problems SGDMA control bits （PARK ）

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 ？？？？