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.Dd April 2, 2014
.Dt EVENTTIMERS 9
.Os
.Sh NAME
.Nm eventtimers
.Nd kernel event timers subsystem
.Sh SYNOPSIS
.In sys/timeet.h
.Bd -literal
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;
};
.Ed
.Ft int
.Fn et_register "struct eventtimer *et"
.Ft int
.Fn et_deregister "struct eventtimer *et"
.Ft void
.Fn et_change_frequency "struct eventtimer *et" "uint64_t newfreq"
.Fn ET_LOCK
.Fn ET_UNLOCK
.Ft struct eventtimer *
.Fn et_find "const char *name" "int check" "int want"
.Ft int
.Fn et_init "struct eventtimer *et" "et_event_cb_t *event" "et_deregister_cb_t *deregister" "void *arg"
.Ft int
.Fn et_start "struct eventtimer *et" "sbintime_t first" "sbintime_t period"
.Ft int
.Fn et_stop "struct eventtimer *et"
.Ft int
.Fn et_ban "struct eventtimer *et"
.Ft int
.Fn et_free "struct eventtimer *et"
.Sh DESCRIPTION
Event timers are responsible for generating interrupts at specified time
or periodically, to run different time-based events.
Subsystem consists of three main parts:
.Bl -tag -width "Consumers"
.It Drivers
Manage hardware to generate requested time events.
.It Consumers
.Pa sys/kern/kern_clocksource.c
uses event timers to supply kernel with
.Fn hardclock ,
.Fn statclock
and
.Fn profclock
time events.
.It Glue code
.Pa sys/sys/timeet.h ,
.Pa sys/kern/kern_et.c
provide APIs for event timer drivers and consumers.
.El
.Sh DRIVER API
Driver API is built around eventtimer structure.
To register its functionality driver allocates that structure and calls
.Fn et_register .
Driver should fill following fields there:
.Bl -tag -width Va
.It Va et_name
Unique name of the event timer for management purposes.
.It Va et_flags
Set of flags, describing timer capabilities:
.Bl -tag -width "ET_FLAGS_PERIODIC" -compact
.It ET_FLAGS_PERIODIC
Periodic mode supported.
.It ET_FLAGS_ONESHOT
One-shot mode supported.
.It ET_FLAGS_PERCPU
Timer is per-CPU.
.It ET_FLAGS_C3STOP
Timer may stop in CPU sleep state.
.It ET_FLAGS_POW2DIV
Timer supports only 2^n divisors.
.El
.It Va et_quality
Abstract value to certify whether this timecounter is better than the others.
Higher value means better.
.It Va 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.
.It Va et_min_period , et_max_period
Minimal and maximal reliably programmable time periods.
.It Va et_start
Driver's timer start function pointer.
.It Va et_stop
Driver's timer stop function pointer.
.It Va et_priv
Driver's private data storage.
.El
.Pp
After the event timer functionality is registered, it is controlled via
.Va et_start
and
.Va et_stop
methods.
.Va 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.
.Va first
argument specifies time period before the first event generated.
In periodic mode NULL value specifies that first period is equal to the
.Va period
argument value.
.Va 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
.Va et_event_cb
callback function, passing
.Va et_arg
as the second argument.
.Va et_stop
method is called to stop the specified event timer.
For the per-CPU event timers
.Va et_start
and
.Va et_stop
methods control timers associated with the current CPU.
.Pp
Driver may deregister its functionality by calling
.Fn et_deregister .
.Pp
If the frequency of the clock hardware can change while it is 
running (for example, during power-saving modes), the driver must call
.Fn et_change_frequency
on each change.
If the given event timer is the active timer,
.Fn et_change_frequency
stops the timer on all CPUs, updates 
.Va 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,
.Fn et_change_frequency
simply updates
.Va et->frequency .
.Sh CONSUMER API
.Fn 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,
.Fn et_init
should be called for it, submitting
.Va event
and optionally
.Va deregister
callbacks functions, and the opaque argument
.Va 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.
.Pp
After the timer is found and initialized, it can be controlled via
.Fn et_start
and
.Fn et_stop .
The arguments are the same as described in driver API.
Per-CPU event timers can be controlled only from specific CPUs.
.Pp
.Fn et_ban
allows consumer to mark event timer as broken via clearing both one-shot and
periodic capability flags, if it was somehow detected.
.Fn et_free
is the opposite to
.Fn et_init .
It releases the event timer for other consumers use.
.Pp
.Fn ET_LOCK
and
.Fn ET_UNLOCK
macros should be used to manage
.Xr mutex 9
lock around
.Fn et_find ,
.Fn et_init
and
.Fn et_free
calls to serialize access to the list of the registered event timers and the
pointers returned by
.Fn et_find .
.Fn et_start
and
.Fn et_stop
calls should be serialized in consumer's internal way to avoid concurrent
timer hardware access.
.Sh SEE ALSO
.Xr eventtimers 4
.Sh AUTHORS
.An Alexander Motin Aq mav@FreeBSD.org