如果建立了异步事件回调，则应当定义回调函数。
static void
ahc_async(void *callback_arg, u_int32_t code, struct cam_path *path, void *arg)
callback_arg - 注册回调时提供的值
code - 标识事件类型
path - 标识事件作用于其上的设备
arg - 事件特定的参量
单一类型事件的实现，AC_LOST_DEVICE，看起来如下：
struct xxx_softc *softc;
struct cam_sim *sim;
int targ;
struct ccb_trans_settings neg;
sim = (struct cam_sim *)callback_arg;
softc = (struct xxx_softc *)cam_sim_softc(sim);
switch (code) {
case AC_LOST_DEVICE:
targ = xpt_path_target_id(path);
if(targ <= OUR_MAX_SUPPORTED_TARGET) {
clean_negotiations(softc, targ);
/* send indication to CAM */
neg.bus_width = 8;
neg.sync_period = neg.sync_offset = 0;
neg.valid = (CCB_TRANS_BUS_WIDTH_VALID
| CCB_TRANS_SYNC_RATE_VALID | CCB_TRANS_SYNC_OFFSET_VALID);
xpt_async(AC_TRANSFER_NEG, path, &neg);
}
break;
default:
break;
}