DMA transfer inside ISR: Problem

Hi everybody,

volatile static int rx_done = 0;
static void done_DMA(void* handle, void* data)
{
	rx_done = 1;
}
void measureIOs()
{
	int rc;
	alt_dma_txchan txchan;
	alt_dma_rxchan rxchan;
	unsigned long* tx_data =   (void*) EXT_DPR_TRISTATE_INTERFACE_0_BASE; // pointer to data to send
	unsigned long* rx_buffer = (void*) SDRAM_BASE + 0x100000; // pointer to rx buffer
	rx_buffer = (void*) SDRAM_BASE + 0x100000;
	rx_done = 0;
	// 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);
	}
	if(alt_dma_txchan_ioctl(txchan,ALT_DMA_SET_MODE_32,NULL)<0)
	{
		exit(1);
	}
	if(alt_dma_rxchan_ioctl(rxchan,ALT_DMA_SET_MODE_32,NULL)<0)
	{
		exit(1);
	}
	if(alt_dma_txchan_ioctl(txchan,ALT_DMA_TX_ONLY_OFF,NULL)<0)
	{
		exit(1);
	}
	if(alt_dma_rxchan_ioctl(rxchan,ALT_DMA_RX_ONLY_OFF,NULL)<0)
	{
		exit(1);
	}
	// Post the transmit request
	if ((rc = alt_dma_txchan_send (txchan,tx_data,4096,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,4096,done_DMA,NULL)) < 0)
	{
		printf ("Failed to post read request, reason = %i\n", rc);
		exit (1);
	}
	// wait for transfer to complete
	while (!rx_done);
	printf ("Transfer successful!\n");
}
/* INTERRUPT VARIABLE DEFINITONS */
volatile int edge_capture;
/* **************** INTERRUPT FUNCTIONS ********************* */
# ifdef PIO_SELECTED_RAM_BASE
# ifdef ALT_ENHANCED_INTERRUPT_API_PRESENT
static void handle_ram_interrupt(void* context)
# else
static void handle_ram_interrupt(void* context, alt_u32 id)
# endif
{
    /* Cast context to edge_capture's type. It is important that this be
     * declared volatile to avoid unwanted compiler optimization.
     */
    volatile int* edge_capture_ptr = (volatile int*) context;
    /* Store the value in the Button's edge capture register in *context. */
    *edge_capture_ptr = IORD_ALTERA_AVALON_PIO_EDGE_CAP(PIO_SELECTED_RAM_BASE);
    /* Reset the Button's edge capture register. */
    IOWR_ALTERA_AVALON_PIO_EDGE_CAP(PIO_SELECTED_RAM_BASE, 0);
    measureIOs();
    /*
     * Read the PIO to delay ISR exit. This is done to prevent a spurious
     * interrupt in systems with high procekssor -> pio latency and fast
     * interrupts.
     */
    IORD_ALTERA_AVALON_PIO_EDGE_CAP(PIO_SELECTED_RAM_BASE);
}
/* Initialize the ram_pio. */
static void init_ram_pio()
{
    /* Recast the edge_capture pointer to match the alt_irq_register() function
     * prototype. */
    void* edge_capture_ptr = (void*) &edge_capture;
    /* Enable all 4 button interrupts. */
    IOWR_ALTERA_AVALON_PIO_IRQ_MASK(PIO_SELECTED_RAM_BASE, 0x1);
    /* Reset the edge capture register. */
    IOWR_ALTERA_AVALON_PIO_EDGE_CAP(PIO_SELECTED_RAM_BASE, 0x0);
    /* Register the interrupt handler. */
# ifdef ALT_ENHANCED_INTERRUPT_API_PRESENT
    alt_ic_isr_register(PIO_SELECTED_RAM_IRQ_INTERRUPT_CONTROLLER_ID,PIO_SELECTED_RAM_IRQ,
    		handle_ram_interrupt, edge_capture_ptr, 0x0);
# else
    alt_irq_register( PIO_SELECTED_RAM_IRQ, edge_capture_ptr,
    		handle_ram_interrupt);
# endif
}
# endif /***************** PIO_SELECTED_RAM_BASE **********************/
int main()
{
  printf("Hello from Nios II!\n");
  init_ram_pio();
  while(1);
  return 0;
}