problem with XAxiDma_BdGetActualLength in Multichannel AXI DMA?

[Port Map]

Hi.
is any one familiar with multichannel AXI Dma On Zynq?

I'm trying to run multichannel AXI DMA. My work is done on SDK default Example by some changes on code.

now I want to use XAxiDma_BdGetActualLength function to get the received packet length. but this function always returns zero!

plz, any one can help me?

static void RxCallBack(XAxiDma *AxiDmaPtr)
{
	int BdCount;
	XAxiDma_Bd *BdPtr;
	XAxiDma_Bd *BdCurPtr;
	u32 BdSts;
	int Index;
	int RingIndex;
	XAxiDma_BdRing *RxRingPtr;

	u8 *RxPacket;


	for (RingIndex = 0;
			RingIndex < AxiDmaPtr->RxNumChannels; RingIndex++) 
	{

		RxRingPtr = XAxiDma_GetRxIndexRing(AxiDmaPtr, RingIndex);
		BdCount = XAxiDma_BdRingFromHw(RxRingPtr, XAXIDMA_ALL_BDS, &BdPtr);

		BdCurPtr = BdPtr;
		for (Index = 0; Index < BdCount; Index++) {

			/*
			 * Check the flags set by the hardware for status
			 * If error happens, processing stops, because the DMA engine
			 * is halted after this BD.
			 */
			BdSts = XAxiDma_BdGetSts(BdCurPtr);
			if ((BdSts & XAXIDMA_BD_STS_ALL_ERR_MASK) ||
			    (!(BdSts & XAXIDMA_BD_STS_COMPLETE_MASK))) {
				Error = 1;
				break;
			}

			//Consume RxPacket Here
			int PackLength=0;
			RxPacket = (u8 *) XAxiDma_BdGetBufAddr(BdCurPtr) ;
			/* Invalidate the DestBuffer before receiving the data, in case the
			 * Data Cache is enabled
			 */
			Xil_DCacheInvalidateRange((u32)RxPacket, MAX_PKT_LEN);

			PackLength=XAxiDma_BdGetActualLength(BdCurPtr,MAX_PKT_Mask);
			//look here  --@@--, PackLength is zero always
			//PackLength=512;



			/* Find the next processed BD */
			BdCurPtr = XAxiDma_BdRingNext(RxRingPtr, BdCurPtr);
			RxDone[RingIndex] += 1;
		}

		if(BdCount!=0)
		{
			//@@// added for test to free RX BDs
			/* Free all processed BDs for future transmission */
			int Status = XAxiDma_BdRingFree(RxRingPtr, BdCount, BdPtr);
			if (Status != XST_SUCCESS) {
				Error = 1;
			}
			/* Return processed BDs to RX channel so we are ready to receive new
			 * packets:
			 *    - Allocate all free RX BDs
			 *    - Pass the BDs to RX channel
			 */
			int FreeBdCount = XAxiDma_BdRingGetFreeCnt(RxRingPtr);
			Status = XAxiDma_BdRingAlloc(RxRingPtr, FreeBdCount, &BdPtr);
			if (Status != XST_SUCCESS) {
				xil_printf("bd alloc failed\r\n");
				return XST_FAILURE;
			}

			Status = XAxiDma_BdRingToHw(RxRingPtr, FreeBdCount, BdPtr);
			if (Status != XST_SUCCESS) {
				xil_printf("Submit %d rx BDs failed %d\r\n", FreeBdCount, Status);
				return XST_FAILURE;
			}

		}

	}
}