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diff --git a/Documentation/timers/hpet.rst b/Documentation/timers/hpet.rst new file mode 100644 index 000000000000..c9d05d3caaca --- /dev/null +++ b/Documentation/timers/hpet.rst @@ -0,0 +1,30 @@ +=========================================== +High Precision Event Timer Driver for Linux +=========================================== + +The High Precision Event Timer (HPET) hardware follows a specification +by 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 has +additional hardware to support periodic interrupts. The comparators are +also called "timers", which can be misleading since usually timers are +independent of each other ... these share a counter, complicating resets. + +HPET devices can support two interrupt routing modes. In one mode, the +comparators are additional interrupt sources with no particular system +role. Many x86 BIOS writers don't route HPET interrupts at all, which +prevents use of that mode. They support the other "legacy replacement" +mode where the first two comparators block interrupts from 8254 timers +and from the RTC. + +The driver supports detection of HPET driver allocation and initialization +of the HPET before the driver module_init routine is called. This enables +platform code which uses timer 0 or 1 as the main timer to intercept HPET +initialization. An example of this initialization can be found in +arch/x86/kernel/hpet.c. + +The driver provides a userspace API which resembles the API found in the +RTC driver framework. An example user space program is provided in +file:samples/timers/hpet_example.c |