TMS570LS3137: CAN发送数据时，如何更新DLC

uint32 canTransmitWithDLC(canBASE_t *node, uint32 messageBox, const uint8 * data,const uint8 dlc)
{
	 uint32 i;
    uint32 success  = 0U;
    uint32 regIndex = (messageBox - 1U) >> 5U;
    uint32 bitIndex = 1U << ((messageBox - 1U) & 0x1FU);

/* USER CODE BEGIN (7) */
/* USER CODE END */


    /** - Check for pending message:
    *     - pending message, return 0
    *     - no pending message, start new transmission 
    */
    if ((node->TXRQx[regIndex] & bitIndex) != 0U)
    {
        success = 0U;
    }

    else
    {
    /** - Wait until IF1 is ready for use */
    /*SAFETYMCUSW 28 D MR:NA <APPROVED> "Potentially infinite loop found - Hardware Status check for execution sequence" */
    while ((node->IF1STAT & 0x80U) ==0x80U)
    { 
    } /* Wait */

    /** - Configure IF1 for
	*     - Message direction - Write
	*     - Data Update
	*     - Start Transmission
	*/	
	node->IF1CMD = 0x97U;
	
    /** - Copy TX data into IF1 */
    for (i = 0U; i < 8U; i++)
    {
#if ((__little_endian__ == 1) || (__LITTLE_ENDIAN__ == 1))
        /*SAFETYMCUSW 45 D MR:21.1 <APPROVED> "Valid non NULL input parameters are only allowed in this driver" */
        node->IF1DATx[i] = *data;
        /*SAFETYMCUSW 45 D MR:21.1 <APPROVED> "Valid non NULL input parameters are only allowed in this driver" */
        /*SAFETYMCUSW 567 S MR:17.1,17.4 <APPROVED> "Pointer increment needed" */
		data++;
#else
        /*SAFETYMCUSW 45 D MR:21.1 <APPROVED> "Valid non NULL input parameters are only allowed in this driver" */
        node->IF1DATx[s_canByteOrder[i]] = *data;
        /*SAFETYMCUSW 45 D MR:21.1 <APPROVED> "Valid non NULL input parameters are only allowed in this driver" */
        /*SAFETYMCUSW 567 S MR:17.1,17.4 <APPROVED> "Pointer increment needed" */
		data++;
#endif
    }

    /** - Copy TX data into message box */
    /*SAFETYMCUSW 93 S MR: 6.1,6.2,10.1,10.2,10.3,10.4 <APPROVED> "LDRA Tool issue" */
    node->IF1NO = (uint8) messageBox;

	node->IF1MCTL &= 0xFFFFFF00;
	node->IF1MCTL |= dlc;

    success = 1U; 
    }
    /**   @note The function canInit has to be called before this function can be used.\n
    *           The user is responsible to initialize the message box.
    */

/* USER CODE BEGIN (8) */
/* USER CODE END */

    return success;
}

/*如果代码这样写，发送的首帧长度依然为8；*/
node->IF1NO = (uint8) messageBox;

node->IF1MCTL &= 0xFFFFFF00;
node->IF1MCTL |= dlc;


/*代码这样写，发送数据的DLC则更新为输入参数dlc；*/
node->IF1MCTL &= 0xFFFFFF00;
node->IF1MCTL |= dlc;
	
node->IF1NO = (uint8) messageBox;