About DMA ,help!

I have a sopc system include a DMA controller called "avalon_bus", which set up a channel to avalon bus.

My purpose is to realize memory-to-memory data transfer and sram is attached to avalon bus.

But once the funcation "alt_dma_txchan_open("/dev/avalon_dma")" runs, the program freezes.

I am not sure where the problem occures,the program or the sopc system.

the code show below is from <<Nios II Software Developer&#39;s Handbook>>.

Please give me some help ,thanks a lot!

int main (int argc, char* argv[], char* envp[])

{

int rc;

alt_dma_txchan txchan;

alt_dma_rxchan rxchan;

void* tx_data = (void*) 0x901000; /* pointer to data to send */

void* rx_buffer = (void*) 0x902000; /* pointer to rx buffer */

/* Create the transmit channel */

if ((txchan = alt_dma_txchan_open("/dev/avalon_dma")) == NULL)

{

printf ("Failed to open transmit channel\n");

exit (1);

}

/* Create the receive channel */

if ((rxchan = alt_dma_rxchan_open("/dev/avalon_dma")) == NULL)

{

printf ("Failed to open receive channel\n");

exit (1);

}

/* Post the transmit request */

if ((rc = alt_dma_txchan_send (txchan, tx_data, 128, NULL, NULL)) < 0)

{

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

exit (1);

}

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