backends/drm: Prefer egl import mode

The egl import mode ensures that there's a local buffer, which is
preferred as it minimizes the number of data transfers over PCIe.

With dmabuf, it's unclear what the driver will do. But the main takeaway
from discussion with mesa developers is that it's undesired for
gbm_bo_import() to migrate or perform data transfers behind the user's
back, it should be done explicitly.

src/backends/drm/drm_egl_layer_surface.cpp

@@ -241,25 +241,24 @@ std::optional<EglGbmLayerSurface::Surface> EglGbmLayerSurface::createSurface(con
     };
     const auto testFormats = [this, &sort, &doTestFormats](QVector<GbmFormat> &formats) -> std::optional<Surface> {
         std::sort(formats.begin(), formats.end(), sort);
+        if (m_gpu == m_eglBackend->gpu()) {
+            return doTestFormats(formats, MultiGpuImportMode::None);
+        }
+        if (const 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;
+        }
         if (const auto surface = doTestFormats(formats, MultiGpuImportMode::Dmabuf)) {
-            if (m_gpu != m_eglBackend->gpu()) {
-                qCDebug(KWIN_DRM) << "chose dmabuf import with format" << formatName(surface->gbmSwapchain->format()).name << "and modifier" << surface->gbmSwapchain->modifier();
-            }
+            qCDebug(KWIN_DRM) << "chose dmabuf import with format" << formatName(surface->gbmSwapchain->format()).name << "and modifier" << surface->gbmSwapchain->modifier();
             return surface;
         }
-        if (m_gpu != m_eglBackend->gpu()) {
-            if (const auto surface = doTestFormats(formats, MultiGpuImportMode::LinearDmabuf)) {
-                qCDebug(KWIN_DRM) << "chose linear dmabuf import with format" << formatName(surface->gbmSwapchain->format()).name << "and modifier" << surface->gbmSwapchain->modifier();
-                return surface;
-            }
-            if (const 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;
-            }
-            if (const auto surface = doTestFormats(formats, MultiGpuImportMode::DumbBuffer)) {
-                qCDebug(KWIN_DRM) << "chose cpu import with format" << formatName(surface->gbmSwapchain->format()).name << "and modifier" << surface->gbmSwapchain->modifier();
-                return surface;
-            }
+        if (const auto surface = doTestFormats(formats, MultiGpuImportMode::LinearDmabuf)) {
+            qCDebug(KWIN_DRM) << "chose linear dmabuf import with format" << formatName(surface->gbmSwapchain->format()).name << "and modifier" << surface->gbmSwapchain->modifier();
+            return surface;
+        }
+        if (const auto surface = doTestFormats(formats, MultiGpuImportMode::DumbBuffer)) {
+            qCDebug(KWIN_DRM) << "chose cpu import with format" << formatName(surface->gbmSwapchain->format()).name << "and modifier" << surface->gbmSwapchain->modifier();
+            return surface;
         }
         return std::nullopt;
     };

src/backends/drm/drm_egl_layer_surface.h

@@ -65,6 +65,7 @@ public:
 
 private:
     enum class MultiGpuImportMode {
+        None,
         Dmabuf,
         LinearDmabuf,
         Egl,