local-display-factory: Provide more flexibility for configuring display server

There's currently a way to disable wayland, but no way to disable Xorg.
We currently prefer wayland if it's not disabled, but have no way to
prefer Xorg without disabling wayland entirely.

There's currently no way use legacy Xorg support at all if user display
server support is enabled at a build time.

This commit adds more flexibility to display server selection. It adds
two new keys: XorgEnable and and PreferredDisplayServer.

XorgEnable=false disables Xorg support entirely on seat 0.

PreferredDisplayServer can be set to "wayland", "xorg", "legacy-xorg" or
"none" to select which display server is used by default. If it's set to
"wayland", it will fall back to "xorg". If it's set to "xorg" it will
fall back to "wayland".

8 files changed, 366 insertions, 124 deletions

diff --git a/common/gdm-settings-keys.h b/common/gdm-settings-keys.h
index f0059b5c..87685d3c 100644
--- a/common/gdm-settings-keys.h
+++ b/common/gdm-settings-keys.h
@@ -33,7 +33,9 @@ G_BEGIN_DECLS
 #define GDM_KEY_TIMED_LOGIN_USER "daemon/TimedLogin"
 #define GDM_KEY_TIMED_LOGIN_DELAY "daemon/TimedLoginDelay"
 #define GDM_KEY_INITIAL_SETUP_ENABLE "daemon/InitialSetupEnable"
+#define GDM_KEY_PREFERRED_DISPLAY_SERVER "daemon/PreferredDisplayServer"
 #define GDM_KEY_WAYLAND_ENABLE "daemon/WaylandEnable"
+#define GDM_KEY_XORG_ENABLE "daemon/XorgEnable"
 
 #define GDM_KEY_DEBUG "debug/Enable"
 
diff --git a/daemon/gdm-display.c b/daemon/gdm-display.c
index 3a260923..34467856 100644
--- a/daemon/gdm-display.c
+++ b/daemon/gdm-display.c
@@ -93,6 +93,8 @@ typedef struct _GdmDisplayPrivate
         guint                 have_existing_user_accounts : 1;
         guint                 doing_initial_setup : 1;
         guint                 session_registered : 1;
+
+        GStrv                 supported_session_types;
 } GdmDisplayPrivate;
 
 enum {
@@ -116,6 +118,7 @@ enum {
         PROP_HAVE_EXISTING_USER_ACCOUNTS,
         PROP_DOING_INITIAL_SETUP,
         PROP_SESSION_REGISTERED,
+        PROP_SUPPORTED_SESSION_TYPES,
 };
 
 static void     gdm_display_class_init  (GdmDisplayClass *klass);
@@ -911,6 +914,23 @@ _gdm_display_set_allow_timed_login (GdmDisplay     *self,
 }
 
 static void
+_gdm_display_set_supported_session_types (GdmDisplay         *self,
+                                          const char * const *supported_session_types)
+
+{
+        GdmDisplayPrivate *priv;
+        g_autofree char *supported_session_types_string = NULL;
+
+        if (supported_session_types != NULL)
+                supported_session_types_string = g_strjoinv (":", (GStrv) supported_session_types);
+
+        priv = gdm_display_get_instance_private (self);
+        g_debug ("GdmDisplay: supported session types: %s", supported_session_types_string);
+        g_strfreev (priv->supported_session_types);
+        priv->supported_session_types = g_strdupv ((GStrv) supported_session_types);
+}
+
+static void
 gdm_display_set_property (GObject        *object,
                           guint           prop_id,
                           const GValue   *value,
@@ -966,6 +986,9 @@ gdm_display_set_property (GObject        *object,
         case PROP_SESSION_REGISTERED:
                 _gdm_display_set_session_registered (self, g_value_get_boolean (value));
                 break;
+        case PROP_SUPPORTED_SESSION_TYPES:
+                _gdm_display_set_supported_session_types (self, g_value_get_boxed (value));
+                break;
         default:
                 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
                 break;
@@ -1044,6 +1067,9 @@ gdm_display_get_property (GObject        *object,
         case PROP_ALLOW_TIMED_LOGIN:
                 g_value_set_boolean (value, priv->allow_timed_login);
                 break;
+        case PROP_SUPPORTED_SESSION_TYPES:
+                g_value_set_boxed (value, priv->supported_session_types);
+                break;
         default:
                 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
                 break;
@@ -1229,6 +1255,7 @@ gdm_display_dispose (GObject *object)
                 priv->finish_idle_id = 0;
         }
         g_clear_object (&priv->launch_environment);
+        g_clear_pointer (&priv->supported_session_types, g_strfreev);
 
         g_warn_if_fail (priv->status != GDM_DISPLAY_MANAGED);
         g_warn_if_fail (priv->user_access_file == NULL);
@@ -1389,6 +1416,14 @@ gdm_display_class_init (GdmDisplayClass *klass)
                                                            G_MAXINT,
                                                            GDM_DISPLAY_UNMANAGED,
                                                            G_PARAM_READWRITE | G_PARAM_CONSTRUCT | G_PARAM_STATIC_STRINGS));
+
+        g_object_class_install_property (object_class,
+                                         PROP_SUPPORTED_SESSION_TYPES,
+                                         g_param_spec_boxed ("supported-session-types",
+                                                             "supported session types",
+                                                             "supported session types",
+                                                             G_TYPE_STRV,
+                                                             G_PARAM_READWRITE | G_PARAM_CONSTRUCT | G_PARAM_STATIC_STRINGS));
 }
 
 static void
@@ -1721,6 +1756,7 @@ gdm_display_start_greeter_session (GdmDisplay *self)
         session = gdm_launch_environment_get_session (priv->launch_environment);
         g_object_set (G_OBJECT (session),
                       "display-is-initial", priv->is_initial,
+                      "supported-session-types", priv->supported_session_types,
                       NULL);
 
         g_free (display_name);
diff --git a/daemon/gdm-launch-environment.c b/daemon/gdm-launch-environment.c
index 87a1c5ff..14ecfac2 100644
--- a/daemon/gdm-launch-environment.c
+++ b/daemon/gdm-launch-environment.c
@@ -161,6 +161,7 @@ build_launch_environment (GdmLaunchEnvironment *launch_environment,
                 NULL
         };
         char *system_data_dirs;
+        g_auto (GStrv) supported_session_types = NULL;
         int i;
 
         /* create a hash table of current environment, then update keys has necessary */
@@ -244,6 +245,14 @@ build_launch_environment (GdmLaunchEnvironment *launch_environment,
                                               system_data_dirs));
         g_free (system_data_dirs);
 
+        g_object_get (launch_environment->priv->session,
+                      "supported-session-types",
+                      &supported_session_types,
+                      NULL);
+        g_hash_table_insert (hash,
+                             g_strdup ("GDM_SUPPORTED_SESSION_TYPES"),
+                             g_strjoinv (":", supported_session_types));
+
         return hash;
 }
 
diff --git a/daemon/gdm-local-display-factory.c b/daemon/gdm-local-display-factory.c
index 8a4ef06c..141d64c6 100644
--- a/daemon/gdm-local-display-factory.c
+++ b/daemon/gdm-local-display-factory.c
@@ -183,6 +183,93 @@ take_next_display_number (GdmLocalDisplayFactory *factory)
         return ret;
 }
 
+static char *
+get_preferred_display_server (GdmLocalDisplayFactory *factory)
+{
+        g_autofree gchar *preferred_display_server = NULL;
+        gboolean wayland_enabled = FALSE, xorg_enabled = FALSE;
+
+        gdm_settings_direct_get_boolean (GDM_KEY_WAYLAND_ENABLE, &wayland_enabled);
+        gdm_settings_direct_get_boolean (GDM_KEY_XORG_ENABLE, &xorg_enabled);
+
+        if (wayland_enabled && !xorg_enabled) {
+                return g_strdup ("wayland");
+        }
+
+        if (!wayland_enabled && !xorg_enabled) {
+                return g_strdup ("none");
+        }
+
+        gdm_settings_direct_get_string (GDM_KEY_PREFERRED_DISPLAY_SERVER, &preferred_display_server);
+
+        if (g_strcmp0 (preferred_display_server, "wayland") == 0) {
+                if (wayland_enabled)
+                        return g_strdup (preferred_display_server);
+                else
+                        return g_strdup ("xorg");
+        }
+
+        if (g_strcmp0 (preferred_display_server, "xorg") == 0) {
+                if (xorg_enabled)
+                        return g_strdup (preferred_display_server);
+                else
+                        return g_strdup ("wayland");
+        }
+
+        if (g_strcmp0 (preferred_display_server, "legacy-xorg") == 0) {
+                if (xorg_enabled)
+                        return g_strdup (preferred_display_server);
+        }
+
+        return g_strdup ("none");
+}
+
+static const char *
+gdm_local_display_factory_get_session_type (GdmLocalDisplayFactory *factory,
+                                            gboolean                should_fall_back)
+{
+        const char *session_types[3] = { NULL };
+        gsize i, session_type_index = 0;
+        g_autofree gchar *preferred_display_server = NULL;
+
+        preferred_display_server = get_preferred_display_server (factory);
+
+        if (g_strcmp0 (preferred_display_server, "wayland") != 0 &&
+            g_strcmp0 (preferred_display_server, "xorg") != 0)
+              return NULL;
+
+        for (i = 0; i < G_N_ELEMENTS (session_types) - 1; i++) {
+#ifdef ENABLE_WAYLAND_SUPPORT
+            if (i > 0 ||
+                g_strcmp0 (preferred_display_server, "wayland") == 0) {
+                    gboolean wayland_enabled = FALSE;
+                    if (gdm_settings_direct_get_boolean (GDM_KEY_WAYLAND_ENABLE, &wayland_enabled)) {
+                            if (wayland_enabled && g_file_test ("/usr/bin/Xwayland", G_FILE_TEST_IS_EXECUTABLE)) {
+                                    session_types[i] = "wayland";
+                                    continue;
+                            }
+                    }
+            }
+#endif
+
+            if (i > 0 ||
+                g_strcmp0 (preferred_display_server, "xorg") == 0) {
+                    gboolean xorg_enabled = FALSE;
+                    if (gdm_settings_direct_get_boolean (GDM_KEY_XORG_ENABLE, &xorg_enabled)) {
+                            if (xorg_enabled && g_file_test ("/usr/bin/Xorg", G_FILE_TEST_IS_EXECUTABLE)) {
+                                    session_types[i] = "x11";
+                                    continue;
+                            }
+                    }
+            }
+        }
+
+        if (should_fall_back)
+                session_type_index++;
+
+        return session_types[session_type_index];
+}
+
 static void
 on_display_disposed (GdmLocalDisplayFactory *factory,
                      GdmDisplay             *display)
@@ -200,19 +287,6 @@ store_display (GdmLocalDisplayFactory *factory,
         gdm_display_store_add (store, display);
 }
 
-static gboolean
-gdm_local_display_factory_use_wayland (void)
-{
-#ifdef ENABLE_WAYLAND_SUPPORT
-        gboolean wayland_enabled = FALSE;
-        if (gdm_settings_direct_get_boolean (GDM_KEY_WAYLAND_ENABLE, &wayland_enabled)) {
-                if (wayland_enabled && g_file_test ("/usr/bin/Xwayland", G_FILE_TEST_IS_EXECUTABLE) )
-                        return TRUE;
-        }
-#endif
-        return FALSE;
-}
-
 /*
   Example:
   dbus-send --system --dest=org.gnome.DisplayManager \
@@ -228,6 +302,8 @@ gdm_local_display_factory_create_transient_display (GdmLocalDisplayFactory *fact
         gboolean         ret;
         GdmDisplay      *display = NULL;
         gboolean         is_initial = FALSE;
+        const char      *session_type;
+        g_autofree gchar *preferred_display_server = NULL;
 
         g_return_val_if_fail (GDM_IS_LOCAL_DISPLAY_FACTORY (factory), FALSE);
 
@@ -235,20 +311,43 @@ gdm_local_display_factory_create_transient_display (GdmLocalDisplayFactory *fact
 
         g_debug ("GdmLocalDisplayFactory: Creating transient display");
 
-#ifdef ENABLE_USER_DISPLAY_SERVER
-        display = gdm_local_display_new ();
-        if (gdm_local_display_factory_use_wayland ())
-                g_object_set (G_OBJECT (display), "session-type", "wayland", NULL);
-        is_initial = TRUE;
-#else
-        if (display == NULL) {
-                guint32 num;
+        preferred_display_server = get_preferred_display_server (factory);
 
-                num = take_next_display_number (factory);
+#ifdef ENABLE_USER_DISPLAY_SERVER
+        if (g_strcmp0 (preferred_display_server, "wayland") == 0 ||
+            g_strcmp0 (preferred_display_server, "xorg") == 0) {
+                session_type = gdm_local_display_factory_get_session_type (factory, FALSE);
+
+                if (session_type == NULL) {
+                        g_set_error_literal (error,
+                                             GDM_DISPLAY_ERROR,
+                                             GDM_DISPLAY_ERROR_GENERAL,
+                                             "Both Wayland and Xorg are unavailable");
+                        return FALSE;
+                }
 
-                display = gdm_legacy_display_new (num);
+                display = gdm_local_display_new ();
+                g_object_set (G_OBJECT (display), "session-type", session_type, NULL);
+                is_initial = TRUE;
         }
 #endif
+        if (g_strcmp0 (preferred_display_server, "legacy-xorg") == 0) {
+                if (display == NULL) {
+                        guint32 num;
+
+                        num = take_next_display_number (factory);
+
+                        display = gdm_legacy_display_new (num);
+                }
+        }
+
+        if (display == NULL) {
+                g_set_error_literal (error,
+                                     GDM_DISPLAY_ERROR,
+                                     GDM_DISPLAY_ERROR_GENERAL,
+                                     "Invalid preferred display server configured");
+                return FALSE;
+        }
 
         g_object_set (display,
                       "seat-id", "seat0",
@@ -481,6 +580,19 @@ ensure_display_for_seat (GdmLocalDisplayFactory *factory,
         GdmDisplayStore *store;
         GdmDisplay      *display = NULL;
         g_autofree char *login_session_id = NULL;
+        gboolean wayland_enabled = FALSE, xorg_enabled = FALSE;
+        g_autofree gchar *preferred_display_server = NULL;
+        gboolean falling_back;
+
+        gdm_settings_direct_get_boolean (GDM_KEY_WAYLAND_ENABLE, &wayland_enabled);
+        gdm_settings_direct_get_boolean (GDM_KEY_XORG_ENABLE, &xorg_enabled);
+
+        preferred_display_server = get_preferred_display_server (factory);
+
+        if (g_strcmp0 (preferred_display_server, "none") == 0) {
+               g_debug ("GdmLocalDisplayFactory: Preferred display server is none, so not creating display");
+               return;
+        }
 
         ret = sd_seat_can_graphical (seat_id);
 
@@ -500,21 +612,18 @@ ensure_display_for_seat (GdmLocalDisplayFactory *factory,
         if (g_strcmp0 (seat_id, "seat0") == 0) {
                 is_seat0 = TRUE;
 
-                /* If we've failed, or are explicitly told to, fall back to legacy X11 support
-                 */
-                if (factory->num_failures > 0 || !gdm_local_display_factory_use_wayland ()) {
-                        session_type = NULL;
-                        g_debug ("GdmLocalDisplayFactory: New displays on seat0 will use X11 fallback");
-                } else {
-                        g_debug ("GdmLocalDisplayFactory: New displays on seat0 will use wayland");
-                }
+                falling_back = factory->num_failures > 0;
+                session_type = gdm_local_display_factory_get_session_type (factory, falling_back);
+
+                g_debug ("GdmLocalDisplayFactory: New displays on seat0 will use %s%s",
+                         session_type, falling_back? " fallback" : "");
         } else {
                 is_seat0 = FALSE;
 
                 g_debug ("GdmLocalDisplayFactory: New displays on seat %s will use X11 fallback", seat_id);
                 /* Force legacy X11 for all auxiliary seats */
                 seat_supports_graphics = TRUE;
-                session_type = NULL;
+                session_type = "x11";
         }
 
         /* For seat0, we have a fallback logic to still try starting it after
@@ -552,7 +661,8 @@ ensure_display_for_seat (GdmLocalDisplayFactory *factory,
                         g_debug ("GdmLocalDisplayFactory: Assuming we can use seat0 for X11 even though system says it doesn't support graphics!");
                         g_debug ("GdmLocalDisplayFactory: This might indicate an issue where the framebuffer device is not tagged as master-of-seat in udev.");
                         seat_supports_graphics = TRUE;
-                        session_type = NULL;
+                        session_type = "x11";
+                        wayland_enabled = FALSE;
                 } else {
                         g_clear_handle_id (&factory->seat0_graphics_check_timeout_id, g_source_remove);
                 }
@@ -561,8 +671,13 @@ ensure_display_for_seat (GdmLocalDisplayFactory *factory,
         if (!seat_supports_graphics)
                 return;
 
-        g_debug ("GdmLocalDisplayFactory: %s login display for seat %s requested",
-                 session_type? : "X11", seat_id);
+        if (session_type != NULL)
+                g_debug ("GdmLocalDisplayFactory: %s login display for seat %s requested",
+                         session_type, seat_id);
+        else if (g_strcmp0 (preferred_display_server, "legacy-xorg") == 0)
+                g_debug ("GdmLocalDisplayFactory: Legacy Xorg login display for seat %s requested",
+                         seat_id);
+
         store = gdm_display_factory_get_display_store (GDM_DISPLAY_FACTORY (factory));
 
         if (is_seat0)
@@ -597,20 +712,53 @@ ensure_display_for_seat (GdmLocalDisplayFactory *factory,
         g_debug ("GdmLocalDisplayFactory: Adding display on seat %s", seat_id);
 
 #ifdef ENABLE_USER_DISPLAY_SERVER
-        if (is_seat0) {
-                display = gdm_local_display_new ();
-                if (session_type != NULL) {
+        if (g_strcmp0 (preferred_display_server, "wayland") == 0 ||
+            g_strcmp0 (preferred_display_server, "xorg") == 0) {
+                if (is_seat0) {
+                        g_autoptr (GPtrArray) supported_session_types = NULL;
+
+                        if (session_type == NULL) {
+                                g_warning ("GdmLocalDisplayFactory: Both Wayland and Xorg sessions are unavailable");
+                                return;
+                        }
+
+                        supported_session_types = g_ptr_array_new ();
+
+                        if (g_strcmp0 (preferred_display_server, "wayland") == 0) {
+                                if (wayland_enabled)
+                                        g_ptr_array_add (supported_session_types, "wayland");
+                        } else {
+                                if (xorg_enabled)
+                                        g_ptr_array_add (supported_session_types, "x11");
+                        }
+
+                        if (!falling_back) {
+                                if (g_strcmp0 (preferred_display_server, "wayland") == 0) {
+                                        if (xorg_enabled)
+                                                g_ptr_array_add (supported_session_types, "x11");
+                                } else {
+                                        if (wayland_enabled)
+                                                g_ptr_array_add (supported_session_types, "wayland");
+                                }
+                        }
+
+                        g_ptr_array_add (supported_session_types, NULL);
+
+                        display = gdm_local_display_new ();
                         g_object_set (G_OBJECT (display), "session-type", session_type, NULL);
+                        g_object_set (G_OBJECT (display), "supported-session-types", supported_session_types->pdata, NULL);
                 }
         }
 #endif
 
         if (display == NULL) {
                 guint32 num;
+                const char *supported_session_types[] = { "x11", NULL };
 
                 num = take_next_display_number (factory);
 
                 display = gdm_legacy_display_new (num);
+                g_object_set (G_OBJECT (display), "supported-session-types", supported_session_types, NULL);
         }
 
         g_object_set (display, "seat-id", seat_id, NULL);
diff --git a/daemon/gdm-manager.c b/daemon/gdm-manager.c
index 9c10adff..4c2752fe 100644
--- a/daemon/gdm-manager.c
+++ b/daemon/gdm-manager.c
@@ -1333,7 +1333,7 @@ set_up_automatic_login_session (GdmManager *manager,
                                 GdmDisplay *display)
 {
         GdmSession *session;
-        char       *display_session_type = NULL;
+        g_auto (GStrv) supported_session_types = NULL;
 
         /* 0 is root user; since the daemon talks to the session object
          * directly, itself, for automatic login
@@ -1342,11 +1342,12 @@ set_up_automatic_login_session (GdmManager *manager,
         session = get_user_session_for_display (display);
 
         g_object_get (G_OBJECT (display),
-                      "session-type", &display_session_type,
+                      "supported-session-types", &supported_session_types,
                       NULL);
 
         g_object_set (G_OBJECT (session),
                       "display-is-initial", FALSE,
+                      "supported-session-types", supported_session_types,
                       NULL);
 
         g_debug ("GdmManager: Starting automatic login conversation");
@@ -2305,10 +2306,7 @@ create_user_session_for_display (GdmManager *manager,
         char       *display_auth_file = NULL;
         char       *display_seat_id = NULL;
         char       *display_id = NULL;
-#if defined(ENABLE_WAYLAND_SUPPORT) && defined(ENABLE_USER_DISPLAY_SERVER)
-        g_autofree char *display_session_type = NULL;
-        gboolean    greeter_is_wayland;
-#endif
+        g_auto (GStrv) supported_session_types = NULL;
 
         g_object_get (G_OBJECT (display),
                       "id", &display_id,
@@ -2317,9 +2315,7 @@ create_user_session_for_display (GdmManager *manager,
                       "remote-hostname", &remote_hostname,
                       "x11-authority-file", &display_auth_file,
                       "seat-id", &display_seat_id,
-#if defined(ENABLE_WAYLAND_SUPPORT) && defined(ENABLE_USER_DISPLAY_SERVER)
-                      "session-type", &display_session_type,
-#endif
+                      "supported-session-types", &supported_session_types,
                       NULL);
         display_device = get_display_device (manager, display);
 
@@ -2332,6 +2328,9 @@ create_user_session_for_display (GdmManager *manager,
                                    display_auth_file,
                                    display_is_local,
                                    NULL);
+        g_object_set (G_OBJECT (session),
+                      "supported-session-types", supported_session_types,
+                      NULL);
 
         g_debug ("GdmSession: Created user session for user %d on display %s (seat %s)",
                  (int) allowed_user,
@@ -2405,11 +2404,6 @@ create_user_session_for_display (GdmManager *manager,
                                 session,
                                 (GDestroyNotify)
                                 clean_user_session);
-
-#if defined(ENABLE_WAYLAND_SUPPORT) && defined(ENABLE_USER_DISPLAY_SERVER)
-        greeter_is_wayland = g_strcmp0 (display_session_type, "wayland") == 0;
-        g_object_set (G_OBJECT (session), "ignore-wayland", !greeter_is_wayland, NULL);
-#endif
 }
 
 static void
diff --git a/daemon/gdm-session.c b/daemon/gdm-session.c
index a010cecf..2b941e5e 100644
--- a/daemon/gdm-session.c
+++ b/daemon/gdm-session.c
@@ -133,11 +133,10 @@ struct _GdmSession
         GDBusServer         *outside_server;
         GHashTable          *environment;
 
+        GStrv                supported_session_types;
+
         guint32              is_program_session : 1;
         guint32              display_is_initial : 1;
-#ifdef ENABLE_WAYLAND_SUPPORT
-        guint32              ignore_wayland : 1;
-#endif
 };
 
 enum {
@@ -154,9 +153,7 @@ enum {
         PROP_DISPLAY_X11_AUTHORITY_FILE,
         PROP_USER_X11_AUTHORITY_FILE,
         PROP_CONVERSATION_ENVIRONMENT,
-#ifdef ENABLE_WAYLAND_SUPPORT
-        PROP_IGNORE_WAYLAND,
-#endif
+        PROP_SUPPORTED_SESSION_TYPES,
 };
 
 enum {
@@ -347,13 +344,24 @@ on_establish_credentials_cb (GdmDBusWorker *proxy,
         g_object_unref (self);
 }
 
+static gboolean
+supports_session_type (GdmSession *self,
+                       const char *session_type)
+{
+        if (session_type == NULL)
+                return TRUE;
+
+        return g_strv_contains ((const char * const *) self->supported_session_types,
+                                session_type);
+}
+
 static char **
 get_system_session_dirs (GdmSession *self,
                          const char *type)
 {
         GArray *search_array = NULL;
         char **search_dirs;
-        int i;
+        int i, j;
         const gchar * const *system_data_dirs = g_get_system_data_dirs ();
 
         static const char *x_search_dirs[] = {
@@ -367,35 +375,32 @@ get_system_session_dirs (GdmSession *self,
 
         search_array = g_array_new (TRUE, TRUE, sizeof (char *));
 
-        if (type == NULL || g_str_equal (type, "x11")) {
-                for (i = 0; system_data_dirs[i]; i++) {
-                        gchar *dir = g_build_filename (system_data_dirs[i], "xsessions", NULL);
-                        g_array_append_val (search_array, dir);
+        for (j = 0; self->supported_session_types[j] != NULL; j++) {
+                const char *supported_type = self->supported_session_types[j];
+
+                if (g_str_equal (supported_type, "x11") &&
+                    (type == NULL || g_str_equal (type, supported_type))) {
+                        for (i = 0; system_data_dirs[i]; i++) {
+                                gchar *dir = g_build_filename (system_data_dirs[i], "xsessions", NULL);
+                                g_array_append_val (search_array, dir);
+                        }
+
+                        g_array_append_vals (search_array, x_search_dirs, G_N_ELEMENTS (x_search_dirs));
                 }
 
-                g_array_append_vals (search_array, x_search_dirs, G_N_ELEMENTS (x_search_dirs));
-        }
 
 #ifdef ENABLE_WAYLAND_SUPPORT
-        if (!self->ignore_wayland &&
-            (type == NULL || g_str_equal (type, "wayland"))) {
-#ifdef ENABLE_USER_DISPLAY_SERVER
-                g_array_prepend_val (search_array, wayland_search_dir);
+                if (g_str_equal (supported_type, "wayland") &&
+                    (type == NULL || g_str_equal (type, supported_type))) {
+                        for (i = 0; system_data_dirs[i]; i++) {
+                                gchar *dir = g_build_filename (system_data_dirs[i], "wayland-sessions", NULL);
+                                g_array_append_val (search_array, dir);
+                        }
 
-                for (i = 0; system_data_dirs[i]; i++) {
-                        gchar *dir = g_build_filename (system_data_dirs[i], "wayland-sessions", NULL);
-                        g_array_insert_val (search_array, i, dir);
+                        g_array_append_val (search_array, wayland_search_dir);
                 }
-#else
-                for (i = 0; system_data_dirs[i]; i++) {
-                        gchar *dir = g_build_filename (system_data_dirs[i], "wayland-sessions", NULL);
-                        g_array_append_val (search_array, dir);
-                }
-
-                g_array_append_val (search_array, wayland_search_dir);
 #endif
         }
-#endif
 
         search_dirs = g_strdupv ((char **) search_array->data);
 
@@ -471,6 +476,12 @@ get_session_command_for_file (GdmSession  *self,
                 *command = NULL;
         }
 
+        if (!supports_session_type (self, type)) {
+                g_debug ("GdmSession: ignoring %s session command request for file '%s'",
+                         type, file);
+                goto out;
+        }
+
         g_debug ("GdmSession: getting session command for file '%s'", file);
         key_file = load_key_file_for_file (self, file, type, NULL);
         if (key_file == NULL) {
@@ -2257,14 +2268,18 @@ stop_conversation_now (GdmSessionConversation *conversation)
         g_clear_object (&conversation->job);
 }
 
-#ifdef ENABLE_WAYLAND_SUPPORT
 void
-gdm_session_set_ignore_wayland (GdmSession *self,
-                                gboolean    ignore_wayland)
+gdm_session_set_supported_session_types (GdmSession         *self,
+                                         const char * const *supported_session_types)
 {
-        self->ignore_wayland = ignore_wayland;
+        const char * const session_types[] = { "wayland", "x11", NULL };
+        g_strfreev (self->supported_session_types);
+
+        if (supported_session_types == NULL)
+                self->supported_session_types = g_strdupv ((GStrv) session_types);
+        else
+                self->supported_session_types = g_strdupv ((GStrv) supported_session_types);
 }
-#endif
 
 gboolean
 gdm_session_start_conversation (GdmSession *self,
@@ -3189,15 +3204,15 @@ gdm_session_is_wayland_session (GdmSession *self)
 {
         GKeyFile   *key_file;
         gboolean    is_wayland_session = FALSE;
-        char       *filename;
-        char       *full_path = NULL;
+        char            *filename;
+        g_autofree char *full_path = NULL;
 
         g_return_val_if_fail (self != NULL, FALSE);
         g_return_val_if_fail (GDM_IS_SESSION (self), FALSE);
 
         filename = get_session_filename (self);
 
-        key_file = load_key_file_for_file (self, filename, "wayland", &full_path);
+        key_file = load_key_file_for_file (self, filename, NULL, &full_path);
 
         if (key_file == NULL) {
                 goto out;
@@ -3540,11 +3555,9 @@ gdm_session_set_property (GObject      *object,
         case PROP_CONVERSATION_ENVIRONMENT:
                 set_conversation_environment (self, g_value_get_pointer (value));
                 break;
-#ifdef ENABLE_WAYLAND_SUPPORT
-        case PROP_IGNORE_WAYLAND:
-                gdm_session_set_ignore_wayland (self, g_value_get_boolean (value));
+        case PROP_SUPPORTED_SESSION_TYPES:
+                gdm_session_set_supported_session_types (self, g_value_get_boxed (value));
                 break;
-#endif
         default:
                 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
                 break;
@@ -3598,11 +3611,9 @@ gdm_session_get_property (GObject    *object,
         case PROP_CONVERSATION_ENVIRONMENT:
                 g_value_set_pointer (value, self->environment);
                 break;
-#ifdef ENABLE_WAYLAND_SUPPORT
-        case PROP_IGNORE_WAYLAND:
-                g_value_set_boolean (value, self->ignore_wayland);
+        case PROP_SUPPORTED_SESSION_TYPES:
+                g_value_set_boxed (value, self->supported_session_types);
                 break;
-#endif
         default:
                 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
                 break;
@@ -3620,6 +3631,8 @@ gdm_session_dispose (GObject *object)
 
         gdm_session_close (self);
 
+        g_clear_pointer (&self->supported_session_types,
+                         g_strfreev);
         g_clear_pointer (&self->conversations,
                          g_hash_table_unref);
 
@@ -4034,15 +4047,13 @@ gdm_session_class_init (GdmSessionClass *session_class)
                                                               NULL,
                                                               G_PARAM_READWRITE | G_PARAM_CONSTRUCT | G_PARAM_STATIC_STRINGS));
 
-#ifdef ENABLE_WAYLAND_SUPPORT
         g_object_class_install_property (object_class,
-                                         PROP_IGNORE_WAYLAND,
-                                         g_param_spec_boolean ("ignore-wayland",
-                                                               "ignore wayland",
-                                                               "ignore wayland",
-                                                               FALSE,
-                                                               G_PARAM_READWRITE | G_PARAM_CONSTRUCT | G_PARAM_STATIC_STRINGS));
-#endif
+                                         PROP_SUPPORTED_SESSION_TYPES,
+                                         g_param_spec_boxed ("supported-session-types",
+                                                             "supported session types",
+                                                             "supported session types",
+                                                             G_TYPE_STRV,
+                                                             G_PARAM_READWRITE | G_PARAM_CONSTRUCT | G_PARAM_STATIC_STRINGS));
 
         /* Ensure we can resolve errors */
         gdm_dbus_error_ensure (GDM_SESSION_WORKER_ERROR);
diff --git a/data/gdm.schemas.in b/data/gdm.schemas.in
index 255bff02..a1035f95 100644
--- a/data/gdm.schemas.in
+++ b/data/gdm.schemas.in
@@ -53,11 +53,21 @@
       <default>true</default>
     </schema>
     <schema>
+      <key>daemon/PreferredDisplayServer</key>
+      <signature>s</signature>
+      <default>wayland</default>
+    </schema>
+    <schema>
       <key>daemon/WaylandEnable</key>
       <signature>b</signature>
       <default>true</default>
     </schema>
     <schema>
+      <key>daemon/XorgEnable</key>
+      <signature>b</signature>
+      <default>true</default>
+    </schema>
+    <schema>
       <key>security/AllowRemoteAutoLogin</key>
       <signature>b</signature>
       <default>false</default>
diff --git a/libgdm/gdm-sessions.c b/libgdm/gdm-sessions.c
index a1322505..f078e04b 100644
--- a/libgdm/gdm-sessions.c
+++ b/libgdm/gdm-sessions.c
@@ -190,6 +190,8 @@ collect_sessions_from_directory (const char *dirname)
 
         gboolean is_x11 = g_getenv ("WAYLAND_DISPLAY") == NULL &&
                           g_getenv ("RUNNING_UNDER_GDM") != NULL;
+        gboolean is_wayland = g_getenv ("WAYLAND_DISPLAY") != NULL &&
+                              g_getenv ("RUNNING_UNDER_GDM") != NULL;
 
         /* FIXME: add file monitor to directory */
 
@@ -206,18 +208,46 @@ collect_sessions_from_directory (const char *dirname)
                         continue;
                 }
 
-                if (is_x11 && g_str_has_suffix (filename, "-xorg.desktop")) {
-                        char *base_name = g_strndup (filename, strlen (filename) - strlen ("-xorg.desktop"));
-                        char *fallback_name = g_strconcat (base_name, ".desktop", NULL);
-                        g_free (base_name);
-                        char *fallback_path = g_build_filename (dirname, fallback_name, NULL);
-                        g_free (fallback_name);
-                        if (g_file_test (fallback_path, G_FILE_TEST_EXISTS)) {
-                                g_free (fallback_path);
-                                g_debug ("Running under X11, ignoring %s", filename);
-                                continue;
+                if (is_wayland) {
+                        if (g_str_has_suffix (filename, "-wayland.desktop")) {
+                                g_autofree char *base_name = g_strndup (filename, strlen (filename) - strlen ("-wayland.desktop"));
+                                g_autofree char *other_name = g_strconcat (base_name, ".desktop", NULL);
+                                g_autofree char *other_path = g_build_filename (dirname, other_name, NULL);
+
+                                if (g_file_test (other_path, G_FILE_TEST_EXISTS)) {
+                                        g_debug ("Running under Wayland, ignoring %s", filename);
+                                        continue;
+                                }
+                        } else {
+                                g_autofree char *base_name = g_strndup (filename, strlen (filename) - strlen (".desktop"));
+                                g_autofree char *other_name = g_strdup_printf ("%s-xorg.desktop", base_name);
+                                g_autofree char *other_path = g_build_filename (dirname, other_name, NULL);
+
+                                if (g_file_test (other_path, G_FILE_TEST_EXISTS)) {
+                                        g_debug ("Running under Wayland, ignoring %s", filename);
+                                        continue;
+                                }
+                        }
+                } else if (is_x11) {
+                        if (g_str_has_suffix (filename, "-xorg.desktop")) {
+                                g_autofree char *base_name = g_strndup (filename, strlen (filename) - strlen ("-xorg.desktop"));
+                                g_autofree char *other_name = g_strconcat (base_name, ".desktop", NULL);
+                                g_autofree char *other_path = g_build_filename (dirname, other_name, NULL);
+
+                                if (g_file_test (other_path, G_FILE_TEST_EXISTS)) {
+                                        g_debug ("Running under X11, ignoring %s", filename);
+                                        continue;
+                                }
+                        } else {
+                                g_autofree char *base_name = g_strndup (filename, strlen (filename) - strlen (".desktop"));
+                                g_autofree char *other_name = g_strdup_printf ("%s-wayland.desktop", base_name);
+                                g_autofree char *other_path = g_build_filename (dirname, other_name, NULL);
+
+                                if (g_file_test (other_path, G_FILE_TEST_EXISTS)) {
+                                        g_debug ("Running under X11, ignoring %s", filename);
+                                        continue;
+                                }
                         }
-                        g_free (fallback_path);
                 }
 
                 id = g_strndup (filename, strlen (filename) - strlen (".desktop"));
@@ -247,6 +277,9 @@ collect_sessions (void)
                 DATADIR "/gdm/BuiltInSessions/",
                 DATADIR "/xsessions/",
         };
+        g_auto (GStrv) supported_session_types = NULL;
+
+        supported_session_types = g_strsplit (g_getenv ("GDM_SUPPORTED_SESSION_TYPES"), ":", -1);
 
         names_seen_before = g_hash_table_new (g_str_hash, g_str_equal);
         xorg_search_array = g_ptr_array_new_with_free_func (g_free);
@@ -284,23 +317,22 @@ collect_sessions (void)
                                                                     g_free, (GDestroyNotify)gdm_session_file_free);
         }
 
-        for (i = 0; i < xorg_search_array->len; i++) {
-                collect_sessions_from_directory (g_ptr_array_index (xorg_search_array, i));
+        if (!supported_session_types || g_strv_contains ((const char * const *) supported_session_types, "x11")) {
+                for (i = 0; i < xorg_search_array->len; i++) {
+                        collect_sessions_from_directory (g_ptr_array_index (xorg_search_array, i));
+                }
         }
 
 #ifdef ENABLE_WAYLAND_SUPPORT
 #ifdef ENABLE_USER_DISPLAY_SERVER
-        if (g_getenv ("WAYLAND_DISPLAY") == NULL && g_getenv ("RUNNING_UNDER_GDM") != NULL) {
-                goto out;
+        if (!supported_session_types  || g_strv_contains ((const char * const *) supported_session_types, "wayland")) {
+                for (i = 0; i < wayland_search_array->len; i++) {
+                        collect_sessions_from_directory (g_ptr_array_index (wayland_search_array, i));
+                }
         }
 #endif
-
-        for (i = 0; i < wayland_search_array->len; i++) {
-                collect_sessions_from_directory (g_ptr_array_index (wayland_search_array, i));
-        }
 #endif
 
-out:
         g_hash_table_foreach_remove (gdm_available_sessions_map,
                                      remove_duplicate_sessions,
                                      names_seen_before);