGE("Failed to open hall sensor device: %s", strerror(errno));
        free(hall_dev);
        return -errno;
    }

    *device = &hall_dev->device.common;

    return 0;
}

static int hall_sensor_close(struct hw_device_t* device) {
    struct hall_sensor_context_t* hall_dev = (struct hall_sensor_context_t*) device;
    close(hall_dev->fd);
    free(hall_dev);
    return 0;
}

static int hall_sensor_activate(struct sensors_poll_device_t *dev, 
        int handle, int enabled) {
    struct hall_sensor_context_t* hall_dev = (struct hall_sensor_context_t*) dev;
    if (handle != SENSOR_HANDLE || (enabled != 0 && enabled != 1)) {
        return -EINVAL;
    }
    return 0;
}

static int hall_sensor_set_delay(struct sensors_poll_device_t *dev, 
        int handle, int64_t ns) {
    struct hall_sensor_context_t* hall_dev = (struct hall_sensor_context_t*) dev;
    if (handle != SENSOR_HANDLE) {
        return -EINVAL;
    }
    return 0;
}

static int hall_sensor_poll(struct sensors_poll_device_t *dev, 
        sensors_event_t* data, int count) {
    struct hall_sensor_context_t* hall_dev = (struct hall_sensor_context_t*) dev;
    ssize_t n = read(hall_dev->fd, &hall_dev->sensor_event.data[0], sizeof(float));
    if (n < 0) {
        ALOGE("Failed to read hall sensor data: %s", strerror(errno));
        return -errno;
    }
    hall_dev->sensor_event.timestamp = getTimestamp();
    *data = hall_dev->sensor_event;
    return 1;
}

这个代码示例定义了一个名为`hall-sensor`的传感器，它返回磁场强度值。在`hall_sensor_open()`函数中，我们打开了`/dev/hall-sensor`设备文件，并初始化了一个`sensors_event_t`结构体来存储传感器事件。在`sensors_poll_device_t`结构体中，我们定义了传感器的`poll()`、`activate()`和`setDelay()`函数。在`hall_sensor_activate()`和`hall_sensor_set_delay()`函数中，我们简单地检查传入的`handle`参数是否是我们定义的传感器句柄，并返回0或-EINVAL。在`hall_sensor_poll()`函数中，我们从设备文件中读取传感器数据，并将其存储在先前初始化的`sensors_event_t`结构体中，最后返回1表示有一个新的传感器事件。