modeventtype_t定义如下：
typedef enum modeventtype {
MOD_LOAD,
MOD_UNLOAD,
MOD_SHUTDOWN,
MOD_QUIESCE
} modeventtype_t;
typedef int (*modeventhand_t)(module_t, int /* modeventtype_t */, void *);
MOD_LOAD，MOD_UNLOAD和MOD_SHUTDOWN都好理解。分别是在加载，卸载模块，还有关机时传入模块处理函数的值。而关于MOD_QUIESCE可以参考FreeBSD Device Drivers：
When one issues the kldunload(8) command, MOD_QUIESCE is run before MOD_UNLOAD . If MOD_QUIESCE returns an error, MOD_UNLOAD does not get executed. In other words, MOD_QUIESCE verifies that it is safe to unload your module.
NOTE The kldunload -f command ignores every error returned by MOD_QUIESCE . So you can always unload a module, but it may not be the best idea.
另外，关于MOD_QUIESCE和MOD_SHUTDOWN区别，也可参考FreeBSD Kernel Developer’s Manual：
The difference between MOD_QUIESCE and MOD_UNLOAD is that the module should fail MOD_QUIESCE if it is currently in use, whereas MOD_UNLOAD should only fail if it is impossible to unload the module, for instance because there are memory references to the module which cannot be revoked.