problems about using Ethernet example in cyclone V GT development kits,help!!

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I use the ethernet example which is in the cyclone V GT installation package( cycloneVGT_5cgtfd9ef35_fpga\examples\board_update_portal ) 

 I download the c5gt_fpga_bup.sof into cyclone V GT Development board,and then debug my simple TSE driver ,I found some problem.  1: i set the TSE into loop mode, RX_SGDMA's ISR is printing the received loopback data.  the follow datas are received loopback data,i found a problem  in first RX interrupt ,the datas are correct, but in the follow RX interrupt ,the datas are always zero ??? why???   RX descriptor reported OK 0 0 ff ff ff ff ff ff 0 1c 23 17 4a cb 8 6 0 1 8 0 6 4 0 1 0 1c 23 17 4a cb c0 a8 1 2 0 0 0 0 0 0 c0 a8 1 72 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc 0 0  RX descriptor reported OK 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  RX descriptor reported OK 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  RX descriptor reported OK 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  RX descriptor reported OK 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  RX descriptor reported OK  my simple drivers:  ethernet_init(); sgdma_tx_dev = alt_avalon_sgdma_open("/dev/sgdma_tx"); sgdma_rx_dev = alt_avalon_sgdma_open("/dev/sgdma_rx"); alt_avalon_sgdma_register_callback(  sgdma_rx_dev,  (alt_avalon_sgdma_callback) &sgdma_rx_isr,  //ALTERA_AVALON_SGDMA_CONTROL_IE_DESC_COMPLETED_MSK &#65372;   ALTERA_AVALON_SGDMA_CONTROL_IE_CHAIN_COMPLETED_MSK |  ALTERA_AVALON_SGDMA_CONTROL_IE_GLOBAL_MSK   ,  0); while(1){  usleep(1000000);  alt_avalon_sgdma_construct_stream_to_mem_desc(  &desc[10],//  &desc[11],//  rx_payload,//  0,//  0);//write_fixed    alt_avalon_sgdma_construct_mem_to_stream_desc(  &desc[2],  &desc[3],  buf,  66,  0,  1,//SOP  1,//EOP  0);  sgdma_rx&#21644;sgdma_tx  alt_avalon_sgdma_do_async_transfer(sgdma_rx_dev, &desc[10]);  usleep(100);  alt_avalon_sgdma_do_sync_transfer (sgdma_tx_dev, &desc[2]);    }   void sgdma_rx_isr(void * context, u_long intnum)  {  int i=0;  printf("RX descriptor reported OK\n");  for(i=0; i<68; i++){  printf("%x ",rx_payload);  rx_payload=0x00;  }  printf("\n");  }