From 8e096916b1dc1aacc17236e8d2bf6eda4edb5edf Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Fri, 27 Nov 2020 18:31:13 -0800
Subject: [PATCH] Qt: Discard additional frame draws if waiting fails

---
 CHANGES                       |  1 +
 src/platform/qt/DisplayGL.cpp | 24 +++++++++++++++++++-----
 src/platform/qt/DisplayGL.h   |  2 ++
 3 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/CHANGES b/CHANGES
index 843d9e530..dc94b316d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -102,6 +102,7 @@ Misc:
  - Qt: Add copy button to GB printer dialog
  - Qt: Window title updates can be disabled (closes mgba.io/i/1912)
  - Qt: Redo OpenGL context thread handling (fixes mgba.io/i/1724)
+ - Qt: Discard additional frame draws if waiting fails
  - Util: Reset vector size on deinit
  - VFS: Change semantics of VFile.sync on mapped files (fixes mgba.io/i/1730)
 
diff --git a/src/platform/qt/DisplayGL.cpp b/src/platform/qt/DisplayGL.cpp
index 17e1ea360..350aec1f1 100644
--- a/src/platform/qt/DisplayGL.cpp
+++ b/src/platform/qt/DisplayGL.cpp
@@ -432,8 +432,18 @@ void PainterGL::draw() {
 	if (!m_active || m_queue.isEmpty()) {
 		return;
 	}
+	if (m_lagging >= 1) {
+		return;
+	}
 	mCoreSync* sync = &m_context->thread()->impl->sync;
-	mCoreSyncWaitFrameStart(sync);
+	if (!mCoreSyncWaitFrameStart(sync)) {
+		mCoreSyncWaitFrameEnd(sync);
+		++m_lagging;
+		if (m_delayTimer.elapsed() < 1000 / m_surface->screen()->refreshRate()) {
+			QTimer::singleShot(1, this, &PainterGL::draw);
+		}
+		return;
+	}
 	dequeue();
 	if (m_videoProxy) {
 		// Only block on the next frame if we're trying to run at roughly 60fps via audio
@@ -445,17 +455,16 @@ void PainterGL::draw() {
 		while (m_delayTimer.nsecsElapsed() + 2000000 < 1000000000 / sync->fpsTarget) {
 			QThread::usleep(500);
 		}
-		m_delayTimer.restart();
 	}
 	mCoreSyncWaitFrameEnd(sync);
 
-	forceDraw();
+	performDraw();
+	m_backend->swap(m_backend);
+	m_delayTimer.restart();
 }
 
 void PainterGL::forceDraw() {
-	m_painter.begin(m_window.get());
 	performDraw();
-	m_painter.end();
 	if (!m_context->thread()->impl->sync.audioWait && !m_context->thread()->impl->sync.videoFrameWait) {
 		if (m_delayTimer.elapsed() < 1000 / m_surface->screen()->refreshRate()) {
 			return;
@@ -486,10 +495,12 @@ void PainterGL::pause() {
 }
 
 void PainterGL::unpause() {
+	m_lagging = 0;
 	m_active = true;
 }
 
 void PainterGL::performDraw() {
+	m_painter.begin(m_window.get());
 	m_painter.beginNativePainting();
 	float r = m_surface->devicePixelRatio();
 	m_backend->resized(m_backend, m_size.width() * r, m_size.height() * r);
@@ -501,6 +512,7 @@ void PainterGL::performDraw() {
 	if (m_showOSD && m_messagePainter) {
 		m_messagePainter->paint(&m_painter);
 	}
+	m_painter.end();
 }
 
 void PainterGL::enqueue(const uint32_t* backing) {
@@ -517,6 +529,7 @@ void PainterGL::enqueue(const uint32_t* backing) {
 			memcpy(buffer, backing, size.width() * size.height() * BYTES_PER_PIXEL);
 		}
 	}
+	m_lagging = 0;
 	m_queue.enqueue(buffer);
 }
 
@@ -531,6 +544,7 @@ void PainterGL::dequeue() {
 		m_buffer = nullptr;
 	}
 	m_buffer = buffer;
+	return;
 }
 
 void PainterGL::dequeueAll() {
diff --git a/src/platform/qt/DisplayGL.h b/src/platform/qt/DisplayGL.h
index be982b006..5af24e478 100644
--- a/src/platform/qt/DisplayGL.h
+++ b/src/platform/qt/DisplayGL.h
@@ -16,6 +16,7 @@
 #endif
 #endif
 
+#include <QAtomicInt>
 #include <QElapsedTimer>
 #include <QOpenGLContext>
 #include <QList>
@@ -130,6 +131,7 @@ private:
 	std::array<std::array<uint32_t, 0x100000>, 3> m_buffers;
 	QList<uint32_t*> m_free;
 	QQueue<uint32_t*> m_queue;
+	QAtomicInt m_lagging = 0;
 	uint32_t* m_buffer;
 	QPainter m_painter;
 	QMutex m_mutex;