ecore_dbus_message_new_method_call(event->server,
				      "/org/freedesktop/Hal/Manager" /*path*/,
				      "org.freedesktop.Hal.Manager" /*interface*/,
				      "GetAllDevices" /*method*/,
				      NULL /*destination*/,
				      ecore_dbus_method_test_cb,
				      ecore_dbus_method_error_cb, NULL,
				      "ao" /*fmt*/,
				      NULL);

and the result : 
[ecore_dbus] dbus message length 193 bytes, still 0
Error: Method "GetAllDevices" with signature "ao" on interface "org.freedesktop.Hal.Manager" doesn't exist