docs: def get_drive_rotational_speed_rpm(system_obj):
      """Gets the set of rotational speed rpms of the disks.
  
      :param system_obj: The HPESystem object.
      :returns the set of rotational speed rpms of the HDD devices.
      """
      speed = set()
      smart_resource = _get_attribute_value_of(system_obj, 'smart_storage')
      if smart_resource is not None:
          speed.update(_get_attribute_value_of(
              smart_resource, 'drive_rotational_speed_rpm', default=set()))
      storage_resource = _get_attribute_value_of(system_obj, 'storages')
      if storage_resource is not None:
          speed.update(_get_attribute_value_of(
              storage_resource, 'drive_rotational_speed_rpm', default=set()))
      return speed