From 4f3adbb52043b2b641d06ca26d14d7a326275773 Mon Sep 17 00:00:00 2001
From: Vlad Zahorodnii <vlad.zahorodnii@kde.org>
Date: Wed, 18 Nov 2020 15:49:25 +0200
Subject: [PATCH] Drop SGI_video_sync based method for vsync

With the new compositing timing, we want to start compositing some time
later after a vsync event. This doesn't go along with the video sync
based method to synchronize buffer swaps with vblank.

Since practically all drivers nowadays provide support for the swap
control extensions (GLX_EXT_swap_control, GLX_SGI_swap_control, or
GLX_MESA_swap_control), it's safe to rely on them for the purpose of
synchronizing buffer swaps to vblank.
---
 .../platforms/x11/standalone/glxbackend.cpp   | 40 +++----------------
 plugins/platforms/x11/standalone/glxbackend.h |  3 --
 2 files changed, 5 insertions(+), 38 deletions(-)

diff --git a/plugins/platforms/x11/standalone/glxbackend.cpp b/plugins/platforms/x11/standalone/glxbackend.cpp
index 336b79f808..fcd8ef5f6b 100644
--- a/plugins/platforms/x11/standalone/glxbackend.cpp
+++ b/plugins/platforms/x11/standalone/glxbackend.cpp
@@ -100,7 +100,6 @@ GlxBackend::GlxBackend(Display *display)
     , glxWindow(None)
     , ctx(nullptr)
     , m_bufferAge(0)
-    , haveSwapInterval(false)
     , m_x11Display(display)
 {
      // Force initialization of GLX integration in the Qt's xcb backend
@@ -204,7 +203,7 @@ void GlxBackend::init()
         glXSelectEvent(display(), glxWindow, GLX_BUFFER_SWAP_COMPLETE_INTEL_MASK);
     }
 
-    haveSwapInterval = m_haveMESASwapControl || m_haveEXTSwapControl || m_haveSGISwapControl;
+    bool haveSwapInterval = m_haveMESASwapControl || m_haveEXTSwapControl || m_haveSGISwapControl;
 
     setSupportsBufferAge(false);
 
@@ -216,21 +215,14 @@ void GlxBackend::init()
     }
 
     setSyncsToVBlank(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 if (hasExtension(QByteArrayLiteral("GLX_SGI_video_sync"))) {
-            unsigned int sync;
-            if (glXGetVideoSyncSGI(&sync) == 0 && glXWaitVideoSyncSGI(1, 0, &sync) == 0) {
-                setSyncsToVBlank(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 {
+            qCWarning(KWIN_X11STANDALONE) << "glSwapInterval is unsupported";
+        }
     } else {
         // disable v-sync (if possible)
         setSwapInterval(0);
@@ -671,23 +663,6 @@ void GlxBackend::setSwapInterval(int interval)
         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(const QRegion &damage)
 {
     if (damage.isEmpty()) {
@@ -702,12 +677,7 @@ void GlxBackend::present(const QRegion &damage)
         if (m_haveINTELSwapEvent)
             Compositor::self()->aboutToSwapBuffers();
 
-        if (haveSwapInterval) {
-            glXSwapBuffers(display(), glxWindow);
-        } else {
-            waitSync();
-            glXSwapBuffers(display(), glxWindow);
-        }
+        glXSwapBuffers(display(), glxWindow);
         if (supportsBufferAge()) {
             glXQueryDrawable(display(), glxWindow, GLX_BACK_BUFFER_AGE_EXT, (GLuint *) &m_bufferAge);
         }
diff --git a/plugins/platforms/x11/standalone/glxbackend.h b/plugins/platforms/x11/standalone/glxbackend.h
index 0a591b6eab..26811c8195 100644
--- a/plugins/platforms/x11/standalone/glxbackend.h
+++ b/plugins/platforms/x11/standalone/glxbackend.h
@@ -73,7 +73,6 @@ private:
     bool initBuffer();
     bool checkVersion();
     void initExtensions();
-    void waitSync();
     bool initRenderingContext();
     bool initFbConfig();
     void initVisualDepthHashTable();
@@ -102,8 +101,6 @@ private:
     bool m_haveEXTSwapControl = false;
     bool m_haveSGISwapControl = false;
     bool m_haveINTELSwapEvent = false;
-    bool haveSwapInterval = false;
-    bool haveWaitSync = false;
     Display *m_x11Display;
     friend class GlxTexture;
 };