backends/drm: add stricter checks for direct scanout

Buffers with implicit modifiers from another GPU must not be imported, as
the layouts may not be compatible.
For buffers with incompatible modifiers, direct scanout can also be rejected
early, saving some computational power.

BUG: 457851

src/backends/drm/drm_backend.cpp

@@ -682,6 +682,11 @@ DrmGpu *DrmBackend::findGpu(dev_t deviceId) const
     return it == m_gpus.end() ? nullptr : it->get();
 }
 
+size_t DrmBackend::gpuCount() const
+{
+    return m_gpus.size();
+}
+
 bool DrmBackend::applyOutputChanges(const OutputConfiguration &config)
 {
     QVector<DrmOutput *> toBeEnabled;

src/backends/drm/drm_backend.h

@@ -68,6 +68,7 @@ public:
 
     DrmGpu *primaryGpu() const;
     DrmGpu *findGpu(dev_t deviceId) const;
+    size_t gpuCount() const;
 
     bool isActive() const;
 

src/backends/drm/drm_buffer_gbm.cpp

@@ -11,6 +11,7 @@
 #include "drm_gbm_surface.h"
 
 #include "config-kwin.h"
+#include "drm_backend.h"
 #include "drm_gpu.h"
 #include "drm_logging.h"
 #include "kwineglutils_p.h"

src/backends/drm/drm_egl_layer.cpp

@@ -143,6 +143,13 @@ bool EglGbmLayer::scanout(SurfaceItem *surfaceItem)
         m_dmabufFeedback.scanoutFailed(surface, formats);
         return false;
     }
+    if (buffer->attributes().modifier == DRM_FORMAT_MOD_INVALID && m_pipeline->gpu()->platform()->gpuCount() > 1) {
+        // importing a buffer from another GPU without an explicit modifier can mess up the buffer format
+        return false;
+    }
+    if (!formats[buffer->format()].contains(buffer->attributes().modifier)) {
+        return false;
+    }
     const auto gbmBuffer = GbmBuffer::importBuffer(m_pipeline->gpu(), buffer);
     if (!gbmBuffer) {
         m_dmabufFeedback.scanoutFailed(surface, formats);