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     eventtimers — kernel event timers subsystem


     #include <sys/timeet.h>

     struct eventtimer;

     typedef int et_start_t(struct eventtimer *et,
         sbintime_t first, sbintime_t period);
     typedef int et_stop_t(struct eventtimer *et);
     typedef void et_event_cb_t(struct eventtimer *et, void *arg);
     typedef int et_deregister_cb_t(struct eventtimer *et, void *arg);

     struct eventtimer {
             SLIST_ENTRY(eventtimer) et_all;
             char                    *et_name;
             int                     et_flags;
     #define ET_FLAGS_PERIODIC       1
     #define ET_FLAGS_ONESHOT        2
     #define ET_FLAGS_PERCPU         4
     #define ET_FLAGS_C3STOP         8
     #define ET_FLAGS_POW2DIV        16
             int                     et_quality;
             int                     et_active;
             uint64_t                et_frequency;
             sbintime_t              et_min_period;
             sbintime_t              et_max_period;
             et_start_t              *et_start;
             et_stop_t               *et_stop;
             et_event_cb_t           *et_event_cb;
             et_deregister_cb_t      *et_deregister_cb;
             void                    *et_arg;
             void                    *et_priv;
             struct sysctl_oid       *et_sysctl;

     et_register(struct eventtimer *et);

     et_deregister(struct eventtimer *et);

     et_change_frequency(struct eventtimer *et, uint64_t newfreq);



     struct eventtimer *
     et_find(const char *name, int check, int want);

     et_init(struct eventtimer *et, et_event_cb_t *event, et_deregister_cb_t *deregister,
         void *arg);

     et_start(struct eventtimer *et, sbintime_t first, sbintime_t period);

     et_stop(struct eventtimer *et);

     et_ban(struct eventtimer *et);

     et_free(struct eventtimer *et);


     Event timers are responsible for generating interrupts at specified time or periodically, to
     run different time-based events.  Subsystem consists of three main parts:

     Drivers    Manage hardware to generate requested time events.

     Consumers  sys/kern/kern_clocksource.c uses event timers to supply kernel with hardclock(),
                statclock() and profclock() time events.

     Glue code  sys/sys/timeet.h, sys/kern/kern_et.c provide APIs for event timer drivers and


     Driver API is built around eventtimer structure.  To register its functionality driver
     allocates that structure and calls et_register().  Driver should fill following fields

     et_name       Unique name of the event timer for management purposes.

     et_flags      Set of flags, describing timer capabilities:
                   ET_FLAGS_PERIODIC  Periodic mode supported.
                   ET_FLAGS_ONESHOT   One-shot mode supported.
                   ET_FLAGS_PERCPU    Timer is per-CPU.
                   ET_FLAGS_C3STOP    Timer may stop in CPU sleep state.
                   ET_FLAGS_POW2DIV   Timer supports only 2^n divisors.

     et_quality    Abstract value to certify whether this timecounter is better than the others.
                   Higher value means better.

     et_frequency  Timer oscillator's base frequency, if applicable and known.  Used by consumers
                   to predict set of possible frequencies that could be obtained by dividing it.
                   Should be zero if not applicable or unknown.

     et_min_period, et_max_period
                   Minimal and maximal reliably programmable time periods.

     et_start      Driver's timer start function pointer.

     et_stop       Driver's timer stop function pointer.

     et_priv       Driver's private data storage.

     After the event timer functionality is registered, it is controlled via et_start and et_stop
     methods.  et_start method is called to start the specified event timer.  The last two
     arguments are used to specify time when events should be generated.  first argument
     specifies time period before the first event generated.  In periodic mode NULL value
     specifies that first period is equal to the period argument value.  period argument
     specifies the time period between following events for the periodic mode.  The NULL value
     there specifies the one-shot mode.  At least one of these two arguments should be not NULL.
     When event time arrive, driver should call et_event_cb callback function, passing et_arg as
     the second argument.  et_stop method is called to stop the specified event timer.  For the
     per-CPU event timers et_start and et_stop methods control timers associated with the current

     Driver may deregister its functionality by calling et_deregister().

     If the frequency of the clock hardware can change while it is running (for example, during
     power-saving modes), the driver must call et_change_frequency() on each change.  If the
     given event timer is the active timer, et_change_frequency() stops the timer on all CPUs,
     updates et->frequency, then restarts the timer on all CPUs so that all current events are
     rescheduled using the new frequency.  If the given timer is not currently active,
     et_change_frequency() simply updates et->frequency.


     et_find() allows consumer to find available event timer, optionally matching specific name
     and/or capability flags.  Consumer may read returned eventtimer structure, but should not
     modify it.  When wanted event timer is found, et_init() should be called for it, submitting
     event and optionally deregister callbacks functions, and the opaque argument arg.  That
     argument will be passed as argument to the callbacks.  Event callback function will be
     called on scheduled time events.  It is called from the hardware interrupt context, so no
     sleep is permitted there.  Deregister callback function may be called to report consumer
     that the event timer functionality is no longer available.  On this call, consumer should
     stop using event timer before the return.

     After the timer is found and initialized, it can be controlled via et_start() and et_stop().
     The arguments are the same as described in driver API.  Per-CPU event timers can be
     controlled only from specific CPUs.

     et_ban() allows consumer to mark event timer as broken via clearing both one-shot and
     periodic capability flags, if it was somehow detected.  et_free() is the opposite to
     et_init().  It releases the event timer for other consumers use.

     ET_LOCK() and ET_UNLOCK() macros should be used to manage mutex(9) lock around et_find(),
     et_init() and et_free() calls to serialize access to the list of the registered event timers
     and the pointers returned by et_find().  et_start() and et_stop() calls should be serialized
     in consumer's internal way to avoid concurrent timer hardware access.




     Alexander Motin <>