problems SGDMA control bits （PARK ）

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I want to test SGDMA park funvtion 

 simple test code   temp1=IORD_ALTERA_AVALON_SGDMA_CONTROL(TX_SGDMA_BASE);  temp1=temp1|ALTERA_AVALON_SGDMA_CONTROL_PARK_MSK;  IOWR_ALTERA_AVALON_SGDMA_CONTROL(TX_SGDMA_BASE, temp1);   alt_avalon_sgdma_construct_mem_to_stream_desc( &desc[2],//主描述字  &desc[2],//次描述字  buf,//发送指针  1468,//发送字数  0,  1,//发送SOP  1,//发送EOP  0); alt_avalon_sgdma_do_sync_transfer (sgdma_tx_dev, &desc[2]);  SGDMA can continuously work as I imagined  if I chang the code  temp1=IORD_ALTERA_AVALON_SGDMA_CONTROL(TX_SGDMA_BASE);  temp1=temp1|ALTERA_AVALON_SGDMA_CONTROL_PARK_MSK;  IOWR_ALTERA_AVALON_SGDMA_CONTROL(TX_SGDMA_BASE, temp1); alt_avalon_sgdma_construct_mem_to_stream_desc( &desc[2],//主描述字  &desc[3],//次描述字  buf,//发送指针  1468,//发送字数  0,  1,//发送SOP  1,//发送EOP  0);    alt_avalon_sgdma_construct_mem_to_stream_desc( &desc[3],//主描述字  &desc[2],//次描述字  buf,//发送指针  1468,//发送字数  0,  1,//发送SOP  1,//发送EOP  0); alt_avalon_sgdma_do_sync_transfer (sgdma_tx_dev, &desc[2]);  SGDMA can not continuously work as I imagined   why？park bit cannot support chain ？？？？