why DMA not done?

Hi,

I have problem to use DMA to transfer data from memory to memory, also memory to peripherals. Each time I start a DMA, it is not done, the DONE bit will not be set, and my problem keep in busy waiting the bit. Here is my program, I think there is no problem. Anyone can help?

#include<stdio.h>
#include<stdlib.h>
#include<stddef.h>
#include<string.h>
#include<system.h>
#include<io.h>
#include<alt_types.h>
#include"sys/alt_dma.h"
#include"sys/alt_cache.h"
#include"sys/alt_alarm.h"
#include"alt_types.h"
static volatile int txrx_done = 0;
//callback function when DMA transfer done
static void txrxDone(void * handle, void * data)
{
    txrx_done = 1;
}
void initMEM(void* base_addr,int len)
{
    int i;
    for (i=0;i<len;i++)
    {
        IOWR_16DIRECT(base_addr,i,i);
    }
}
static alt_16 txbuff;
static alt_16 rxbuff;
int main()
{
    int i;
printf("testing sdram to sdram : DMA operation\n");
    initMEM((void*)txbuff, 0x10);
    memset(rxbuff, 0, sizeof(rxbuff));
printf("content of txbuff:before DMA operation\n");
    for (i=0;i<0x10;i++)
    {
        printf("%d: %x\n",i,IORD_16DIRECT(txbuff,i));
    }
printf("content of rxbuff:before DMA operation\n");
    for (i=0;i<0x10;i++)
    {
        printf("%d: %x\n",i,IORD_16DIRECT(rxbuff,i));
    }
    int rc;    //request
alt_dma_txchan txchan;
alt_dma_rxchan rxchan;
    void* tx_data = (void*)alt_remap_uncached(txbuff, sizeof(txbuff)); /* pointer to data to send */
    void* rx_data = (void*)alt_remap_uncached(rxbuff, sizeof(rxbuff)); /* pointer to rx buffer */
/* Create the transmit channel */
    if ((txchan = alt_dma_txchan_open("/dev/dma_3")) == NULL)
    {
printf ("Failed to open transmit channel\n");
        exit(1);
    }
/* Create the receive channel */
    if ((rxchan = alt_dma_rxchan_open("/dev/dma_3")) == NULL)
    {
printf ("Failed to open receive channel\n");
        exit(1);
    }
/* Post the transmit request */
    if ((rc = alt_dma_txchan_send (txchan,
            tx_data,
            0x20,
            NULL,
            NULL)) < 0)
    {
printf ("Failed to post transmit request, reason = %i\n", rc);
        exit (1);
    }
/* Post the receive request */
    if ((rc = alt_dma_rxchan_prepare (rxchan,
            rx_data,
            0x20,
            txrxDone,
            NULL)) < 0)
    {
printf ("Failed to post read request, reason = %i\n", rc);
        exit (1);
    }
/* wait for transfer to complete */
    while (!txrx_done);
printf ("Transfer successful!\n");
printf("content of rxbuff:after DMA operation\n");
    for (i=0;i<0x10;i++)
    {
        printf("%d: %x\n",i,IORD_16DIRECT(txbuff,i));
    }
printf("content of txbuff:after DMA operation\n");
    for (i=0;i<0x10;i++)
    {
        printf("%d: %x\n",i,IORD_16DIRECT(rxbuff,i));
    }
    alt_dma_rxchan_close (rxchan);
    alt_dma_txchan_close (txchan);
    return 0;
}

Also I attached the qsys file.