HAL

分析hardware/akm/AK8975_FS
大致有4类文件，有着不同的分工
1：hardware\akm\ak8975_fs\libsensors\Sensors.cpp
hal中一般支持多类设备，起框架作用，主要关注poll方法
2：AkmSensor.cpp
具体sensor的驱动代码
3：SensorBase.cpp
sensor的共有方法，比如open /dev/input/eventx...
4：InputEventReader.cpp
读取数据等方法

• hardware\akm\ak8975_fs\libsensors\Sensors.cpp
  起总领作用

  static int open_sensors(const struct hw_module_t* module, const char* id,
                      struct hw_device_t** device)
  {
  sensors_poll_context_t *dev = new sensors_poll_context_t();
      // sensors_poll_context_t::sensors_poll_context_t
      mSensors[akm] = new AkmSensor();
      mPollFds[akm].fd = mSensors[akm]->getFd();
  
  // 填充回调函数
  dev->device.common.module   = const_cast<hw_module_t*>(module);
  dev->device.common.close    = poll__close;
  dev->device.activate        = poll__activate;
  dev->device.setDelay        = poll__setDelay;
  dev->device.poll            = poll__poll;
  
  *device = &dev->device.common;
  }
  static struct hw_module_methods_t sensors_module_methods = {
  .open = open_sensors
  };
  // HAL入口
  struct sensors_module_t HAL_MODULE_INFO_SYM = {
  .common = {
      .id = SENSORS_HARDWARE_MODULE_ID,
      .methods = &sensors_module_methods,
  },
  .get_sensors_list = sensors__get_sensors_list,
  };

  重点分析一下poll方法：对每个sensor使用sensor->readEvents读取数据

  static int poll__poll(struct sensors_poll_device_t *dev, sensors_event_t* data, int count) {
  sensors_poll_context_t *ctx = (sensors_poll_context_t *)dev;
  return ctx->pollEvents(data, count);    // sensors_poll_context_t::pollEvents
      do {
          for (int i=0 ; count && i<numSensorDrivers ; i++) {
              SensorBase* const sensor(mSensors[i]);  // 转化为基类
              if ((mPollFds[i].revents & POLLIN) || (sensor->hasPendingEvents())) {
                  int nb = sensor->readEvents(data, count);   // 多态机制，读取数据
  
                  count -= nb;
                  nbEvents += nb;
                  data += nb;
              }
          }
      } while (n && count);
  }

• hardware\akm\ak8975_fs\libsensors\AkmSensor.cpp
  具体sensor驱动

  // 得到指定设备的fd
  AkmSensor::AkmSensor()
  : SensorBase(NULL, "compass"),  // 对/dev/input/eventx进行ioctl(EVIOCGNAME)得到name，如果是"compass"，则能使用该HAL code
  {
  mPendingEvents[MagneticField].version = sizeof(sensors_event_t);
  mPendingEvents[MagneticField].sensor = ID_M;
  mPendingEvents[MagneticField].type = SENSOR_TYPE_MAGNETIC_FIELD;    // 上报的数据类型
  mPendingEvents[MagneticField].magnetic.status = SENSOR_STATUS_ACCURACY_HIGH;
  }

再看数据读取函数，对接上Sensors.cpp

int AkmSensor::readEvents(sensors_event_t* data, int count)
    // 读取数据
    ssize_t n = mInputReader.fill(data_fd); // 
    // 处理数据
    while (count && mInputReader.readEvent(&event)) {
        int type = event->type;
        if (type == EV_ABS) {
            processEvent(event->code, event->value);    // 稍加处理
                case EVENT_TYPE_MAGV_X:
                    mPendingMask |= 1<<MagneticField;
                    mPendingEvents[MagneticField].magnetic.x = value * CONVERT_M;
                    break;
            mInputReader.next();
        }
    }

• hardware\akm\ak8975_fs\libsensors\SensorBase.cpp
  sensor的共有部分抽象

  // 遍历"/dev/input"下的所有节点，使用ioctl(EVIOCGNAME)得到name，看能否匹配上，匹配上则记录fd
  SensorBase::SensorBase(
  const char* dev_name,
  const char* data_name)
  : dev_name(dev_name), data_name(data_name),
    dev_fd(-1), data_fd(-1)
  data_fd = openInput(data_name);
      const char *dirname = "/dev/input";
      dir = opendir(dirname);
      strcpy(devname, dirname);
      filename = devname + strlen(devname);
      *filename++ = ‘/‘;
      while((de = readdir(dir))) {
          strcpy(filename, de->d_name);
          fd = open(devname, O_RDONLY);
          if (fd>=0) {
              char name[80];
              if (ioctl(fd, EVIOCGNAME(sizeof(name) - 1), &name) < 1) {
                  name[0] = ‘\0‘;
              }
              if (!strcmp(name, inputName)) {
                  strcpy(input_name, filename);
                  break;
              }
          }
      }
      closedir(dir);
      return fd;

• hardware\akm\ak8975_fs\libsensors\InputEventReader.cpp
  维护sensor数据
  ```
  // 读取"/dev/input/eventx"数据
  ssize_t InputEventCircularReader::fill(int fd)
  {
  // read name为compass的"/dev/input/eventx"
  const ssize_t nread = read(fd, mHead, mFreeSpace * sizeof(input_event));

  numEventsRead = nread / sizeof(input_event);
  if (numEventsRead) {
  mHead += numEventsRead;
  mFreeSpace -= numEventsRead;
  if (mHead > mBufferEnd) {
  size_t s = mHead - mBufferEnd;
  memcpy(mBuffer, mBufferEnd, s * sizeof(input_event));
  mHead = mBuffer + s;
  }
  }

  return numEventsRead;
  }

// 更新数据指针位置
ssize_t InputEventCircularReader::readEvent(input_event const** events)
{
*events = mCurr;
}
void InputEventCircularReader::next()
{
mCurr++;
}
```