| =========================================== |
| 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 |
