[mutter] output/kms: Only add common modes for single mode connectors

[Marge Bot <marge-bot src gnome org>]

commit 4c7a846dc89098149cab0da6c1ac7573eb8e465c
Author: Jonas Ådahl <jadahl gmail com>
Date:   Wed Apr 14 09:03:12 2021 +0200

    output/kms: Only add common modes for single mode connectors
    
    If there was only a single mode, add the common modes to provide options
    to select other resolutions than the built in default. This avoids
    issues where the connector listed multiple supported modes, but where
    the common modes added would exceed the possible bandwidth. We could
    probably make an attempt to filter out more modes from the common mode
    list to avoid these issues, but it's likely that the driver already
    lists suitable modes, meaning there is no point in adding the common
    modes.
    
    The common modes were initially added[0] to add modes to connectors with
    a single bundled mode, so we shouldn't regress the original bug fix.
    
    [0] https://bugzilla.gnome.org/show_bug.cgi?id=744544
    
    Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1232
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1824>

 src/backends/native/meta-output-kms.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)
---
diff --git a/src/backends/native/meta-output-kms.c b/src/backends/native/meta-output-kms.c
index 024344a3cc..fa265bcad6 100644
--- a/src/backends/native/meta-output-kms.c
+++ b/src/backends/native/meta-output-kms.c
@@ -245,11 +245,14 @@ init_output_modes (MetaOutputInfo    *output_info,
         output_info->preferred_mode = output_info->modes[i];
     }
 
-  /* Presume that if the output supports scaling, then we have
-   * a panel fitter capable of adjusting any mode to suit.
-   */
-  if (connector_state->has_scaling)
-    add_common_modes (output_info, gpu_kms);
+  if (connector_state->has_scaling &&
+      g_list_length (connector_state->modes) == 1)
+    {
+      meta_topic (META_DEBUG_KMS, "Adding common modes to connector %u on %s",
+                  meta_kms_connector_get_id (kms_connector),
+                  meta_gpu_kms_get_file_path (gpu_kms));
+      add_common_modes (output_info, gpu_kms);
+    }
 
   if (!output_info->modes)
     {