tc_init man page on OpenBSD

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TC_INIT(9)		     OpenBSD Kernel Manual		    TC_INIT(9)

NAME
     tc_init - machine-independent binary timescale

SYNOPSIS
     #include <sys/timetc.h>

     void
     tc_init(struct timecounter *tc);

DESCRIPTION
     The timecounter interface is a machine-independent implementation of a
     binary timescale using whatever hardware support is at hand for tracking
     time.

     A timecounter is a binary counter which has two properties:

	   o   it runs at a fixed, known frequency

	   o   it has sufficient bits to not roll over in less than
	       approximately max(2 msec, 2/HZ seconds) (the value 2 here is
	       really 1 + delta, for some indeterminate value of delta)

     The interface between the hardware which implements a timecounter and the
     machine-independent code which uses this to keep track of time is a
     timecounter structure:

	   struct timecounter {
		   timecounter_get_t	   *tc_get_timecount;
		   timecounter_pps_t	   *tc_poll_pps;
		   u_int		   tc_counter_mask;
		   u_int64_t		   tc_frequency;
		   char			   *tc_name;
		   int			   tc_quality;
		   void			   *tc_priv;
		   struct timecounter	   *tc_next;
	   }

     The fields of the timecounter structure are described below.

     u_int (*tc_get_timecount)(struct timecounter *)
	     This function reads the counter.  It is not required to mask any
	     unimplemented bits out, as long as they are constant.

     void (*tc_poll_pps)(struct timecounter *)
	     This function is optional and can be set to NULL.	It will be
	     called whenever the timecounter is rewound, and is intended to
	     check for PPS events.  Normal hardware does not need it but
	     timecounters which latch PPS in hardware do.

     tc_counter_mask
	     This mask should mask off any unimplemented bits.

     tc_frequency
	     Frequency of the counter in Hz.

     tc_name
	     Name of the timecounter.  Can be any null-terminated string.

     tc_quality
	     Used to determine if this timecounter is better than another
	     timecounter - higher means better.	 Negative means ``only use at
	     explicit request''.

     tc_priv
	     Pointer to the timecounter's private parts.

     tc_next
	     For internal use.

     To register a new timecounter, the hardware device driver should fill a
     timecounter structure with appropriate values and call the tc_init()
     function, giving a pointer to the structure as a tc parameter.

CODE REFERENCES
     The timecounter framework is implemented in the file sys/kern/kern_tc.c.

SEE ALSO
     amdpm(4), elansc(4), gscpm(4), ichpcib(4), viaenv(4), hz(9), microtime(9)

     Poul-Henning Kamp, "Timecounter: Efficient and precise timekeeping in SMP
     kernels", http://phk.freebsd.dk/pubs/timecounter.pdf, The FreeBSD
     Project.

HISTORY
     The timecounter interface first appeared in OpenBSD 3.6.

OpenBSD 4.9			 May 31, 2007			   OpenBSD 4.9
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