linux/Documentation/rtc.txt
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   2        Real Time Clock (RTC) Drivers for Linux
   3        =======================================
   4
   5When Linux developers talk about a "Real Time Clock", they usually mean
   6something that tracks wall clock time and is battery backed so that it
   7works even with system power off.  Such clocks will normally not track
   8the local time zone or daylight savings time -- unless they dual boot
   9with MS-Windows -- but will instead be set to Coordinated Universal Time
  10(UTC, formerly "Greenwich Mean Time").
  11
  12The newest non-PC hardware tends to just count seconds, like the time(2)
  13system call reports, but RTCs also very commonly represent time using
  14the Gregorian calendar and 24 hour time, as reported by gmtime(3).
  15
  16Linux has two largely-compatible userspace RTC API families you may
  17need 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
  25Programmers need to understand that the PC/AT functionality is not
  26always available, and some systems can do much more.  That is, the
  27RTCs use the same API to make requests in both RTC frameworks (using
  28different filenames of course), but the hardware may not offer the
  29same functionality.  For example, not every RTC is hooked up to an
  30IRQ, so they can't all issue alarms; and where standard PC RTCs can
  31only issue an alarm up to 24 hours in the future, other hardware may
  32be able to schedule one any time in the upcoming century.
  33
  34
  35        Old PC/AT-Compatible driver:  /dev/rtc
  36        --------------------------------------
  37
  38All PCs (even Alpha machines) have a Real Time Clock built into them.
  39Usually they are built into the chipset of the computer, but some may
  40actually have a Motorola MC146818 (or clone) on the board. This is the
  41clock that keeps the date and time while your computer is turned off.
  42
  43ACPI has standardized that MC146818 functionality, and extended it in
  44a few ways (enabling longer alarm periods, and wake-from-hibernate).
  45That functionality is NOT exposed in the old driver.
  46
  47However it can also be used to generate signals from a slow 2Hz to a
  48relatively fast 8192Hz, in increments of powers of two. These signals
  49are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
  50for...) It can also function as a 24hr alarm, raising IRQ 8 when the
  51alarm goes off. The alarm can also be programmed to only check any
  52subset of the three programmable values, meaning that it could be set to
  53ring on the 30th second of the 30th minute of every hour, for example.
  54The clock can also be set to generate an interrupt upon every clock
  55update, thus generating a 1Hz signal.
  56
  57The interrupts are reported via /dev/rtc (major 10, minor 135, read only
  58character device) in the form of an unsigned long. The low byte contains
  59the type of interrupt (update-done, alarm-rang, or periodic) that was
  60raised, and the remaining bytes contain the number of interrupts since
  61the last read.  Status information is reported through the pseudo-file
  62/proc/driver/rtc if the /proc filesystem was enabled.  The driver has
  63built in locking so that only one process is allowed to have the /dev/rtc
  64interface open at a time.
  65
  66A user process can monitor these interrupts by doing a read(2) or a
  67select(2) on /dev/rtc -- either will block/stop the user process until
  68the next interrupt is received. This is useful for things like
  69reasonably high frequency data acquisition where one doesn't want to
  70burn up 100% CPU by polling gettimeofday etc. etc.
  71
  72At high frequencies, or under high loads, the user process should check
  73the number of interrupts received since the last read to determine if
  74there has been any interrupt "pileup" so to speak. Just for reference, a
  75typical 486-33 running a tight read loop on /dev/rtc will start to suffer
  76occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
  77frequencies above 1024Hz. So you really should check the high bytes
  78of the value you read, especially at frequencies above that of the
  79normal timer interrupt, which is 100Hz.
  80
  81Programming and/or enabling interrupt frequencies greater than 64Hz is
  82only allowed by root. This is perhaps a bit conservative, but we don't want
  83an evil user generating lots of IRQs on a slow 386sx-16, where it might have
  84a negative impact on performance. This 64Hz limit can be changed by writing
  85a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
  86interrupt handler is only a few lines of code to minimize any possibility
  87of this effect.
  88
  89Also, if the kernel time is synchronized with an external source, the 
  90kernel will write the time back to the CMOS clock every 11 minutes. In 
  91the process of doing this, the kernel briefly turns off RTC periodic 
  92interrupts, so be aware of this if you are doing serious work. If you
  93don't synchronize the kernel time with an external source (via ntp or
  94whatever) then the kernel will keep its hands off the RTC, allowing you
  95exclusive access to the device for your applications.
  96
  97The alarm and/or interrupt frequency are programmed into the RTC via
  98various ioctl(2) calls as listed in ./include/linux/rtc.h
  99Rather than write 50 pages describing the ioctl() and so on, it is
 100perhaps more useful to include a small test program that demonstrates
 101how to use them, and demonstrates the features of the driver. This is
 102probably a lot more useful to people interested in writing applications
 103that 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
 111Because Linux supports many non-ACPI and non-PC platforms, some of which
 112have more than one RTC style clock, it needed a more portable solution
 113than expecting a single battery-backed MC146818 clone on every system.
 114Accordingly, a new "RTC Class" framework has been defined.  It offers
 115three 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
 127The RTC Class framework supports a wide variety of RTCs, ranging from those
 128integrated into embeddable system-on-chip (SOC) processors to discrete chips
 129using I2C, SPI, or some other bus to communicate with the host CPU.  There's
 130even support for PC-style RTCs ... including the features exposed on newer PCs
 131through ACPI.
 132
 133The new framework also removes the "one RTC per system" restriction.  For
 134example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
 135a high functionality RTC is integrated into the SOC.  That system might read
 136the system clock from the discrete RTC, but use the integrated one for all
 137other tasks, because of its greater functionality.
 138
 139SYSFS INTERFACE
 140---------------
 141
 142The sysfs interface under /sys/class/rtc/rtcN provides access to various
 143rtc attributes without requiring the use of ioctls. All dates and times
 144are in the RTC's timezone, rather than in system time.
 145
 146date:            RTC-provided date
 147hctosys:         1 if the RTC provided the system time at boot via the
 148                 CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
 149max_user_freq:   The maximum interrupt rate an unprivileged user may request
 150                 from this RTC.
 151name:            The name of the RTC corresponding to this sysfs directory
 152since_epoch:     The number of seconds since the epoch according to the RTC
 153time:            RTC-provided time
 154wakealarm:       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 seconds since
 157                 the epoch by default, or if there's a leading +, seconds in the
 158                 future, or if there is a leading +=, seconds ahead of the current
 159                 alarm.
 160
 161IOCTL INTERFACE
 162---------------
 163
 164The ioctl() calls supported by /dev/rtc are also supported by the RTC class
 165framework.  However, because the chips and systems are not standardized,
 166some PC/AT functionality might not be provided.  And in the same way, some
 167newer features -- including those enabled by ACPI -- are exposed by the
 168RTC class framework, but can't be supported by the older driver.
 169
 170    *   RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading
 171        time, returning the result as a Gregorian calendar date and 24 hour
 172        wall clock time.  To be most useful, this time may also be updated.
 173
 174    *   RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
 175        is connected to an IRQ line, it can often issue an alarm IRQ up to
 176        24 hours in the future.  (Use RTC_WKALM_* by preference.)
 177
 178    *   RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
 179        the next 24 hours use a slightly more powerful API, which supports
 180        setting the longer alarm time and enabling its IRQ using a single
 181        request (using the same model as EFI firmware).
 182
 183    *   RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework
 184        will emulate this mechanism.
 185
 186    *   RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
 187        are emulated via a kernel hrtimer.
 188
 189In many cases, the RTC alarm can be a system wake event, used to force
 190Linux out of a low power sleep state (or hibernation) back to a fully
 191operational state.  For example, a system could enter a deep power saving
 192state until it's time to execute some scheduled tasks.
 193
 194Note that many of these ioctls are handled by the common rtc-dev interface.
 195Some common examples:
 196
 197    *   RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
 198        called with appropriate values.
 199
 200    *   RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets
 201        the alarm rtc_timer. May call the set_alarm driver function.
 202
 203    *   RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
 204
 205    *   RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
 206
 207If all else fails, check out the rtc-test.c driver!
 208
 209
 210-------------------- 8< ---------------- 8< -----------------------------
 211
 212/*
 213 *      Real Time Clock Driver Test/Example Program
 214 *
 215 *      Compile with:
 216 *                   gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
 217 *
 218 *      Copyright (C) 1996, Paul Gortmaker.
 219 *
 220 *      Released under the GNU General Public License, version 2,
 221 *      included herein by reference.
 222 *
 223 */
 224
 225#include <stdio.h>
 226#include <linux/rtc.h>
 227#include <sys/ioctl.h>
 228#include <sys/time.h>
 229#include <sys/types.h>
 230#include <fcntl.h>
 231#include <unistd.h>
 232#include <stdlib.h>
 233#include <errno.h>
 234
 235
 236/*
 237 * This expects the new RTC class driver framework, working with
 238 * clocks that will often not be clones of what the PC-AT had.
 239 * Use the command line to specify another RTC if you need one.
 240 */
 241static const char default_rtc[] = "/dev/rtc0";
 242
 243
 244int main(int argc, char **argv)
 245{
 246        int i, fd, retval, irqcount = 0;
 247        unsigned long tmp, data;
 248        struct rtc_time rtc_tm;
 249        const char *rtc = default_rtc;
 250
 251        switch (argc) {
 252        case 2:
 253                rtc = argv[1];
 254                /* FALLTHROUGH */
 255        case 1:
 256                break;
 257        default:
 258                fprintf(stderr, "usage:  rtctest [rtcdev]\n");
 259                return 1;
 260        }
 261
 262        fd = open(rtc, O_RDONLY);
 263
 264        if (fd ==  -1) {
 265                perror(rtc);
 266                exit(errno);
 267        }
 268
 269        fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
 270
 271        /* Turn on update interrupts (one per second) */
 272        retval = ioctl(fd, RTC_UIE_ON, 0);
 273        if (retval == -1) {
 274                if (errno == ENOTTY) {
 275                        fprintf(stderr,
 276                                "\n...Update IRQs not supported.\n");
 277                        goto test_READ;
 278                }
 279                perror("RTC_UIE_ON ioctl");
 280                exit(errno);
 281        }
 282
 283        fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
 284                        rtc);
 285        fflush(stderr);
 286        for (i=1; i<6; i++) {
 287                /* This read will block */
 288                retval = read(fd, &data, sizeof(unsigned long));
 289                if (retval == -1) {
 290                        perror("read");
 291                        exit(errno);
 292                }
 293                fprintf(stderr, " %d",i);
 294                fflush(stderr);
 295                irqcount++;
 296        }
 297
 298        fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
 299        fflush(stderr);
 300        for (i=1; i<6; i++) {
 301                struct timeval tv = {5, 0};     /* 5 second timeout on select */
 302                fd_set readfds;
 303
 304                FD_ZERO(&readfds);
 305                FD_SET(fd, &readfds);
 306                /* The select will wait until an RTC interrupt happens. */
 307                retval = select(fd+1, &readfds, NULL, NULL, &tv);
 308                if (retval == -1) {
 309                        perror("select");
 310                        exit(errno);
 311                }
 312                /* This read won't block unlike the select-less case above. */
 313                retval = read(fd, &data, sizeof(unsigned long));
 314                if (retval == -1) {
 315                        perror("read");
 316                        exit(errno);
 317                }
 318                fprintf(stderr, " %d",i);
 319                fflush(stderr);
 320                irqcount++;
 321        }
 322
 323        /* Turn off update interrupts */
 324        retval = ioctl(fd, RTC_UIE_OFF, 0);
 325        if (retval == -1) {
 326                perror("RTC_UIE_OFF ioctl");
 327                exit(errno);
 328        }
 329
 330test_READ:
 331        /* Read the RTC time/date */
 332        retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
 333        if (retval == -1) {
 334                perror("RTC_RD_TIME ioctl");
 335                exit(errno);
 336        }
 337
 338        fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
 339                rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
 340                rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 341
 342        /* Set the alarm to 5 sec in the future, and check for rollover */
 343        rtc_tm.tm_sec += 5;
 344        if (rtc_tm.tm_sec >= 60) {
 345                rtc_tm.tm_sec %= 60;
 346                rtc_tm.tm_min++;
 347        }
 348        if (rtc_tm.tm_min == 60) {
 349                rtc_tm.tm_min = 0;
 350                rtc_tm.tm_hour++;
 351        }
 352        if (rtc_tm.tm_hour == 24)
 353                rtc_tm.tm_hour = 0;
 354
 355        retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
 356        if (retval == -1) {
 357                if (errno == ENOTTY) {
 358                        fprintf(stderr,
 359                                "\n...Alarm IRQs not supported.\n");
 360                        goto test_PIE;
 361                }
 362                perror("RTC_ALM_SET ioctl");
 363                exit(errno);
 364        }
 365
 366        /* Read the current alarm settings */
 367        retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
 368        if (retval == -1) {
 369                perror("RTC_ALM_READ ioctl");
 370                exit(errno);
 371        }
 372
 373        fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
 374                rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 375
 376        /* Enable alarm interrupts */
 377        retval = ioctl(fd, RTC_AIE_ON, 0);
 378        if (retval == -1) {
 379                perror("RTC_AIE_ON ioctl");
 380                exit(errno);
 381        }
 382
 383        fprintf(stderr, "Waiting 5 seconds for alarm...");
 384        fflush(stderr);
 385        /* This blocks until the alarm ring causes an interrupt */
 386        retval = read(fd, &data, sizeof(unsigned long));
 387        if (retval == -1) {
 388                perror("read");
 389                exit(errno);
 390        }
 391        irqcount++;
 392        fprintf(stderr, " okay. Alarm rang.\n");
 393
 394        /* Disable alarm interrupts */
 395        retval = ioctl(fd, RTC_AIE_OFF, 0);
 396        if (retval == -1) {
 397                perror("RTC_AIE_OFF ioctl");
 398                exit(errno);
 399        }
 400
 401test_PIE:
 402        /* Read periodic IRQ rate */
 403        retval = ioctl(fd, RTC_IRQP_READ, &tmp);
 404        if (retval == -1) {
 405                /* not all RTCs support periodic IRQs */
 406                if (errno == ENOTTY) {
 407                        fprintf(stderr, "\nNo periodic IRQ support\n");
 408                        goto done;
 409                }
 410                perror("RTC_IRQP_READ ioctl");
 411                exit(errno);
 412        }
 413        fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
 414
 415        fprintf(stderr, "Counting 20 interrupts at:");
 416        fflush(stderr);
 417
 418        /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
 419        for (tmp=2; tmp<=64; tmp*=2) {
 420
 421                retval = ioctl(fd, RTC_IRQP_SET, tmp);
 422                if (retval == -1) {
 423                        /* not all RTCs can change their periodic IRQ rate */
 424                        if (errno == ENOTTY) {
 425                                fprintf(stderr,
 426                                        "\n...Periodic IRQ rate is fixed\n");
 427                                goto done;
 428                        }
 429                        perror("RTC_IRQP_SET ioctl");
 430                        exit(errno);
 431                }
 432
 433                fprintf(stderr, "\n%ldHz:\t", tmp);
 434                fflush(stderr);
 435
 436                /* Enable periodic interrupts */
 437                retval = ioctl(fd, RTC_PIE_ON, 0);
 438                if (retval == -1) {
 439                        perror("RTC_PIE_ON ioctl");
 440                        exit(errno);
 441                }
 442
 443                for (i=1; i<21; i++) {
 444                        /* This blocks */
 445                        retval = read(fd, &data, sizeof(unsigned long));
 446                        if (retval == -1) {
 447                                perror("read");
 448                                exit(errno);
 449                        }
 450                        fprintf(stderr, " %d",i);
 451                        fflush(stderr);
 452                        irqcount++;
 453                }
 454
 455                /* Disable periodic interrupts */
 456                retval = ioctl(fd, RTC_PIE_OFF, 0);
 457                if (retval == -1) {
 458                        perror("RTC_PIE_OFF ioctl");
 459                        exit(errno);
 460                }
 461        }
 462
 463done:
 464        fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
 465
 466        close(fd);
 467
 468        return 0;
 469}
 470
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