From a8a39ba24e36119dce48a01c2139e0e5a25674e1 Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Tue, 25 Feb 2025 19:29:33 -0800
Subject: Add mailbox.

---
 src/mailbox.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 src/mailbox.c

(limited to 'src/mailbox.c')

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);
+}
+
-- 
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