Calling wglMakeCurrent on an already current context gives 100% cpu load

The problem occurs at least with nvidia cards when vsync is enabled
in the control panel.

Task-number: QTBUG-29686
Change-Id: I6fd2a3560a5baeeac7c3fe0440db85904c45026d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/src/plugins/platforms/windows/qwindowsglcontext.cpp b/src/plugins/platforms/windows/qwindowsglcontext.cpp
index da3e2a6a6a..ae66ef8a3d 100644
--- a/src/plugins/platforms/windows/qwindowsglcontext.cpp
+++ b/src/plugins/platforms/windows/qwindowsglcontext.cpp
@@ -1052,8 +1052,16 @@ bool QWindowsGLContext::makeCurrent(QPlatformSurface *surface)
     // Do we already have a DC entry for that window?
     QWindowsWindow *window = static_cast<QWindowsWindow *>(surface);
     const HWND hwnd = window->handle();
-    if (const QOpenGLContextData *contextData = findByHWND(m_windowContexts, hwnd))
+    if (const QOpenGLContextData *contextData = findByHWND(m_windowContexts, hwnd)) {
+        // Repeated calls to wglMakeCurrent when vsync is enabled in the driver will
+        // often result in 100% cpuload. This check is cheap and avoids the problem.
+        // This is reproducable on NVidia cards and Intel onboard chips.
+        if (wglGetCurrentContext() == contextData->renderingContext
+                && wglGetCurrentDC() == contextData->hdc) {
+            return true;
+        }
         return wglMakeCurrent(contextData->hdc, contextData->renderingContext);
+    }
     // Create a new entry.
     const QOpenGLContextData newContext(m_renderingContext, hwnd, GetDC(hwnd));
     if (!newContext.hdc)