Add logging (#492)

* Add logger implementation for mgba debugging

* Add logging details to contributors guide

* Clang-format

* update docs real quick

* Cleanup for PR

* Update for PR

* Fix bad rebase

* Fix docs

source/mgba_logger.c

@@ -0,0 +1,53 @@
+#include "mgba_logger.h"
+
+#ifdef MGBA_LOGGING
+
+#include <stdarg.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+#include <tonc.h>
+
+#define MGBA_REG_DEBUG_ENABLE ((vu16*)0x4FFF780)
+#define MGBA_REG_DEBUG_FLAGS  ((vu16*)0x4FFF700)
+#define MGBA_REG_DEBUG_STRING ((char*)0x4FFF600)
+
+static const u32 MGBA_ENABLE_MAGIC = 0xC0DE;
+static const u32 MGBA_ENABLE_OK = 0x1DEA;
+static const u32 MGBA_LOG_SEND = 0x100;
+static const u32 MGBA_LOG_BUFFER_SIZE = 0x100;
+
+static bool mgba_logger_available = false;
+
+bool mgba_logger_init(void)
+{
+    *MGBA_REG_DEBUG_ENABLE = MGBA_ENABLE_MAGIC;
+    mgba_logger_available = (*MGBA_REG_DEBUG_ENABLE == MGBA_ENABLE_OK);
+    return mgba_logger_available;
+}
+
+void mgba_printf(MgbaLogLevel level, const char* fmt, ...)
+{
+    if (!mgba_logger_available || fmt == NULL)
+        return;
+
+    va_list args;
+    va_start(args, fmt);
+    vsnprintf(MGBA_REG_DEBUG_STRING, MGBA_LOG_BUFFER_SIZE, fmt, args);
+    va_end(args);
+
+    *MGBA_REG_DEBUG_FLAGS = ((uint16_t)level & 0x7) | MGBA_LOG_SEND;
+}
+#else
+
+// Noop stubs
+bool mgba_logger_init(void)
+{
+    return false;
+}
+
+void mgba_printf(MgbaLogLevel level, const char* fmt, ...)
+{
+}
+
+#endif