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| High Precision Event Timer Driver for Linux | |
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| 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 |