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HPET(4)                FreeBSD 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
     hardware.

     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 register.
     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
     "LegacyReplacement Route" HPET can steal IRQ0 of i8254 and IRQ8 of the
     RTC.  Interrupt can be either edge- or level-triggered. In last case they
     could be safely shared with PCI IRQs.  Driver prefers to use FSB
     interrupts, 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
     manually 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 several
     per-CPU event timers. In last case interrupt of every of those
     comparators within group is bound to specific CPU core. This is possible
     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 11.0-PRERELEASE       September 14, 2010       FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | HISTORY

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