platforms/drm: Add buffer age support in qpainter backend

Currently, the entire screen will be repainted when using the qpainter
render backend. With this change, kwin will repaint only the dirty parts
on the screen.

src/plugins/platforms/drm/dumb_swapchain.cpp

@@ -26,26 +26,42 @@ DumbSwapchain::DumbSwapchain(DrmGpu *gpu, const QSize &size)
             break;
         }
         buffer->image()->fill(Qt::black);
-        m_buffers << buffer;
+        m_slots.append(Slot{.buffer = buffer, .age = 0,});
     }
-    if (m_buffers.count() < 2) {
+    if (m_slots.count() < 2) {
         qCWarning(KWIN_DRM) << "Failed to create dumb buffers for swapchain!";
-        m_buffers.clear();
+        m_slots.clear();
     }
 }
 
-QSharedPointer<DrmDumbBuffer> DumbSwapchain::acquireBuffer()
+QSharedPointer<DrmDumbBuffer> DumbSwapchain::acquireBuffer(int *age)
 {
-    if (m_buffers.isEmpty()) {
+    if (m_slots.isEmpty()) {
         return nullptr;
     }
-    index = (index + 1) % m_buffers.count();
-    return m_buffers[index];
+    index = (index + 1) % m_slots.count();
+    if (age) {
+        *age = m_slots[index].age;
+    }
+    return m_slots[index].buffer;
 }
 
 QSharedPointer<DrmDumbBuffer> DumbSwapchain::currentBuffer() const
 {
-    return m_buffers[index];
+    return m_slots[index].buffer;
+}
+
+void DumbSwapchain::releaseBuffer(QSharedPointer<DrmDumbBuffer> buffer)
+{
+    Q_ASSERT(m_slots[index].buffer == buffer);
+
+    for (qsizetype i = 0; i < m_slots.count(); ++i) {
+        if (m_slots[i].buffer == buffer) {
+            m_slots[i].age = 1;
+        } else if (m_slots[i].age > 0) {
+            m_slots[i].age++;
+        }
+    }
 }
 
 }

src/plugins/platforms/drm/dumb_swapchain.h

@@ -24,22 +24,32 @@ class DumbSwapchain
 public:
     DumbSwapchain(DrmGpu *gpu, const QSize &size);
 
-    QSharedPointer<DrmDumbBuffer> acquireBuffer();
+    QSharedPointer<DrmDumbBuffer> acquireBuffer(int *age = nullptr);
     QSharedPointer<DrmDumbBuffer> currentBuffer() const;
+    void releaseBuffer(QSharedPointer<DrmDumbBuffer> buffer);
+
+    qsizetype slotCount() const {
+        return m_slots.count();
+    }
 
     QSize size() const {
         return m_size;
     }
 
     bool isEmpty() const {
-        return m_buffers.isEmpty();
+        return m_slots.isEmpty();
     }
 
 private:
-    QSize m_size;
+    struct Slot
+    {
+        QSharedPointer<DrmDumbBuffer> buffer;
+        int age = 0;
+    };
 
+    QSize m_size;
     int index = 0;
-    QVector<QSharedPointer<DrmDumbBuffer>> m_buffers;
+    QVector<Slot> m_slots;
 };
 
 }

src/plugins/platforms/drm/scene_qpainter_drm_backend.cpp

@@ -57,6 +57,7 @@ void DrmQPainterBackend::initOutput(DrmOutput *output)
                 return;
             }
             it->swapchain = QSharedPointer<DumbSwapchain>::create(m_gpu, output->pixelSize());
+            it->damageJournal.setCapacity(it->swapchain->slotCount());
         }
     );
 }
@@ -68,21 +69,30 @@ QImage *DrmQPainterBackend::bufferForScreen(int screenId)
 
 QRegion DrmQPainterBackend::beginFrame(int screenId)
 {
-    m_outputs[screenId].swapchain->acquireBuffer();
-    return m_outputs[screenId].output->geometry();
+    Output *rendererOutput = &m_outputs[screenId];
+
+    int bufferAge;
+    rendererOutput->swapchain->acquireBuffer(&bufferAge);
+
+    return rendererOutput->damageJournal.accumulate(bufferAge, rendererOutput->output->geometry());
 }
 
 void DrmQPainterBackend::endFrame(int screenId, const QRegion &damage)
 {
     Q_UNUSED(damage)
 
-    const Output &rendererOutput = m_outputs[screenId];
+    Output &rendererOutput = m_outputs[screenId];
     DrmOutput *drmOutput = rendererOutput.output;
 
-    if (!drmOutput->present(rendererOutput.swapchain->currentBuffer(), drmOutput->geometry())) {
+    QSharedPointer<DrmDumbBuffer> back = rendererOutput.swapchain->currentBuffer();
+    rendererOutput.swapchain->releaseBuffer(back);
+
+    if (!drmOutput->present(back, drmOutput->geometry())) {
         RenderLoopPrivate *renderLoopPrivate = RenderLoopPrivate::get(drmOutput->renderLoop());
         renderLoopPrivate->notifyFrameFailed();
     }
+
+    rendererOutput.damageJournal.add(damage);
 }
 
 }

src/plugins/platforms/drm/scene_qpainter_drm_backend.h

@@ -9,6 +9,7 @@
 #ifndef KWIN_SCENE_QPAINTER_DRM_BACKEND_H
 #define KWIN_SCENE_QPAINTER_DRM_BACKEND_H
 #include "qpainterbackend.h"
+#include "utils.h"
 
 #include <QObject>
 #include <QVector>
@@ -39,6 +40,7 @@ private:
     struct Output {
         DrmOutput *output;
         QSharedPointer<DumbSwapchain> swapchain;
+        DamageJournal damageJournal;
     };
     QVector<Output> m_outputs;
     DrmBackend *m_backend;