Device drivers usually allocate a mutex for each driver data structure. The mutex is typically a field in the structure of type kmutex_t. mutex_init(9F) is called to prepare the mutex for use. This call is usually made at attach(9E) time for per-device mutexes and _init(9E) time for global driver mutexes.
struct xxstate *xsp; /* ... */ mutex_init(&xsp->mu, NULL, MUTEX_DRIVER, NULL); /* ... */
For a more complete example of mutex initialization, see Chapter 6, Driver Autoconfiguration.
The driver must destroy the mutex with mutex_destroy(9F) before being unloaded. Destroying the mutex is usually done at detach(9E) time for per-device mutexes and _fini(9E) time for global driver mutexes.