Kernel: Add a MMIO class for aarch64

It doesn't do anything yet except figure out the peripheral base
address.

Very likely belongs in Kernel, not Prekernel, eventually.

Kernel/Prekernel/Arch/aarch64/MMIO.cpp

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2021, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Prekernel/Arch/aarch64/MMIO.h>
+#include <Kernel/Prekernel/Arch/aarch64/MainIdRegister.h>
+
+namespace Prekernel {
+
+MMIO::MMIO()
+    : m_base_address(0xFE00'0000)
+{
+    MainIdRegister id;
+    if (id.part_num() <= MainIdRegister::RaspberryPi3)
+        m_base_address = 0x3F00'0000;
+}
+
+MMIO& MMIO::the()
+{
+    static MMIO instance;
+    return instance;
+}
+
+}

Kernel/Prekernel/Arch/aarch64/MMIO.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2021, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Prekernel {
+
+// Knows about memory-mapped IO addresses on the Broadcom family of SOCs used in Raspberry Pis.
+// RPi3 is the first Raspberry Pi that supports aarch64.
+// https://datasheets.raspberrypi.org/bcm2836/bcm2836-peripherals.pdf (RPi3)
+// https://datasheets.raspberrypi.org/bcm2711/bcm2711-peripherals.pdf (RPi4 Model B)
+class MMIO {
+public:
+    static MMIO& the();
+
+private:
+    MMIO();
+
+    unsigned int m_base_address;
+};
+
+}

Kernel/Prekernel/Arch/aarch64/init.cpp

@@ -5,15 +5,14 @@
  */
 
 #include <AK/Types.h>
-#include <Kernel/Prekernel/Arch/aarch64/MainIdRegister.h>
+#include <Kernel/Prekernel/Arch/aarch64/MMIO.h>
 
 extern "C" [[noreturn]] void halt();
 
 extern "C" [[noreturn]] void init();
 extern "C" [[noreturn]] void init()
 {
-    Prekernel::MainIdRegister id;
+    [[maybe_unused]] auto& MMIO = Prekernel::MMIO::the();
-    [[maybe_unused]] unsigned part_num = id.part_num();
     halt();
 }
 

Kernel/Prekernel/CMakeLists.txt

@@ -13,6 +13,7 @@ if ("${SERENITY_ARCH}" STREQUAL "aarch64")
 
         ${SOURCES}
         Arch/aarch64/MainIdRegister.cpp
+        Arch/aarch64/MMIO.cpp
         Arch/aarch64/init.cpp
     )
 else()