MBOX_DOWN_TX_SIZE
mbox->tx_size = MBOX_DOWN_TX_SIZE;
mbox->rx_size = MBOX_DOWN_TX_SIZE;
mbox->tx_size = MBOX_DOWN_TX_SIZE;
mbox->rx_size = MBOX_DOWN_TX_SIZE;
#define MBOX_UP_RX_START (MBOX_DOWN_TX_START + MBOX_DOWN_TX_SIZE)