MAX_TX_PAGES
#define MAX_TX_BD_AVAIL (MAX_TX_PAGES * TOTAL_TX_BD_PER_PAGE)
bus_dmamap_t tx_bd_chain_map[MAX_TX_PAGES];
struct tx_bd *tx_bd_chain[MAX_TX_PAGES];
bus_addr_t tx_bd_chain_paddr[MAX_TX_PAGES];