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
  81 .BR ioctl (2)
  82 requests listed below.
  83 .PP
  84 Besides tracking the date and time, many RTCs can also generate
  85 interrupts
  86 .IP * 3
  87 on every clock update (i.e., once per second);
  88 .IP *
  89 at periodic intervals with a frequency that can be set to
  90 any power-of-2 multiple in the range 2 Hz to 8192 Hz;
  91 .IP *
  92 on reaching a previously specified alarm time.
  93 .PP
  94 Each of those interrupt sources can be enabled or disabled separately.
  95 On many systems, the alarm interrupt can be configured as a system wakeup
  96 event, which can resume the system from a low power state such as
  97 Suspend-to-RAM (STR, called S3 in ACPI systems),
  98 Hibernation (called S4 in ACPI systems),
  99 or even "off" (called S5 in ACPI systems).
 100 On some systems, the battery backed RTC can't issue
 101 interrupts, but another one can.
 102 .PP
 103 The
 104 .I /dev/rtc
 105 (or
 106 .IR /dev/rtc0 ,
 107 .IR /dev/rtc1 ,
 108 etc.)
 109 device can be opened only once (until it is closed) and it is read-only.
 110 On
 111 .BR read (2)
 112 and
 113 .BR select (2)
 114 the calling process is blocked until the next interrupt from that RTC
 115 is received.
 116 Following the interrupt, the process can read a long integer, of which
 117 the least significant byte contains a bit mask encoding
 118 the types of interrupt that occurred,
 119 while the remaining 3 bytes contain the number of interrupts since the
 120 last
 121 .BR read (2).
 122 .SS ioctl(2) interface
 123 The following
 124 .BR ioctl (2)
 125 requests are defined on file descriptors connected to RTC devices:
 126 .TP
 127 .B RTC_RD_TIME
 128 Returns this RTC's time in the following structure:
 129 .IP
 130 .in +4n
 131 .EX
 132 struct rtc_time {
 133     int tm_sec;
 134     int tm_min;
 135     int tm_hour;
 136     int tm_mday;
 137     int tm_mon;
 138     int tm_year;
 139     int tm_wday;     /* unused */
 140     int tm_yday;     /* unused */
 141     int tm_isdst;    /* unused */
 142 };
 143 .EE
 144 .in
 145 .IP
 146 The fields in this structure have the same meaning and ranges as for the
 147 .I tm
 148 structure described in
 149 .BR gmtime (3).
 150 A pointer to this structure should be passed as the third
 151 .BR ioctl (2)
 152 argument.
 153 .TP
 154 .B RTC_SET_TIME
 155 Sets this RTC's time to the time specified by the
 156 .I rtc_time
 157 structure pointed to by the third
 158 .BR ioctl (2)
 159 argument.
 160 To set the
 161 RTC's time the process must be privileged (i.e., have the
 162 .B CAP_SYS_TIME
 163 capability).
 164 .TP
 165 .BR RTC_ALM_READ ", " RTC_ALM_SET
 166 Read and set the alarm time, for RTCs that support alarms.
 167 The alarm interrupt must be separately enabled or disabled using the
 168 .BR RTC_AIE_ON ", " RTC_AIE_OFF
 169 requests.
 170 The third
 171 .BR ioctl (2)
 172 argument is a pointer to an
 173 .I rtc_time
 174 structure.
 175 Only the
 176 .IR tm_sec ,
 177 .IR tm_min ,
 178 and
 179 .I tm_hour
 180 fields of this structure are used.
 181 .TP
 182 .BR RTC_IRQP_READ ", " RTC_IRQP_SET
 183 Read and set the frequency for periodic interrupts,
 184 for RTCs that support periodic interrupts.
 185 The periodic interrupt must be separately enabled or disabled using the
 186 .BR RTC_PIE_ON ", " RTC_PIE_OFF
 187 requests.
 188 The third
 189 .BR ioctl (2)
 190 argument is an
 191 .I "unsigned long\ *"
 192 or an
 193 .IR "unsigned long" ,
 194 respectively.
 195 The value is the frequency in interrupts per second.
 196 The set of allowable frequencies is the multiples of two
 197 in the range 2 to 8192.
 198 Only a privileged process (i.e., one having the
 199 .B CAP_SYS_RESOURCE
 200 capability) can set frequencies above the value specified in
 201 .IR /proc/sys/dev/rtc/max-user-freq .
 202 (This file contains the value 64 by default.)
 203 .TP
 204 .BR RTC_AIE_ON ", " RTC_AIE_OFF
 205 Enable or disable the alarm interrupt, for RTCs that support alarms.
 206 The third
 207 .BR ioctl (2)
 208 argument is ignored.
 209 .TP
 210 .BR RTC_UIE_ON ", " RTC_UIE_OFF
 211 Enable or disable the interrupt on every clock update,
 212 for RTCs that support this once-per-second interrupt.
 213 The third
 214 .BR ioctl (2)
 215 argument is ignored.
 216 .TP
 217 .BR RTC_PIE_ON ", " RTC_PIE_OFF
 218 Enable or disable the periodic interrupt,
 219 for RTCs that support these periodic interrupts.
 220 The third
 221 .BR ioctl (2)
 222 argument is ignored.
 223 Only a privileged process (i.e., one having the
 224 .B CAP_SYS_RESOURCE
 225 capability) can enable the periodic interrupt if the frequency is
 226 currently set above the value specified in
 227 .IR /proc/sys/dev/rtc/max-user-freq .
 228 .TP
 229 .BR RTC_EPOCH_READ ", " RTC_EPOCH_SET
 230 Many RTCs encode the year in an 8-bit register which is either
 231 interpreted as an 8-bit binary number or as a BCD number.
 232 In both cases,
 233 the number is interpreted relative to this RTC's Epoch.
 234 The RTC's Epoch is
 235 initialized to 1900 on most systems but on Alpha and MIPS it might
 236 also be initialized to 1952, 1980, or 2000, depending on the value of
 237 an RTC register for the year.
 238 With some RTCs,
 239 these operations can be used to read or to set the RTC's Epoch,
 240 respectively.
 241 The third
 242 .BR ioctl (2)
 243 argument is an
 244 .I "unsigned long\ *"
 245 or an
 246 .IR "unsigned long" ,
 247 respectively, and the value returned (or assigned) is the Epoch.
 248 To set the RTC's Epoch the process must be privileged (i.e., have the
 249 .B CAP_SYS_TIME
 250 capability).
 251 .TP
 252 .BR RTC_WKALM_RD ", " RTC_WKALM_SET
 253 Some RTCs support a more powerful alarm interface, using these ioctls
 254 to read or write the RTC's alarm time (respectively) with this structure:
 255 .PP
 256 .RS
 257 .in +4n
 258 .EX
 259 struct rtc_wkalrm {
 260     unsigned char enabled;
 261     unsigned char pending;
 262     struct rtc_time time;
 263 };
 264 .EE
 265 .in
 266 .RE
 267 .IP
 268 The
 269 .I enabled
 270 flag is used to enable or disable the alarm interrupt,
 271 or to read its current status; when using these calls,
 272 .BR RTC_AIE_ON " and " RTC_AIE_OFF
 273 are not used.
 274 The
 275 .I pending
 276 flag is used by
 277 .B RTC_WKALM_RD
 278 to report a pending interrupt
 279 (so it's mostly useless on Linux, except when talking
 280 to the RTC managed by EFI firmware).
 281 The
 282 .I time
 283 field is as used with
 284 .B RTC_ALM_READ
 285 and
 286 .B RTC_ALM_SET
 287 except that the
 288 .IR tm_mday ,
 289 .IR tm_mon ,
 290 and
 291 .I tm_year
 292 fields are also valid.
 293 A pointer to this structure should be passed as the third
 294 .BR ioctl (2)
 295 argument.
 296 .SH FILES
 297 .TP
 298 .IR /dev/rtc ", " /dev/rtc0 ", " /dev/rtc1 ", etc."
 299 RTC special character device files.
 300 .TP
 301 .IR /proc/driver/rtc
 302 status of the (first) RTC.
 303 .SH NOTES
 304 When the kernel's system time is synchronized with an external
 305 reference using
 306 .BR adjtimex (2)
 307 it will update a designated RTC periodically every 11 minutes.
 308 To do so, the kernel has to briefly turn off periodic interrupts;
 309 this might affect programs using that RTC.
 310 .PP
 311 An RTC's Epoch has nothing to do with the POSIX Epoch which is
 312 used only for the system clock.
 313 .PP
 314 If the year according to the RTC's Epoch and the year register is
 315 less than 1970 it is assumed to be 100 years later, that is, between 2000
 316 and 2069.
 317 .PP
 318 Some RTCs support "wildcard" values in alarm fields, to support
 319 scenarios like periodic alarms at fifteen minutes after every hour,
 320 or on the first day of each month.
 321 Such usage is nonportable;
 322 portable user-space code expects only a single alarm interrupt, and
 323 will either disable or reinitialize the alarm after receiving it.
 324 .PP
 325 Some RTCs support periodic interrupts with periods that are multiples
 326 of a second rather than fractions of a second;
 327 multiple alarms;
 328 programmable output clock signals;
 329 nonvolatile memory;
 330 and other hardware
 331 capabilities that are not currently exposed by this API.
 332 .SH SEE ALSO
 333 .BR date (1),
 334 .BR adjtimex (2),
 335 .BR gettimeofday (2),
 336 .BR settimeofday (2),
 337 .BR stime (2),
 338 .BR time (2),
 339 .BR gmtime (3),
 340 .BR time (7),
 341 .BR hwclock (8)
 342 .PP
 343 .I Documentation/rtc.txt
 344 in the Linux kernel source tree