Use display scaling to improve performance on slow GPUs

This dramatically improves performance for lower resolution
streams on slow GPUs when using the GL and Vulkan renderers.

app/settings/streamingpreferences.cpp

@@ -109,8 +109,10 @@ void StreamingPreferences::reload()
 #ifdef Q_OS_DARWIN
     recommendedFullScreenMode = WindowMode::WM_FULLSCREEN_DESKTOP;
 #else
-    // Wayland doesn't support modesetting, so use fullscreen desktop mode.
-    if (WMUtils::isRunningWayland()) {
+    // Wayland doesn't support modesetting, so use fullscreen desktop mode
+    // unless we have a slow GPU (which can take advantage of wp_viewporter
+    // to reduce GPU load with lower resolution video streams).
+    if (WMUtils::isRunningWayland() && !WMUtils::isGpuSlow()) {
         recommendedFullScreenMode = WindowMode::WM_FULLSCREEN_DESKTOP;
     }
     else {

app/streaming/session.cpp

@@ -904,6 +904,13 @@ bool Session::initialize(QQuickWindow* qtWindow)
     switch (m_Preferences->windowMode)
     {
     default:
+        // Normally we'd default to fullscreen desktop when starting in windowed
+        // mode, but in the case of a slow GPU, we want to use real fullscreen
+        // to allow the display to assist with the video scaling work.
+        if (WMUtils::isGpuSlow()) {
+            m_FullScreenFlag = SDL_WINDOW_FULLSCREEN;
+            break;
+        }
     case StreamingPreferences::WM_FULLSCREEN_DESKTOP:
         // Only use full-screen desktop mode if we're running a desktop environment
         if (WMUtils::isRunningDesktopEnvironment()) {
@@ -1422,19 +1429,28 @@ void Session::updateOptimalWindowDisplayMode()
         return;
     }
 
-    // Start with the native desktop resolution and try to find
-    // the highest refresh rate that our stream FPS evenly divides.
+    // On devices with slow GPUs, we will try to match the display mode
+    // to the video stream to offload the scaling work to the display.
+    bool matchVideo;
+    if (!Utils::getEnvironmentVariableOverride("MATCH_DISPLAY_MODE_TO_VIDEO", &matchVideo)) {
+        matchVideo = WMUtils::isGpuSlow();
+    }
+
     bestMode = desktopMode;
     bestMode.refresh_rate = 0;
-    for (int i = 0; i < SDL_GetNumDisplayModes(displayIndex); i++) {
-        if (SDL_GetDisplayMode(displayIndex, i, &mode) == 0) {
-            if (mode.w == desktopMode.w && mode.h == desktopMode.h &&
+    if (!matchVideo) {
+        // Start with the native desktop resolution and try to find
+        // the highest refresh rate that our stream FPS evenly divides.
+        for (int i = 0; i < SDL_GetNumDisplayModes(displayIndex); i++) {
+            if (SDL_GetDisplayMode(displayIndex, i, &mode) == 0) {
+                if (mode.w == desktopMode.w && mode.h == desktopMode.h &&
                     mode.refresh_rate % m_StreamConfig.fps == 0) {
-                SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION,
-                            "Found display mode with desktop resolution: %dx%dx%d",
-                            mode.w, mode.h, mode.refresh_rate);
-                if (mode.refresh_rate > bestMode.refresh_rate) {
-                    bestMode = mode;
+                    SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION,
+                                "Found display mode with desktop resolution: %dx%dx%d",
+                                mode.w, mode.h, mode.refresh_rate);
+                    if (mode.refresh_rate > bestMode.refresh_rate) {
+                        bestMode = mode;
+                    }
                 }
             }
         }

app/utils.h

@@ -13,6 +13,7 @@ namespace WMUtils {
     bool isRunningWindowManager();
     bool isRunningDesktopEnvironment();
     QString getDrmCardOverride();
+    bool isGpuSlow();
 }
 
 namespace Utils {

app/wm.cpp

@@ -205,6 +205,21 @@ bool WMUtils::isRunningDesktopEnvironment()
 #endif
 }
 
+bool WMUtils::isGpuSlow()
+{
+    bool ret;
+
+    if (!Utils::getEnvironmentVariableOverride("GL_IS_SLOW", &ret)) {
+#ifdef GL_IS_SLOW
+        ret = true;
+#else
+        ret = false;
+#endif
+    }
+
+    return ret;
+}
+
 QString WMUtils::getDrmCardOverride()
 {
 #ifdef HAVE_DRM