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.

Debian                                         September 14, 2010                                        HPET(4)