Documentation/rtc.txt

   1
   2         Real Time Clock (RTC) Drivers for Linux
   3         =======================================
   4
   5 When Linux developers talk about a "Real Time Clock", they usually mean
   6 something that tracks wall clock time and is battery backed so that it
   7 works even with system power off.  Such clocks will normally not track
   8 the local time zone or daylight savings time -- unless they dual boot
   9 with MS-Windows -- but will instead be set to Coordinated Universal Time
  10 (UTC, formerly "Greenwich Mean Time").
  11
  12 The newest non-PC hardware tends to just count seconds, like the time(2)
  13 system call reports, but RTCs also very commonly represent time using
  14 the Gregorian calendar and 24 hour time, as reported by gmtime(3).
  15
  16 Linux has two largely-compatible userspace RTC API families you may
  17 need to know about:
  18
  19     *   /dev/rtc ... is the RTC provided by PC compatible systems,
  20         so it's not very portable to non-x86 systems.
  21
  22     *   /dev/rtc0, /dev/rtc1 ... are part of a framework that's
  23         supported by a wide variety of RTC chips on all systems.
  24
  25 Programmers need to understand that the PC/AT functionality is not
  26 always available, and some systems can do much more.  That is, the
  27 RTCs use the same API to make requests in both RTC frameworks (using
  28 different filenames of course), but the hardware may not offer the
  29 same functionality.  For example, not every RTC is hooked up to an
  30 IRQ, so they can't all issue alarms; and where standard PC RTCs can
  31 only issue an alarm up to 24 hours in the future, other hardware may
  32 be able to schedule one any time in the upcoming century.
  33
  34
  35         Old PC/AT-Compatible driver:  /dev/rtc
  36         --------------------------------------
  37
  38 All PCs (even Alpha machines) have a Real Time Clock built into them.
  39 Usually they are built into the chipset of the computer, but some may
  40 actually have a Motorola MC146818 (or clone) on the board. This is the
  41 clock that keeps the date and time while your computer is turned off.
  42
  43 ACPI has standardized that MC146818 functionality, and extended it in
  44 a few ways (enabling longer alarm periods, and wake-from-hibernate).
  45 That functionality is NOT exposed in the old driver.
  46
  47 However it can also be used to generate signals from a slow 2Hz to a
  48 relatively fast 8192Hz, in increments of powers of two. These signals
  49 are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
  50 for...) It can also function as a 24hr alarm, raising IRQ 8 when the
  51 alarm goes off. The alarm can also be programmed to only check any
  52 subset of the three programmable values, meaning that it could be set to
  53 ring on the 30th second of the 30th minute of every hour, for example.
  54 The clock can also be set to generate an interrupt upon every clock
  55 update, thus generating a 1Hz signal.
  56
  57 The interrupts are reported via /dev/rtc (major 10, minor 135, read only
  58 character device) in the form of an unsigned long. The low byte contains
  59 the type of interrupt (update-done, alarm-rang, or periodic) that was
  60 raised, and the remaining bytes contain the number of interrupts since
  61 the last read.  Status information is reported through the pseudo-file
  62 /proc/driver/rtc if the /proc filesystem was enabled.  The driver has
  63 built in locking so that only one process is allowed to have the /dev/rtc
  64 interface open at a time.
  65
  66 A user process can monitor these interrupts by doing a read(2) or a
  67 select(2) on /dev/rtc -- either will block/stop the user process until
  68 the next interrupt is received. This is useful for things like
  69 reasonably high frequency data acquisition where one doesn't want to
  70 burn up 100% CPU by polling gettimeofday etc. etc.
  71
  72 At high frequencies, or under high loads, the user process should check
  73 the number of interrupts received since the last read to determine if
  74 there has been any interrupt "pileup" so to speak. Just for reference, a
  75 typical 486-33 running a tight read loop on /dev/rtc will start to suffer
  76 occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
  77 frequencies above 1024Hz. So you really should check the high bytes
  78 of the value you read, especially at frequencies above that of the
  79 normal timer interrupt, which is 100Hz.
  80
  81 Programming and/or enabling interrupt frequencies greater than 64Hz is
  82 only allowed by root. This is perhaps a bit conservative, but we don't want
  83 an evil user generating lots of IRQs on a slow 386sx-16, where it might have
  84 a negative impact on performance. This 64Hz limit can be changed by writing
  85 a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
  86 interrupt handler is only a few lines of code to minimize any possibility
  87 of this effect.
  88
  89 Also, if the kernel time is synchronized with an external source, the
  90 kernel will write the time back to the CMOS clock every 11 minutes. In
  91 the process of doing this, the kernel briefly turns off RTC periodic
  92 interrupts, so be aware of this if you are doing serious work. If you
  93 don't synchronize the kernel time with an external source (via ntp or
  94 whatever) then the kernel will keep its hands off the RTC, allowing you
  95 exclusive access to the device for your applications.
  96
  97 The alarm and/or interrupt frequency are programmed into the RTC via
  98 various ioctl(2) calls as listed in ./include/linux/rtc.h
  99 Rather than write 50 pages describing the ioctl() and so on, it is
 100 perhaps more useful to include a small test program that demonstrates
 101 how to use them, and demonstrates the features of the driver. This is
 102 probably a lot more useful to people interested in writing applications
 103 that will be using this driver.  See the code at the end of this document.
 104
 105 (The original /dev/rtc driver was written by Paul Gortmaker.)
 106
 107
 108         New portable "RTC Class" drivers:  /dev/rtcN
 109         --------------------------------------------
 110
 111 Because Linux supports many non-ACPI and non-PC platforms, some of which
 112 have more than one RTC style clock, it needed a more portable solution
 113 than expecting a single battery-backed MC146818 clone on every system.
 114 Accordingly, a new "RTC Class" framework has been defined.  It offers
 115 three different userspace interfaces:
 116
 117     *   /dev/rtcN ... much the same as the older /dev/rtc interface
 118
 119     *   /sys/class/rtc/rtcN ... sysfs attributes support readonly
 120         access to some RTC attributes.
 121
 122     *   /proc/driver/rtc ... the first RTC (rtc0) may expose itself
 123         using a procfs interface.  More information is (currently) shown
 124         here than through sysfs.
 125
 126 The RTC Class framework supports a wide variety of RTCs, ranging from those
 127 integrated into embeddable system-on-chip (SOC) processors to discrete chips
 128 using I2C, SPI, or some other bus to communicate with the host CPU.  There's
 129 even support for PC-style RTCs ... including the features exposed on newer PCs
 130 through ACPI.
 131
 132 The new framework also removes the "one RTC per system" restriction.  For
 133 example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
 134 a high functionality RTC is integrated into the SOC.  That system might read
 135 the system clock from the discrete RTC, but use the integrated one for all
 136 other tasks, because of its greater functionality.
 137
 138 SYSFS INTERFACE
 139 ---------------
 140
 141 The sysfs interface under /sys/class/rtc/rtcN provides access to various
 142 rtc attributes without requiring the use of ioctls. All dates and times
 143 are in the RTC's timezone, rather than in system time.
 144
 145 date:            RTC-provided date
 146 hctosys:         1 if the RTC provided the system time at boot via the
 147                  CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
 148 max_user_freq:   The maximum interrupt rate an unprivileged user may request
 149                  from this RTC.
 150 name:            The name of the RTC corresponding to this sysfs directory
 151 since_epoch:     The number of seconds since the epoch according to the RTC
 152 time:            RTC-provided time
 153 wakealarm:       The time at which the clock will generate a system wakeup
 154                  event. This is a one shot wakeup event, so must be reset
 155                  after wake if a daily wakeup is required. Format is either
 156                  seconds since the epoch or, if there's a leading +, seconds
 157                  in the future.
 158
 159 IOCTL INTERFACE
 160 ---------------
 161
 162 The ioctl() calls supported by /dev/rtc are also supported by the RTC class
 163 framework.  However, because the chips and systems are not standardized,
 164 some PC/AT functionality might not be provided.  And in the same way, some
 165 newer features -- including those enabled by ACPI -- are exposed by the
 166 RTC class framework, but can't be supported by the older driver.
 167
 168     *   RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading
 169         time, returning the result as a Gregorian calendar date and 24 hour
 170         wall clock time.  To be most useful, this time may also be updated.
 171
 172     *   RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
 173         is connected to an IRQ line, it can often issue an alarm IRQ up to
 174         24 hours in the future.  (Use RTC_WKALM_* by preference.)
 175
 176     *   RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
 177         the next 24 hours use a slightly more powerful API, which supports
 178         setting the longer alarm time and enabling its IRQ using a single
 179         request (using the same model as EFI firmware).
 180
 181     *   RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework
 182         will emulate this mechanism.
 183
 184     *   RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
 185         are emulated via a kernel hrtimer.
 186
 187 In many cases, the RTC alarm can be a system wake event, used to force
 188 Linux out of a low power sleep state (or hibernation) back to a fully
 189 operational state.  For example, a system could enter a deep power saving
 190 state until it's time to execute some scheduled tasks.
 191
 192 Note that many of these ioctls are handled by the common rtc-dev interface.
 193 Some common examples:
 194
 195     *   RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
 196         called with appropriate values.
 197
 198     *   RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets
 199         the alarm rtc_timer. May call the set_alarm driver function.
 200
 201     *   RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
 202
 203     *   RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
 204
 205 If all else fails, check out the rtc-test.c driver!
 206
 207
 208 -------------------- 8< ---------------- 8< -----------------------------
 209
 210 /*
 211  *      Real Time Clock Driver Test/Example Program
 212  *
 213  *      Compile with:
 214  *                   gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
 215  *
 216  *      Copyright (C) 1996, Paul Gortmaker.
 217  *
 218  *      Released under the GNU General Public License, version 2,
 219  *      included herein by reference.
 220  *
 221  */
 222
 223 #include <stdio.h>
 224 #include <linux/rtc.h>
 225 #include <sys/ioctl.h>
 226 #include <sys/time.h>
 227 #include <sys/types.h>
 228 #include <fcntl.h>
 229 #include <unistd.h>
 230 #include <stdlib.h>
 231 #include <errno.h>
 232
 233
 234 /*
 235  * This expects the new RTC class driver framework, working with
 236  * clocks that will often not be clones of what the PC-AT had.
 237  * Use the command line to specify another RTC if you need one.
 238  */
 239 static const char default_rtc[] = "/dev/rtc0";
 240
 241
 242 int main(int argc, char **argv)
 243 {
 244         int i, fd, retval, irqcount = 0;
 245         unsigned long tmp, data;
 246         struct rtc_time rtc_tm;
 247         const char *rtc = default_rtc;
 248
 249         switch (argc) {
 250         case 2:
 251                 rtc = argv[1];
 252                 /* FALLTHROUGH */
 253         case 1:
 254                 break;
 255         default:
 256                 fprintf(stderr, "usage:  rtctest [rtcdev]\n");
 257                 return 1;
 258         }
 259
 260         fd = open(rtc, O_RDONLY);
 261
 262         if (fd ==  -1) {
 263                 perror(rtc);
 264                 exit(errno);
 265         }
 266
 267         fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
 268
 269         /* Turn on update interrupts (one per second) */
 270         retval = ioctl(fd, RTC_UIE_ON, 0);
 271         if (retval == -1) {
 272                 if (errno == ENOTTY) {
 273                         fprintf(stderr,
 274                                 "\n...Update IRQs not supported.\n");
 275                         goto test_READ;
 276                 }
 277                 perror("RTC_UIE_ON ioctl");
 278                 exit(errno);
 279         }
 280
 281         fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
 282                         rtc);
 283         fflush(stderr);
 284         for (i=1; i<6; i++) {
 285                 /* This read will block */
 286                 retval = read(fd, &data, sizeof(unsigned long));
 287                 if (retval == -1) {
 288                         perror("read");
 289                         exit(errno);
 290                 }
 291                 fprintf(stderr, " %d",i);
 292                 fflush(stderr);
 293                 irqcount++;
 294         }
 295
 296         fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
 297         fflush(stderr);
 298         for (i=1; i<6; i++) {
 299                 struct timeval tv = {5, 0};     /* 5 second timeout on select */
 300                 fd_set readfds;
 301
 302                 FD_ZERO(&readfds);
 303                 FD_SET(fd, &readfds);
 304                 /* The select will wait until an RTC interrupt happens. */
 305                 retval = select(fd+1, &readfds, NULL, NULL, &tv);
 306                 if (retval == -1) {
 307                         perror("select");
 308                         exit(errno);
 309                 }
 310                 /* This read won't block unlike the select-less case above. */
 311                 retval = read(fd, &data, sizeof(unsigned long));
 312                 if (retval == -1) {
 313                         perror("read");
 314                         exit(errno);
 315                 }
 316                 fprintf(stderr, " %d",i);
 317                 fflush(stderr);
 318                 irqcount++;
 319         }
 320
 321         /* Turn off update interrupts */
 322         retval = ioctl(fd, RTC_UIE_OFF, 0);
 323         if (retval == -1) {
 324                 perror("RTC_UIE_OFF ioctl");
 325                 exit(errno);
 326         }
 327
 328 test_READ:
 329         /* Read the RTC time/date */
 330         retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
 331         if (retval == -1) {
 332                 perror("RTC_RD_TIME ioctl");
 333                 exit(errno);
 334         }
 335
 336         fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
 337                 rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
 338                 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 339
 340         /* Set the alarm to 5 sec in the future, and check for rollover */
 341         rtc_tm.tm_sec += 5;
 342         if (rtc_tm.tm_sec >= 60) {
 343                 rtc_tm.tm_sec %= 60;
 344                 rtc_tm.tm_min++;
 345         }
 346         if (rtc_tm.tm_min == 60) {
 347                 rtc_tm.tm_min = 0;
 348                 rtc_tm.tm_hour++;
 349         }
 350         if (rtc_tm.tm_hour == 24)
 351                 rtc_tm.tm_hour = 0;
 352
 353         retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
 354         if (retval == -1) {
 355                 if (errno == ENOTTY) {
 356                         fprintf(stderr,
 357                                 "\n...Alarm IRQs not supported.\n");
 358                         goto test_PIE;
 359                 }
 360                 perror("RTC_ALM_SET ioctl");
 361                 exit(errno);
 362         }
 363
 364         /* Read the current alarm settings */
 365         retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
 366         if (retval == -1) {
 367                 perror("RTC_ALM_READ ioctl");
 368                 exit(errno);
 369         }
 370
 371         fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
 372                 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 373
 374         /* Enable alarm interrupts */
 375         retval = ioctl(fd, RTC_AIE_ON, 0);
 376         if (retval == -1) {
 377                 perror("RTC_AIE_ON ioctl");
 378                 exit(errno);
 379         }
 380
 381         fprintf(stderr, "Waiting 5 seconds for alarm...");
 382         fflush(stderr);
 383         /* This blocks until the alarm ring causes an interrupt */
 384         retval = read(fd, &data, sizeof(unsigned long));
 385         if (retval == -1) {
 386                 perror("read");
 387                 exit(errno);
 388         }
 389         irqcount++;
 390         fprintf(stderr, " okay. Alarm rang.\n");
 391
 392         /* Disable alarm interrupts */
 393         retval = ioctl(fd, RTC_AIE_OFF, 0);
 394         if (retval == -1) {
 395                 perror("RTC_AIE_OFF ioctl");
 396                 exit(errno);
 397         }
 398
 399 test_PIE:
 400         /* Read periodic IRQ rate */
 401         retval = ioctl(fd, RTC_IRQP_READ, &tmp);
 402         if (retval == -1) {
 403                 /* not all RTCs support periodic IRQs */
 404                 if (errno == ENOTTY) {
 405                         fprintf(stderr, "\nNo periodic IRQ support\n");
 406                         goto done;
 407                 }
 408                 perror("RTC_IRQP_READ ioctl");
 409                 exit(errno);
 410         }
 411         fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
 412
 413         fprintf(stderr, "Counting 20 interrupts at:");
 414         fflush(stderr);
 415
 416         /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
 417         for (tmp=2; tmp<=64; tmp*=2) {
 418
 419                 retval = ioctl(fd, RTC_IRQP_SET, tmp);
 420                 if (retval == -1) {
 421                         /* not all RTCs can change their periodic IRQ rate */
 422                         if (errno == ENOTTY) {
 423                                 fprintf(stderr,
 424                                         "\n...Periodic IRQ rate is fixed\n");
 425                                 goto done;
 426                         }
 427                         perror("RTC_IRQP_SET ioctl");
 428                         exit(errno);
 429                 }
 430
 431                 fprintf(stderr, "\n%ldHz:\t", tmp);
 432                 fflush(stderr);
 433
 434                 /* Enable periodic interrupts */
 435                 retval = ioctl(fd, RTC_PIE_ON, 0);
 436                 if (retval == -1) {
 437                         perror("RTC_PIE_ON ioctl");
 438                         exit(errno);
 439                 }
 440
 441                 for (i=1; i<21; i++) {
 442                         /* This blocks */
 443                         retval = read(fd, &data, sizeof(unsigned long));
 444                         if (retval == -1) {
 445                                 perror("read");
 446                                 exit(errno);
 447                         }
 448                         fprintf(stderr, " %d",i);
 449                         fflush(stderr);
 450                         irqcount++;
 451                 }
 452
 453                 /* Disable periodic interrupts */
 454                 retval = ioctl(fd, RTC_PIE_OFF, 0);
 455                 if (retval == -1) {
 456                         perror("RTC_PIE_OFF ioctl");
 457                         exit(errno);
 458                 }
 459         }
 460
 461 done:
 462         fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
 463
 464         close(fd);
 465
 466         return 0;
 467 }