core/syncobjtimeline: make explicit sync use SYNC_IOC_MERGE instead of waiting on the CPU side

This brings some performance benefits, because the application can potentially reuse
the buffer earlier, and it simplifies the code a bit

src/core/syncobjtimeline.cpp

@@ -6,8 +6,24 @@
 #include "syncobjtimeline.h"
 
 #include <sys/eventfd.h>
+#include <sys/ioctl.h>
 #include <xf86drm.h>
 
+#if defined(Q_OS_LINUX)
+#include <linux/sync_file.h>
+#else
+struct sync_merge_data
+{
+    char name[32];
+    __s32 fd2;
+    __s32 fence;
+    __u32 flags;
+    __u32 pad;
+};
+#define SYNC_IOC_MAGIC '>'
+#define SYNC_IOC_MERGE _IOWR(SYNC_IOC_MAGIC, 3, struct sync_merge_data)
+#endif
+
 namespace KWin
 {
 
@@ -19,7 +35,39 @@ SyncReleasePoint::SyncReleasePoint(const std::shared_ptr<SyncTimeline> &timeline
 
 SyncReleasePoint::~SyncReleasePoint()
 {
-    m_timeline->signal(m_timelinePoint);
+    if (m_releaseFence.isValid()) {
+        m_timeline->moveInto(m_timelinePoint, m_releaseFence);
+    } else {
+        m_timeline->signal(m_timelinePoint);
+    }
+}
+
+static FileDescriptor mergeSyncFds(const FileDescriptor &fd1, const FileDescriptor &fd2)
+{
+    struct sync_merge_data data
+    {
+        .name = "merged release fence",
+        .fd2 = fd2.get(),
+        .fence = -1,
+    };
+    int err = -1;
+    do {
+        err = ioctl(fd1.get(), SYNC_IOC_MERGE, &data);
+    } while (err == -1 && (errno == EINTR || errno == EAGAIN));
+    if (err < 0) {
+        return FileDescriptor{};
+    } else {
+        return FileDescriptor(data.fence);
+    }
+}
+
+void SyncReleasePoint::addReleaseFence(const FileDescriptor &fd)
+{
+    if (m_releaseFence.isValid()) {
+        m_releaseFence = mergeSyncFds(m_releaseFence, fd);
+    } else {
+        m_releaseFence = fd.duplicate();
+    }
 }
 
 SyncTimeline *SyncReleasePoint::timeline() const
@@ -60,17 +108,8 @@ void SyncTimeline::signal(uint64_t timelinePoint)
     drmSyncobjTimelineSignal(m_drmFd, &m_handle, &timelinePoint, 1);
 }
 
-SyncReleasePointHolder::SyncReleasePointHolder(FileDescriptor &&requirement, std::unordered_set<std::shared_ptr<SyncReleasePoint>> &&releasePoints)
-    : m_fence(std::move(requirement))
-    , m_notifier(m_fence.get(), QSocketNotifier::Type::Read)
-    , m_releasePoints(std::move(releasePoints))
-{
-    connect(&m_notifier, &QSocketNotifier::activated, this, &SyncReleasePointHolder::signaled);
-    m_notifier.setEnabled(true);
-}
-
-void SyncReleasePointHolder::signaled()
+void SyncTimeline::moveInto(uint64_t timelinePoint, const FileDescriptor &fd)
 {
-    delete this;
+    drmSyncobjImportSyncFile(m_drmFd, m_handle, fd.get());
 }
 }

src/core/syncobjtimeline.h

@@ -7,11 +7,8 @@
 #include "kwin_export.h"
 #include "utils/filedescriptor.h"
 
-#include <QObject>
-#include <QSocketNotifier>
 #include <memory>
 #include <stdint.h>
-#include <unordered_set>
 
 namespace KWin
 {
@@ -30,9 +27,16 @@ public:
     SyncTimeline *timeline() const;
     uint64_t timelinePoint() const;
 
+    /**
+     * Adds the fence of a graphics job that this release point should wait for
+     * before the timeline point is signaled
+     */
+    void addReleaseFence(const FileDescriptor &fd);
+
 private:
     const std::shared_ptr<SyncTimeline> m_timeline;
     const uint64_t m_timelinePoint;
+    FileDescriptor m_releaseFence;
 };
 
 class KWIN_EXPORT SyncTimeline
@@ -47,26 +51,10 @@ public:
     FileDescriptor eventFd(uint64_t timelinePoint) const;
 
     void signal(uint64_t timelinePoint);
+    void moveInto(uint64_t timelinePoint, const FileDescriptor &fd);
 
 private:
     const int32_t m_drmFd;
     const uint32_t m_handle;
 };
-
-class SyncReleasePointHolder : public QObject
-{
-    Q_OBJECT
-public:
-    /**
-     * @param requirement the filedescriptor that needs to be readable before the release points may be signalled. Once that's happened, this object deletes itself!'
-     */
-    explicit SyncReleasePointHolder(FileDescriptor &&requirement, std::unordered_set<std::shared_ptr<SyncReleasePoint>> &&releasePoints);
-
-private:
-    void signaled();
-
-    const FileDescriptor m_fence;
-    QSocketNotifier m_notifier;
-    const std::unordered_set<std::shared_ptr<SyncReleasePoint>> m_releasePoints;
-};
 }

src/scene/itemrenderer_opengl.cpp

@@ -53,7 +53,10 @@ void ItemRendererOpenGL::endFrame()
     if (m_eglDisplay) {
         EGLNativeFence fence(m_eglDisplay);
         if (fence.isValid()) {
-            new SyncReleasePointHolder(std::move(fence.fileDescriptor()), std::move(m_releasePoints));
+            for (const auto &releasePoint : m_releasePoints) {
+                releasePoint->addReleaseFence(fence.fileDescriptor());
+            }
+            m_releasePoints.clear();
         }
     }
     m_releasePoints.clear();

src/wayland/surface.h

@@ -344,8 +344,10 @@ public:
     void setPreferredColorDescription(const ColorDescription &descr);
 
     /**
-     * @returns the release point that should be referenced as long as the buffer on this surface
-     *          is, or may still be used by the compositor
+     * Returns the current release point for the buffer on this surface. The buffer keeps the
+     * release point referenced as long as it's referenced itself; for synchronization on the
+     * GPU side, the compositor has to either keep the release point referenced as long as the
+     * GPU task is running, or add a fence for each GPU task to the release point
      */
     std::shared_ptr<SyncReleasePoint> bufferReleasePoint() const;