man4/rtc.4

   1 .\" rtc.4
   2 .\" Copyright 2002 Urs Thuermann (urs@isnogud.escape.de)
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  25 .\" $Id: rtc.4,v 1.4 2005/12/05 17:19:49 urs Exp $
  26 .\"
  27 .\" 2006-02-08 Various additions by mtk
  28 .\" 2006-11-26 cleanup, cover the generic rtc framework; David Brownell
  29 .\"
  30 .TH RTC 4 2017-09-15 "Linux" "Linux Programmer's Manual"
  31 .SH NAME
  32 rtc \- real-time clock
  33 .SH SYNOPSIS
  34 #include <linux/rtc.h>
  35 .PP
  36 .BI "int ioctl(" fd ", RTC_" request ", " param ");"
  37 .SH DESCRIPTION
  38 This is the interface to drivers for real-time clocks (RTCs).
  39 .PP
  40 Most computers have one or more hardware clocks which record the
  41 current "wall clock" time.
  42 These are called "Real Time Clocks" (RTCs).
  43 One of these usually has battery backup power so that it tracks the time
  44 even while the computer is turned off.
  45 RTCs often provide alarms and other interrupts.
  46 .PP
  47 All i386 PCs, and ACPI-based systems, have an RTC that is compatible with
  48 the Motorola MC146818 chip on the original PC/AT.
  49 Today such an RTC is usually integrated into the mainboard's chipset
  50 (south bridge), and uses a replaceable coin-sized backup battery.
  51 .PP
  52 Non-PC systems, such as embedded systems built around system-on-chip
  53 processors, use other implementations.
  54 They usually won't offer the same functionality as the RTC from a PC/AT.
  55 .SS RTC vs system clock
  56 RTCs should not be confused with the system clock, which is
  57 a software clock maintained by the kernel and used to implement
  58 .BR gettimeofday (2)
  59 and
  60 .BR time (2),
  61 as well as setting timestamps on files, and so on.
  62 The system clock reports seconds and microseconds since a start point,
  63 defined to be the POSIX Epoch: 1970-01-01 00:00:00 +0000 (UTC).
  64 (One common implementation counts timer interrupts, once
  65 per "jiffy", at a frequency of 100, 250, or 1000 Hz.)
  66 That is, it is supposed to report wall clock time, which RTCs also do.
  67 .PP
  68 A key difference between an RTC and the system clock is that RTCs
  69 run even when the system is in a low power state (including "off"),
  70 and the system clock can't.
  71 Until it is initialized, the system clock can only report time since
  72 system boot ... not since the POSIX Epoch.
  73 So at boot time, and after resuming from a system low power state, the
  74 system clock will often be set to the current wall clock time using an RTC.
  75 Systems without an RTC need to set the system clock using another clock,
  76 maybe across the network or by entering that data manually.
  77 .SS RTC functionality
  78 RTCs can be read and written with
  79 .BR hwclock (8),
  80 or directly with the ioctl requests listed below.
  81 .PP
  82 Besides tracking the date and time, many RTCs can also generate
  83 interrupts
  84 .IP * 3
  85 on every clock update (i.e., once per second);
  86 .IP *
  87 at periodic intervals with a frequency that can be set to
  88 any power-of-2 multiple in the range 2 Hz to 8192 Hz;
  89 .IP *
  90 on reaching a previously specified alarm time.
  91 .PP
  92 Each of those interrupt sources can be enabled or disabled separately.
  93 On many systems, the alarm interrupt can be configured as a system wakeup
  94 event, which can resume the system from a low power state such as
  95 Suspend-to-RAM (STR, called S3 in ACPI systems),
  96 Hibernation (called S4 in ACPI systems),
  97 or even "off" (called S5 in ACPI systems).
  98 On some systems, the battery backed RTC can't issue
  99 interrupts, but another one can.
 100 .PP
 101 The
 102 .I /dev/rtc
 103 (or
 104 .IR /dev/rtc0 ,
 105 .IR /dev/rtc1 ,
 106 etc.)
 107 device can be opened only once (until it is closed) and it is read-only.
 108 On
 109 .BR read (2)
 110 and
 111 .BR select (2)
 112 the calling process is blocked until the next interrupt from that RTC
 113 is received.
 114 Following the interrupt, the process can read a long integer, of which
 115 the least significant byte contains a bit mask encoding
 116 the types of interrupt that occurred,
 117 while the remaining 3 bytes contain the number of interrupts since the
 118 last
 119 .BR read (2).
 120 .SS ioctl(2) interface
 121 The following
 122 .BR ioctl (2)
 123 requests are defined on file descriptors connected to RTC devices:
 124 .TP
 125 .B RTC_RD_TIME
 126 Returns this RTC's time in the following structure:
 127 .IP
 128 .in +4n
 129 .EX
 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 .EE
 142 .in
 143 .IP
 144 The fields in this structure have the same meaning and ranges as for the
 145 .I tm
 146 structure described in
 147 .BR gmtime (3).
 148 A pointer to this structure should be passed as the third
 149 .BR ioctl (2)
 150 argument.
 151 .TP
 152 .B RTC_SET_TIME
 153 Sets this RTC's time to the time specified by the
 154 .I rtc_time
 155 structure pointed to by the third
 156 .BR ioctl (2)
 157 argument.
 158 To set the
 159 RTC's time the process must be privileged (i.e., have the
 160 .B CAP_SYS_TIME
 161 capability).
 162 .TP
 163 .BR RTC_ALM_READ ", " RTC_ALM_SET
 164 Read and set the alarm time, for RTCs that support alarms.
 165 The alarm interrupt must be separately enabled or disabled using the
 166 .BR RTC_AIE_ON ", " RTC_AIE_OFF
 167 requests.
 168 The third
 169 .BR ioctl (2)
 170 argument is a pointer to an
 171 .I rtc_time
 172 structure.
 173 Only the
 174 .IR tm_sec ,
 175 .IR tm_min ,
 176 and
 177 .I tm_hour
 178 fields of this structure are used.
 179 .TP
 180 .BR RTC_IRQP_READ ", " RTC_IRQP_SET
 181 Read and set the frequency for periodic interrupts,
 182 for RTCs that support periodic interrupts.
 183 The periodic interrupt must be separately enabled or disabled using the
 184 .BR RTC_PIE_ON ", " RTC_PIE_OFF
 185 requests.
 186 The third
 187 .BR ioctl (2)
 188 argument is an
 189 .I "unsigned long\ *"
 190 or an
 191 .IR "unsigned long" ,
 192 respectively.
 193 The value is the frequency in interrupts per second.
 194 The set of allowable frequencies is the multiples of two
 195 in the range 2 to 8192.
 196 Only a privileged process (i.e., one having the
 197 .B CAP_SYS_RESOURCE
 198 capability) can set frequencies above the value specified in
 199 .IR /proc/sys/dev/rtc/max-user-freq .
 200 (This file contains the value 64 by default.)
 201 .TP
 202 .BR RTC_AIE_ON ", " RTC_AIE_OFF
 203 Enable or disable the alarm interrupt, for RTCs that support alarms.
 204 The third
 205 .BR ioctl (2)
 206 argument is ignored.
 207 .TP
 208 .BR RTC_UIE_ON ", " RTC_UIE_OFF
 209 Enable or disable the interrupt on every clock update,
 210 for RTCs that support this once-per-second interrupt.
 211 The third
 212 .BR ioctl (2)
 213 argument is ignored.
 214 .TP
 215 .BR RTC_PIE_ON ", " RTC_PIE_OFF
 216 Enable or disable the periodic interrupt,
 217 for RTCs that support these periodic interrupts.
 218 The third
 219 .BR ioctl (2)
 220 argument is ignored.
 221 Only a privileged process (i.e., one having the
 222 .B CAP_SYS_RESOURCE
 223 capability) can enable the periodic interrupt if the frequency is
 224 currently set above the value specified in
 225 .IR /proc/sys/dev/rtc/max-user-freq .
 226 .TP
 227 .BR RTC_EPOCH_READ ", " RTC_EPOCH_SET
 228 Many RTCs encode the year in an 8-bit register which is either
 229 interpreted as an 8-bit binary number or as a BCD number.
 230 In both cases,
 231 the number is interpreted relative to this RTC's Epoch.
 232 The RTC's Epoch is
 233 initialized to 1900 on most systems but on Alpha and MIPS it might
 234 also be initialized to 1952, 1980, or 2000, depending on the value of
 235 an RTC register for the year.
 236 With some RTCs,
 237 these operations can be used to read or to set the RTC's Epoch,
 238 respectively.
 239 The third
 240 .BR ioctl (2)
 241 argument is an
 242 .I "unsigned long\ *"
 243 or an
 244 .IR "unsigned long" ,
 245 respectively, and the value returned (or assigned) is the Epoch.
 246 To set the RTC's Epoch the process must be privileged (i.e., have the
 247 .B CAP_SYS_TIME
 248 capability).
 249 .TP
 250 .BR RTC_WKALM_RD ", " RTC_WKALM_SET
 251 Some RTCs support a more powerful alarm interface, using these ioctls
 252 to read or write the RTC's alarm time (respectively) with this structure:
 253 .PP
 254 .RS
 255 .in +4n
 256 .EX
 257 struct rtc_wkalrm {
 258     unsigned char enabled;
 259     unsigned char pending;
 260     struct rtc_time time;
 261 };
 262 .EE
 263 .in
 264 .RE
 265 .IP
 266 The
 267 .I enabled
 268 flag is used to enable or disable the alarm interrupt,
 269 or to read its current status; when using these calls,
 270 .BR RTC_AIE_ON " and " RTC_AIE_OFF
 271 are not used.
 272 The
 273 .I pending
 274 flag is used by
 275 .B RTC_WKALM_RD
 276 to report a pending interrupt
 277 (so it's mostly useless on Linux, except when talking
 278 to the RTC managed by EFI firmware).
 279 The
 280 .I time
 281 field is as used with
 282 .B RTC_ALM_READ
 283 and
 284 .B RTC_ALM_SET
 285 except that the
 286 .IR tm_mday ,
 287 .IR tm_mon ,
 288 and
 289 .I tm_year
 290 fields are also valid.
 291 A pointer to this structure should be passed as the third
 292 .BR ioctl (2)
 293 argument.
 294 .SH FILES
 295 .TP
 296 .IR /dev/rtc ", " /dev/rtc0 ", " /dev/rtc1 ", etc."
 297 RTC special character device files.
 298 .TP
 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 .PP
 309 An RTC's Epoch has nothing to do with the POSIX Epoch which is
 310 used only for the system clock.
 311 .PP
 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, that is, between 2000
 314 and 2069.
 315 .PP
 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 nonportable;
 320 portable user-space code expects only a single alarm interrupt, and
 321 will either disable or reinitialize the alarm after receiving it.
 322 .PP
 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 nonvolatile memory;
 328 and other hardware
 329 capabilities that are not currently exposed by this API.
 330 .SH SEE ALSO
 331 .BR date (1),
 332 .BR adjtimex (2),
 333 .BR gettimeofday (2),
 334 .BR settimeofday (2),
 335 .BR stime (2),
 336 .BR time (2),
 337 .BR gmtime (3),
 338 .BR time (7),
 339 .BR hwclock (8)
 340 .PP
 341 .I Documentation/rtc.txt
 342 in the Linux kernel source tree