From 21b6b529f36c635820c0e650025e7b0c23121968 Mon Sep 17 00:00:00 2001
From: lifehackerhansol <lifehackerhansol@ds-homebrew.com>
Date: Sun, 13 Oct 2024 04:46:43 -0700
Subject: [PATCH] Work around broken FIFO setup

Not entirely sure why, but FIFOs were being sent and received AFTER the
interrupts were cleared, causing them to not send.

It's unknown how this ever worked on older libnds/devkitARM, but FIFOs
need interrupts to work.

This workaround, instead, hijacks an area of the ROM header as a
temporary variable to wait for ARM7.
---
 arm7/source/main.cpp                     |  3 ++-
 arm9/source/launcher/AcekardLauncher.cpp | 32 +++++++-----------------
 2 files changed, 11 insertions(+), 24 deletions(-)

diff --git a/arm7/source/main.cpp b/arm7/source/main.cpp
index 12f3520..9f5e72f 100644
--- a/arm7/source/main.cpp
+++ b/arm7/source/main.cpp
@@ -61,7 +61,8 @@ static void prepairReset() {
     REG_IE = 0;
     REG_IF = ~0;
 
-    fifoSendValue32(FIFO_USER_01, MENU_MSG_ARM7_READY_BOOT);
+    // instruct arm9 to reset
+    *((vu32*)0x02FFFE04) = MENU_MSG_ARM7_READY_BOOT;
     swiDelay(1);
 }
 
diff --git a/arm9/source/launcher/AcekardLauncher.cpp b/arm9/source/launcher/AcekardLauncher.cpp
index 4fc9b1c..2651689 100644
--- a/arm9/source/launcher/AcekardLauncher.cpp
+++ b/arm9/source/launcher/AcekardLauncher.cpp
@@ -21,35 +21,21 @@
 // 256 UCS-2 characters encoded into UTF-8 can use up to 768 UTF-8 chars
 #define MAX_FILENAME_LENGTH 768
 
-// FIFO_CHANNEL_BITS - number of bits used to specify the channel in a packet - default=4
-#define FIFO_CHANNEL_BITS 4
-#define FIFO_CHANNEL_SHIFT (32 - FIFO_CHANNEL_BITS)
-#define FIFO_IMMEDIATEBIT_SHIFT (FIFO_CHANNEL_SHIFT - 2)
-#define FIFO_IMMEDIATEBIT (1 << FIFO_IMMEDIATEBIT_SHIFT)
-#define FIFO_EXTRABIT_SHIFT (FIFO_CHANNEL_SHIFT - 3)
-#define FIFO_EXTRABIT (1 << FIFO_EXTRABIT_SHIFT)
-#define FIFO_VALUE32_MASK (FIFO_EXTRABIT - 1)
-
-#define FIFO_PACK_VALUE32(channel, value32) \
-    (((channel) << FIFO_CHANNEL_SHIFT) | FIFO_IMMEDIATEBIT | (((value32)) & FIFO_VALUE32_MASK))
-
 static void resetAndLoop() {
+    DC_FlushAll();
+    DC_InvalidateAll();
+
+    fifoSendValue32(FIFO_USER_01, MENU_MSG_ARM7_REBOOT);
+    *((vu32*)0x02FFFE04) = 0;
+
     // Interrupt
     REG_IME = 0;
     REG_IE = 0;
     REG_IF = ~0;
 
-    DC_FlushAll();
-    DC_InvalidateAll();
-
-    fifoSendValue32(FIFO_USER_01, MENU_MSG_ARM7_REBOOT);
-    while (true) {
-        while (REG_IPC_FIFO_CR & IPC_FIFO_RECV_EMPTY)
-            ;
-        u32 res = REG_IPC_FIFO_RX;
-        if (FIFO_PACK_VALUE32(FIFO_USER_01, MENU_MSG_ARM7_READY_BOOT) == res) break;
-    }
-
+    // wait for arm7
+    while (*((vu32*)0x02FFFE04) == 0)
+        ;
     swiSoftReset();
 }