[mutter/gbsneto/underscan: 3/4] monitor-manager: check for underscan setting validity upfront

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 483c26af1e67dc9f12237458dc71da5e0ac594f2
Author: Cosimo Cecchi <cosimo endlessm com>
Date:   Tue Jun 28 13:26:36 2016 -0700

    monitor-manager: check for underscan setting validity upfront
    
    Instead of doing a roundtrip to the X server before setting it, rely on
    the previous value fetched before the configuration was sent over DBus.
    This matches the argument check we already do elsewhere, and will allow
    us to more easily add an additional condition to determine if underscan
    is supported.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=767804
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/549

 src/backends/meta-monitor-manager.c   | 13 ++++++++++++-
 src/backends/x11/meta-output-xrandr.c |  4 +---
 2 files changed, 13 insertions(+), 4 deletions(-)
---
diff --git a/src/backends/meta-monitor-manager.c b/src/backends/meta-monitor-manager.c
index 2d898c757..6f2460f27 100644
--- a/src/backends/meta-monitor-manager.c
+++ b/src/backends/meta-monitor-manager.c
@@ -1736,6 +1736,7 @@ create_monitor_config_from_variant (MetaMonitorManager *manager,
   MetaMonitor *monitor;
   MetaMonitorSpec *monitor_spec;
   MetaMonitorModeSpec *monitor_mode_spec;
+  MetaOutput *output;
   g_autoptr (GVariant) properties_variant = NULL;
   gboolean enable_underscanning = FALSE;
 
@@ -1760,7 +1761,17 @@ create_monitor_config_from_variant (MetaMonitorManager *manager,
       return NULL;
     }
 
-  g_variant_lookup (properties_variant, "underscanning", "b", &enable_underscanning);
+  output = meta_monitor_get_main_output (monitor);
+  if (g_variant_lookup (properties_variant, "underscanning", "b", &enable_underscanning))
+    {
+      if (enable_underscanning && !output->supports_underscanning)
+        {
+          g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                       "Underscanning requested but unsupported");
+          return NULL;
+
+        }
+    }
 
   monitor_spec = meta_monitor_spec_clone (meta_monitor_get_spec (monitor));
 
diff --git a/src/backends/x11/meta-output-xrandr.c b/src/backends/x11/meta-output-xrandr.c
index 26a32f406..e97890a49 100644
--- a/src/backends/x11/meta-output-xrandr.c
+++ b/src/backends/x11/meta-output-xrandr.c
@@ -166,9 +166,7 @@ meta_output_xrandr_apply_mode (MetaOutput *output)
     }
 
   output_set_presentation_xrandr (output, output->is_presentation);
-
-  if (output->supports_underscanning)
-    output_set_underscanning_xrandr (output, output->is_underscanning);
+  output_set_underscanning_xrandr (output, output->is_underscanning);
 }
 
 static int