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 system clock RTC may expose itself
 123         using a procfs interface. If there is no RTC for the system clock,
 124         rtc0 is used by default. More information is (currently) shown
 125         here than through sysfs.
 126
 127 The RTC Class framework supports a wide variety of RTCs, ranging from those
 128 integrated into embeddable system-on-chip (SOC) processors to discrete chips
 129 using I2C, SPI, or some other bus to communicate with the host CPU.  There's
 130 even support for PC-style RTCs ... including the features exposed on newer PCs
 131 through ACPI.
 132
 133 The new framework also removes the "one RTC per system" restriction.  For
 134 example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
 135 a high functionality RTC is integrated into the SOC.  That system might read
 136 the system clock from the discrete RTC, but use the integrated one for all
 137 other tasks, because of its greater functionality.
 138
 139 SYSFS INTERFACE
 140 ---------------
 141
 142 The sysfs interface under /sys/class/rtc/rtcN provides access to various
 143 rtc attributes without requiring the use of ioctls. All dates and times
 144 are in the RTC's timezone, rather than in system time.
 145
 146 date:            RTC-provided date
 147 hctosys:         1 if the RTC provided the system time at boot via the
 148                  CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
 149 max_user_freq:   The maximum interrupt rate an unprivileged user may request
 150                  from this RTC.
 151 name:            The name of the RTC corresponding to this sysfs directory
 152 since_epoch:     The number of seconds since the epoch according to the RTC
 153 time:            RTC-provided time
 154 wakealarm:       The time at which the clock will generate a system wakeup
 155                  event. This is a one shot wakeup event, so must be reset
 156                  after wake if a daily wakeup is required. Format is either
 157                  seconds since the epoch or, if there's a leading +, seconds
 158                  in the future.
 159
 160 IOCTL INTERFACE
 161 ---------------
 162
 163 The ioctl() calls supported by /dev/rtc are also supported by the RTC class
 164 framework.  However, because the chips and systems are not standardized,
 165 some PC/AT functionality might not be provided.  And in the same way, some
 166 newer features -- including those enabled by ACPI -- are exposed by the
 167 RTC class framework, but can't be supported by the older driver.
 168
 169     *   RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading
 170         time, returning the result as a Gregorian calendar date and 24 hour
 171         wall clock time.  To be most useful, this time may also be updated.
 172
 173     *   RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
 174         is connected to an IRQ line, it can often issue an alarm IRQ up to
 175         24 hours in the future.  (Use RTC_WKALM_* by preference.)
 176
 177     *   RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
 178         the next 24 hours use a slightly more powerful API, which supports
 179         setting the longer alarm time and enabling its IRQ using a single
 180         request (using the same model as EFI firmware).
 181
 182     *   RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework
 183         will emulate this mechanism.
 184
 185     *   RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
 186         are emulated via a kernel hrtimer.
 187
 188 In many cases, the RTC alarm can be a system wake event, used to force
 189 Linux out of a low power sleep state (or hibernation) back to a fully
 190 operational state.  For example, a system could enter a deep power saving
 191 state until it's time to execute some scheduled tasks.
 192
 193 Note that many of these ioctls are handled by the common rtc-dev interface.
 194 Some common examples:
 195
 196     *   RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
 197         called with appropriate values.
 198
 199     *   RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets
 200         the alarm rtc_timer. May call the set_alarm driver function.
 201
 202     *   RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
 203
 204     *   RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
 205
 206 If all else fails, check out the rtc-test.c driver!
 207
 208
 209 -------------------- 8< ---------------- 8< -----------------------------
 210
 211 /*
 212  *      Real Time Clock Driver Test/Example Program
 213  *
 214  *      Compile with:
 215  *                   gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
 216  *
 217  *      Copyright (C) 1996, Paul Gortmaker.
 218  *
 219  *      Released under the GNU General Public License, version 2,
 220  *      included herein by reference.
 221  *
 222  */
 223
 224 #include <stdio.h>
 225 #include <linux/rtc.h>
 226 #include <sys/ioctl.h>
 227 #include <sys/time.h>
 228 #include <sys/types.h>
 229 #include <fcntl.h>
 230 #include <unistd.h>
 231 #include <stdlib.h>
 232 #include <errno.h>
 233
 234
 235 /*
 236  * This expects the new RTC class driver framework, working with
 237  * clocks that will often not be clones of what the PC-AT had.
 238  * Use the command line to specify another RTC if you need one.
 239  */
 240 static const char default_rtc[] = "/dev/rtc0";
 241
 242
 243 int main(int argc, char **argv)
 244 {
 245         int i, fd, retval, irqcount = 0;
 246         unsigned long tmp, data;
 247         struct rtc_time rtc_tm;
 248         const char *rtc = default_rtc;
 249
 250         switch (argc) {
 251         case 2:
 252                 rtc = argv[1];
 253                 /* FALLTHROUGH */
 254         case 1:
 255                 break;
 256         default:
 257                 fprintf(stderr, "usage:  rtctest [rtcdev]\n");
 258                 return 1;
 259         }
 260
 261         fd = open(rtc, O_RDONLY);
 262
 263         if (fd ==  -1) {
 264                 perror(rtc);
 265                 exit(errno);
 266         }
 267
 268         fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
 269
 270         /* Turn on update interrupts (one per second) */
 271         retval = ioctl(fd, RTC_UIE_ON, 0);
 272         if (retval == -1) {
 273                 if (errno == ENOTTY) {
 274                         fprintf(stderr,
 275                                 "\n...Update IRQs not supported.\n");
 276                         goto test_READ;
 277                 }
 278                 perror("RTC_UIE_ON ioctl");
 279                 exit(errno);
 280         }
 281
 282         fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
 283                         rtc);
 284         fflush(stderr);
 285         for (i=1; i<6; i++) {
 286                 /* This read will block */
 287                 retval = read(fd, &data, sizeof(unsigned long));
 288                 if (retval == -1) {
 289                         perror("read");
 290                         exit(errno);
 291                 }
 292                 fprintf(stderr, " %d",i);
 293                 fflush(stderr);
 294                 irqcount++;
 295         }
 296
 297         fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
 298         fflush(stderr);
 299         for (i=1; i<6; i++) {
 300                 struct timeval tv = {5, 0};     /* 5 second timeout on select */
 301                 fd_set readfds;
 302
 303                 FD_ZERO(&readfds);
 304                 FD_SET(fd, &readfds);
 305                 /* The select will wait until an RTC interrupt happens. */
 306                 retval = select(fd+1, &readfds, NULL, NULL, &tv);
 307                 if (retval == -1) {
 308                         perror("select");
 309                         exit(errno);
 310                 }
 311                 /* This read won't block unlike the select-less case above. */
 312                 retval = read(fd, &data, sizeof(unsigned long));
 313                 if (retval == -1) {
 314                         perror("read");
 315                         exit(errno);
 316                 }
 317                 fprintf(stderr, " %d",i);
 318                 fflush(stderr);
 319                 irqcount++;
 320         }
 321
 322         /* Turn off update interrupts */
 323         retval = ioctl(fd, RTC_UIE_OFF, 0);
 324         if (retval == -1) {
 325                 perror("RTC_UIE_OFF ioctl");
 326                 exit(errno);
 327         }
 328
 329 test_READ:
 330         /* Read the RTC time/date */
 331         retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
 332         if (retval == -1) {
 333                 perror("RTC_RD_TIME ioctl");
 334                 exit(errno);
 335         }
 336
 337         fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
 338                 rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
 339                 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 340
 341         /* Set the alarm to 5 sec in the future, and check for rollover */
 342         rtc_tm.tm_sec += 5;
 343         if (rtc_tm.tm_sec >= 60) {
 344                 rtc_tm.tm_sec %= 60;
 345                 rtc_tm.tm_min++;
 346         }
 347         if (rtc_tm.tm_min == 60) {
 348                 rtc_tm.tm_min = 0;
 349                 rtc_tm.tm_hour++;
 350         }
 351         if (rtc_tm.tm_hour == 24)
 352                 rtc_tm.tm_hour = 0;
 353
 354         retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
 355         if (retval == -1) {
 356                 if (errno == ENOTTY) {
 357                         fprintf(stderr,
 358                                 "\n...Alarm IRQs not supported.\n");
 359                         goto test_PIE;
 360                 }
 361                 perror("RTC_ALM_SET ioctl");
 362                 exit(errno);
 363         }
 364
 365         /* Read the current alarm settings */
 366         retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
 367         if (retval == -1) {
 368                 perror("RTC_ALM_READ ioctl");
 369                 exit(errno);
 370         }
 371
 372         fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
 373                 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 374
 375         /* Enable alarm interrupts */
 376         retval = ioctl(fd, RTC_AIE_ON, 0);
 377         if (retval == -1) {
 378                 perror("RTC_AIE_ON ioctl");
 379                 exit(errno);
 380         }
 381
 382         fprintf(stderr, "Waiting 5 seconds for alarm...");
 383         fflush(stderr);
 384         /* This blocks until the alarm ring causes an interrupt */
 385         retval = read(fd, &data, sizeof(unsigned long));
 386         if (retval == -1) {
 387                 perror("read");
 388                 exit(errno);
 389         }
 390         irqcount++;
 391         fprintf(stderr, " okay. Alarm rang.\n");
 392
 393         /* Disable alarm interrupts */
 394         retval = ioctl(fd, RTC_AIE_OFF, 0);
 395         if (retval == -1) {
 396                 perror("RTC_AIE_OFF ioctl");
 397                 exit(errno);
 398         }
 399
 400 test_PIE:
 401         /* Read periodic IRQ rate */
 402         retval = ioctl(fd, RTC_IRQP_READ, &tmp);
 403         if (retval == -1) {
 404                 /* not all RTCs support periodic IRQs */
 405                 if (errno == ENOTTY) {
 406                         fprintf(stderr, "\nNo periodic IRQ support\n");
 407                         goto done;
 408                 }
 409                 perror("RTC_IRQP_READ ioctl");
 410                 exit(errno);
 411         }
 412         fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
 413
 414         fprintf(stderr, "Counting 20 interrupts at:");
 415         fflush(stderr);
 416
 417         /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
 418         for (tmp=2; tmp<=64; tmp*=2) {
 419
 420                 retval = ioctl(fd, RTC_IRQP_SET, tmp);
 421                 if (retval == -1) {
 422                         /* not all RTCs can change their periodic IRQ rate */
 423                         if (errno == ENOTTY) {
 424                                 fprintf(stderr,
 425                                         "\n...Periodic IRQ rate is fixed\n");
 426                                 goto done;
 427                         }
 428                         perror("RTC_IRQP_SET ioctl");
 429                         exit(errno);
 430                 }
 431
 432                 fprintf(stderr, "\n%ldHz:\t", tmp);
 433                 fflush(stderr);
 434
 435                 /* Enable periodic interrupts */
 436                 retval = ioctl(fd, RTC_PIE_ON, 0);
 437                 if (retval == -1) {
 438                         perror("RTC_PIE_ON ioctl");
 439                         exit(errno);
 440                 }
 441
 442                 for (i=1; i<21; i++) {
 443                         /* This blocks */
 444                         retval = read(fd, &data, sizeof(unsigned long));
 445                         if (retval == -1) {
 446                                 perror("read");
 447                                 exit(errno);
 448                         }
 449                         fprintf(stderr, " %d",i);
 450                         fflush(stderr);
 451                         irqcount++;
 452                 }
 453
 454                 /* Disable periodic interrupts */
 455                 retval = ioctl(fd, RTC_PIE_OFF, 0);
 456                 if (retval == -1) {
 457                         perror("RTC_PIE_OFF ioctl");
 458                         exit(errno);
 459                 }
 460         }
 461
 462 done:
 463         fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
 464
 465         close(fd);
 466
 467         return 0;
 468 }