Avoid painting unchanged scene layers

Currently when we move the mouse the one render loop triggers a repaint.
When the cursor layer needs a new update we end up in the compositor
repainting the main content.

Even though painting should mostly no-op it still goes through all
existing items and effects to collect damage, still potentially making
the GL context current which could stall. A waste when we know we
haven't got anything to do. It's enough to cause noticable mouse lag on
some hardware.

Co-authored-by: Vlad Zahorodnii <vlad.zahorodnii@kde.org>

autotests/drm/drmTest.cpp

@@ -404,7 +404,8 @@ void DrmTest::testModeset()
     const auto output = gpu->drmOutputs().front();
     const auto layer = renderBackend->primaryLayer(output);
     layer->beginFrame();
-    output->renderLoop()->beginFrame();
+    output->renderLoop()->prepareNewFrame();
+    output->renderLoop()->beginPaint();
     layer->endFrame(infiniteRegion(), infiniteRegion());
     QVERIFY(gpu->drmOutputs().front()->present());
 

src/composite.cpp

@@ -31,11 +31,9 @@
 #include "scene/surfaceitem_x11.h"
 #include "scene/workspacescene_opengl.h"
 #include "scene/workspacescene_qpainter.h"
-#include "shadow.h"
 #include "useractions.h"
 #include "utils/common.h"
 #include "utils/xcbutils.h"
-#include "wayland/surface_interface.h"
 #include "wayland_server.h"
 #include "workspace.h"
 #include "x11syncmanager.h"
@@ -685,44 +683,51 @@ void Compositor::composite(RenderLoop *renderLoop)
     fTraceDuration("Paint (", output->name(), ")");
 
     RenderLayer *superLayer = m_superlayers[renderLoop];
-    prePaintPass(superLayer);
     superLayer->setOutputLayer(primaryLayer);
 
-    SurfaceItem *scanoutCandidate = superLayer->delegate()->scanoutCandidate();
-    renderLoop->setFullscreenSurface(scanoutCandidate);
-    output->setContentType(scanoutCandidate ? scanoutCandidate->contentType() : ContentType::None);
+    renderLoop->prepareNewFrame();
 
-    renderLoop->beginFrame();
-    bool directScanout = false;
-    if (scanoutCandidate) {
-        const auto sublayers = superLayer->sublayers();
-        const bool scanoutPossible = std::none_of(sublayers.begin(), sublayers.end(), [](RenderLayer *sublayer) {
-            return sublayer->isVisible();
-        });
-        if (scanoutPossible && !output->directScanoutInhibited()) {
-            directScanout = primaryLayer->scanout(scanoutCandidate);
+    if (superLayer->needsRepaint()) {
+        renderLoop->beginPaint();
+        prePaintPass(superLayer);
+
+        SurfaceItem *scanoutCandidate = superLayer->delegate()->scanoutCandidate();
+        renderLoop->setFullscreenSurface(scanoutCandidate);
+        output->setContentType(scanoutCandidate ? scanoutCandidate->contentType() : ContentType::None);
+
+        bool directScanout = false;
+        if (scanoutCandidate) {
+            const auto sublayers = superLayer->sublayers();
+            const bool scanoutPossible = std::none_of(sublayers.begin(), sublayers.end(), [](RenderLayer *sublayer) {
+                return sublayer->isVisible();
+            });
+            if (scanoutPossible && !output->directScanoutInhibited()) {
+                directScanout = primaryLayer->scanout(scanoutCandidate);
+            }
         }
-    }
 
-    if (!directScanout) {
-        QRegion surfaceDamage = primaryLayer->repaints();
-        primaryLayer->resetRepaints();
-        preparePaintPass(superLayer, &surfaceDamage);
+        if (!directScanout) {
+            QRegion surfaceDamage = primaryLayer->repaints();
+            primaryLayer->resetRepaints();
+            preparePaintPass(superLayer, &surfaceDamage);
 
-        if (auto beginInfo = primaryLayer->beginFrame()) {
-            auto &[renderTarget, repaint] = beginInfo.value();
+            if (auto beginInfo = primaryLayer->beginFrame()) {
+                auto &[renderTarget, repaint] = beginInfo.value();
 
-            const QRegion bufferDamage = surfaceDamage.united(repaint).intersected(superLayer->rect().toAlignedRect());
+                const QRegion bufferDamage = surfaceDamage.united(repaint).intersected(superLayer->rect().toAlignedRect());
 
-            paintPass(superLayer, renderTarget, bufferDamage);
-            primaryLayer->endFrame(bufferDamage, surfaceDamage);
+                paintPass(superLayer, renderTarget, bufferDamage);
+                primaryLayer->endFrame(bufferDamage, surfaceDamage);
+            }
         }
-    }
 
-    postPaintPass(superLayer);
+        postPaintPass(superLayer);
+    }
 
     m_backend->present(output);
 
+    framePass(superLayer);
+
     // TODO: Put it inside the cursor layer once the cursor layer can be backed by a real output layer.
     if (waylandServer()) {
         const std::chrono::milliseconds frameTime =
@@ -737,6 +742,15 @@ void Compositor::composite(RenderLoop *renderLoop)
     }
 }
 
+void Compositor::framePass(RenderLayer *layer)
+{
+    layer->delegate()->frame();
+    const auto sublayers = layer->sublayers();
+    for (RenderLayer *sublayer : sublayers) {
+        framePass(sublayer);
+    }
+}
+
 void Compositor::prePaintPass(RenderLayer *layer)
 {
     layer->delegate()->prePaint();

src/composite.h

@@ -203,6 +203,7 @@ private:
     void postPaintPass(RenderLayer *layer);
     void preparePaintPass(RenderLayer *layer, QRegion *repaint);
     void paintPass(RenderLayer *layer, const RenderTarget &renderTarget, const QRegion &region);
+    void framePass(RenderLayer *layer);
 
     State m_state = State::Off;
     std::unique_ptr<CompositorSelectionOwner> m_selectionOwner;

src/core/renderlayer.cpp

@@ -134,6 +134,25 @@ void RenderLayer::setGeometry(const QRectF &geometry)
     }
 }
 
+void RenderLayer::scheduleRepaint(Item *item)
+{
+    m_repaintScheduled = true;
+    m_loop->scheduleRepaint(item);
+}
+
+bool RenderLayer::needsRepaint() const
+{
+    if (m_repaintScheduled) {
+        return true;
+    }
+    if (!m_repaints.isEmpty()) {
+        return true;
+    }
+    return std::any_of(m_sublayers.constBegin(), m_sublayers.constEnd(), [](RenderLayer *layer) {
+        return layer->needsRepaint();
+    });
+}
+
 void RenderLayer::updateBoundingRect()
 {
     QRectF boundingRect = rect();
@@ -183,6 +202,7 @@ QRegion RenderLayer::repaints() const
 void RenderLayer::resetRepaints()
 {
     m_repaints = QRegion();
+    m_repaintScheduled = false;
 }
 
 bool RenderLayer::isVisible() const

src/core/renderlayer.h

@@ -21,6 +21,7 @@ namespace KWin
 class OutputLayer;
 class RenderLayerDelegate;
 class RenderLoop;
+class Item;
 
 /**
  * The RenderLayer class represents a composited layer.
@@ -72,6 +73,16 @@ public:
     QRectF geometry() const;
     void setGeometry(const QRectF &rect);
 
+    /**
+     * Mark this layer as dirty and trigger a repaint
+     * on the render loop
+     */
+    void scheduleRepaint(Item *item);
+    /**
+     * Returns true if this or a sublayer has requested an update
+     */
+    bool needsRepaint() const;
+
     void addRepaint(const QRegion &region);
     void addRepaint(const QRect &rect);
     void addRepaint(int x, int y, int width, int height);
@@ -87,6 +98,7 @@ private:
     bool computeEffectiveVisibility() const;
 
     RenderLoop *m_loop;
+    bool m_repaintScheduled = false;
     std::unique_ptr<RenderLayerDelegate> m_delegate;
     QRegion m_repaints;
     QRectF m_boundingRect;

src/core/renderlayerdelegate.cpp

@@ -24,6 +24,10 @@ QRegion RenderLayerDelegate::repaints() const
     return QRegion();
 }
 
+void RenderLayerDelegate::frame()
+{
+}
+
 void RenderLayerDelegate::prePaint()
 {
 }

src/core/renderlayerdelegate.h

@@ -35,13 +35,18 @@ public:
     virtual QRegion repaints() const;
 
     /**
-     * This function is called by the compositor before starting compositing. Reimplement
+     * This function is called by the compositor after compositing the frame.
+     */
+    virtual void frame();
+
+    /**
+     * This function is called by the compositor before starting painting. Reimplement
      * this function to do frame initialization.
      */
     virtual void prePaint();
 
     /**
-     * This function is called by the compositor after finishing compositing. Reimplement
+     * This function is called by the compositor after finishing painting. Reimplement
      * this function to do post frame cleanup.
      */
     virtual void postPaint();

src/core/renderloop.cpp

@@ -198,12 +198,16 @@ void RenderLoop::uninhibit()
     }
 }
 
-void RenderLoop::beginFrame()
+void RenderLoop::prepareNewFrame()
 {
-    d->pendingRepaint = false;
     d->pendingFrameCount++;
 }
 
+void RenderLoop::beginPaint()
+{
+    d->pendingRepaint = false;
+}
+
 int RenderLoop::refreshRate() const
 {
     return d->refreshRate;

src/core/renderloop.h

@@ -46,11 +46,17 @@ public:
      */
     void uninhibit();
 
+    /**
+     * This function must be called before the Compositor sumbits the next
+     * frame.
+     */
+    void prepareNewFrame();
+
     /**
      * This function must be called before the Compositor starts rendering the next
      * frame.
      */
-    void beginFrame();
+    void beginPaint();
 
     /**
      * Returns the refresh rate at which the output is being updated, in millihertz.

src/scene/item.cpp

@@ -6,7 +6,6 @@
 
 #include "scene/item.h"
 #include "core/renderlayer.h"
-#include "core/renderloop.h"
 #include "scene/scene.h"
 #include "utils/common.h"
 
@@ -310,7 +309,7 @@ void Item::scheduleRepaintInternal(const QRegion &region)
         const QRegion dirtyRegion = globalRegion & delegate->viewport();
         if (!dirtyRegion.isEmpty()) {
             m_repaints[delegate] += dirtyRegion;
-            delegate->layer()->loop()->scheduleRepaint(this);
+            delegate->layer()->scheduleRepaint(this);
         }
     }
 }
@@ -321,7 +320,7 @@ void Item::scheduleRepaintInternal(SceneDelegate *delegate, const QRegion &regio
     const QRegion dirtyRegion = globalRegion & delegate->viewport();
     if (!dirtyRegion.isEmpty()) {
         m_repaints[delegate] += dirtyRegion;
-        delegate->layer()->loop()->scheduleRepaint(this);
+        delegate->layer()->scheduleRepaint(this);
     }
 }
 
@@ -334,7 +333,7 @@ void Item::scheduleFrame()
     const QList<SceneDelegate *> delegates = m_scene->delegates();
     for (SceneDelegate *delegate : delegates) {
         if (delegate->viewport().intersects(geometry)) {
-            delegate->layer()->loop()->scheduleRepaint(this);
+            delegate->layer()->scheduleRepaint(this);
         }
     }
 }

src/scene/scene.cpp

@@ -49,6 +49,11 @@ void SceneDelegate::paint(const RenderTarget &renderTarget, const QRegion &regio
     m_scene->paint(renderTarget, region == infiniteRegion() ? infiniteRegion() : region.translated(viewport().topLeft()));
 }
 
+void SceneDelegate::frame()
+{
+    m_scene->frame(this);
+}
+
 Output *SceneDelegate::output() const
 {
     return m_output;
@@ -139,6 +144,10 @@ SurfaceItem *Scene::scanoutCandidate() const
     return nullptr;
 }
 
+void Scene::frame(SceneDelegate *delegate)
+{
+}
+
 } // namespace KWin
 
 #include "moc_scene.cpp"

src/scene/scene.h

@@ -31,6 +31,7 @@ public:
 
     QRegion repaints() const override;
     SurfaceItem *scanoutCandidate() const override;
+    void frame() override;
     void prePaint() override;
     void postPaint() override;
     void paint(const RenderTarget &renderTarget, const QRegion &region) override;
@@ -85,6 +86,7 @@ public:
     virtual void prePaint(SceneDelegate *delegate) = 0;
     virtual void postPaint() = 0;
     virtual void paint(const RenderTarget &renderTarget, const QRegion &region) = 0;
+    virtual void frame(SceneDelegate *delegate);
 
 Q_SIGNALS:
     void delegateRemoved(SceneDelegate *delegate);

src/scene/workspacescene.cpp

@@ -184,6 +184,33 @@ SurfaceItem *WorkspaceScene::scanoutCandidate() const
     return candidate;
 }
 
+void WorkspaceScene::frame(SceneDelegate *delegate)
+{
+    if (waylandServer()) {
+        Output *output = delegate->output();
+        const std::chrono::milliseconds frameTime =
+            std::chrono::duration_cast<std::chrono::milliseconds>(output->renderLoop()->lastPresentationTimestamp());
+
+        const QList<Item *> items = m_containerItem->sortedChildItems();
+        for (Item *item : items) {
+            if (!item->isVisible()) {
+                continue;
+            }
+            Window *window = static_cast<WindowItem *>(item)->window();
+            if (!window->isOnOutput(output)) {
+                continue;
+            }
+            if (auto surface = window->surface()) {
+                surface->frameRendered(frameTime.count());
+            }
+        }
+
+        if (m_dndIcon) {
+            m_dndIcon->frameRendered(frameTime.count());
+        }
+    }
+}
+
 void WorkspaceScene::prePaint(SceneDelegate *delegate)
 {
     createStackingOrder();
@@ -320,25 +347,6 @@ void WorkspaceScene::preparePaintSimpleScreen()
 
 void WorkspaceScene::postPaint()
 {
-    if (waylandServer()) {
-        const std::chrono::milliseconds frameTime =
-            std::chrono::duration_cast<std::chrono::milliseconds>(painted_screen->renderLoop()->lastPresentationTimestamp());
-
-        for (WindowItem *windowItem : std::as_const(stacking_order)) {
-            Window *window = windowItem->window();
-            if (!window->isOnOutput(painted_screen)) {
-                continue;
-            }
-            if (auto surface = window->surface()) {
-                surface->frameRendered(frameTime.count());
-            }
-        }
-
-        if (m_dndIcon) {
-            m_dndIcon->frameRendered(frameTime.count());
-        }
-    }
-
     for (WindowItem *w : std::as_const(stacking_order)) {
         effects->postPaintWindow(w->window()->effectWindow());
     }

src/scene/workspacescene.h

@@ -60,6 +60,7 @@ public:
     void prePaint(SceneDelegate *delegate) override;
     void postPaint() override;
     void paint(const RenderTarget &renderTarget, const QRegion &region) override;
+    void frame(SceneDelegate *delegate) override;
 
     virtual bool makeOpenGLContextCurrent();
     virtual void doneOpenGLContextCurrent();