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HPET(4)			    Kernel Interfaces Manual		       HPET(4)

NAME
       hpet -- High Precision Event Timer driver

SYNOPSIS
       To  compile  this  driver into the kernel, place	the following lines in
       your kernel configuration file:

	     device acpi

       The following tunables are settable from	the loader(8):

       hint.hpet.X.allowed_irqs
       is a 32bit mask.	 Each set bit allows driver to use respective IRQ,  if
       BIOS  also  set respective capability bit in comparator's configuration
       register.  Default value	is 0xffff0000, except some known broken	 hard-
       ware.

       hint.hpet.X.clock
       controls	 event	timers	functionality support.	Setting	to 0, disables
       it.  Default value is 1.

       hint.hpet.X.legacy_route
       controls	"LegacyReplacement Route" mode.	 If enabled, HPET  will	 steal
       IRQ0  of	 i8254 timer and IRQ8 of RTC.  Before using it,	make sure that
       respective drivers are not using	interrupts, by setting also:

       hint.attimer.0.clock=0
       hint.atrtc.0.clock=0
       Default value is	0.

       hint.hpet.X.per_cpu
       controls	how much per-CPU event timers should driver attempt to	regis-
       ter.   This  functionality requires every comparator in a group to have
       own unshared IRQ, so it depends on hardware capabilities	and interrupts
       configuration.  Default value is	1.

DESCRIPTION
       This driver uses	High Precision	Event  Timer  hardware	(part  of  the
       chipset,	 usually  enumerated  via ACPI)	to supply kernel with one time
       counter and several (usually from 3 to 8) event timers.	This  hardware
       includes	 single	 main counter with known increment frequency (10MHz or
       more), and several programmable comparators (optionally with  automatic
       reload  feature).  When value of	the main counter matches current value
       of any comparator, interrupt can	be generated.  Depending  on  hardware
       capabilities  and  configuration, interrupt can be delivered as regular
       I/O APIC	interrupt (ISA or PCI) in range	from 0 to 31, or as Front Side
       Bus interrupt, alike to PCI MSI interrupts, or in so called  "LegacyRe-
       placement Route"	HPET can steal IRQ0 of i8254 and IRQ8 of the RTC.  In-
       terrupt	can  be	 either	 edge-	or level-triggered.  In	last case they
       could be	safely shared with PCI IRQs.  Driver prefers to	use FSB	inter-
       rupts, if supported, to avoid sharing.  If it is	not possible, it  uses
       single sharable IRQ from	PCI range.  Other modes	(LegacyReplacement and
       ISA  IRQs) require special care to setup, but could be configured manu-
       ally via	device hints.

       Event timers provided by	the driver support both	one-shot  an  periodic
       modes and irrelevant to CPU power states.

       Depending on hardware capabilities and configuration, driver can	expose
       each  comparator	as separate event timer	or group them into one or sev-
       eral per-CPU event timers.  In last case	interrupt of  every  of	 those
       comparators within group	is bound to specific CPU core.	This is	possi-
       ble only	when each of these comparators has own unsharable IRQ.

SEE ALSO
       acpi(4),	apic(4), atrtc(4), attimer(4), eventtimers(4), timecounters(4)

HISTORY
       The  hpet  driver  first	 appeared  in  FreeBSD 6.3.  Support for event
       timers was added	in FreeBSD 9.0.

FreeBSD	13.2		      September	14, 2010		       HPET(4)

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | HISTORY

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