questions about DMA transmition

/* begin receive data*/

if((rx= alt_avalon_dma_prepare(rxchan,rx_buffer,1024,done_t,NULL))<0)

{

printf("rxchan receive failed.reason: %d",rx);

exit(1);

}

after running,niosII console:

rx=-28;

I check the meaning of "28" in altera_avalon_dma.h:

start = priv->tx_start;

end = priv->tx_end;

slot = &priv->tx_buf[end];

next = (end + 1) & ALT_AVALON_DMA_NSLOTS_MSK;

if (next == start)

{

return -ENOSPC;

}

so I guess that there is no room in memory,but I am not sure which memory has no room and why it has no room,so I delete my signaltapII file so as to let more room off,but the error is still there.

I will keep trying~