About DMA problem..please help me

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/*
 * NIOSimg.cpp
 *
 *  Created on: Apr 26, 2010
 *      Author: suki
 */
# 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"
# define DEPTH 0x4b000
static volatile int txrx_done = 0;
//callback function when DMA transfer done
static void txrxDone(void * handle, void * data)
{
    txrx_done = 1;
}
void initMEM(int base_addr,int len)
{
    for (int i=0;i<len;i++)
    {
        IOWR_8DIRECT(base_addr,i,i);
    }
}
int main()
{
    printf("testing memory to memory DMA transfer operation of 0x%x bytes\n",DEPTH);
    alt_8 buffer;
    //memset((void *)SSRAM_0_BASE,0x7a,0x10);//this write base on byte
    initMEM(SDRAM_1_BASE,DEPTH);
    //memset((void *)(SDRAM_1_BASE+0x10),0x33,0x10);
    memset(buffer,0x33,DEPTH);
    int rc;    //request
    alt_dma_txchan txchan;
    alt_dma_rxchan rxchan;
    void* tx_data = (void*)SDRAM_1_BASE; /* pointer to data to send */
    void* rx_buffer = (void*)&buffer; /* pointer to rx buffer */
    /* Create the transmit channel */
    if ((txchan = alt_dma_txchan_open("/dev/dma_0")) == NULL)
    {
        printf ("Failed to open transmit channel\n");
        exit (1);
    }
    /* Create the receive channel */
    if ((rxchan = alt_dma_rxchan_open("/dev/dma_0")) == NULL)
    {
        printf ("Failed to open receive channel\n");
        exit (1);
    }
    /* Post the transmit request */
    if ((rc = alt_dma_txchan_send (txchan,
            tx_data,
            DEPTH,
            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_buffer,
            DEPTH,
            txrxDone,
            NULL)) < 0)
    {
        printf ("Failed to post read request, reason = %i\n", rc);
        //exit (1);
    }
    printf("DMA transfer running. . .");
    /* wait for transfer to complete */
    while (!txrx_done);
    printf ("Transfer finish!\n");
    printf("Verifying transfered data using memcmp. . .");
    if (!(memcmp((void*)SDRAM_1_BASE,(void*)&buffer,DEPTH)))
        printf("Verification succesful!\n");
    else
        printf("Verification failed\n");
    //    printf("content of buffer\n");
    //    for (int i=0;i<sizeof(buffer)/2;i++)
    //        printf("%d: %x\n",i,buffer); // this output width ,base on what we declare
    /*printf("content of ssram_0\n");
    for (int i=0;i<sizeof(buffer);i=i+2)
    {
        printf("%d: %x\n",i/2,IORD_16DIRECT(SSRAM_0_BASE,i));
    }
    printf("content of sdram_1\n");
    for (int i=0;i<sizeof(buffer);i=i+2)
    {
        printf("%d: %x\n",i/2,IORD_16DIRECT(SDRAM_1_BASE,i));
    }
    memcpy((void*)SDRAM_1_BASE,(void *)SSRAM_0_BASE,20);
    printf("content of sdram_1: after memcpy operation\n");
    for (int i=0;i<sizeof(buffer);i=i+2)
    {
        printf("%d: %x\n",i/2,IORD_16DIRECT(SDRAM_1_BASE,i));
    }*/
    return 0;
}

testing memory to memory DMA transfer operation of 0x4b000 bytes
DMA transfer running. . .Transfer finish!
Verifying transfered data using memcmp. . .Verification succesful!