High Precision Event Timer Driver for Linux¶

The High Precision Event Timer (HPET) hardware follows a specificationby Intel and Microsoft, revision 1.

Each HPET has one fixed-rate counter (at 10+ MHz, hence “High Precision”)and up to 32 comparators. Normally three or more comparators are provided,each of which can generate oneshot interrupts and at least one of which hasadditional hardware to support periodic interrupts. The comparators arealso called “timers”, which can be misleading since usually timers areindependent of each other … these share a counter, complicating resets.

HPET devices can support two interrupt routing modes. In one mode, thecomparators are additional interrupt sources with no particular systemrole. Many x86 BIOS writers don’t route HPET interrupts at all, whichprevents use of that mode. They support the other “legacy replacement”mode where the first two comparators block interrupts from 8254 timersand from the RTC.

The driver supports detection of HPET driver allocation and initializationof the HPET before the driver module_init routine is called. This enablesplatform code which uses timer 0 or 1 as the main timer to intercept HPETinitialization. An example of this initialization can be found inarch/x86/kernel/hpet.c.

The driver provides a userspace API which resembles the API found in theRTC driver framework. An example user space program is provided infile:samples/timers/hpet_example.c