如果建立了异步事件回调，则应当定义回调函数。
static voidahc_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; }