Display off is done by a callback into backlight controller

signal_callback.h

 // Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef POWER_MANAGER_SIGNAL_CALLBACK_H_
 #define POWER_MANAGER_SIGNAL_CALLBACK_H_
 
 // These macros provide a wrapper around class methods so they can be used as
 // callbacks.
 
@@ skipping 90 matching lines @@
   static RETURN METHOD ## Thunk(void* data) {                        \
     METHOD ## Args* args = reinterpret_cast<METHOD ## Args*>(data);  \
     CLASS* obj = args->obj;                                          \
     TYPE0 arg0 = args->arg0;                                         \
     TYPE0 arg1 = args->arg1;                                         \
     delete args;                                                     \
     return obj->METHOD(arg0, arg1);                                  \
   }                                                                  \
   RETURN METHOD(TYPE0, TYPE1);
 
+// This macro defines a function pointer for the thunk functions defined by
+// the above macros.
+//
+// Usage example:
+//   SIGNAL_CALLBACK_PTR(bool, callback_func);
+// is equivalent to:
+//   bool (*callback_func)((void*)
+
+#define SIGNAL_CALLBACK_PTR(RETURN, NAME) RETURN (*NAME)(void*)
+
 #endif  // POWER_MANAGER_SIGNAL_CALLBACK_H_