BufferDesc
((i+1)%NUM_TX_DESC)*sizeof(BufferDesc);
((i+1)%NUM_RX_DESC)*sizeof(BufferDesc);
BufferDesc *tx_ring;
BufferDesc *rx_ring;
#define TX_TOTAL_SIZE NUM_TX_DESC*sizeof(BufferDesc)
#define RX_TOTAL_SIZE NUM_RX_DESC*sizeof(BufferDesc)