6 files changed, 127 insertions, 17 deletions

diff --git a/src/kernel.c b/src/kernel.c
index a9a2d11..4296aa7 100644
--- a/src/kernel.c
+++ b/src/kernel.c
@@ -1,3 +1,4 @@
+#include <mailbox.h>
 #include <mmio.h>
 #include <raspi.h>
 #include <uart.h>
@@ -10,7 +11,9 @@ static void halt() {
 
 void main() {
   const int raspi = raspi_init();
-  mmio_init(raspi);
+  mmio_init(raspi); // Must be initialized before other peripherals.
+  
+  mbox_init();
   uart_init(raspi);
   
   uart_print("Hello world!\n");
diff --git a/src/mailbox.c b/src/mailbox.c
new file mode 100644
index 0000000..eaf0955
--- /dev/null
+++ b/src/mailbox.c
@@ -0,0 +1,73 @@
+#include <mailbox.h>
+
+#include <mmio.h>
+
+#include <stdbool.h>
+#include <stdint.h>
+
+enum
+{
+  MAILBOX = 0xB880 // Mailbox address relative to MMIO base address.
+};
+
+typedef uint32_t Message;
+
+static inline uint8_t msg_channel(Message msg) {
+  return msg & 0xf;
+}
+
+static inline const void* msg_data(Message msg) {
+  // The data field is actually a pointer to the data.
+  return (const void*)((uintptr_t)(msg >> 4));
+}
+
+static inline Message msg_make(uint8_t channel, const void* data) {
+  return ((uintptr_t)(data) << 4) | (channel & 0xf);
+}
+
+typedef struct Mailbox {
+  Message inbox;     // 0x00
+  uint32_t unused_1; // 0x04
+  uint32_t unused_2; // 0x08
+  uint32_t unused_3; // 0x0C
+  uint32_t unused_4; // 0x10
+  uint32_t unused_5; // 0x14
+  uint32_t status;   // 0x18
+  uint32_t unused_6; // 0x1C
+  Message outbox;    // 0x20
+} Mailbox;
+
+static inline bool inbox_empty(const volatile Mailbox* pMailbox) {
+  return (pMailbox->status & 0x40000000) != 0;
+}
+
+static inline bool outbox_full(const volatile Mailbox* pMailbox) {
+  return (pMailbox->status & 0x80000000) != 0;
+}
+
+static volatile Mailbox* pMailbox;
+
+void mbox_init() {
+  pMailbox = mmio_get_peripheral(MAILBOX);
+}
+
+const Mail* mbox_read(uint8_t channel) {
+  Message msg;
+  // Read until we get a message from the desired channel.
+  // Discard any other messages.
+  do {
+    // Wait until there is mail to receive.
+    while (inbox_empty(pMailbox));
+    // Read the mail.
+    msg = pMailbox->inbox;
+  } while (msg_channel(msg) != channel);
+  return (const Mail*)(msg_data(msg));
+}
+
+void mbox_write(uint8_t channel, const void* mail) {
+  // Wait until the outbox is clear.
+  while (outbox_full(pMailbox));
+  // Send the mail.
+  pMailbox->outbox = msg_make(channel, mail);
+}
+
diff --git a/src/mailbox.h b/src/mailbox.h
new file mode 100644
index 0000000..e96a4ed
--- /dev/null
+++ b/src/mailbox.h
@@ -0,0 +1,38 @@
+#pragma once
+
+#include <stdint.h>
+
+// The channel takes the lower 4 bits; the data pointer the upper 28. So a mail
+// must be aligned to a 16-byte boundary.
+#define MAIL_ALIGN 16
+
+enum
+{
+  PROPERTY_CHANNEL = 8,
+};
+
+typedef struct Tag {
+  union {
+    uint32_t all;
+    struct {
+      uint16_t command : 12; // Command.
+      uint8_t  type    :  4; // Command type.
+      uint8_t  device  :  4; // Hardware device.
+      uint16_t zeroes  : 12; // Reserved.
+    } id;
+  };
+  uint32_t size;    // Buffer size.
+  uint32_t code;    // Request/response code.
+  uint32_t data[1]; // Buffer data.
+} Tag;
+
+typedef struct __attribute__((aligned(MAIL_ALIGN))) Mail {
+  uint32_t size;    // Buffer size.
+  uint32_t code;    // Request/response code.
+  Tag      tags[1]; // Variable quantity.
+} Mail;
+
+void        mbox_init();
+const Mail* mbox_read(uint8_t channel);
+void        mbox_write(uint8_t channel, const void* mail);
+
diff --git a/src/mmio.c b/src/mmio.c
index e082d66..b2abe98 100644
--- a/src/mmio.c
+++ b/src/mmio.c
@@ -30,3 +30,7 @@ void mmio_write(uint32_t reg, uint32_t val) {
   *REG_ADDR(reg) = val;
 }
 
+void* mmio_get_peripheral(uint32_t peripheral_base) {
+  return MMIO_BASE + peripheral_base;
+}
+
diff --git a/src/mmio.h b/src/mmio.h
index b566bbe..4d111d6 100644
--- a/src/mmio.h
+++ b/src/mmio.h
@@ -2,7 +2,8 @@
 
 #include <stdint.h>
 
-void mmio_init(int raspi);
+void     mmio_init(int raspi);
 uint32_t mmio_read(uint32_t reg);
-void mmio_write(uint32_t reg, uint32_t val);
+void     mmio_write(uint32_t reg, uint32_t val);
+void*    mmio_get_peripheral(uint32_t peripheral_base);
 
diff --git a/src/uart.c b/src/uart.c
index bb2a0ea..3b107f4 100644
--- a/src/uart.c
+++ b/src/uart.c
@@ -5,6 +5,7 @@ References:
 #include <uart.h>
 
 #include <gpio.h>
+#include <mailbox.h>
 #include <mmio.h>
 
 enum
@@ -31,15 +32,10 @@ enum
     UART0_ITIP   = (UART0_BASE + 0x84),
     UART0_ITOP   = (UART0_BASE + 0x88),
     UART0_TDR    = (UART0_BASE + 0x8C),
-    // The offsets for Mailbox registers.
-    MBOX_BASE    = 0xB880,
-    MBOX_READ    = (MBOX_BASE + 0x00),
-    MBOX_STATUS  = (MBOX_BASE + 0x18),
-    MBOX_WRITE   = (MBOX_BASE + 0x20)
 };
 
 // A mailbox message with set clock rate of PL011 to 3MHz tag.
-static volatile unsigned int  __attribute__((aligned(16))) mbox[9] = {
+static const uint32_t __attribute__((aligned(MAIL_ALIGN))) UART_SET_CLK[9] = {
   9*4, 0, 0x38002, 12, 8, 2, 3000000, 0, 0
 };
 
@@ -77,14 +73,9 @@ void uart_init(int raspi) {
   // For Raspi3 and 4 the UART_CLOCK is system-clock dependent by default.
   // Set it to 3Mhz so that we can consistently set the baud rate
   if (raspi >= 3) {
-    // UART_CLOCK = 30000000;
-    unsigned int r = (unsigned int) (((uint64_t)(&mbox) & ~0xF) | 8);
-    // Wait until we can talk to the VC.
-    while (mmio_read(MBOX_STATUS) & 0x80000000);
-    // Send our message to property channel and wait for the response.
-    mmio_write(MBOX_WRITE, r);
-    while ((mmio_read(MBOX_STATUS) & 0x40000000) ||
-           (mmio_read(MBOX_READ) != r));
+    // Send message over property channel to configure UART clock.
+    mbox_write(PROPERTY_CHANNEL, UART_SET_CLK);
+    mbox_read(PROPERTY_CHANNEL);
   }
 
   // Divider = 3000000 / (16 * 115200) = 1.627 = ~1.