linux/Documentation/timers/hpet.txt
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   1                High Precision Event Timer Driver for Linux
   2
   3The High Precision Event Timer (HPET) hardware follows a specification
   4by Intel and Microsoft which can be found at
   5
   6        http://www.intel.com/hardwaredesign/hpetspec_1.pdf
   7
   8Each HPET has one fixed-rate counter (at 10+ MHz, hence "High Precision")
   9and up to 32 comparators.  Normally three or more comparators are provided,
  10each of which can generate oneshot interrupts and at least one of which has
  11additional hardware to support periodic interrupts.  The comparators are
  12also called "timers", which can be misleading since usually timers are
  13independent of each other ... these share a counter, complicating resets.
  14
  15HPET devices can support two interrupt routing modes.  In one mode, the
  16comparators are additional interrupt sources with no particular system
  17role.  Many x86 BIOS writers don't route HPET interrupts at all, which
  18prevents use of that mode.  They support the other "legacy replacement"
  19mode where the first two comparators block interrupts from 8254 timers
  20and from the RTC.
  21
  22The driver supports detection of HPET driver allocation and initialization
  23of the HPET before the driver module_init routine is called.  This enables
  24platform code which uses timer 0 or 1 as the main timer to intercept HPET
  25initialization.  An example of this initialization can be found in
  26arch/x86/kernel/hpet.c.
  27
  28The driver provides a userspace API which resembles the API found in the
  29RTC driver framework.  An example user space program is provided in
  30file:Documentation/timers/hpet_example.c
  31
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