RT2573_TX_DESC_SIZE
m = allocb(RAL_TXBUF_SIZE + RT2573_TX_DESC_SIZE, BPRI_MED);
m->b_rptr += RT2573_TX_DESC_SIZE; /* skip TX descriptor */
m->b_wptr += RT2573_TX_DESC_SIZE;
m->b_rptr -= RT2573_TX_DESC_SIZE; /* restore */
pktlen = msgdsize(m) - RT2573_TX_DESC_SIZE;
xferlen = (RT2573_TX_DESC_SIZE + pktlen + 3) & ~3;