RX_BD_USABLE_PER_PAGE
#define RX_BD_USABLE (RX_BD_USABLE_PER_PAGE * RX_BD_NUM_PAGES)
((((x) & RX_BD_PER_PAGE_MASK) == (RX_BD_USABLE_PER_PAGE - 1)) ? \
((NUM_BD_REQ(sc) + RX_BD_USABLE_PER_PAGE - 1) / RX_BD_USABLE_PER_PAGE)