IMXUART_RING_SIZE
#ifndef IMXUART_RING_SIZE
u_int imxuart_rbuf_size = IMXUART_RING_SIZE;
u_int imxuart_rbuf_hiwat = (IMXUART_RING_SIZE * 1) / 4;
u_int imxuart_rbuf_lowat = (IMXUART_RING_SIZE * 3) / 4;