Add workaround for broken Qt EGLFS card selection logic

app/main.cpp

@@ -10,7 +10,7 @@
 #include <QFont>
 #include <QCursor>
 #include <QElapsedTimer>
-#include <QFile>
+#include <QTemporaryFile>
 
 // Don't let SDL hook our main function, since Qt is already
 // doing the same thing. This needs to be before any headers
@@ -351,6 +351,11 @@ int main(int argc, char *argv[])
     SSL_free(nullptr);
 #endif
 
+    // We keep this at function scope to ensure it stays around while we're running,
+    // becaue the Qt QPA will need to read it. Since the temporary file is only
+    // created when open() is called, this doesn't do any harm for other platforms.
+    QTemporaryFile eglfsConfigFile("eglfs_override_XXXXXX.conf");
+
     // Avoid using High DPI on EGLFS. It breaks font rendering.
     // https://bugreports.qt.io/browse/QTBUG-64377
     //
@@ -372,6 +377,7 @@ int main(int argc, char *argv[])
         if (!qEnvironmentVariableIsSet("QT_QPA_PLATFORM")) {
             qInfo() << "Unable to detect Wayland or X11, so EGLFS will be used by default. Set QT_QPA_PLATFORM to override this.";
             qputenv("QT_QPA_PLATFORM", "eglfs");
+            qputenv("SDL_VIDEODRIVER", "kmsdrm");
 
             if (!qEnvironmentVariableIsSet("QT_QPA_EGLFS_ALWAYS_SET_MODE")) {
                 qInfo() << "Setting display mode by default. Set QT_QPA_EGLFS_ALWAYS_SET_MODE=0 to override this.";
@@ -384,6 +390,17 @@ int main(int argc, char *argv[])
                 qWarning() << "Unable to find a KMSDRM display device!";
                 qWarning() << "On the Raspberry Pi, you must enable the 'fake KMS' driver in raspi-config to use Moonlight outside of the GUI environment.";
             }
+            else if (!qEnvironmentVariableIsSet("QT_QPA_EGLFS_KMS_CONFIG")) {
+                // HACK: Remove this when Qt is fixed to properly check for display support before picking a card
+                QString cardOverride = WMUtils::getDrmCardOverride();
+                if (!cardOverride.isEmpty()) {
+                    if (eglfsConfigFile.open()) {
+                        qInfo() << "Overriding default Qt EGLFS card selection to" << cardOverride;
+                        QTextStream(&eglfsConfigFile) << "{ \"device\": \"" << cardOverride << "\" }";
+                        qputenv("QT_QPA_EGLFS_KMS_CONFIG", eglfsConfigFile.fileName().toUtf8());
+                    }
+                }
+            }
         }
 
         // EGLFS uses OpenGLES 2.0, so we will too. Some embedded platforms may not

app/utils.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <QString>
+
 #define THROW_BAD_ALLOC_IF_NULL(x) \
     if ((x) == nullptr) throw std::bad_alloc()
 
@@ -8,4 +10,5 @@ namespace WMUtils {
     bool isRunningWayland();
     bool isRunningWindowManager();
     bool isRunningDesktopEnvironment();
+    QString getDrmCardOverride();
 }

app/wm.cpp

@@ -1,4 +1,5 @@
 #include <QtGlobal>
+#include <QDir>
 
 #include "utils.h"
 
@@ -12,6 +13,11 @@
 #include <wayland-client.h>
 #endif
 
+#ifdef HAVE_DRM
+#include <xf86drm.h>
+#include <xf86drmMode.h>
+#endif
+
 #define VALUE_SET 0x01
 #define VALUE_TRUE 0x02
 
@@ -96,3 +102,43 @@ bool WMUtils::isRunningDesktopEnvironment()
     return isRunningWindowManager();
 #endif
 }
+
+QString WMUtils::getDrmCardOverride()
+{
+#ifdef HAVE_DRM
+    QDir dir("/dev/dri");
+    QStringList cardList = dir.entryList(QStringList("card*"), QDir::Files | QDir::System);
+    if (cardList.length() == 0) {
+        return QString();
+    }
+
+    bool needsOverride = false;
+    for (const QString& card : cardList) {
+        QFile cardFd(dir.filePath(card));
+        if (!cardFd.open(QFile::ReadOnly)) {
+            continue;
+        }
+
+        auto resources = drmModeGetResources(cardFd.handle());
+        if (resources == nullptr) {
+            // If we find a card that doesn't have a display before a card that
+            // has one, we'll need to override Qt's EGLFS config because they
+            // don't properly handle cards without displays.
+            needsOverride = true;
+        }
+        else {
+            // We found a card with a display
+            drmModeFreeResources(resources);
+            if (needsOverride) {
+                // Override the default card with this one
+                return dir.filePath(card);
+            }
+            else {
+                return QString();
+            }
+        }
+    }
+#endif
+
+    return QString();
+}