MBOX_UP_TX_START
mbox->tx_start = MBOX_UP_TX_START;
mbox->rx_start = MBOX_UP_TX_START;
mbox->tx_start = MBOX_UP_TX_START;
mbox->rx_start = MBOX_UP_TX_START;
#if MBOX_UP_TX_SIZE + MBOX_UP_TX_START != MBOX_SIZE