to_string
.to_string = attr_speed_to_string,
.to_string = attr_string_to_string,
.to_string = attr_bool_to_string,
attr_str = kunit_attr_list[i].to_string(attr, &to_free);
const char *(*to_string)(void *attr, bool *to_free);
(object)->name = (type) read_attr_value(dev, to_string(name), \
dbg("%-20s = %x", to_string(name), (int) udev->name)
dbg("%-20s = %x", to_string(name), (int) uinf->name)