X86 timing

• Linux clocksources vs clockevents
• PIT -- Programmable Interval Timer (eg Intel 8253)
• ACPI counter
• LAPIC counter
• HPET. Unprivileged programmability is limited by /proc/sys/dev/hpet/max-user-freq (in Hz):

[recombinator](1) $ cat /proc/sys/dev/hpet/max-user-freq 
64
[recombinator](0) $ 

• TSC since Pentium

Real Time Clocks

• Linux docs
  • ACPI standardized the cmos-rtc type, extending the Motorola MC146818 (CONFIG_RTC_DRV_CMOS at the LKDD)

[recombinator](0) $ cat /proc/driver/rtc 
rtc_time	: 07:43:30
rtc_date	: 2009-06-27
alrm_time	: **:00:00
alrm_date	: ****-**-**
alarm_IRQ	: no
alrm_pending	: no
24hr		: yes
periodic_IRQ	: no
update_IRQ	: no
HPET_emulated	: yes
DST_enable	: no
periodic_freq	: 1024
batt_status	: okay
[recombinator](0) $ 

• Multiple RTC's might exist in a machine, thus Linux's "RTC Class"
  • Each RTC gets a device node (by default, /dev/rtc*
  • If [sysfs] is enabled, each gets an entry at /sys/class/rtc/*/:

[recombinator](0) $ ls /sys/class/rtc/rtc0/
date  device         name   since_epoch  time    wakealarm
dev   max_user_freq  power  subsystem    uevent
[recombinator](0) $ 

See Also

• Vojtech Pavlik's 2005-11-18 LKML post