platforms/drm: Use a heuristic to determine if EGLDevice backend can be used

Currently, in order to use the EGLStreams backend, you need to set a
kernel parameter and an environment variable. If you set only the kernel
parameter, performance-wise, kwin will be unusable.

This change makes using the EGLStreams backend easier by making the env
var optional. If you omit KWIN_DRM_USE_EGL_STREAMS=1, kwin will check the
driver name to determine whether the EGLStreams can be enabled.

plugins/platforms/drm/drm_backend.cpp

@@ -66,11 +66,6 @@ DrmBackend::DrmBackend(QObject *parent)
     , m_udevMonitor(m_udev->monitor())
     , m_dpmsFilter()
 {
-#if HAVE_EGL_STREAMS
-    if (qEnvironmentVariableIsSet("KWIN_DRM_USE_EGL_STREAMS")) {
-        m_useEglStreams = true;
-    }
-#endif
     setSupportsGammaControl(true);
     supportsOutputChanges();
 }
@@ -271,6 +266,17 @@ void DrmBackend::openDrm()
     );
     m_drmId = device->sysNum();
 
+#if HAVE_EGL_STREAMS
+    if (qEnvironmentVariableIsSet("KWIN_DRM_USE_EGL_STREAMS")) {
+        m_useEglStreams = true;
+    } else {
+        // If KWIN_DRM_USE_EGL_STREAMS is not set and we know that we are running with
+        // the nvidia proprietary driver, enable the EGLStreams backend anyway.
+        DrmScopedPointer<drmVersion> version(drmGetVersion(fd));
+        m_useEglStreams = version->name == QByteArrayLiteral("nvidia-drm");
+    }
+#endif
+
     // trying to activate Atomic Mode Setting (this means also Universal Planes)
     if (!qEnvironmentVariableIsSet("KWIN_DRM_NO_AMS")) {
         if (drmSetClientCap(m_fd, DRM_CLIENT_CAP_ATOMIC, 1) == 0) {

plugins/platforms/drm/drm_pointer.h

@@ -12,6 +12,7 @@
 
 #include <QScopedPointer>
 
+#include <xf86drm.h>
 #include <xf86drmMode.h>
 
 namespace KWin
@@ -128,6 +129,15 @@ struct DrmDeleter<drmModeRes>
     }
 };
 
+template <>
+struct DrmDeleter<drmVersion>
+{
+    static void cleanup(drmVersion *version)
+    {
+        drmFreeVersion(version);
+    }
+};
+
 template <typename T>
 using DrmScopedPointer = QScopedPointer<T, DrmDeleter<T>>;