MBOX_UP_RX_SIZE
mbox->rx_size = MBOX_UP_RX_SIZE;
mbox->tx_size = MBOX_UP_RX_SIZE;
mbox->rx_size = MBOX_UP_RX_SIZE;
mbox->tx_size = MBOX_UP_RX_SIZE;
#define MBOX_UP_TX_START (MBOX_UP_RX_START + MBOX_UP_RX_SIZE)