串口设备可以正常找到且能正常发送，但是串口接受回调函数却无法进入？

ret_uart2 = rt_device_open(serial_u2, RT_DEVICE_OFLAG_RDWR | RT_DEVICE_FLAG_INT_RX);
   if(ret_uart2 < 0)
   {
   LOG_E("can't open uart2\r\n");
   return RT_ERROR;
   }
   rt_device_set_rx_indicate(serial_u2, RxCallBackUart2);
   ret_uart2rxsem = rt_sem_init(&uart2rx_sem, "rx_sem", 0, RT_IPC_FLAG_PRIO);
   if(ret_uart2rxsem < 0)
   {
   LOG_E("ret_uart2rxsem create failed!\r\n");
   return RT_ERROR;
   }
   rt_device_write(serial_u2, 0, str, sizeof(str));
static rt_err_t RxCallBackUart2(rt_device_t dev, rt_size_t size)   //接受回调函数
{
   LOG_D("JION RxCallBackUart2\r\n");
   rt_sem_release(&uart2rx_sem);
   return RT_EOK;
}