From a15624dcc5fe1a9ce0df8380ea22c2d6df5eb27a Mon Sep 17 00:00:00 2001
From: Roman Gilg <subdiff@gmail.com>
Date: Thu, 9 Jan 2020 18:18:25 +0100
Subject: [PATCH] Revert "[platforms/x11/standalone] Remove SGI swap control
 and video sync"

This reverts commit 285adc1950c98a197ff51d2e82ec928e404a0533.

See: https://mail.kde.org/pipermail/kwin/2020-January/002999.html
---
 .../platforms/x11/standalone/glxbackend.cpp   | 39 ++++++++++++++++---
 plugins/platforms/x11/standalone/glxbackend.h |  3 ++
 2 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/plugins/platforms/x11/standalone/glxbackend.cpp b/plugins/platforms/x11/standalone/glxbackend.cpp
index e632109543..611d3e3996 100644
--- a/plugins/platforms/x11/standalone/glxbackend.cpp
+++ b/plugins/platforms/x11/standalone/glxbackend.cpp
@@ -212,6 +212,7 @@ void GlxBackend::init()
     m_haveMESACopySubBuffer = hasExtension(QByteArrayLiteral("GLX_MESA_copy_sub_buffer"));
     m_haveMESASwapControl   = hasExtension(QByteArrayLiteral("GLX_MESA_swap_control"));
     m_haveEXTSwapControl    = hasExtension(QByteArrayLiteral("GLX_EXT_swap_control"));
+    m_haveSGISwapControl    = hasExtension(QByteArrayLiteral("GLX_SGI_swap_control"));
     // only enable Intel swap event if env variable is set, see BUG 342582
     m_haveINTELSwapEvent    = hasExtension(QByteArrayLiteral("GLX_INTEL_swap_event"))
                                 && qgetenv("KWIN_USE_INTEL_SWAP_EVENT") == QByteArrayLiteral("1");
@@ -221,7 +222,7 @@ void GlxBackend::init()
         glXSelectEvent(display(), glxWindow, GLX_BUFFER_SWAP_COMPLETE_INTEL_MASK);
     }
 
-    haveSwapInterval = m_haveMESASwapControl || m_haveEXTSwapControl;
+    haveSwapInterval = m_haveMESASwapControl || m_haveEXTSwapControl || m_haveSGISwapControl;
 
     setSupportsBufferAge(false);
 
@@ -233,15 +234,23 @@ void GlxBackend::init()
     }
 
     setSyncsToVBlank(false);
-    setBlocksForRetrace(true);
+    setBlocksForRetrace(false);
+    haveWaitSync = false;
     const bool wantSync = options->glPreferBufferSwap() != Options::NoSwapEncourage;
     if (wantSync && glXIsDirect(display(), ctx)) {
         if (haveSwapInterval) { // glXSwapInterval is preferred being more reliable
             setSwapInterval(1);
             setSyncsToVBlank(true);
-        } else {
-            qCWarning(KWIN_X11STANDALONE) << "NO VSYNC! glSwapInterval is not supported";
-        }
+        } else if (hasExtension(QByteArrayLiteral("GLX_SGI_video_sync"))) {
+            unsigned int sync;
+            if (glXGetVideoSyncSGI(&sync) == 0 && glXWaitVideoSyncSGI(1, 0, &sync) == 0) {
+                setSyncsToVBlank(true);
+                setBlocksForRetrace(true);
+                haveWaitSync = true;
+            } else
+                qCWarning(KWIN_X11STANDALONE) << "NO VSYNC! glXSwapInterval is not supported, glXWaitVideoSync is supported but broken";
+        } else
+            qCWarning(KWIN_X11STANDALONE) << "NO VSYNC! neither glSwapInterval nor glXWaitVideoSync are supported";
     } else {
         // disable v-sync (if possible)
         setSwapInterval(0);
@@ -678,6 +687,25 @@ void GlxBackend::setSwapInterval(int interval)
         glXSwapIntervalEXT(display(), glxWindow, interval);
     else if (m_haveMESASwapControl)
         glXSwapIntervalMESA(interval);
+    else if (m_haveSGISwapControl)
+        glXSwapIntervalSGI(interval);
+}
+
+void GlxBackend::waitSync()
+{
+    // NOTE that vsync has no effect with indirect rendering
+    if (haveWaitSync) {
+        uint sync;
+#if 0
+        // TODO: why precisely is this important?
+        // the sync counter /can/ perform multiple steps during glXGetVideoSync & glXWaitVideoSync
+        // but this only leads to waiting for two frames??!?
+        glXGetVideoSync(&sync);
+        glXWaitVideoSync(2, (sync + 1) % 2, &sync);
+#else
+        glXWaitVideoSyncSGI(1, 0, &sync);
+#endif
+    }
 }
 
 void GlxBackend::present()
@@ -696,6 +724,7 @@ void GlxBackend::present()
         if (haveSwapInterval) {
             glXSwapBuffers(display(), glxWindow);
         } else {
+            waitSync();
             glXSwapBuffers(display(), glxWindow);
         }
         if (supportsBufferAge()) {
diff --git a/plugins/platforms/x11/standalone/glxbackend.h b/plugins/platforms/x11/standalone/glxbackend.h
index ec55ac49b1..33914337fb 100644
--- a/plugins/platforms/x11/standalone/glxbackend.h
+++ b/plugins/platforms/x11/standalone/glxbackend.h
@@ -86,6 +86,7 @@ private:
     bool initBuffer();
     bool checkVersion();
     void initExtensions();
+    void waitSync();
     bool initRenderingContext();
     bool initFbConfig();
     void initVisualDepthHashTable();
@@ -112,8 +113,10 @@ private:
     bool m_haveMESACopySubBuffer = false;
     bool m_haveMESASwapControl = false;
     bool m_haveEXTSwapControl = false;
+    bool m_haveSGISwapControl = false;
     bool m_haveINTELSwapEvent = false;
     bool haveSwapInterval = false;
+    bool haveWaitSync = false;
     Display *m_x11Display;
     friend class GlxTexture;
 };