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===========================================
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High Precision Event Timer Driver for Linux
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===========================================
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The High Precision Event Timer (HPET) hardware follows a specification
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by Intel and Microsoft, revision 1.
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Each HPET has one fixed-rate counter (at 10+ MHz, hence "High Precision")
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and up to 32 comparators. Normally three or more comparators are provided,
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each of which can generate oneshot interrupts and at least one of which has
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additional hardware to support periodic interrupts. The comparators are
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also called "timers", which can be misleading since usually timers are
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independent of each other ... these share a counter, complicating resets.
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HPET devices can support two interrupt routing modes. In one mode, the
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comparators are additional interrupt sources with no particular system
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role. Many x86 BIOS writers don't route HPET interrupts at all, which
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prevents use of that mode. They support the other "legacy replacement"
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mode where the first two comparators block interrupts from 8254 timers
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and from the RTC.
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The driver supports detection of HPET driver allocation and initialization
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of the HPET before the driver module_init routine is called. This enables
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platform code which uses timer 0 or 1 as the main timer to intercept HPET
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initialization. An example of this initialization can be found in
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arch/x86/kernel/hpet.c.
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The driver provides a userspace API which resembles the API found in the
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RTC driver framework. An example user space program is provided in
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file:samples/timers/hpet_example.c
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