man4/rtc.4

   1 .\" rtc.4
   2 .\" Copyright 2002 Urs Thuermann (urs@isnogud.escape.de)
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  24 .\" $Id: rtc.4,v 1.4 2005/12/05 17:19:49 urs Exp $
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  26 .\" 2006-02-08 Various additions by mtk
  27 .\" 2006-11-26 cleanup, cover the generic rtc framework; David Brownell
  28 .\"
  29 .TH RTC 4 2006-11-26 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 rtc \- real-time clock
  32 .SH SYNOPSIS
  33 #include <linux/rtc.h>
  34 .sp
  35 .BI "int ioctl(" fd ", RTC_" request ", " param ");"
  36 .SH DESCRIPTION
  37 This is the interface to drivers for real-time clocks (RTCs).
  38
  39 Most computers have one or more hardware clocks which record the
  40 current "wall clock" time.
  41 These are called "Real Time Clocks" (RTCs).
  42 One of these usually has battery backup power so that it tracks the time
  43 even while the computer is turned off.
  44 RTCs often provide alarms and other interrupts.
  45
  46 All x86 PCs, and ACPI based systems, have an RTC that is compatible with
  47 the Motorola MC146818 chip on the original PC/AT.
  48 Today such an RTC is usually integrated into the mainboard's chipset
  49 (south bridge), and uses a replaceable coin-sized backup battery.
  50
  51 Non-PC systems, such as embedded systems built around system-on-chip
  52 processors, use other implementations.
  53 They usually won't offer the same functionality as the RTC from a PC/AT.
  54 .SS RTC vs System Clock
  55 RTCs should not be confused with the system clock, which is
  56 a software clock maintained by the kernel and used to implement
  57 .BR gettimeofday (2)
  58 and
  59 .BR time (2),
  60 as well as setting timestamps on files, etc.
  61 The system clock reports seconds and microseconds since a start point,
  62 defined to be the POSIX Epoch: Jan 1, 1970, 0:00 UTC.
  63 (One common implementation counts timer interrupts, once
  64 per "jiffy", at a frequency of 100, 250, or 1000 Hz.)
  65 That is, it's supposed to report wall clock time, which RTCs also do.
  66
  67 A key difference between an RTC and the system clock is that RTCs
  68 run even when the system is in a low power state (including "off"),
  69 and the system clock can't.
  70 Until it's initialized, the system clock can only report time since
  71 system boot ... not since the POSIX Epoch.
  72 So at boot time, and after resuming from a system low power state, the
  73 system clock will often be set to the current wall clock time using an RTC.
  74 Systems without an RTC need to set the system clock using another clock,
  75 maybe across the network or by entering that data manually.
  76 .SS RTC functionality
  77 RTCs can be read and written with
  78 .BR hwclock (8),
  79 or directly with the ioctl requests listed below.
  80
  81 Besides tracking the date and time, many RTCs can also generate
  82 interrupts
  83 .IP *
  84 on every clock update (i.e. once per second);
  85 .IP *
  86 at periodic intervals with a frequency that can be set to
  87 any power-of-2 multiple in the range 2 Hz to 8192 Hz;
  88 .IP *
  89 on reaching a previously specified alarm time.
  90 .PP
  91 Each of those interrupt sources can be enabled or disabled separately.
  92 On many systems, the alarm interrupt can be configured as a system wakeup
  93 event, which can resume the system from a low power state such as
  94 Suspend-to-RAM (STR, called S3 in ACPI systems),
  95 Hibernation (called S4 in ACPI systems),
  96 or even "off" (called S5 in ACPI systems).
  97 On some systems, the battery backed RTC can't issue
  98 interrupts, but another one can.
  99
 100 The
 101 .B /dev/rtc
 102 (or
 103 .BR /dev/rtc0,
 104 .BR /dev/rtc1,
 105 etc)
 106 device can be opened only once (until it is closed) and it is read-only.
 107 On
 108 .BR read (2)
 109 and
 110 .BR select (2)
 111 the calling process is blocked until the next interrupt from that RTC
 112 is received.
 113 Following the interrupt, the process can read a long integer, of which
 114 the least significant byte contains a bit mask encoding
 115 the types of interrupt that occurred,
 116 while the remaining 3 bytes contain the number of interrupts since the
 117 last
 118 .BR read (2).
 119 .SS ioctl(2) interface
 120 The following
 121 .BR ioctl (2)
 122 requests are defined on file descriptors connected to RTC devices:
 123 .TP
 124 .B RTC_RD_TIME
 125 Returns this RTC's time in the following structure:
 126 .PP
 127 .RS
 128 .in +0.5i
 129 .nf
 130 struct rtc_time {
 131     int tm_sec;
 132     int tm_min;
 133     int tm_hour;
 134     int tm_mday;
 135     int tm_mon;
 136     int tm_year;
 137     int tm_wday;     /* unused */
 138     int tm_yday;     /* unused */
 139     int tm_isdst;    /* unused */
 140 };
 141 .fi
 142 .in -0.5i
 143 .RE
 144 .IP
 145 The fields in this structure have the same meaning and ranges as for the
 146 .I tm
 147 structure described in
 148 .BR gmtime (3).
 149 A pointer to this structure should be passed as the third
 150 .BR ioctl (2)
 151 argument.
 152 .TP
 153 .B RTC_SET_TIME
 154 Sets this RTC's time to the time specified by the
 155 .I rtc_time
 156 structure pointed to by the third
 157 .BR ioctl (2)
 158 argument.
 159 To set the
 160 RTC's time the process must be privileged (i.e., have the
 161 .B CAP_SYS_TIME
 162 capability).
 163 .TP
 164 .BR RTC_ALM_READ ", " RTC_ALM_SET
 165 Read and set the alarm time, for RTCs that support alarms.
 166 The alarm interrupt must be separately enabled or disabled using the
 167 .BR RTC_AIE_ON ", " RTC_AIE_OFF
 168 requests.
 169 The third
 170 .BR ioctl (2)
 171 argument is a pointer to an
 172 .I rtc_time
 173 structure.
 174 Only the
 175 .IR tm_sec ,
 176 .IR tm_min ,
 177 and
 178 .IR tm_hour
 179 fields of this structure are used.
 180 .TP
 181 .BR RTC_IRQP_READ ", " RTC_IRQP_SET
 182 Read and set the frequency for periodic interrupts,
 183 for RTCs that support periodic interrupts.
 184 The periodic interrupt must be separately enabled or disabled using the
 185 .BR RTC_PIE_ON ", " RTC_PIE_OFF
 186 requests.
 187 The third
 188 .BR ioctl (2)
 189 argument is a
 190 .I "unsigned long\ *"
 191 or a
 192 .IR "unsigned long" ,
 193 respectively.
 194 The value is the frequency in interrupts per second.
 195 The set of allowable frequencies is the multiples of two
 196 in the range 2 to 8192.
 197 Only a privileged process (i.e., one having the
 198 .B CAP_SYS_RESOURCE
 199 capability) can set frequencies above the value specified in
 200 .IR /proc/sys/dev/rtc/max-user-freq .
 201 (This file contains the value 64 by default.)
 202 .TP
 203 .BR RTC_AIE_ON ", " RTC_AIE_OFF
 204 Enable or disable the alarm interrupt, for RTCs that support alarms.
 205 The third
 206 .BR ioctl (2)
 207 argument is ignored.
 208 .TP
 209 .BR RTC_UIE_ON ", " RTC_UIE_OFF
 210 Enable or disable the interrupt on every clock update,
 211 for RTCs that support this once-per-second interrupt.
 212 The third
 213 .BR ioctl (2)
 214 argument is ignored.
 215 .TP
 216 .BR RTC_PIE_ON ", " RTC_PIE_OFF
 217 Enable or disable the periodic interrupt,
 218 for RTCs that support these periodic interrupts.
 219 The third
 220 .BR ioctl (2)
 221 argument is ignored.
 222 Only a privileged process (i.e., one having the
 223 .B CAP_SYS_RESOURCE
 224 capability) can enable the periodic interrupt if the frequency is
 225 currently set above the value specified in
 226 .IR /proc/sys/dev/rtc/max-user-freq .
 227 .TP
 228 .BR RTC_EPOCH_READ ", " RTC_EPOCH_SET
 229 Many RTCs encode the year in an 8-bit register which is either
 230 interpreted as an 8-bit binary number or as a BCD number.
 231 In both cases,
 232 the number is interpreted relative to this RTC's Epoch.
 233 The RTC's Epoch is
 234 initialized to 1900 on most systems but on Alpha and MIPS it might
 235 also be initialized to 1952, 1980, or 2000, depending on the value of
 236 an RTC register for the year.
 237 With some RTCs,
 238 these operations can be used to read or to set the RTC's Epoch,
 239 respectively.
 240 The third
 241 .BR ioctl (2)
 242 argument is a
 243 .I "unsigned long\ *"
 244 or a
 245 .IR "unsigned long" ,
 246 respectively, and the value returned (or assigned) is the epoch.
 247 To set the RTC's Epoch the process must be privileged (i.e., have the
 248 .B CAP_SYS_TIME
 249 capability).
 250 .TP
 251 .BR RTC_WKALM_RD ", " RTC_WKALM_SET
 252 Some RTCs support a more powerful alarm interface, using these ioctls
 253 to read or write the RTC's alarm time (respectively) with this structure:
 254 .PP
 255 .RS
 256 .in +0.5i
 257 .nf
 258 struct rtc_wkalrm {
 259     unsigned char enabled;
 260     unsigned char pending;
 261     struct rtc_time time;
 262 };
 263 .fi
 264 .in -0.5i
 265 .RE
 266 .IP
 267 The
 268 .I enabled
 269 flag is used to enable or disable the alarm interrupt,
 270 or to read its current status; when using these calls,
 271 .BR RTC_AIE_ON " and " RTC_AIE_OFF
 272 are not used.
 273 The
 274 .I pending
 275 flag is used by RTC_WKALM_RD to report a pending interrupt
 276 (so it's mostly useless on Linux, except when talking
 277 to the RTC managed by EFI firmware).
 278 The
 279 .I time
 280 field is as used with
 281 .B RTC_ALM_READ
 282 and
 283 .B RTC_ALM_SET
 284 except that the
 285 .IR tm_mday ,
 286 .IR tm_mon ,
 287 and
 288 .IR tm_year
 289 fields are also valid.
 290 A pointer to this structure should be passed as the third
 291 .BR ioctl (2)
 292 argument.
 293 .SH FILES
 294 .IR /dev/rtc ", "
 295 .IR /dev/rtc0 ", "
 296 .IR /dev/rtc1 ", "
 297 etc: RTC special character device files.
 298
 299 .IR /proc/driver/rtc :
 300 status of the (first) RTC.
 301 .SH NOTES
 302 When the kernel's system time is synchronized with an external
 303 reference using
 304 .BR adjtimex (2)
 305 it will update a designated RTC periodically every 11 minutes.
 306 To do so, the kernel has to briefly turn off periodic interrupts;
 307 this might affect programs using that RTC.
 308
 309 An RTC's Epoch has nothing to do with the POSIX Epoch which is only
 310 used for the system clock.
 311
 312 If the year according to the RTC's Epoch and the year register is
 313 less than 1970 it is assumed to be 100 years later, i.e. between 2000
 314 and 2069.
 315
 316 Some RTCs support "wildcard" values in alarm fields, to support
 317 scenarios like periodic alarms at fifteen minutes after every hour,
 318 or on the first day of each month.
 319 Such usage is non portable;
 320 portable user space code only expects a single alarm interrupt, and
 321 will either disable or reinitialize the alarm after receiving it.
 322
 323 Some RTCs support periodic interrupts with periods that are multiples
 324 of a second rather than fractions of a second;
 325 multiple alarms;
 326 programmable output clock signals;
 327 non-volatile memory;
 328 and other hardware
 329 capabilities that are not currently exposed by this API.
 330 .SH "SEE ALSO"
 331 .BR hwclock (8),
 332 .BR date (1),
 333 .BR time (2),
 334 .BR stime (2),
 335 .BR gettimeofday (2),
 336 .BR settimeofday (2),
 337 .BR adjtimex (2),
 338 .BR gmtime (3),
 339 .BR time (7),
 340 /usr/src/linux/Documentation/rtc.txt