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Update the RTC man page to reflect the new RTC class framework: 

- Generalize ... it's not just for PC/AT style RTCs, and there
   may be more than one RTC per system.

 - Not all RTCs expose the same feature set as PC/AT ones; most
   of these ioctls will be rejected by some RTCs.

 - Be explicit about when {A,P}IE_{ON,OFF} calls are needed.

 - Describe the parameter to the get/set epoch request; correct
   the description of the get/set frequency parameter.

 - Document RTC_WKALM_{RD,SET}, which don't need AIE_{ON,OFF} and
   which support longer alarm periods.

 - Hey, not all system clock implementations count timer irqs any
   more now that the new RT-derived clock support is merging.
This commit is contained in:
Michael Kerrisk 2006-11-28 04:59:58 +00:00
parent 9e9d4ed3e8
commit 92aebf8f75
1 changed files with 163 additions and 62 deletions
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225
man4/rtc.4
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@ -24,47 +24,61 @@
.\" $Id: rtc.4,v 1.4 2005/12/05 17:19:49 urs Exp $
.\"
.\" 2006-02-08 Various additions by mtk
.\" 2006-11-26 cleanup, cover the generic rtc framework; David Brownell
.\"
.TH RTC 4 "2005-12-05" "Linux" "Linux Programmer's Manual"
.TH RTC 4 "2006-11-26" "Linux" "Linux Programmer's Manual"
.SH NAME
rtc \- real-time clock
.SH SYNOPSIS
#include <linux/rtc.h>
.sp
.BI "int ioctl(" fd ", RTC_" request ", " param ");"
.SH DESCRIPTION
This is the driver for the real-time clock (RTC).
This is the interface to drivers for real-time clocks (RTCs).
Most computers have a built-in hardware clock, usually called the
real-time clock.
This clock is normally battery powered so
that it keeps the time even while the computer is switched off.
It represents the current time as year, month, day of month, hour,
minute, and second.
Most computers have one or more hardware clocks which record the
current "wall clock" time.
These are called "Real Time Clocks" (RTCs).
One of these usually has battery backup power so that it tracks the time
even while the computer is turned off.
RTCs often provide alarms and other interrupts.
The RTC is a chip that maintains the time and date and is able to
generate interrupts at specified times. This chip typically used to
be a Motorola MC146818, a Dallas DS12887, or similar,
but today it is usually implemented in the mainboard's chipset.
All x86 PCs, and ACPI based systems, have an RTC that is compatible with
the Motorola MC146818 chip on the original PC/AT.
Today such an RTC is usually integrated into the mainboard's chipset
(south bridge), and uses a replaceable coin-sized backup battery.
The RTC should not be confused with the system time which is an
independent, interrupt-driven software clock maintained by the kernel.
The software clock is maintained by an interrupt routine that
typically has a frequency of 100, 250, or 1000 Hz.
The software clock counts seconds and microsecond since the POSIX
Epoch, i.e., Jan 1, 1970, 0:00 UTC.
This clock does not involve any special hardware.
The RTC can be read and set with
.BR hwclock (8).
The RTC is almost never used by the Linux kernel. Instead,
the kernel uses the software clock time for
Non-PC systems, such as embedded systems built around system-on-chip
processors, use other implementations.
They usually won't offer the same functionality as the RTC from a PC/AT.
.SS RTC vs System Clock
RTCs should not be confused with the system clock, which is
a software clock maintained by the kernel and used to implement
.BR gettimeofday (2)
and
.BR time (2),
.BR gettimeofday (2),
timestamps on files, etc.
However, at boot time the kernel initializes its software clock by
reading the RTC.
as well as setting timestamps on files, etc.
The system clock reports seconds and microseconds since a start point,
defined to be the POSIX Epoch: Jan 1, 1970, 0:00 UTC.
(One common implementation counts timer interrupts, once
per "jiffy", at a frequency of 100, 250, or 1000 Hz.)
That is, it's supposed to report wall clock time, which RTCs also do.
Besides counting the date and time, the RTC can also generate
A key difference between an RTC and the system clock is that RTCs
run even when the system is in a low power state (including "off"),
and the system clock can't.
Until it's initialized, the system clock can only report time since
system boot ... not since the POSIX Epoch.
So at boot time, and after resuming from a system low power state, the
system clock will often be set to the current wall clock time using an RTC.
Systems without an RTC need to set the system clock using another clock,
maybe across the network or by entering that data manually.
.SS RTC functionality
RTCs can be read and written with
.BR hwclock (8),
or directly with the ioctl requests listed below.
Besides tracking the date and time, many RTCs can also generate
interrupts
.IP *
on every clock update (i.e. once per second);
@ -74,28 +88,41 @@ any power-of-2 multiple in the range 2 Hz to 8192 Hz;
.IP *
on reaching a previously specified alarm time.
.PP
Each of these interrupt sources can be enabled or disabled separately.
Each of those interrupt sources can be enabled or disabled separately.
On many systems, the alarm interrupt can be configured as a system wakeup
event, which can resume the system from a low power state such as
Suspend-to-RAM (STR, called S3 in ACPI systems),
Hibernation (called S4 in ACPI systems),
or even "off" (called S5 in ACPI systems).
On some systems, the battery backed RTC can't issue
interrupts, but another one can.
The
.B /dev/rtc
device can be opened only once simultaneously and it is read-only. On
(or
.BR /dev/rtc0,
.BR /dev/rtc1,
etc)
device can be opened only once (until it is closed) and it is read-only.
On
.BR read (2)
and
.BR select (2)
the calling process is blocked until the next interrupt from the RTC
the calling process is blocked until the next interrupt from that RTC
is received.
Following the interrupt, the process can read a long integer, of which
the least significant byte contains the type of interrupt that occurred,
the least significant byte contains a bit mask encoding
the types of interrupt that occurred,
while the remaining 3 bytes contain the number of interrupts since the
last
.BR read (2).
.SS ioctl() interface
The following
.BR ioctl (2)
operations are provided:
requests are defined on file descriptors connected to RTC devices:
.TP
.B RTC_RD_TIME
Returns the RTC time in the following structure:
Returns this RTC's time in the following structure:
.PP
.RS
.in +0.5i
@ -124,18 +151,21 @@ A pointer to this structure should be passed as the third
argument.
.TP
.B RTC_SET_TIME
Sets the RTC time to the time specified by the
Sets this RTC's time to the time specified by the
.I rtc_time
structure pointed to by the third
.BR ioctl ()
argument.
To set the
RTC time the process must be privileged (i.e., have the
RTC's time the process must be privileged (i.e., have the
.B CAP_SYS_TIME
capability).
.TP
.BR RTC_ALM_READ ", " RTC_ALM_SET
Read and set the alarm time.
Read and set the alarm time, for RTCs that support alarms.
The alarm interrupt must be separately enabled or disabled using the
.BR RTC_AIE_ON ", " RTC_AIE_OFF
requests.
The third \fBioctl\fP() argument is a pointer to an
.I rtc_time
structure.
@ -147,11 +177,15 @@ and
fields of this structure are used.
.TP
.BR RTC_IRQP_READ ", " RTC_IRQP_SET
Read and set the frequency for periodic interrupts.
Read and set the frequency for periodic interrupts,
for RTCs that support periodic interrupts.
The periodic interrupt must be separately enabled or disabled using the
.BR RTC_PIE_ON ", " RTC_PIE_OFF
requests.
The third \fBioctl\fP() argument is a
.I "long\ *"
.I "unsigned long\ *"
or a
.IR long ,
.IR "unsigned long" ,
respectively.
The value is the frequency in interrupts per second.
The set of allowable frequencies is the multiples of two
@ -163,15 +197,17 @@ capability) can set frequencies above the value specified in
(This file contains the value 64 by default.)
.TP
.BR RTC_AIE_ON ", " RTC_AIE_OFF
Enable or disable the alarm interrupt.
Enable or disable the alarm interrupt, for RTCs that support alarms.
The third \fBioctl\fP() argument is ignored.
.TP
.BR RTC_UIE_ON ", " RTC_UIE_OFF
Enable or disable the interrupt on every clock update.
Enable or disable the interrupt on every clock update,
for RTCs that support this once-per-second interrupt.
The third \fBioctl\fP() argument is ignored.
.TP
.BR RTC_PIE_ON ", " RTC_PIE_OFF
Enable or disable the periodic interrupt.
Enable or disable the periodic interrupt,
for RTCs that support these periodic interrupts.
The third \fBioctl\fP() argument is ignored.
Only a privileged process (i.e., one having the
.B CAP_SYS_RESOURCE
@ -180,38 +216,103 @@ currently set above the value specified in
.IR /proc/sys/dev/rtc/max-user-freq .
.TP
.BR RTC_EPOCH_READ ", " RTC_EPOCH_SET
The RTC encodes the year in an 8-bit register which is either
Many RTCs encode the year in an 8-bit register which is either
interpreted as an 8-bit binary number or as a BCD number.
In both cases,
the number is interpreted relative to the RTC Epoch.
The RTC Epoch is
initialized to 1900 on most systems but on Alpha and Mips it might
the number is interpreted relative to this RTC's Epoch.
The RTC's Epoch is
initialized to 1900 on most systems but on Alpha and MIPS it might
also be initialized to 1952, 1980, or 2000, depending on the value of
RTC register for the year.
These operations can be used to read or to set the RTC Epoch, respectively.
To set the RTC Epoch the process must be privileged (i.e., have the
an RTC register for the year.
With some RTCs,
these operations can be used to read or to set the RTC's Epoch,
respectively.
The third \fBioctl\fP() argument is a
.I "unsigned long\ *"
or a
.IR "unsigned long" ,
respectively, and the value returned (or assigned) is the epoch.
To set the RTC's Epoch the process must be privileged (i.e., have the
.B CAP_SYS_TIME
capability).
.TP
.BR RTC_WKALM_RD ", " RTC_WKALM_SET
Some RTCs support a more powerful alarm interface, using these ioctls
to read or write the RTC's alarm time (respectively) with this structure:
.PP
.RS
.in +0.5i
.nf
struct rtc_wkalrm {
unsigned char enabled;
unsigned char pending;
struct rtc_time time;
};
.fi
.in -0.5i
.RE
.IP
The
.I enabled
flag is used to enable or disable the alarm interrupt,
or to read its current status; when using these calls,
.BR RTC_AIE_ON " and " RTC_AIE_OFF
are not used. The
.I pending
flag is used by RTC_WKALM_RD to report a pending interrupt
(so it's mostly useless on Linux, except when talking
to the RTC managed by EFI firmware).
The
.I time
field is as used with
.B RTC_ALM_READ
and
.B RTC_ALM_SET
except that the
.IR tm_mday ,
.IR tm_mon ,
and
.IR tm_year
fields are also valid.
A pointer to this structure should be passed as the third
.BR ioctl ()
argument.
.SH FILES
.IR /dev/rtc :
the RTC special character device file.
.IR /dev/rtc ", "
.IR /dev/rtc0 ", "
.IR /dev/rtc1 ", "
etc: RTC special character device files.
.IR /proc/driver/rtc :
status of the RTC.
.IR /proc/driver/rtc :
status of the (first) RTC.
.SH NOTES
When the kernel's system time is synchronized with an external
reference using
.BR adjtimex (2)
it will update the RTC periodically every 11 minutes. To
do so, the kernel has to briefly turn off periodic interrupts;
this might affect programs using the RTC.
it will update a designated RTC periodically every 11 minutes.
To do so, the kernel has to briefly turn off periodic interrupts;
this might affect programs using that RTC.
The RTC Epoch has nothing to do with the POSIX Epoch which is only
An RTC's Epoch has nothing to do with the POSIX Epoch which is only
used for the system clock.
If the year according to the RTC Epoch and the RTC's year register is
If the year according to the RTC's Epoch and the year register is
less than 1970 it is assumed to be 100 years later, i.e. between 2000
and 2069.
Some RTCs support "wildcard" values in alarm fields, to support
scenarios like periodic alarms at fifteen minutes after every hour,
or on the first day of each month. Such usage is non portable;
portable user space code only expects a single alarm interrupt, and
will either disable or reinitialize the alarm after receiving it.
Some RTCs support periodic interrupts with periods that are multiples
of a second rather than fractions of a second;
multiple alarms;
programmable output clock signals;
non-volatile memory;
and other hardware
capabilities that are not currently exposed by this API.
.SH "SEE ALSO"
.BR hwclock (8),
.BR date (1),