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Commit | Line | Data |
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1 | High Precision Event Timer Driver for Linux | |
2 | ||
3 | The High Precision Event Timer (HPET) hardware follows a specification | |
4 | by Intel and Microsoft which can be found at | |
5 | ||
6 | http://www.intel.com/hardwaredesign/hpetspec_1.pdf | |
7 | ||
8 | Each HPET has one fixed-rate counter (at 10+ MHz, hence "High Precision") | |
9 | and up to 32 comparators. Normally three or more comparators are provided, | |
10 | each of which can generate oneshot interrupts and at least one of which has | |
11 | additional hardware to support periodic interrupts. The comparators are | |
12 | also called "timers", which can be misleading since usually timers are | |
13 | independent of each other ... these share a counter, complicating resets. | |
14 | ||
15 | HPET devices can support two interrupt routing modes. In one mode, the | |
16 | comparators are additional interrupt sources with no particular system | |
17 | role. Many x86 BIOS writers don't route HPET interrupts at all, which | |
18 | prevents use of that mode. They support the other "legacy replacement" | |
19 | mode where the first two comparators block interrupts from 8254 timers | |
20 | and from the RTC. | |
21 | ||
22 | The driver supports detection of HPET driver allocation and initialization | |
23 | of the HPET before the driver module_init routine is called. This enables | |
24 | platform code which uses timer 0 or 1 as the main timer to intercept HPET | |
25 | initialization. An example of this initialization can be found in | |
26 | arch/x86/kernel/hpet.c. | |
27 | ||
28 | The driver provides a userspace API which resembles the API found in the | |
29 | RTC driver framework. An example user space program is provided in | |
30 | file:Documentation/timers/hpet_example.c |