GBA Audio: Redo channel 4 batching for GBA only

CHANGES

@@ -110,6 +110,7 @@ Other fixes:
  - Qt: Fix inability to clear default keybindings
 Misc:
  - GB Memory: Support manual SRAM editing (fixes mgba.io/i/1580)
+ - GBA Audio: Redo channel 4 batching for GBA only
  - GBA I/O: Stop logging several harmless invalid register reads
  - Debugger: Separate aliases from main commands
  - Debugger: Print break-/watchpoint ID when breaking in CLI

include/mgba/internal/gb/audio.h

@@ -139,6 +139,7 @@ struct GBAudioNoiseChannel {
 	uint32_t lfsr;
 	int nSamples;
 	int samples;
+	uint32_t lastEvent;
 
 	int8_t sample;
 };

include/mgba/internal/gb/serialize.h

@@ -79,7 +79,7 @@ mLOG_DECLARE_CATEGORY(GB_STATE);
  *   | bits 0 - 2: Remaining length
  *   | bits 3 - 5: Next step
  *   | bits 6 - 31: Reserved
- * | 0x00098 - 0x0009F: Reserved
+ * | 0x0009C - 0x0009F: Last event
  * | 0x000A0 - 0x000A3: Next event
  * 0x000A4 - 0x000B7: Audio miscellaneous state
  * | TODO: Fix this, they're in big-endian order, but field is little-endian
@@ -224,7 +224,7 @@ struct GBSerializedPSGState {
 	struct {
 		int32_t lfsr;
 		GBSerializedAudioEnvelope envelope;
-		int32_t reserved;
+		int32_t lastEvent;
 		uint32_t nextEvent;
 	} ch4;
 };

include/mgba/internal/gba/serialize.h

@@ -61,7 +61,7 @@ mLOG_DECLARE_CATEGORY(GBA_STATE);
  *   | bits 0 - 2: Remaining length
  *   | bits 3 - 5: Next step
  *   | bits 6 - 31: Reserved
- * | 0x00184 - 0x00187: Reserved
+ * | 0x00184 - 0x00187: Last event
  * | 0x00188 - 0x0018B: Next event
  * 0x0018C - 0x001AB: Audio FIFO 1
  * 0x001AC - 0x001CB: Audio FIFO 2

src/gb/audio.c

@@ -359,6 +359,7 @@ void GBAudioWriteNR42(struct GBAudio* audio, uint8_t value) {
 }
 
 void GBAudioWriteNR43(struct GBAudio* audio, uint8_t value) {
+	// TODO: Reschedule event
 	audio->ch4.ratio = GBAudioRegisterNoiseFeedbackGetRatio(value);
 	audio->ch4.frequency = GBAudioRegisterNoiseFeedbackGetFrequency(value);
 	audio->ch4.power = GBAudioRegisterNoiseFeedbackGetPower(value);
@@ -933,14 +934,30 @@ static void _updateChannel4(struct mTiming* timing, void* user, uint32_t cyclesL
 	cycles <<= ch->frequency;
 	cycles *= 8 * audio->timingFactor;
 
-	int lsb = ch->lfsr & 1;
-	ch->sample = lsb * ch->envelope.currentVolume;
-	++ch->nSamples;
-	ch->samples += ch->sample;
-	ch->lfsr >>= 1;
-	ch->lfsr ^= (lsb * 0x60) << (ch->power ? 0 : 8);
+	uint32_t last = 0;
+	uint32_t now = cycles;
+	uint32_t next = cycles - cyclesLate;
 
-	mTimingSchedule(timing, &audio->ch4Event, cycles - cyclesLate);
+	if (audio->style == GB_AUDIO_GBA) {
+		last = ch->lastEvent;
+		now = mTimingCurrentTime(timing) - cyclesLate;
+		ch->lastEvent = now;
+		now -= last;
+		last = 0;
+		// TODO: Make batching work when descheduled
+		next = audio->sampleInterval;
+	}
+
+	for (; last < now; last += cycles) {
+		int lsb = ch->lfsr & 1;
+		ch->sample = lsb * ch->envelope.currentVolume;
+		++ch->nSamples;
+		ch->samples += ch->sample;
+		ch->lfsr >>= 1;
+		ch->lfsr ^= (lsb * 0x60) << (ch->power ? 0 : 8);
+	}
+
+	mTimingSchedule(timing, &audio->ch4Event, next);
 }
 
 void GBAudioPSGSerialize(const struct GBAudio* audio, struct GBSerializedPSGState* state, uint32_t* flagsOut) {
@@ -984,6 +1001,7 @@ void GBAudioPSGSerialize(const struct GBAudio* audio, struct GBSerializedPSGStat
 	ch4Flags = GBSerializedAudioEnvelopeSetLength(ch4Flags, audio->ch4.length);
 	ch4Flags = GBSerializedAudioEnvelopeSetNextStep(ch4Flags, audio->ch4.envelope.nextStep);
 	STORE_32LE(ch4Flags, 0, &state->ch4.envelope);
+	STORE_32LE(audio->ch4.lastEvent, 0, &state->ch4.lastEvent);
 	STORE_32LE(audio->ch4Event.when - mTimingCurrentTime(audio->timing), 0, &state->ch4.nextEvent);
 
 	STORE_32LE(flags, 0, flagsOut);
@@ -1055,8 +1073,13 @@ void GBAudioPSGDeserialize(struct GBAudio* audio, const struct GBSerializedPSGSt
 	audio->ch4.length = GBSerializedAudioEnvelopeGetLength(ch4Flags);
 	audio->ch4.envelope.nextStep = GBSerializedAudioEnvelopeGetNextStep(ch4Flags);
 	LOAD_32LE(audio->ch4.lfsr, 0, &state->ch4.lfsr);
+	LOAD_32LE(audio->ch4.lastEvent, 0, &state->ch4.lastEvent);
 	LOAD_32LE(when, 0, &state->ch4.nextEvent);
 	if (audio->ch4.envelope.dead < 2 && audio->playingCh4) {
+		if (when - audio->ch4.lastEvent > (uint32_t) audio->sampleInterval) {
+			// Back-compat: fake this value
+			audio->ch4.lastEvent = when - audio->sampleInterval;
+		}
 		mTimingSchedule(audio->timing, &audio->ch4Event, when);
 	}
 }