TX_NEXT
#define TX_NEXT(n) (((n) + 1) & (TX_DESC_COUNT - 1))
#define TX_NEXT(n, count) (((n) + 1) & ((count) - 1))
#define TX_NEXT(n) (((n) + 1 ) % TX_DESC_COUNT)
#define TX_NEXT 0x0c