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1da177e4 LT |
1 | High Precision Event Timer Driver for Linux |
2 | ||
64a76f66 | 3 | The High Precision Event Timer (HPET) hardware follows a specification |
4e7f9df2 | 4 | by Intel and Microsoft, revision 1. |
64a76f66 DB |
5 | |
6 | Each HPET has one fixed-rate counter (at 10+ MHz, hence "High Precision") | |
7 | and up to 32 comparators. Normally three or more comparators are provided, | |
19f59460 | 8 | each of which can generate oneshot interrupts and at least one of which has |
64a76f66 DB |
9 | additional hardware to support periodic interrupts. The comparators are |
10 | also called "timers", which can be misleading since usually timers are | |
11 | independent of each other ... these share a counter, complicating resets. | |
12 | ||
13 | HPET devices can support two interrupt routing modes. In one mode, the | |
14 | comparators are additional interrupt sources with no particular system | |
15 | role. Many x86 BIOS writers don't route HPET interrupts at all, which | |
16 | prevents use of that mode. They support the other "legacy replacement" | |
17 | mode where the first two comparators block interrupts from 8254 timers | |
18 | and from the RTC. | |
8a0d4900 RD |
19 | |
20 | The driver supports detection of HPET driver allocation and initialization | |
21 | of the HPET before the driver module_init routine is called. This enables | |
22 | platform code which uses timer 0 or 1 as the main timer to intercept HPET | |
23 | initialization. An example of this initialization can be found in | |
64a76f66 | 24 | arch/x86/kernel/hpet.c. |
1da177e4 | 25 | |
64a76f66 | 26 | The driver provides a userspace API which resembles the API found in the |
e6186309 RD |
27 | RTC driver framework. An example user space program is provided in |
28 | file:Documentation/timers/hpet_example.c |