DMA problem on alt_dma_txchan_send

Hi every body.

I have a problem with the NIOS DMA setting.

In my system, I need to read data from a device by a MM-Avalon interface, and write the data in a other device with the MM-Avalon interface. The memoryes in the devices are FIFO. The part of my code about DMA is:

# include "system.h"

# include <io.h>

# include <stdio.h>

# include <sys/alt_irq.h>

....

int rc;

alt_dma_txchan txchan;

alt_dma_rxchan rxchan;

void* tx_data = (void*) (MC_BASE+MC_READ);

void* rx_buffer = (void*) (GIGAINT_BASE);

if ((txchan = alt_dma_txchan_open(DMA_0_NAME)) == null)

ret++;

alt_dma_txchan_reg (*pTxchan);

if(alt_dma_txchan_ioctl(txchan,ALT_DMA_SET_MODE_32,null)<0)

printf ("Failed to set ALT_DMA_SET_MODE_32 in TX\r\n");

if(alt_dma_txchan_ioctl(txchan,ALT_DMA_RX_ONLY_OFF,tx_data)<0)

printf ("Failed to set ALT_DMA_RX_ONLY_OFF in TX \r\n");

if(alt_dma_txchan_ioctl(txchan,ALT_DMA_TX_ONLY_ON,tx_data)<0)

printf ("Failed to set ALT_DMA_TX_ONLY_ON in TX \r\n");

if ((rxchan = alt_dma_rxchan_open(DMA_0_NAME)) == null)

ret++;

if(alt_dma_rxchan_ioctl(rxchan,ALT_DMA_SET_MODE_32,null))

printf ("Failed to set ALT_DMA_SET_MODE_32 in RX\r\n");

if(alt_dma_rxchan_ioctl(rxchan,ALT_DMA_TX_ONLY_OFF,rx_buffer))

printf ("Failed to set ALT_DMA_TX_ONLY_OFF in RX \r\n");

if(alt_dma_rxchan_ioctl(rxchan,ALT_DMA_RX_ONLY_ON,rx_buffer))

printf ("Failed to set ALT_DMA_RX_ONLY_ON in RX \r\n");

rc = alt_dma_txchan_send ( txchan,tx_data,length,null,null);

if (rc < 0)

printf ("Failed to post transmit request, reason = %i\r\n", rc);

rc = alt_dma_rxchan_prepare (rxchan,rx_buffer,length,done,null);

if (rc < 0)

printf ("Failed to post read request, reason = %i\r\n", rc);

while (!rx_done);

....

"done" is the function to call when the DMA end.

"length" is 256, and teh DMA dimension is 1024

The problem is the function < alt_dma_txchan_send > return -22, but i don't understand where is the bug.

Help me!!!

Thank you