monitor-manager: add a property to discover if underscan is supported

So that clients such as the control center can decide to hide an
underscanning checkbutton when the output does not support it.

Support in the KMS / native backend to come later...

src/backends/meta-monitor-manager-private.h

@@ -145,6 +145,7 @@ struct _MetaOutput
   gboolean is_primary;
   gboolean is_presentation;
   gboolean is_underscanning;
+  gboolean supports_underscanning;
 
   gpointer driver_private;
   GDestroyNotify driver_notify;

src/backends/meta-monitor-manager.c

@@ -647,6 +647,8 @@ meta_monitor_manager_handle_get_resources (MetaDBusDisplayConfig *skeleton,
                              g_variant_new_string (get_connector_type_name (output->connector_type)));
       g_variant_builder_add (&properties, "{sv}", "underscanning",
                              g_variant_new_boolean (output->is_underscanning));
+      g_variant_builder_add (&properties, "{sv}", "supports-underscanning",
+                             g_variant_new_boolean (output->supports_underscanning));
 
       edid_file = manager_class->get_edid_file (manager, output);
       if (edid_file)

src/backends/x11/meta-monitor-manager-xrandr.c

@@ -246,6 +246,51 @@ output_get_underscanning_xrandr (MetaMonitorManagerXrandr *manager_xrandr,
   return (strcmp (str, "on") == 0);
 }
 
+static gboolean
+output_get_supports_underscanning_xrandr (MetaMonitorManagerXrandr *manager_xrandr,
+                                          MetaOutput               *output)
+{
+  Atom atom, actual_type;
+  int actual_format, i;
+  unsigned long nitems, bytes_after;
+  g_autofree unsigned char *buffer = NULL;
+  XRRPropertyInfo *property_info;
+  Atom *values;
+  gboolean supports_underscanning = FALSE;
+
+  atom = XInternAtom (manager_xrandr->xdisplay, "underscan", False);
+  XRRGetOutputProperty (manager_xrandr->xdisplay,
+                        (XID)output->winsys_id,
+                        atom,
+                        0, G_MAXLONG, False, False, XA_ATOM,
+                        &actual_type, &actual_format,
+                        &nitems, &bytes_after, &buffer);
+
+  if (actual_type != XA_ATOM || actual_format != 32 || nitems < 1)
+    return FALSE;
+
+  property_info = XRRQueryOutputProperty (manager_xrandr->xdisplay,
+                                          (XID) output->winsys_id,
+                                          atom);
+  values = (Atom *) property_info->values;
+
+  for (i = 0; i < property_info->num_values; i++)
+    {
+      /* The output supports underscanning if "on" is a valid value
+       * for the underscan property.
+       */
+      char *name = XGetAtomName (manager_xrandr->xdisplay, values[i]);
+      if (strcmp (name, "on") == 0)
+        supports_underscanning = TRUE;
+
+      XFree (name);
+    }
+
+  XFree (property_info);
+
+  return supports_underscanning;
+}
+
 static int
 normalize_backlight (MetaOutput *output,
                      int         hw_value)
@@ -804,6 +849,7 @@ meta_monitor_manager_xrandr_read_current (MetaMonitorManager *manager)
 	  meta_output->is_primary = ((XID)meta_output->winsys_id == primary_output);
 	  meta_output->is_presentation = output_get_presentation_xrandr (manager_xrandr, meta_output);
 	  meta_output->is_underscanning = output_get_underscanning_xrandr (manager_xrandr, meta_output);
+          meta_output->supports_underscanning = output_get_supports_underscanning_xrandr (manager_xrandr, meta_output);
 	  output_get_backlight_limits_xrandr (manager_xrandr, meta_output);
 
 	  if (!(meta_output->backlight_min == 0 && meta_output->backlight_max == 0))