RX_NEXT
#define RX_NEXT(n) (((n) + 1) & (RX_DESC_COUNT - 1))
#define RX_NEXT(n, count) (((n) + 1) & ((count) - 1))
#define RX_NEXT(n) (((n) + 1) % RX_DESC_COUNT)
#define RX_NEXT 0x1c