rpi-vc-mailbox.c

#define IOREG(X)  (*(volatile uint32_t *) (X))
#define MAILBOX0_FIFO   IOREG(0x2000B880)
#define MAILBOX0_POLL   IOREG(0x2000B890)
#define MAILBOX0_SENDER IOREG(0x2000B894)
#define MAILBOX0_STATUS IOREG(0x2000B898)
#define MAILBOX0_CONFIG IOREG(0x2000B89C)
#define MAILBOX1_FIFO   IOREG(0x2000B8A0)
#define MAILBOX1_POLL   IOREG(0x2000B8B0)
#define MAILBOX1_SENDER IOREG(0x2000B8B4)
#define MAILBOX1_STATUS IOREG(0x2000B8B8)
#define MAILBOX1_CONFIG IOREG(0x2000B8BC)

#define MAIL_FULL      0x80000000
#define MAIL_EMPTY     0x40000000

void mailbox_write(uint8_t chan, uint32_t msg) {
    if ((msg & 0xfU) == 0) {
        while ((MAILBOX1_STATUS & MAIL_FULL) != 0) {
        }
	MAILBOX1_FIFO = msg | chan;
    }
}

uint32_t mailbox_read(uint8_t chan) {
    uint32_t data;
    do {
        while (MAILBOX0_STATUS & MAIL_EMPTY) {
        }
    } while (((data = MAILBOX0_FIFO) & 0xfU) != chan);
    return data >> 4;
}