passasync
status = xpt_register_async(AC_FOUND_DEVICE, passasync, NULL, NULL);
xpt_register_async(0, passasync, periph, periph->path);
passasync, AC_FOUND_DEVICE, cgd);
xpt_register_async(AC_LOST_DEVICE, passasync, periph, periph->path);
static void passasync(void *callback_arg, u_int32_t code,