Failed to post DMA transmit request

Hi,

I tried to use the general Avalon-MM DMA controller in Qsys and use the following C code to implement the DMA transmit:

# include <stdio.h># include "system.h"# include "io.h"# include "sys/alt_dma.h"
# define CUSTOM_BASE_ADDR 0x00800000
int i = 0;
alt_u32 data ;
alt_dma_txchan tx;
int main()
{
    /*fill the array*/
        for(i=0;i<10;i++)
        {
            data = i;
        }
    /*DMA*/
    tx = alt_dma_txchan_open("/dev/dma_import");
    if(tx != NULL)
    {
        printf("DMA TX channel open\n");
        if(alt_dma_txchan_ioctl(tx, ALT_DMA_TX_ONLY_ON, CUSTOM_BASE_ADDR)<0)
        {
            printf("Error: Failed to configure the DMA\n");
        }
        if(alt_dma_txchan_send(tx, data , 5, NULL, NULL)<0)
        {
            printf("Error: failed to post transmit request\n");
        }
        else
        {
            printf("DMA done\n");
        }
        alt_dma_txchan_close(tx);
    }
    else
    {
        printf("Error: Failed to open tx channel!\n");
    }
  return 0;
}

Where the array is stored in an external memory while the CUSTOM_BASE_ADDR is a separate peripheral. The connection among NIOS, external memory and custom peripheral should be fine since I've tested with general reading/writing.

Turned out "alt_dma_txchan_send(tx, data , 5, NULL, NULL)" always failed.

Currently I'm not sure if the problem lies in the hardware part or the software. Please suggest if there's any problem in the above code.

Thanks!