backends/drm: use sync files for checking buffer readiness and use IN_FENCE_FD

This shouldn't change anything in practice, but with some badly behaving
drivers it can prevent oversynchronization.

src/backends/drm/drm_buffer.cpp

@@ -19,9 +19,13 @@
 #include <cerrno>
 // drm
 #include <drm_fourcc.h>
+#include <sys/ioctl.h>
 #include <unistd.h>
 #include <xf86drm.h>
 #include <xf86drmMode.h>
+#ifdef Q_OS_LINUX
+#include <linux/dma-buf.h>
+#endif
 
 #ifndef DRM_IOCTL_MODE_CLOSEFB
 #define DRM_IOCTL_MODE_CLOSEFB 0xD0
@@ -42,6 +46,16 @@ DrmFramebuffer::DrmFramebuffer(DrmGpu *gpu, uint32_t fbId, GraphicsBuffer *buffe
         // buffer readability checks cause frames to be wrongly delayed on some Intel laptops
         // See https://gitlab.freedesktop.org/drm/intel/-/issues/9415
         m_readable = true;
+    } else {
+#ifdef Q_OS_LINUX
+        dma_buf_export_sync_file req{
+            .flags = DMA_BUF_SYNC_READ,
+            .fd = -1,
+        };
+        if (drmIoctl(buffer->dmabufAttributes()->fd[0].get(), DMA_BUF_IOCTL_EXPORT_SYNC_FILE, &req) == 0) {
+            m_syncFd = FileDescriptor{req.fd};
+        }
+#endif
     }
 }
 
@@ -68,10 +82,17 @@ void DrmFramebuffer::releaseBuffer()
     m_bufferRef = nullptr;
 }
 
+const FileDescriptor &DrmFramebuffer::syncFd() const
+{
+    return m_syncFd;
+}
+
 bool DrmFramebuffer::isReadable()
 {
     if (m_readable) {
         return true;
+    } else if (m_syncFd.isValid()) {
+        return m_readable = m_syncFd.isReadable();
     } else {
         const auto &fds = m_bufferRef->dmabufAttributes()->fd;
         return m_readable = std::all_of(fds.begin(), fds.end(), [](const auto &fd) {

src/backends/drm/drm_buffer.h

@@ -10,6 +10,7 @@
 #pragma once
 
 #include "core/graphicsbuffer.h"
+#include "utils/filedescriptor.h"
 
 namespace KWin
 {
@@ -32,11 +33,14 @@ public:
     void releaseBuffer();
     bool isReadable();
 
+    const FileDescriptor &syncFd() const;
+
 protected:
     const uint32_t m_framebufferId;
     DrmGpu *const m_gpu;
     GraphicsBufferRef m_bufferRef;
     bool m_readable = false;
+    FileDescriptor m_syncFd;
 };
 
 }

src/backends/drm/drm_commit.cpp

@@ -57,6 +57,9 @@ void DrmAtomicCommit::addBuffer(DrmPlane *plane, const std::shared_ptr<DrmFrameb
 {
     addProperty(plane->fbId, buffer ? buffer->framebufferId() : 0);
     m_buffers[plane] = buffer;
+    if (plane->inFenceFd.isValid()) {
+        addProperty(plane->inFenceFd, buffer ? buffer->syncFd().get() : -1);
+    }
     m_planes.emplace(plane);
 }
 

src/backends/drm/drm_plane.cpp

@@ -65,6 +65,7 @@ DrmPlane::DrmPlane(DrmGpu *gpu, uint32_t planeId)
                                                          })
     , vmHotspotX(this, QByteArrayLiteral("HOTSPOT_X"))
     , vmHotspotY(this, QByteArrayLiteral("HOTSPOT_Y"))
+    , inFenceFd(this, QByteArrayLiteral("IN_FENCE_FD"))
 {
 }
 
@@ -95,6 +96,7 @@ bool DrmPlane::updateProperties()
     colorRange.update(props);
     vmHotspotX.update(props);
     vmHotspotY.update(props);
+    inFenceFd.update(props);
 
     if (!type.isValid() || !srcX.isValid() || !srcY.isValid() || !srcW.isValid() || !srcH.isValid()
         || !crtcX.isValid() || !crtcY.isValid() || !crtcW.isValid() || !crtcH.isValid() || !fbId.isValid()) {

src/backends/drm/drm_plane.h

@@ -90,6 +90,7 @@ public:
     DrmEnumProperty<ColorRange> colorRange;
     DrmProperty vmHotspotX;
     DrmProperty vmHotspotY;
+    DrmProperty inFenceFd;
 
     static int32_t transformationToDegrees(Transformations transformation);