monitor: Add API to get GAMMA LUT size

Will be used to generate look up tables.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2141>

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

@@ -459,4 +459,11 @@ GList * meta_monitor_manager_get_virtual_monitors (MetaMonitorManager *manager);
 
 void meta_monitor_manager_maybe_emit_privacy_screen_change (MetaMonitorManager *manager);
 
+void meta_monitor_manager_get_crtc_gamma (MetaMonitorManager  *manager,
+                                          MetaCrtc            *crtc,
+                                          size_t              *size,
+                                          unsigned short     **red,
+                                          unsigned short     **green,
+                                          unsigned short     **blue);
+
 #endif /* META_MONITOR_MANAGER_PRIVATE_H */

src/backends/meta-monitor-manager.c

@@ -1152,7 +1152,7 @@ update_has_builtin_panel (MetaMonitorManager *manager)
                             obj_props[PROP_HAS_BUILTIN_PANEL]);
 }
 
-static void
+void
 meta_monitor_manager_get_crtc_gamma (MetaMonitorManager  *manager,
                                      MetaCrtc            *crtc,
                                      size_t              *size,

src/backends/meta-monitor.c

@@ -618,6 +618,36 @@ meta_monitor_create_spec (MetaMonitor  *monitor,
   };
 }
 
+/**
+ * meta_monitor_get_gamma_lut_size:
+ * @monitor: The MetaMonitor instance to retrieve the size from.
+ *
+ * Get the size of the look-up tables (LUTs) for the monitor.
+ *
+ * Retrieve the size of the LUT used to implement the encoding or decoding
+ * transfer functions ("gamma", "degamma") for the CRTC or CRTCs that backs
+ * this monitor.
+ *
+ * Returns: The number of look-up table entries possible for the monitor. It is
+ *   assumed that each CRTC of a monitor has identical gamma LUT sizes.
+ */
+size_t
+meta_monitor_get_gamma_lut_size (MetaMonitor *monitor)
+{
+  MetaMonitorPrivate *priv = meta_monitor_get_instance_private (monitor);
+  MetaMonitorManager *monitor_manager =
+    meta_backend_get_monitor_manager (priv->backend);
+  MetaOutput *output;
+  MetaCrtc *crtc;
+  size_t size;
+
+  output = meta_monitor_get_main_output (monitor);
+  crtc = meta_output_get_assigned_crtc (output);
+  meta_monitor_manager_get_crtc_gamma (monitor_manager, crtc,
+                                       &size, NULL, NULL, NULL);
+  return size;
+}
+
 static void
 meta_monitor_normal_generate_modes (MetaMonitorNormal *monitor_normal)
 {

src/backends/meta-monitor.h

@@ -295,4 +295,6 @@ gboolean meta_monitor_set_privacy_screen_enabled (MetaMonitor  *monitor,
                                                   gboolean      enabled,
                                                   GError      **error);
 
+size_t meta_monitor_get_gamma_lut_size (MetaMonitor *monitor);
+
 #endif /* META_MONITOR_H */