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EVENTTIMERS(9) 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;
};

int
et_register(struct eventtimer *et);

int
et_deregister(struct eventtimer *et);

void
et_change_frequency(struct eventtimer *et, uint64_t newfreq);

ET_LOCK();

ET_UNLOCK();

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

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

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

int
et_stop(struct eventtimer *et);

int
et_ban(struct eventtimer *et);

int
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 (), () and () 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 (). 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 ().

If the frequency of the clock hardware can change while it is running (for example, during power-saving modes), the driver must call () 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 () and et_stop(). The arguments are the same as described in driver API. Per-CPU event timers can be controlled only from specific CPUs.

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

() and () macros should be used to manage mutex(9) lock around (), 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.

eventtimers(4)

Alexander Motin <mav@FreeBSD.org>

April 2, 2014 FreeBSD-12.0