backends: use explicit sync for reusing graphics buffers with EGL

Otherwise there can be glitches on NVidia, and this also makes some future
code changes around multi gpu copies and shadow buffers easier

src/backends/drm/drm_egl_layer_surface.cpp

@@ -177,7 +177,6 @@ bool EglGbmLayerSurface::endRendering(const QRegion &damagedRegion)
         GLFramebuffer::popFramebuffer();
     }
     m_surface->damageJournal.add(damagedRegion);
-    m_surface->gbmSwapchain->release(m_surface->currentSlot);
     m_surface->timeQuery->end();
     glFlush();
     EGLNativeFence sourceFence(m_eglBackend->eglDisplayObject());
@@ -186,6 +185,7 @@ bool EglGbmLayerSurface::endRendering(const QRegion &damagedRegion)
         // and NVidia doesn't support implicit sync
         glFinish();
     }
+    m_surface->gbmSwapchain->release(m_surface->currentSlot, sourceFence.fileDescriptor().duplicate());
     const auto buffer = importBuffer(m_surface.get(), m_surface->currentSlot.get(), sourceFence.fileDescriptor());
     m_surface->renderEnd = std::chrono::steady_clock::now();
     if (buffer) {
@@ -579,7 +579,7 @@ std::shared_ptr<DrmFramebuffer> EglGbmLayerSurface::importWithEgl(Surface *surfa
     if (!endFence.isValid()) {
         glFinish();
     }
-    surface->importGbmSwapchain->release(slot);
+    surface->importGbmSwapchain->release(slot, endFence.fileDescriptor().duplicate());
     surface->importTimeQuery->end();
 
     // restore the old context

src/backends/drm/drm_virtual_egl_layer.cpp

@@ -11,6 +11,7 @@
 #include "drm_gpu.h"
 #include "drm_logging.h"
 #include "drm_virtual_output.h"
+#include "opengl/eglnativefence.h"
 #include "opengl/eglswapchain.h"
 #include "opengl/glrendertimequery.h"
 #include "scene/surfaceitem_wayland.h"
@@ -89,7 +90,9 @@ bool VirtualEglGbmLayer::endFrame(const QRegion &renderedRegion, const QRegion &
     glFlush();
     m_currentDamage = damagedRegion;
     m_damageJournal.add(damagedRegion);
-    m_gbmSwapchain->release(m_currentSlot);
+
+    EGLNativeFence releaseFence{m_eglBackend->eglDisplayObject()};
+    m_gbmSwapchain->release(m_currentSlot, releaseFence.fileDescriptor().duplicate());
     return true;
 }
 

src/backends/wayland/wayland_egl_backend.cpp

@@ -106,6 +106,10 @@ bool WaylandEglPrimaryLayer::endFrame(const QRegion &renderedRegion, const QRegi
     m_query->end();
     // Flush rendering commands to the dmabuf.
     glFlush();
+    EGLNativeFence releaseFence{m_backend->eglDisplayObject()};
+
+    m_presentationBuffer = m_backend->backend()->importBuffer(m_buffer->buffer());
+    m_swapchain->release(m_buffer, releaseFence.fileDescriptor().duplicate());
 
     m_damageJournal.add(damagedRegion);
     return true;
@@ -127,10 +131,6 @@ bool WaylandEglPrimaryLayer::scanout(SurfaceItem *surfaceItem)
 
 void WaylandEglPrimaryLayer::present()
 {
-    if (!m_presentationBuffer) {
-        m_presentationBuffer = m_backend->backend()->importBuffer(m_buffer->buffer());
-        Q_ASSERT(m_presentationBuffer);
-    }
 
     KWayland::Client::Surface *surface = m_waylandOutput->surface();
     surface->attachBuffer(m_presentationBuffer);
@@ -139,8 +139,6 @@ void WaylandEglPrimaryLayer::present()
     surface->commit();
     Q_EMIT m_waylandOutput->outputChange(m_damageJournal.lastDamage());
     m_presentationBuffer = nullptr;
-
-    m_swapchain->release(m_buffer);
 }
 
 std::chrono::nanoseconds WaylandEglPrimaryLayer::queryRenderTime() const
@@ -217,7 +215,8 @@ bool WaylandEglCursorLayer::endFrame(const QRegion &renderedRegion, const QRegio
 
     m_output->cursor()->update(buffer, scale(), hotspot().toPoint());
 
-    m_swapchain->release(m_buffer);
+    EGLNativeFence releaseFence{m_backend->eglDisplayObject()};
+    m_swapchain->release(m_buffer, releaseFence.fileDescriptor().duplicate());
     return true;
 }
 

src/backends/wayland/wayland_egl_backend.h

@@ -10,6 +10,7 @@
 #pragma once
 
 #include "core/outputlayer.h"
+#include "opengl/eglnativefence.h"
 #include "platformsupport/scenes/opengl/abstract_egl_backend.h"
 #include "utils/damagejournal.h"
 

src/backends/x11/windowed/x11_windowed_egl_backend.cpp

@@ -8,6 +8,7 @@
 */
 #include "x11_windowed_egl_backend.h"
 #include "core/gbmgraphicsbufferallocator.h"
+#include "opengl/eglnativefence.h"
 #include "opengl/eglswapchain.h"
 #include "opengl/glrendertimequery.h"
 #include "platformsupport/scenes/opengl/basiceglsurfacetexture_wayland.h"
@@ -104,7 +105,8 @@ void X11WindowedEglPrimaryLayer::present()
 
     Q_EMIT m_output->outputChange(infiniteRegion());
 
-    m_swapchain->release(m_buffer);
+    EGLNativeFence releaseFence{m_backend->eglDisplayObject()};
+    m_swapchain->release(m_buffer, releaseFence.fileDescriptor().duplicate());
 }
 
 std::shared_ptr<GLTexture> X11WindowedEglPrimaryLayer::texture() const

src/opengl/eglswapchain.cpp

@@ -54,6 +54,11 @@ int EglSwapchainSlot::age() const
     return m_age;
 }
 
+bool EglSwapchainSlot::isBusy() const
+{
+    return m_buffer->isReferenced() || (m_releaseFd.isValid() && !m_releaseFd.isReadable());
+}
+
 std::shared_ptr<EglSwapchainSlot> EglSwapchainSlot::create(EglContext *context, GraphicsBuffer *buffer)
 {
     auto texture = context->importDmaBufAsTexture(*buffer->dmabufAttributes());
@@ -100,10 +105,11 @@ uint64_t EglSwapchain::modifier() const
 
 std::shared_ptr<EglSwapchainSlot> EglSwapchain::acquire()
 {
-    for (const auto &slot : std::as_const(m_slots)) {
-        if (!slot->buffer()->isReferenced()) {
-            return slot;
-        }
+    const auto it = std::ranges::find_if(std::as_const(m_slots), [](const auto &slot) {
+        return !slot->isBusy();
+    });
+    if (it != m_slots.cend()) {
+        return *it;
     }
 
     GraphicsBuffer *buffer = m_allocator->allocate(GraphicsBufferOptions{
@@ -124,8 +130,9 @@ std::shared_ptr<EglSwapchainSlot> EglSwapchain::acquire()
     return slot;
 }
 
-void EglSwapchain::release(std::shared_ptr<EglSwapchainSlot> slot)
+void EglSwapchain::release(std::shared_ptr<EglSwapchainSlot> slot, FileDescriptor &&releaseFence)
 {
+    slot->m_releaseFd = std::move(releaseFence);
     for (qsizetype i = 0; i < m_slots.count(); ++i) {
         if (m_slots[i] == slot) {
             m_slots[i]->m_age = 1;

src/opengl/eglswapchain.h

@@ -9,6 +9,7 @@
 #pragma once
 
 #include "kwin_export.h"
+#include "utils/filedescriptor.h"
 
 #include <QList>
 #include <QSize>
@@ -40,10 +41,13 @@ public:
     static std::shared_ptr<EglSwapchainSlot> create(EglContext *context, GraphicsBuffer *buffer);
 
 private:
+    bool isBusy() const;
+
     GraphicsBuffer *m_buffer;
     std::unique_ptr<GLFramebuffer> m_framebuffer;
     std::shared_ptr<GLTexture> m_texture;
     int m_age = 0;
+    FileDescriptor m_releaseFd;
     friend class EglSwapchain;
 };
 
@@ -58,7 +62,7 @@ public:
     uint64_t modifier() const;
 
     std::shared_ptr<EglSwapchainSlot> acquire();
-    void release(std::shared_ptr<EglSwapchainSlot> slot);
+    void release(std::shared_ptr<EglSwapchainSlot> slot, FileDescriptor &&releaseFence);
 
     static std::shared_ptr<EglSwapchain> create(GraphicsBufferAllocator *allocator, EglContext *context, const QSize &size, uint32_t format, const QList<uint64_t> &modifiers);