TOTAL_TX_BD_PER_PAGE
#define USABLE_TX_BD_PER_PAGE (TOTAL_TX_BD_PER_PAGE - 1)
#define MAX_TX_BD_AVAIL (MAX_TX_PAGES * TOTAL_TX_BD_PER_PAGE)
#define TOTAL_TX_BD_ALLOC (TOTAL_TX_BD_PER_PAGE * sc->tx_pages)