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EVENTTIMERS(9)         FreeBSD Kernel Developer's Manual        EVENTTIMERS(9)

     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 consumers.

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

     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 CPU.

     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

     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 <[email protected]>

FreeBSD 11.1-RELEASE-p4          April 2, 2014         FreeBSD 11.1-RELEASE-p4
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