backends/drm: don't do multi gpu copies if we'd copy from and to the same render device

Some hardware has multiple display devices but only one render device; doing a copy in that
case reduces performance for no reason.

src/backends/drm/drm_egl_layer_surface.cpp

@@ -407,6 +407,13 @@ std::unique_ptr<EglGbmLayerSurface::Surface> EglGbmLayerSurface::createSurface(c
         if (m_gpu == m_eglBackend->gpu()) {
             return doTestFormats(formats, MultiGpuImportMode::None);
         }
+        // special case, we're using different display devices but the same render device
+        const auto display = m_eglBackend->displayForGpu(m_gpu);
+        if (display && !display->renderNode().isEmpty() && display->renderNode() == m_eglBackend->eglDisplayObject()->renderNode()) {
+            if (auto surface = doTestFormats(formats, MultiGpuImportMode::None)) {
+                return surface;
+            }
+        }
         if (auto surface = doTestFormats(formats, MultiGpuImportMode::Egl)) {
             qCDebug(KWIN_DRM) << "chose egl import with format" << formatName(surface->gbmSwapchain->format()).name << "and modifier" << surface->gbmSwapchain->modifier();
             return surface;

src/opengl/egldisplay.cpp

@@ -77,6 +77,7 @@ EglDisplay::EglDisplay(::EGLDisplay display, const QList<QByteArray> &extensions
     : m_handle(display)
     , m_extensions(extensions)
     , m_owning(owning)
+    , m_renderNode(determineRenderNode())
     , m_supportsBufferAge(extensions.contains(QByteArrayLiteral("EGL_EXT_buffer_age")) && qgetenv("KWIN_USE_BUFFER_AGE") != "0")
     , m_supportsNativeFence(extensions.contains(QByteArrayLiteral("EGL_ANDROID_native_fence_sync")))
 {
@@ -135,27 +136,7 @@ static bool checkExtension(const QByteArrayView extensions, const QByteArrayView
 
 QString EglDisplay::renderNode() const
 {
-    const char *clientExtensions = eglQueryString(EGL_NO_DISPLAY, EGL_EXTENSIONS);
-    if (checkExtension(clientExtensions, "EGL_EXT_device_query")) {
-        EGLAttrib eglDeviceAttrib;
-        if (eglQueryDisplayAttribEXT(m_handle, EGL_DEVICE_EXT, &eglDeviceAttrib)) {
-            EGLDeviceEXT eglDevice = reinterpret_cast<EGLDeviceEXT>(eglDeviceAttrib);
-
-            const char *deviceExtensions = eglQueryDeviceStringEXT(eglDevice, EGL_EXTENSIONS);
-            if (checkExtension(deviceExtensions, "EGL_EXT_device_drm_render_node")) {
-                if (const char *node = eglQueryDeviceStringEXT(eglDevice, EGL_DRM_RENDER_NODE_FILE_EXT)) {
-                    return QString::fromLocal8Bit(node);
-                }
-            }
-            if (checkExtension(deviceExtensions, "EGL_EXT_device_drm")) {
-                // Fallback to display device.
-                if (const char *node = eglQueryDeviceStringEXT(eglDevice, EGL_DRM_DEVICE_FILE_EXT)) {
-                    return QString::fromLocal8Bit(node);
-                }
-            }
-        }
-    }
-    return QString();
+    return m_renderNode;
 }
 
 bool EglDisplay::supportsBufferAge() const
@@ -328,6 +309,31 @@ QHash<uint32_t, EglDisplay::DrmFormatInfo> EglDisplay::queryImportFormats() cons
     return ret;
 }
 
+QString EglDisplay::determineRenderNode() const
+{
+    const char *clientExtensions = eglQueryString(EGL_NO_DISPLAY, EGL_EXTENSIONS);
+    if (checkExtension(clientExtensions, "EGL_EXT_device_query")) {
+        EGLAttrib eglDeviceAttrib;
+        if (eglQueryDisplayAttribEXT(m_handle, EGL_DEVICE_EXT, &eglDeviceAttrib)) {
+            EGLDeviceEXT eglDevice = reinterpret_cast<EGLDeviceEXT>(eglDeviceAttrib);
+
+            const char *deviceExtensions = eglQueryDeviceStringEXT(eglDevice, EGL_EXTENSIONS);
+            if (checkExtension(deviceExtensions, "EGL_EXT_device_drm_render_node")) {
+                if (const char *node = eglQueryDeviceStringEXT(eglDevice, EGL_DRM_RENDER_NODE_FILE_EXT)) {
+                    return QString::fromLocal8Bit(node);
+                }
+            }
+            if (checkExtension(deviceExtensions, "EGL_EXT_device_drm")) {
+                // Fallback to display device.
+                if (const char *node = eglQueryDeviceStringEXT(eglDevice, EGL_DRM_DEVICE_FILE_EXT)) {
+                    return QString::fromLocal8Bit(node);
+                }
+            }
+        }
+    }
+    return QString();
+}
+
 EGLImageKHR EglDisplay::createImage(EGLContext ctx, EGLenum target, EGLClientBuffer buffer, const EGLint *attrib_list) const
 {
     Q_ASSERT(m_functions.createImageKHR);

src/opengl/egldisplay.h

@@ -59,10 +59,12 @@ public:
 
 private:
     QHash<uint32_t, DrmFormatInfo> queryImportFormats() const;
+    QString determineRenderNode() const;
 
     const ::EGLDisplay m_handle;
     const QList<QByteArray> m_extensions;
     const bool m_owning;
+    const QString m_renderNode;
 
     const bool m_supportsBufferAge;
     const bool m_supportsNativeFence;