From 0786d7fe3b6c78f46236cbb8fe2b91793d5304be Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Tue, 4 Jul 2017 09:59:26 -0700
Subject: [PATCH] GBA Timer: Fix reload timing regression

---
 include/mgba/internal/gba/timer.h | 1 -
 src/gba/io.c                      | 2 --
 src/gba/timer.c                   | 6 +++++-
 3 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/include/mgba/internal/gba/timer.h b/include/mgba/internal/gba/timer.h
index 5cb23605b..664842e6b 100644
--- a/include/mgba/internal/gba/timer.h
+++ b/include/mgba/internal/gba/timer.h
@@ -23,7 +23,6 @@ struct GBATimer {
 	uint16_t reload;
 	int32_t lastEvent;
 	struct mTimingEvent event;
-	int32_t overflowInterval;
 	GBATimerFlags flags;
 };
 
diff --git a/src/gba/io.c b/src/gba/io.c
index ebcb30d95..bae8236f3 100644
--- a/src/gba/io.c
+++ b/src/gba/io.c
@@ -928,7 +928,6 @@ void GBAIOSerialize(struct GBA* gba, struct GBASerializedState* state) {
 		STORE_16(gba->timers[i].reload, 0, &state->timers[i].reload);
 		STORE_32(gba->timers[i].lastEvent - mTimingCurrentTime(&gba->timing), 0, &state->timers[i].lastEvent);
 		STORE_32(gba->timers[i].event.when - mTimingCurrentTime(&gba->timing), 0, &state->timers[i].nextEvent);
-		STORE_32(gba->timers[i].overflowInterval, 0, &state->timers[i].overflowInterval);
 		STORE_32(gba->timers[i].flags, 0, &state->timers[i].flags);
 		STORE_32(gba->memory.dma[i].nextSource, 0, &state->dma[i].nextSource);
 		STORE_32(gba->memory.dma[i].nextDest, 0, &state->dma[i].nextDest);
@@ -958,7 +957,6 @@ void GBAIODeserialize(struct GBA* gba, const struct GBASerializedState* state) {
 	mTimingSchedule(&gba->timing, &gba->timerMaster, when);
 	for (i = 0; i < 4; ++i) {
 		LOAD_16(gba->timers[i].reload, 0, &state->timers[i].reload);
-		LOAD_32(gba->timers[i].overflowInterval, 0, &state->timers[i].overflowInterval);
 		LOAD_32(gba->timers[i].flags, 0, &state->timers[i].flags);
 		if (i > 0 && GBATimerFlagsIsCountUp(gba->timers[i].flags)) {
 			// Overwrite invalid values in savestate
diff --git a/src/gba/timer.c b/src/gba/timer.c
index ee965aa78..d68fcbef4 100644
--- a/src/gba/timer.c
+++ b/src/gba/timer.c
@@ -13,7 +13,11 @@
 static void GBATimerUpdate(struct GBA* gba, int timerId, uint32_t cyclesLate) {
 	struct GBATimer* timer = &gba->timers[timerId];
 	gba->memory.io[(REG_TM0CNT_LO >> 1) + (timerId << 1)] = timer->reload;
-	GBATimerUpdateRegister(gba, timerId, cyclesLate);
+	int32_t currentTime = mTimingCurrentTime(&gba->timing) - cyclesLate;
+	int32_t tickMask = (1 << GBATimerFlagsGetPrescaleBits(timer->flags)) - 1;
+	currentTime &= ~tickMask;
+	timer->lastEvent = currentTime;
+	GBATimerUpdateRegister(gba, timerId, 0);
 
 	if (GBATimerFlagsIsDoIrq(timer->flags)) {
 		GBARaiseIRQ(gba, IRQ_TIMER0 + timerId);