i8254(4) [freebsd man page]

ATTIMER(4)						   BSD Kernel Interfaces Manual 						ATTIMER(4)

NAME

     attimer -- i8254 Programmable Interval Timer (AT Timer) driver

SYNOPSIS

     This driver is a mandatory part of x86 kernels.

     The following tunables are settable from the loader(8):

     hint.attimer.X.clock
     controls support for the event timer functionality.  Setting this value to 0 disables it.	The default value is 1.

     hint.attimer.X.timecounter
     controls support for the time counter functionality.  Setting this value to 0 disables it.  The default value is 1.

     hw.i8254.freq
     allows overriding the default counter frequency.  The same value is also available at run-time via the machdep.i8254_freq sysctl.

DESCRIPTION

     This driver uses i8254 Programmable Interval Timer (AT Timer) hardware to supply the kernel with one timecounter and one event timer, and to
     generate sound tones for the system speaker.  This hardware includes three channels.  Each channel includes a 16 bit counter which decreases
     with a known, platform-dependent frequency.  Counters can operate in several different modes, including periodic and one-shot.  The output of
     each channel has platform-defined wiring: one channel is wired to the interrupt controller and may be used as event timer, one channel is
     wired to the speaker and used to generate sound tones, and one timer is reserved for platform purposes.

     The attimer driver uses a single hardware channel to provide both time counter and event timer functionality.  To make this possible, the
     respective counter must be running in periodic mode.  As a result, the one-shot event timer mode is supported only when time counter func-
     tionality is disabled.

     The event timer provided by the driver is irrelevant to CPU power states.

SEE ALSO

     apic(4), atrtc(4), eventtimers(4), hpet(4), timecounters(4)

BSD
								   May 26, 2014 							       BSD

TIMECOUNTERS(4) 					   BSD Kernel Interfaces Manual 					   TIMECOUNTERS(4)

NAME

     timecounters -- kernel time counters subsystem

SYNOPSIS

     The kernel uses several types of time-related devices, such as: real time clocks, time counters and event timers.	Real time clocks are
     responsible for tracking real world time, mostly when the system is down.	Time counters are responsible for tracking purposes, when the sys-
     tem is running.  Event timers are responsible for generating interrupts at a specified time or periodically, to run different time-based
     events.  This page is about the second.

DESCRIPTION

     Time counters are the lowest level of time tracking in the kernel.  They provide monotonically increasing timestamps with known width and
     update frequency.	They can overflow, drift, etc and so in raw form can be used only in very limited performance-critical places like the
     process scheduler.

     More usable time is created by scaling the values read from the selected time counter and combining it with some offset, regularly updated by
     tc_windup() on hardclock() invocation.

     Different platforms provide different kinds of timer hardware.  The goal of the time counters subsystem is to provide a unified way to access
     that hardware.

     Each driver implementing time counters registers them with the subsystem.	It is possible to see the list of present time counters, via the
     kern.timecounter sysctl(8) variable:

     kern.timecounter.choice: TSC-low(-100) HPET(950) i8254(0) ACPI-fast(900) dummy(-1000000)
     kern.timecounter.tc.ACPI-fast.mask: 16777215
     kern.timecounter.tc.ACPI-fast.counter: 13467909
     kern.timecounter.tc.ACPI-fast.frequency: 3579545
     kern.timecounter.tc.ACPI-fast.quality: 900
     kern.timecounter.tc.i8254.mask: 65535
     kern.timecounter.tc.i8254.counter: 62692
     kern.timecounter.tc.i8254.frequency: 1193182
     kern.timecounter.tc.i8254.quality: 0
     kern.timecounter.tc.HPET.mask: 4294967295
     kern.timecounter.tc.HPET.counter: 3013495652
     kern.timecounter.tc.HPET.frequency: 14318180
     kern.timecounter.tc.HPET.quality: 950
     kern.timecounter.tc.TSC-low.mask: 4294967295
     kern.timecounter.tc.TSC-low.counter: 4067509463
     kern.timecounter.tc.TSC-low.frequency: 11458556
     kern.timecounter.tc.TSC-low.quality: -100

     The output nodes are defined as follows:

     kern.timecounter.tc.X.mask is a bitmask, defining valid counter bits,

     kern.timecounter.tc.X.counter is a present counter value,

     kern.timecounter.tc.X.frequency is a counter update frequency,

     kern.timecounter.tc.X.quality is an integral value, defining the quality of this time counter compared to others.	A negative value means
     this time counter is broken and should not be used.

     The time management code of the kernel chooses one time counter from that list.  The current choice can be read and affected via the
     kern.timecounter.hardware tunable/sysctl.

SEE ALSO

     attimer(4), eventtimers(4), ffclock(4), hpet(4)

BSD
								  April 12, 2014							       BSD