scene: Reduce the call cost of Platform::supportsQpaContext()

Every time Platform::supportsQpaContext() is called, we go through the
list of supported extensions and perform a string comparison op. This is
not really cheap.

platform.cpp

@@ -416,7 +416,7 @@ void Platform::warpPointer(const QPointF &globalPos)
 bool Platform::supportsQpaContext() const
 {
     if (Compositor *c = Compositor::self()) {
-        return c->scene()->openGLPlatformInterfaceExtensions().contains(QByteArrayLiteral("EGL_KHR_surfaceless_context"));
+        return c->scene()->supportsSurfacelessContext();
     }
     return false;
 }

platformsupport/scenes/opengl/abstract_egl_backend.cpp

@@ -103,6 +103,7 @@ bool AbstractEglBackend::initEglAPI()
     qCDebug(KWIN_OPENGL) << "EGL version: " << major << "." << minor;
     const QByteArray eglExtensions = eglQueryString(m_display, EGL_EXTENSIONS);
     setExtensions(eglExtensions.split(' '));
+    setSupportsSurfacelessContext(hasExtension(QByteArrayLiteral("EGL_KHR_surfaceless_context")));
     return true;
 }
 

platformsupport/scenes/opengl/backend.h

@@ -166,11 +166,9 @@ public:
         return m_haveSwapBuffersWithDamage;
     }
 
-    /**
-     * @returns whether the context is surfaceless
-     */
-    bool isSurfaceLessContext() const {
-        return m_surfaceLessContext;
+    bool supportsSurfacelessContext() const
+    {
+        return m_haveSurfacelessContext;
     }
 
     /**
@@ -267,6 +265,11 @@ protected:
         m_haveSwapBuffersWithDamage = value;
     }
 
+    void setSupportsSurfacelessContext(bool value)
+    {
+        m_haveSurfacelessContext = value;
+    }
+
     /**
      * @return const QRegion& Damage of previously rendered frame
      */
@@ -285,13 +288,6 @@ protected:
         m_renderTimer.start();
     }
 
-    /**
-     * @param set whether the context is surface less
-     */
-    void setSurfaceLessContext(bool set) {
-        m_surfaceLessContext = set;
-    }
-
     /**
      * Sets the platform-specific @p extensions.
      *
@@ -323,6 +319,10 @@ private:
      */
     bool m_havePartialUpdate;
     bool m_haveSwapBuffersWithDamage = false;
+    /**
+     * @brief Whether the backend supports EGL_KHR_surfaceless_context.
+     */
+    bool m_haveSurfacelessContext = false;
     /**
      * @brief Whether the initialization failed, of course default to @c false.
      */
@@ -339,7 +339,6 @@ private:
      * @brief Timer to measure how long a frame renders.
      */
     QElapsedTimer m_renderTimer;
-    bool m_surfaceLessContext = false;
 
     QList<QByteArray> m_extensions;
 };

plugins/platforms/virtual/egl_gbm_backend.cpp

@@ -103,16 +103,14 @@ bool EglGbmBackend::initRenderingContext()
 {
     initBufferConfigs();
 
-    const char* eglExtensionsCString = eglQueryString(eglDisplay(), EGL_EXTENSIONS);
-    const QList<QByteArray> extensions = QByteArray::fromRawData(eglExtensionsCString, qstrlen(eglExtensionsCString)).split(' ');
-    if (!extensions.contains(QByteArrayLiteral("EGL_KHR_surfaceless_context"))) {
+    if (!supportsSurfacelessContext()) {
+        qCWarning(KWIN_VIRTUAL) << "EGL_KHR_surfaceless_context extension is unavailable";
         return false;
     }
 
     if (!createContext()) {
         return false;
     }
-    setSurfaceLessContext(true);
 
     return makeCurrent();
 }

plugins/scenes/opengl/scene_opengl.cpp

@@ -860,6 +860,11 @@ void SceneOpenGL::doneOpenGLContextCurrent()
     m_backend->doneCurrent();
 }
 
+bool SceneOpenGL::supportsSurfacelessContext() const
+{
+    return m_backend->supportsSurfacelessContext();
+}
+
 Scene::EffectFrame *SceneOpenGL::createEffectFrame(EffectFrameImpl *frame)
 {
     return new SceneOpenGL::EffectFrame(frame, this);

plugins/scenes/opengl/scene_opengl.h

@@ -45,6 +45,7 @@ public:
     bool syncsToVBlank() const override;
     bool makeOpenGLContextCurrent() override;
     void doneOpenGLContextCurrent() override;
+    bool supportsSurfacelessContext() const override;
     Decoration::Renderer *createDecorationRenderer(Decoration::DecoratedClientImpl *impl) override;
     void triggerFence() override;
     virtual QMatrix4x4 projectionMatrix() const = 0;

scene.cpp

@@ -681,6 +681,11 @@ void Scene::doneOpenGLContextCurrent()
 {
 }
 
+bool Scene::supportsSurfacelessContext() const
+{
+    return false;
+}
+
 void Scene::triggerFence()
 {
 }

scene.h

@@ -139,6 +139,7 @@ public:
 
     virtual bool makeOpenGLContextCurrent();
     virtual void doneOpenGLContextCurrent();
+    virtual bool supportsSurfacelessContext() const;
 
     virtual QMatrix4x4 screenProjectionMatrix() const;