kms: Only emit resources-changed signal if we recorded a change

Since we cache already all the KMS parameters we care about let's check at
each device update if anything has really changed and only in such case
emit a resources-changed signal.

In this way we can also filter out the DRM parameters that when changed
don't require a full monitors rebuild.

Examples are the gamma settings or the privacy screen parameters, that
emits an udev "hotplug" event when changed, but we want to register those
only when we handle the changed property.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1964>

src/backends/meta-output.c

@@ -463,3 +463,37 @@ meta_output_class_init (MetaOutputClass *klass)
                         G_PARAM_STATIC_STRINGS);
   g_object_class_install_properties (object_class, N_PROPS, obj_props);
 }
+
+gboolean
+meta_tile_info_equal (MetaTileInfo *a,
+                      MetaTileInfo *b)
+{
+  if (a == b)
+    return TRUE;
+
+  if (a->group_id != b->group_id)
+    return FALSE;
+
+  if (a->flags != b->flags)
+    return FALSE;
+
+  if (a->max_h_tiles != b->max_h_tiles)
+    return FALSE;
+
+  if (a->max_v_tiles != b->max_v_tiles)
+    return FALSE;
+
+  if (a->loc_h_tile != b->loc_h_tile)
+    return FALSE;
+
+  if (a->loc_v_tile != b->loc_v_tile)
+    return FALSE;
+
+  if (a->tile_w != b->tile_w)
+    return FALSE;
+
+  if (a->tile_h != b->tile_h)
+    return FALSE;
+
+  return TRUE;
+}

src/backends/meta-output.h

@@ -106,6 +106,10 @@ typedef struct _MetaOutputInfo
   MetaTileInfo tile_info;
 } MetaOutputInfo;
 
+gboolean
+meta_tile_info_equal (MetaTileInfo *a,
+                      MetaTileInfo *b);
+
 #define META_TYPE_OUTPUT_INFO (meta_output_info_get_type ())
 META_EXPORT_TEST
 GType meta_output_info_get_type (void);

src/backends/native/meta-gpu-kms.h

@@ -62,9 +62,6 @@ void meta_gpu_kms_set_power_save_mode (MetaGpuKms    *gpu_kms,
 MetaCrtcMode * meta_gpu_kms_get_mode_from_kms_mode (MetaGpuKms  *gpu_kms,
                                                     MetaKmsMode *kms_mode);
 
-gboolean meta_drm_mode_equal (const drmModeModeInfo *one,
-                              const drmModeModeInfo *two);
-
 MetaGpuKmsFlipClosureContainer * meta_gpu_kms_wrap_flip_closure (MetaGpuKms *gpu_kms,
                                                                  MetaCrtc   *crtc,
                                                                  GClosure   *flip_closure);

src/backends/native/meta-kms-connector-private.h

@@ -38,7 +38,7 @@ uint32_t meta_kms_connector_get_prop_id (MetaKmsConnector     *connector,
 const char * meta_kms_connector_get_prop_name (MetaKmsConnector     *connector,
                                                MetaKmsConnectorProp  prop);
 
-void meta_kms_connector_update_state (MetaKmsConnector *connector,
+MetaKmsUpdateChanges meta_kms_connector_update_state (MetaKmsConnector *connector,
                                                       drmModeRes       *drm_resources);
 
 void meta_kms_connector_predict_state (MetaKmsConnector *connector,

src/backends/native/meta-kms-connector.c

@@ -46,6 +46,7 @@ struct _MetaKmsConnector
   uint32_t type_id;
   char *name;
 
+  drmModeConnection connection;
   MetaKmsConnectorState *current_state;
 
   MetaKmsConnectorPropTable prop_table;
@@ -467,18 +468,116 @@ meta_kms_connector_state_free (MetaKmsConnectorState *state)
   g_free (state);
 }
 
-static void
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (MetaKmsConnectorState,
+                               meta_kms_connector_state_free);
+
+static gboolean
+kms_modes_equal (GList *modes,
+                 GList *other_modes)
+{
+  GList *l;
+
+  if (g_list_length (modes) != g_list_length (other_modes))
+    return FALSE;
+
+  for (l = modes; l; l = l->next)
+    {
+      GList *k;
+      MetaKmsMode *mode = l->data;
+
+      for (k = other_modes; k; k = k->next)
+        {
+          MetaKmsMode *other_mode = k->data;
+
+          if (!meta_kms_mode_equal (mode, other_mode))
+            return FALSE;
+        }
+    }
+
+  return TRUE;
+}
+
+static MetaKmsUpdateChanges
+meta_kms_connector_state_changes (MetaKmsConnectorState *state,
+                                  MetaKmsConnectorState *new_state)
+{
+  if (state->current_crtc_id != new_state->current_crtc_id)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->common_possible_crtcs != new_state->common_possible_crtcs)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->common_possible_clones != new_state->common_possible_clones)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->encoder_device_idxs != new_state->encoder_device_idxs)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->width_mm != new_state->width_mm)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->height_mm != new_state->height_mm)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->has_scaling != new_state->has_scaling)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->non_desktop != new_state->non_desktop)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->subpixel_order != new_state->subpixel_order)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->suggested_x != new_state->suggested_x)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->suggested_y != new_state->suggested_y)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->hotplug_mode_update != new_state->hotplug_mode_update)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->panel_orientation_transform !=
+      new_state->panel_orientation_transform)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (!meta_tile_info_equal (&state->tile_info, &new_state->tile_info))
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if ((state->edid_data && !new_state->edid_data) || !state->edid_data ||
+      !g_bytes_equal (state->edid_data, new_state->edid_data))
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (!kms_modes_equal (state->modes, new_state->modes))
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  return META_KMS_UPDATE_CHANGE_NONE;
+}
+
+static MetaKmsUpdateChanges
 meta_kms_connector_read_state (MetaKmsConnector  *connector,
                                MetaKmsImplDevice *impl_device,
                                drmModeConnector  *drm_connector,
                                drmModeRes        *drm_resources)
 {
-  MetaKmsConnectorState *state;
+  g_autoptr (MetaKmsConnectorState) state = NULL;
+  g_autoptr (MetaKmsConnectorState) current_state = NULL;
+  MetaKmsUpdateChanges connector_changes;
+  MetaKmsUpdateChanges changes;
 
-  g_clear_pointer (&connector->current_state, meta_kms_connector_state_free);
+  current_state = g_steal_pointer (&connector->current_state);
+  changes = META_KMS_UPDATE_CHANGE_NONE;
 
-  if (!drm_connector || drm_connector->connection != DRM_MODE_CONNECTED)
+  if (drm_connector->connection != DRM_MODE_CONNECTED)
-    return;
+    {
+      if (drm_connector->connection != connector->connection)
+        {
+          connector->connection = drm_connector->connection;
+          changes |= META_KMS_UPDATE_CHANGE_FULL;
+        }
+
+      return changes;
+    }
 
   state = meta_kms_connector_state_new ();
 
@@ -495,26 +594,54 @@ meta_kms_connector_read_state (MetaKmsConnector  *connector,
 
   state_set_crtc_state (state, drm_connector, impl_device, drm_resources);
 
-  connector->current_state = state;
+  if (drm_connector->connection != connector->connection)
-
+    {
-  sync_fd_held (connector, impl_device);
+      connector->connection = drm_connector->connection;
+      changes |= META_KMS_UPDATE_CHANGE_FULL;
     }
 
-void
+  if (!current_state)
+    connector_changes = META_KMS_UPDATE_CHANGE_FULL;
+  else
+    connector_changes = meta_kms_connector_state_changes (current_state, state);
+
+  if (connector_changes == META_KMS_UPDATE_CHANGE_NONE)
+    {
+      connector->current_state = g_steal_pointer (&current_state);
+    }
+  else
+    {
+      connector->current_state = g_steal_pointer (&state);
+      changes |= connector_changes;
+    }
+
+  if (changes != META_KMS_UPDATE_CHANGE_NONE)
+    sync_fd_held (connector, impl_device);
+
+  return changes;
+}
+
+MetaKmsUpdateChanges
 meta_kms_connector_update_state (MetaKmsConnector *connector,
                                  drmModeRes       *drm_resources)
 {
   MetaKmsImplDevice *impl_device;
   drmModeConnector *drm_connector;
+  MetaKmsUpdateChanges changes;
 
   impl_device = meta_kms_device_get_impl_device (connector->device);
   drm_connector = drmModeGetConnector (meta_kms_impl_device_get_fd (impl_device),
                                        connector->id);
-  meta_kms_connector_read_state (connector, impl_device,
+
+  if (!drm_connector)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  changes = meta_kms_connector_read_state (connector, impl_device,
                                            drm_connector,
                                            drm_resources);
-  if (drm_connector)
   drmModeFreeConnector (drm_connector);
+
+  return changes;
 }
 
 void
@@ -651,6 +778,7 @@ meta_kms_connector_new (MetaKmsImplDevice *impl_device,
 {
   MetaKmsConnector *connector;
 
+  g_assert (drm_connector);
   connector = g_object_new (META_TYPE_KMS_CONNECTOR, NULL);
   connector->device = meta_kms_impl_device_get_device (impl_device);
   connector->id = drm_connector->connector_id;

src/backends/native/meta-kms-crtc-private.h

@@ -23,6 +23,7 @@
 #include <xf86drmMode.h>
 
 #include "backends/native/meta-kms-crtc.h"
+#include "backends/native/meta-kms-update-private.h"
 
 typedef enum _MetaKmsCrtcProp
 {
@@ -37,7 +38,7 @@ MetaKmsCrtc * meta_kms_crtc_new (MetaKmsImplDevice  *impl_device,
                                  int                 idx,
                                  GError            **error);
 
-void meta_kms_crtc_update_state (MetaKmsCrtc *crtc);
+MetaKmsUpdateChanges meta_kms_crtc_update_state (MetaKmsCrtc *crtc);
 
 void meta_kms_crtc_predict_state (MetaKmsCrtc   *crtc,
                                   MetaKmsUpdate *update);

src/backends/native/meta-kms-crtc.c

@@ -94,32 +94,62 @@ meta_kms_crtc_is_active (MetaKmsCrtc *crtc)
 
 static void
 read_gamma_state (MetaKmsCrtc       *crtc,
+                  MetaKmsCrtcState  *crtc_state,
                   MetaKmsImplDevice *impl_device,
                   drmModeCrtc       *drm_crtc)
 {
-  MetaKmsCrtcState *current_state = &crtc->current_state;
+  if (crtc_state->gamma.size != drm_crtc->gamma_size)
-
-  if (current_state->gamma.size != drm_crtc->gamma_size)
     {
-      current_state->gamma.size = drm_crtc->gamma_size;
+      crtc_state->gamma.size = drm_crtc->gamma_size;
 
-      current_state->gamma.red = g_realloc_n (current_state->gamma.red,
+      crtc_state->gamma.red = g_realloc_n (crtc_state->gamma.red,
                                            drm_crtc->gamma_size,
                                            sizeof (uint16_t));
-      current_state->gamma.green = g_realloc_n (current_state->gamma.green,
+      crtc_state->gamma.green = g_realloc_n (crtc_state->gamma.green,
                                              drm_crtc->gamma_size,
                                              sizeof (uint16_t));
-      current_state->gamma.blue = g_realloc_n (current_state->gamma.blue,
+      crtc_state->gamma.blue = g_realloc_n (crtc_state->gamma.blue,
                                             drm_crtc->gamma_size,
                                             sizeof (uint16_t));
     }
 
   drmModeCrtcGetGamma (meta_kms_impl_device_get_fd (impl_device),
                        crtc->id,
-                       current_state->gamma.size,
+                       crtc_state->gamma.size,
-                       current_state->gamma.red,
+                       crtc_state->gamma.red,
-                       current_state->gamma.green,
+                       crtc_state->gamma.green,
-                       current_state->gamma.blue);
+                       crtc_state->gamma.blue);
+}
+
+static MetaKmsUpdateChanges
+meta_kms_crtc_state_changes (MetaKmsCrtcState *state,
+                             MetaKmsCrtcState *other_state)
+{
+  if (!meta_rectangle_equal (&state->rect, &other_state->rect))
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->is_drm_mode_valid != other_state->is_drm_mode_valid)
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (!meta_drm_mode_equal (&state->drm_mode, &other_state->drm_mode))
+    return META_KMS_UPDATE_CHANGE_FULL;
+
+  if (state->gamma.size != other_state->gamma.size)
+    return META_KMS_UPDATE_CHANGE_GAMMA;
+
+  if (memcmp (state->gamma.blue, other_state->gamma.blue,
+              state->gamma.size * sizeof (uint16_t)) != 0)
+    return META_KMS_UPDATE_CHANGE_GAMMA;
+
+  if (memcmp (state->gamma.green, other_state->gamma.green,
+              state->gamma.size * sizeof (uint16_t)) != 0)
+    return META_KMS_UPDATE_CHANGE_GAMMA;
+
+  if (memcmp (state->gamma.red, other_state->gamma.red,
+              state->gamma.size * sizeof (uint16_t)) != 0)
+    return META_KMS_UPDATE_CHANGE_GAMMA;
+
+  return META_KMS_UPDATE_CHANGE_NONE;
 }
 
 static int
@@ -141,23 +171,35 @@ find_prop_idx (MetaKmsProp *prop,
 }
 
 static void
+clear_gamma_state (MetaKmsCrtcState *crtc_state)
+{
+  crtc_state->gamma.size = 0;
+  g_clear_pointer (&crtc_state->gamma.red, g_free);
+  g_clear_pointer (&crtc_state->gamma.green, g_free);
+  g_clear_pointer (&crtc_state->gamma.blue, g_free);
+}
+
+static MetaKmsUpdateChanges
 meta_kms_crtc_read_state (MetaKmsCrtc             *crtc,
                           MetaKmsImplDevice       *impl_device,
                           drmModeCrtc             *drm_crtc,
                           drmModeObjectProperties *drm_props)
 {
+  MetaKmsCrtcState crtc_state = {0};
+  MetaKmsUpdateChanges changes = META_KMS_UPDATE_CHANGE_NONE;
   MetaKmsProp *active_prop;
   int active_idx;
 
-  crtc->current_state.rect = (MetaRectangle) {
+  crtc_state.rect = (MetaRectangle) {
     .x = drm_crtc->x,
     .y = drm_crtc->y,
     .width = drm_crtc->width,
     .height = drm_crtc->height,
   };
 
-  crtc->current_state.is_drm_mode_valid = drm_crtc->mode_valid;
+  crtc_state.is_drm_mode_valid = drm_crtc->mode_valid;
-  crtc->current_state.drm_mode = drm_crtc->mode;
+  crtc_state.drm_mode = drm_crtc->mode;
+  crtc_state.gamma.size = 0;
 
   active_prop = &crtc->prop_table.props[META_KMS_CRTC_PROP_ACTIVE];
   if (active_prop->prop_id)
@@ -165,13 +207,36 @@ meta_kms_crtc_read_state (MetaKmsCrtc             *crtc,
       active_idx = find_prop_idx (active_prop,
                                   drm_props->props,
                                   drm_props->count_props);
-      crtc->current_state.is_active = !!drm_props->prop_values[active_idx];
+      crtc_state.is_active = !!drm_props->prop_values[active_idx];
     }
   else
     {
-      crtc->current_state.is_active = drm_crtc->mode_valid;
+      crtc_state.is_active = drm_crtc->mode_valid;
     }
 
+  if (crtc_state.is_active != crtc->current_state.is_active)
+    {
+      changes |= META_KMS_UPDATE_CHANGE_FULL;
+    }
+  else if (!crtc_state.is_active)
+    {
+      clear_gamma_state (&crtc_state);
+      return changes;
+    }
+
+  read_gamma_state (crtc, &crtc_state, impl_device, drm_crtc);
+
+  changes |= meta_kms_crtc_state_changes (&crtc->current_state, &crtc_state);
+
+  if (changes == META_KMS_UPDATE_CHANGE_NONE)
+    {
+      clear_gamma_state (&crtc_state);
+      return changes;
+    }
+
+  clear_gamma_state (&crtc->current_state);
+  crtc->current_state = crtc_state;
+
   meta_topic (META_DEBUG_KMS,
               "Read CRTC %u state: active: %d, mode: %s",
               crtc->id, crtc->current_state.is_active,
@@ -179,13 +244,14 @@ meta_kms_crtc_read_state (MetaKmsCrtc             *crtc,
                 ? crtc->current_state.drm_mode.name
                 : "(nil)");
 
-  read_gamma_state (crtc, impl_device, drm_crtc);
+  return changes;
 }
 
-void
+MetaKmsUpdateChanges
 meta_kms_crtc_update_state (MetaKmsCrtc *crtc)
 {
   MetaKmsImplDevice *impl_device;
+  MetaKmsUpdateChanges changes;
   int fd;
   drmModeCrtc *drm_crtc;
   drmModeObjectProperties *drm_props;
@@ -201,23 +267,17 @@ meta_kms_crtc_update_state (MetaKmsCrtc *crtc)
       crtc->current_state.is_active = FALSE;
       crtc->current_state.rect = (MetaRectangle) { };
       crtc->current_state.is_drm_mode_valid = FALSE;
+      changes = META_KMS_UPDATE_CHANGE_FULL;
       goto out;
     }
 
-  meta_kms_crtc_read_state (crtc, impl_device, drm_crtc, drm_props);
+  changes = meta_kms_crtc_read_state (crtc, impl_device, drm_crtc, drm_props);
 
 out:
   g_clear_pointer (&drm_props, drmModeFreeObjectProperties);
   g_clear_pointer (&drm_crtc, drmModeFreeCrtc);
-}
 
-static void
+  return changes;
-clear_gamma_state (MetaKmsCrtc *crtc)
-{
-  crtc->current_state.gamma.size = 0;
-  g_clear_pointer (&crtc->current_state.gamma.red, g_free);
-  g_clear_pointer (&crtc->current_state.gamma.green, g_free);
-  g_clear_pointer (&crtc->current_state.gamma.blue, g_free);
 }
 
 void
@@ -270,7 +330,7 @@ meta_kms_crtc_predict_state (MetaKmsCrtc   *crtc,
       if (gamma->crtc != crtc)
         continue;
 
-      clear_gamma_state (crtc);
+      clear_gamma_state (&crtc->current_state);
       crtc->current_state.gamma.size = gamma->size;
       crtc->current_state.gamma.red =
         g_memdup2 (gamma->red, gamma->size * sizeof (uint16_t));
@@ -380,7 +440,7 @@ meta_kms_crtc_finalize (GObject *object)
 {
   MetaKmsCrtc *crtc = META_KMS_CRTC (object);
 
-  clear_gamma_state (crtc);
+  clear_gamma_state (&crtc->current_state);
 
   G_OBJECT_CLASS (meta_kms_crtc_parent_class)->finalize (object);
 }
@@ -388,6 +448,7 @@ meta_kms_crtc_finalize (GObject *object)
 static void
 meta_kms_crtc_init (MetaKmsCrtc *crtc)
 {
+  crtc->current_state.gamma.size = 0;
 }
 
 static void

src/backends/native/meta-kms-device-private.h

@@ -21,10 +21,11 @@
 #define META_KMS_DEVICE_PRIVATE_H
 
 #include "backends/native/meta-kms-types.h"
+#include "backends/native/meta-kms-update-private.h"
 
 MetaKmsImplDevice * meta_kms_device_get_impl_device (MetaKmsDevice *device);
 
-void meta_kms_device_update_states_in_impl (MetaKmsDevice *device);
+MetaKmsUpdateChanges meta_kms_device_update_states_in_impl (MetaKmsDevice *device);
 
 void meta_kms_device_predict_states_in_impl (MetaKmsDevice *device,
                                              MetaKmsUpdate *update);

src/backends/native/meta-kms-device.c

@@ -186,15 +186,19 @@ meta_kms_device_get_fallback_modes (MetaKmsDevice *device)
   return device->fallback_modes;
 }
 
-void
+MetaKmsUpdateChanges
 meta_kms_device_update_states_in_impl (MetaKmsDevice *device)
 {
   MetaKmsImplDevice *impl_device = meta_kms_device_get_impl_device (device);
+  MetaKmsUpdateChanges changes;
 
   meta_assert_in_kms_impl (device->kms);
   meta_assert_is_waiting_for_kms_impl_task (device->kms);
 
-  meta_kms_impl_device_update_states (impl_device);
+  changes = meta_kms_impl_device_update_states (impl_device);
+
+  if (changes == META_KMS_UPDATE_CHANGE_NONE)
+    return changes;
 
   g_list_free (device->crtcs);
   device->crtcs = meta_kms_impl_device_copy_crtcs (impl_device);
@@ -204,6 +208,8 @@ meta_kms_device_update_states_in_impl (MetaKmsDevice *device)
 
   g_list_free (device->planes);
   device->planes = meta_kms_impl_device_copy_planes (impl_device);
+
+  return changes;
 }
 
 void

src/backends/native/meta-kms-impl-device.c

@@ -36,7 +36,6 @@
 #include "backends/native/meta-kms-plane-private.h"
 #include "backends/native/meta-kms-plane.h"
 #include "backends/native/meta-kms-private.h"
-#include "backends/native/meta-kms-update-private.h"
 
 #include "meta-default-modes.h"
 #include "meta-private-enum-types.h"
@@ -390,13 +389,14 @@ find_existing_connector (MetaKmsImplDevice *impl_device,
   return NULL;
 }
 
-static void
+static MetaKmsUpdateChanges
 update_connectors (MetaKmsImplDevice *impl_device,
                    drmModeRes        *drm_resources)
 {
   MetaKmsImplDevicePrivate *priv =
     meta_kms_impl_device_get_instance_private (impl_device);
-  GList *connectors = NULL;
+  g_autolist (MetaKmsConnector) connectors = NULL;
+  gboolean needs_full_change = FALSE;
   unsigned int i;
   int fd;
 
@@ -413,17 +413,28 @@ update_connectors (MetaKmsImplDevice *impl_device,
 
       connector = find_existing_connector (impl_device, drm_connector);
       if (connector)
+        {
           connector = g_object_ref (connector);
+        }
       else
+        {
           connector = meta_kms_connector_new (impl_device, drm_connector,
                                               drm_resources);
+          needs_full_change = TRUE;
+        }
+
       drmModeFreeConnector (drm_connector);
 
       connectors = g_list_prepend (connectors, connector);
     }
 
+  if (!needs_full_change)
+    return META_KMS_UPDATE_CHANGE_NONE;
+
   g_list_free_full (priv->connectors, g_object_unref);
-  priv->connectors = g_list_reverse (connectors);
+  priv->connectors = g_list_reverse (g_steal_pointer (&connectors));
+
+  return META_KMS_UPDATE_CHANGE_FULL;
 }
 
 static MetaKmsPlaneType
@@ -690,7 +701,7 @@ clear_latched_fd_hold (MetaKmsImplDevice *impl_device)
     }
 }
 
-void
+MetaKmsUpdateChanges
 meta_kms_impl_device_update_states (MetaKmsImplDevice *impl_device)
 {
   MetaKmsImplDevicePrivate *priv =
@@ -698,6 +709,8 @@ meta_kms_impl_device_update_states (MetaKmsImplDevice *impl_device)
   g_autoptr (GError) error = NULL;
   int fd;
   drmModeRes *drm_resources;
+  MetaKmsUpdateChanges changes;
+  GList *l;
 
   meta_assert_in_kms_impl (meta_kms_impl_get_kms (priv->impl));
 
@@ -720,20 +733,25 @@ meta_kms_impl_device_update_states (MetaKmsImplDevice *impl_device)
       goto err;
     }
 
-  update_connectors (impl_device, drm_resources);
+  changes = update_connectors (impl_device, drm_resources);
 
-  g_list_foreach (priv->crtcs, (GFunc) meta_kms_crtc_update_state,
+  for (l = priv->crtcs; l; l = l->next)
-                  NULL);
+    changes |= meta_kms_crtc_update_state (META_KMS_CRTC (l->data));
-  g_list_foreach (priv->connectors, (GFunc) meta_kms_connector_update_state,
+
+  for (l = priv->connectors; l; l = l->next)
+    changes |= meta_kms_connector_update_state (META_KMS_CONNECTOR (l->data),
                                                 drm_resources);
+
   drmModeFreeResources (drm_resources);
 
-  return;
+  return changes;
 
 err:
   g_clear_list (&priv->planes, g_object_unref);
   g_clear_list (&priv->crtcs, g_object_unref);
   g_clear_list (&priv->connectors, g_object_unref);
+
+  return META_KMS_UPDATE_CHANGE_FULL;
 }
 
 void

src/backends/native/meta-kms-impl-device.h

@@ -135,7 +135,7 @@ void meta_kms_impl_device_hold_fd (MetaKmsImplDevice *impl_device);
 
 void meta_kms_impl_device_unhold_fd (MetaKmsImplDevice *impl_device);
 
-void meta_kms_impl_device_update_states (MetaKmsImplDevice *impl_device);
+MetaKmsUpdateChanges meta_kms_impl_device_update_states (MetaKmsImplDevice *impl_device);
 
 void meta_kms_impl_device_predict_states (MetaKmsImplDevice *impl_device,
                                           MetaKmsUpdate     *update);

src/backends/native/meta-kms-mode.c

@@ -73,7 +73,7 @@ meta_kms_mode_get_drm_mode (MetaKmsMode *mode)
   return &mode->drm_mode;
 }
 
-static gboolean
+gboolean
 meta_drm_mode_equal (const drmModeModeInfo *one,
                      const drmModeModeInfo *two)
 {

src/backends/native/meta-kms-mode.h

@@ -42,6 +42,9 @@ const drmModeModeInfo * meta_kms_mode_get_drm_mode (MetaKmsMode *mode);
 gboolean meta_kms_mode_equal (MetaKmsMode *mode,
                               MetaKmsMode *other_mode);
 
+gboolean meta_drm_mode_equal (const drmModeModeInfo *one,
+                              const drmModeModeInfo *two);
+
 unsigned int meta_kms_mode_hash (MetaKmsMode *mode);
 
 #endif /* META_KMS_MODE_H */

src/backends/native/meta-kms-types.h

@@ -64,6 +64,13 @@ typedef enum _MetaKmsDeviceFlag
   META_KMS_DEVICE_FLAG_NO_MODE_SETTING = 1 << 4,
 } MetaKmsDeviceFlag;
 
+typedef enum _MetaKmsUpdateChanges
+{
+  META_KMS_UPDATE_CHANGE_NONE = 0,
+  META_KMS_UPDATE_CHANGE_GAMMA = 1 << 0,
+  META_KMS_UPDATE_CHANGE_FULL = -1,
+} MetaKmsUpdateChanges;
+
 typedef enum _MetaKmsPlaneType MetaKmsPlaneType;
 
 typedef enum _MetaKmsPropType

src/backends/native/meta-kms.c

@@ -29,6 +29,8 @@
 #include "backends/native/meta-udev.h"
 #include "cogl/cogl.h"
 
+#include "meta-private-enum-types.h"
+
 /**
  * SECTION:kms
  * @short description: KMS abstraction
@@ -563,17 +565,25 @@ meta_kms_is_waiting_for_impl_task (MetaKms *kms)
   return kms->waiting_for_impl_task;
 }
 
-static void
+static MetaKmsUpdateChanges
 meta_kms_update_states_in_impl (MetaKms  *kms)
 {
+  MetaKmsUpdateChanges changes = META_KMS_UPDATE_CHANGE_NONE;
+  GList *l;
+
   COGL_TRACE_BEGIN_SCOPED (MetaKmsUpdateStates,
                            "KMS (update states)");
 
   meta_assert_in_kms_impl (kms);
 
-  g_list_foreach (kms->devices,
+  for (l = kms->devices; l; l = l->next)
-                  (GFunc) meta_kms_device_update_states_in_impl,
+    {
-                  NULL);
+      MetaKmsDevice *kms_device = META_KMS_DEVICE (l->data);
+
+      changes |= meta_kms_device_update_states_in_impl (kms_device);
+    }
+
+  return changes;
 }
 
 static gpointer
@@ -583,30 +593,27 @@ update_states_in_impl (MetaKmsImpl  *impl,
 {
   MetaKms *kms = meta_kms_impl_get_kms (impl);
 
-  meta_kms_update_states_in_impl (kms);
+  return GUINT_TO_POINTER (meta_kms_update_states_in_impl (kms));
-
-  return GINT_TO_POINTER (TRUE);
 }
 
-static gboolean
+static MetaKmsUpdateChanges
-meta_kms_update_states_sync (MetaKms  *kms,
+meta_kms_update_states_sync (MetaKms  *kms)
-                             GError  **error)
 {
   gpointer ret;
 
-  ret = meta_kms_run_impl_task_sync (kms, update_states_in_impl, NULL, error);
+  ret = meta_kms_run_impl_task_sync (kms, update_states_in_impl, NULL, NULL);
-  return GPOINTER_TO_INT (ret);
+  return GPOINTER_TO_UINT (ret);
 }
 
 static void
 handle_hotplug_event (MetaKms *kms)
 {
-  g_autoptr (GError) error = NULL;
+  MetaKmsUpdateChanges changes;
 
-  if (!meta_kms_update_states_sync (kms, &error))
+  changes = meta_kms_update_states_sync (kms);
-    g_warning ("Updating KMS state failed: %s", error->message);
 
-  g_signal_emit (kms, signals[RESOURCES_CHANGED], 0);
+  if (changes != META_KMS_UPDATE_CHANGE_NONE)
+    g_signal_emit (kms, signals[RESOURCES_CHANGED], 0, changes);
 }
 
 void
@@ -750,5 +757,6 @@ meta_kms_class_init (MetaKmsClass *klass)
                   G_SIGNAL_RUN_LAST,
                   0,
                   NULL, NULL, NULL,
-                  G_TYPE_NONE, 0);
+                  G_TYPE_NONE, 1,
+                  META_TYPE_KMS_UPDATE_CHANGES);
 }

src/backends/native/meta-monitor-manager-native.c

@@ -509,8 +509,11 @@ handle_hotplug_event (MetaMonitorManager *manager)
 
 static void
 on_kms_resources_changed (MetaKms              *kms,
+                          MetaKmsUpdateChanges  changes,
                           MetaMonitorManager   *manager)
 {
+  g_assert (changes != META_KMS_UPDATE_CHANGE_NONE);
+
   handle_hotplug_event (manager);
 }