usb host msc模式下的USBH_Process函数疑惑，求解释

yuer19900712 · 发表于 2017-6-23 11:22:08

在usb host msc模式中，在轮询处理函数中void USBH_Process(USB_OTG_CORE_HANDLE *pdev , USBH_HOST *phost)
对枚举完成后的程序不太理解，尤其是这里

[mw_shl_code=applescript,true]case HOST_CLASS:
  /* process class state machine */
  status = phost->class_cb->Machine(pdev, phost);
  USBH_ErrorHandle(phost, status);
  break;[/mw_shl_code]
第3行进入对应的函数后，经过
USBH_MSC_BOT_INIT_STATE
USBH_MSC_BOT_RESET
USBH_MSC_GET_MAX_LUN
进入case USBH_MSC_TEST_UNIT_READY
[mw_shl_code=applescript,true]case USBH_MSC_TEST_UNIT_READY:
  /* Issue SCSI command TestUnitReady */
  mscStatus = USBH_MSC_TestUnitReady(pdev);

  if(mscStatus == USBH_MSC_OK )
  {
USBH_MSC_BOTXferParam.MSCState = USBH_MSC_READ_CAPACITY10;
MSCErrorCount = 0;
status = USBH_OK;
  }
  else
  {
USBH_MSC_ErrorHandle(mscStatus);
  }
  break;[/mw_shl_code]
进入第3行的子函数
[mw_shl_code=applescript,true]uint8_t USBH_MSC_TestUnitReady (USB_OTG_CORE_HANDLE *pdev)
{
  uint8_t index;
  USBH_MSC_Status_TypeDef status = USBH_MSC_BUSY;

  if(HCD_IsDeviceConnected(pdev))
  {
switch(USBH_MSC_BOTXferParam.CmdStateMachine)
{
case CMD_SEND_STATE:
   /*Prepare the CBW and relevent field*/
   USBH_MSC_CBWData.field.CBWTransferLength = 0;    /* No Data Transfer */
   USBH_MSC_CBWData.field.CBWFlags = USB_EP_DIR_OUT;
   USBH_MSC_CBWData.field.CBWLength = CBW_LENGTH_TEST_UNIT_READY;
   USBH_MSC_BOTXferParam.pRxTxBuff = USBH_MSC_CSWData.CSWArray;
   USBH_MSC_BOTXferParam.DataLength = USBH_MSC_CSW_MAX_LENGTH;
   USBH_MSC_BOTXferParam.MSCStateCurrent = USBH_MSC_TEST_UNIT_READY;

   for(index = CBW_CB_LENGTH; index != 0; index--)
   {
      USBH_MSC_CBWData.field.CBWCB[index] = 0x00;
   }

   USBH_MSC_CBWData.field.CBWCB[0]  = OPCODE_TEST_UNIT_READY;
   USBH_MSC_BOTXferParam.BOTState = USBH_MSC_SEND_CBW;
   /* Start the transfer, then let the state
   machine magage the other transactions */
   USBH_MSC_BOTXferParam.MSCState = USBH_MSC_BOT_USB_TRANSFERS;
   USBH_MSC_BOTXferParam.BOTXferStatus = USBH_MSC_BUSY;
   USBH_MSC_BOTXferParam.CmdStateMachine = CMD_WAIT_STATUS;

   status = USBH_MSC_BUSY;
   break;

case CMD_WAIT_STATUS:
   if(USBH_MSC_BOTXferParam.BOTXferStatus == USBH_MSC_OK)
   {
      /* Commands successfully sent and Response Received  */
      USBH_MSC_BOTXferParam.CmdStateMachine = CMD_SEND_STATE;

      status = USBH_MSC_OK;
   }
   else if ( USBH_MSC_BOTXferParam.BOTXferStatus == USBH_MSC_FAIL )
   {
      /* Failure Mode */
      USBH_MSC_BOTXferParam.CmdStateMachine = CMD_SEND_STATE;
      status = USBH_MSC_FAIL;
   }

   else if ( USBH_MSC_BOTXferParam.BOTXferStatus == USBH_MSC_PHASE_ERROR )
   {
      /* Failure Mode */
      USBH_MSC_BOTXferParam.CmdStateMachine = CMD_SEND_STATE;
      status = USBH_MSC_PHASE_ERROR;
   }
   break;

default:
   break;
}
  }
  return status;
}[/mw_shl_code]
执行完第一个case CMD_SEND_STATE后，返回的状态不就成了USBH_MSC_BUSY了，返回到上一层
[mw_shl_code=applescript,true]case USBH_MSC_TEST_UNIT_READY:
  /* Issue SCSI command TestUnitReady */
  mscStatus = USBH_MSC_TestUnitReady(pdev);

  if(mscStatus == USBH_MSC_OK )
  {
USBH_MSC_BOTXferParam.MSCState = USBH_MSC_READ_CAPACITY10;
MSCErrorCount = 0;
status = USBH_OK;
  }
  else
  {
USBH_MSC_ErrorHandle(mscStatus);
  }
  break;[/mw_shl_code]
这不是要执行USBH_MSC_ErrorHandle(mscStatus);这个语句了，还如何往下执行呢？
我看process这个程序是一直要执行到 case USBH_MSC_DEFAULT_APPLI_STATE:
才能转换到用户自己的应用程序当中，这如何过来的？