EDMA 乒乓模式，看不懂，求解释

如下所示的程序：

// **************************************************************************************
EDMA3_DRV_Result edmaInitiateXferDoubleChain(const void *dst, const void *src, const int aCnt,
									   const int bCnt, const int cCnt, const int srcBIdx,
									   const int destBIdx, const int srcCIdx,
									   const int destCIdx, const int chCnt)
{
  EDMA3_DRV_PaRAMRegs paramSet, *gParamSet;
  uint32_t addr;
  EDMA3_DRV_Result edmaResult = EDMA3_DRV_SOK;
  signed char *srcBuf, *dstBuf;
  volatile int coreCnt = determineProcId();
  int bIndex; //, cIndex;

  if((chCnt<0) || (chCnt>2) || ((chCnt&1) != 0))
  {
	System_printf("DMA channel (%d) not valid", chCnt);
	System_abort("\n");
  }

  if(bCnt * cCnt > MAX_PARAMS-1)
  {
	System_printf("Can't handle bCnt(%d)*cCnt(%); max allowed is %d", bCnt, cCnt, MAX_PARAMS-1);
	System_abort("\n");
  }
  srcBuf = (signed char*) getGlobalAddr((signed char *) src);
  dstBuf = (signed char*) getGlobalAddr((signed char *) dst);

  // set paramSet
  paramSet.srcAddr  = (unsigned int) srcBuf;
  paramSet.destAddr = (unsigned int) dstBuf;
  paramSet.aCnt     = aCnt;
  paramSet.bCnt     = 1;
  paramSet.cCnt     = 1;
  paramSet.srcBIdx  = 0;
  paramSet.destBIdx = 0;
  paramSet.srcCIdx  = 0;
  paramSet.destCIdx = 0;
  paramSet.bCntReload = 0;
  paramSet.linkAddr   = 0xFFFFu;
  paramSet.opt = 0x00000000u;
  // enable final TCC chaining; only one transfer
  paramSet.opt |= (1 << OPT_TCCHEN_SHIFT);
  // set TCC to chained channel
  paramSet.opt |= ((gEdmaXfer[coreCnt][chCnt+1].tcc << OPT_TCC_SHIFT) & OPT_TCC_MASK);

  gParamSet = &globalParamSet[(coreCnt*(EDMA_CHANNELS>>1)+(chCnt>>1))*MAX_PARAMS];
  // now write the paramsets to be used by the first of the doubly chained channels
  for(bIndex = 0; bIndex <= bCnt; bIndex++)
  {
	// table of paramSet
	gParamSet[bIndex] = paramSet;
	// update src and dst address;
	paramSet.srcAddr += srcBIdx;
	paramSet.destAddr += destBIdx;
	if(bIndex == (bCnt-1)) // last in the set
	{
		// set to be a dummy transfer
		paramSet.aCnt = 1;
		paramSet.bCnt = 0;
		paramSet.cCnt = 0;
		// disable chaining
		paramSet.opt &= ~(1 << OPT_TCCHEN_SHIFT);
	}
  }

  // ensure gParamSet is written to cache
  Cache_wbInv(gParamSet, (bCnt+1)*sizeof(EDMA3_DRV_PaRAMRegs), Cache_Type_ALLD, 1);
  // set paramSet; src is global param set; dst is chained PARAMset;
  // src increment is size of PARAMset and dst address is always the same
  paramSet.srcAddr  = (uint32_t) getGlobalAddr((signed char *) &gParamSet[0]);
  EDMA3_DRV_getPaRAMPhyAddr(gHEdma[coreCnt/4], gEdmaXfer[coreCnt][chCnt].chId, &addr);
  paramSet.destAddr = (uint32_t) getGlobalAddr((signed char *) addr);
  paramSet.aCnt     = sizeof(EDMA3_DRV_PaRAMRegs);
  paramSet.bCnt     = bCnt+1;
  paramSet.cCnt     = 1;
  paramSet.srcBIdx  = sizeof(EDMA3_DRV_PaRAMRegs);
  paramSet.destBIdx = 0;
  paramSet.srcCIdx  = 0;
  paramSet.destCIdx = 0;
  paramSet.bCntReload = 0;
  paramSet.linkAddr   = 0xFFFFu;
  paramSet.opt = 0x00000000u;
  // both intermediate and final chaining enabled
  paramSet.opt |= (1 << OPT_TCCHEN_SHIFT);
  paramSet.opt |= (1 << OPT_ITCCHEN_SHIFT);
  // final interrupt enabled
  paramSet.opt |= (1 << OPT_TCINTEN_SHIFT);
  // set TCC to chained channel
  paramSet.opt |= ((gEdmaXfer[coreCnt][chCnt].tcc << OPT_TCC_SHIFT) & OPT_TCC_MASK);

  /* Now, write the PaRAM Set to the second of the doubly chained channels */
  edmaResult = EDMA3_DRV_setPaRAM(gHEdma[coreCnt/4], gEdmaXfer[coreCnt][chCnt+1].chId, (EDMA3_DRV_PaRAMRegs *) &paramSet);


  // start transfer on the second of the doubly chained channels
  if(edmaResult == EDMA3_DRV_SOK)
  {
    edmaResult = EDMA3_DRV_enableTransfer (gHEdma[coreCnt/4],
			                               gEdmaXfer[coreCnt][chCnt+1].chId,
	                                       EDMA3_DRV_TRIG_MODE_MANUAL);
  }
  return(edmaResult);

} // edmaInitiateXferDoubleChain

应该是乒乓模式， 但是我看不懂，希望可以得到解释，困在这里好多天了