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-rw-r--r--Documentation/timers/hpet.txt43
1 files changed, 21 insertions, 22 deletions
diff --git a/Documentation/timers/hpet.txt b/Documentation/timers/hpet.txt
index 6ad52d9dad6c..e7c09abcfab4 100644
--- a/Documentation/timers/hpet.txt
+++ b/Documentation/timers/hpet.txt
@@ -1,21 +1,32 @@
High Precision Event Timer Driver for Linux
-The High Precision Event Timer (HPET) hardware is the future replacement
-for the 8254 and Real Time Clock (RTC) periodic timer functionality.
-Each HPET can have up to 32 timers. It is possible to configure the
-first two timers as legacy replacements for 8254 and RTC periodic timers.
-A specification done by Intel and Microsoft can be found at
-<http://www.intel.com/technology/architecture/hpetspec.htm>.
+The High Precision Event Timer (HPET) hardware follows a specification
+by Intel and Microsoft which can be found at
+
+ http://www.intel.com/technology/architecture/hpetspec.htm
+
+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 interupts 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/i386/kernel/time_hpet.c.
+arch/x86/kernel/hpet.c.
-The driver provides two APIs which are very similar to the API found in
-the rtc.c driver. There is a user space API and a kernel space API.
-An example user space program is provided below.
+The driver provides a userspace API which resembles the API found in the
+RTC driver framework. An example user space program is provided below.
#include <stdio.h>
#include <stdlib.h>
@@ -286,15 +297,3 @@ out:
return;
}
-
-The kernel API has three interfaces exported from the driver:
-
- hpet_register(struct hpet_task *tp, int periodic)
- hpet_unregister(struct hpet_task *tp)
- hpet_control(struct hpet_task *tp, unsigned int cmd, unsigned long arg)
-
-The kernel module using this interface fills in the ht_func and ht_data
-members of the hpet_task structure before calling hpet_register.
-hpet_control simply vectors to the hpet_ioctl routine and has the same
-commands and respective arguments as the user API. hpet_unregister
-is used to terminate usage of the HPET timer reserved by hpet_register.