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 first RTC (rtc0) may expose itself
 123        using a procfs interface.  More information is (currently) shown
 124        here than through sysfs.
 125
 126The RTC Class framework supports a wide variety of RTCs, ranging from those
 127integrated into embeddable system-on-chip (SOC) processors to discrete chips
 128using I2C, SPI, or some other bus to communicate with the host CPU.  There's
 129even support for PC-style RTCs ... including the features exposed on newer PCs
 130through ACPI.
 131
 132The new framework also removes the "one RTC per system" restriction.  For
 133example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
 134a high functionality RTC is integrated into the SOC.  That system might read
 135the system clock from the discrete RTC, but use the integrated one for all
 136other tasks, because of its greater functionality.
 137
 138SYSFS INTERFACE
 139---------------
 140
 141The sysfs interface under /sys/class/rtc/rtcN provides access to various
 142rtc attributes without requiring the use of ioctls. All dates and times
 143are in the RTC's timezone, rather than in system time.
 144
 145date:            RTC-provided date
 146hctosys:         1 if the RTC provided the system time at boot via the
 147                 CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
 148max_user_freq:   The maximum interrupt rate an unprivileged user may request
 149                 from this RTC.
 150name:            The name of the RTC corresponding to this sysfs directory
 151since_epoch:     The number of seconds since the epoch according to the RTC
 152time:            RTC-provided time
 153wakealarm:       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
 159IOCTL INTERFACE
 160---------------
 161
 162The ioctl() calls supported by /dev/rtc are also supported by the RTC class
 163framework.  However, because the chips and systems are not standardized,
 164some PC/AT functionality might not be provided.  And in the same way, some
 165newer features -- including those enabled by ACPI -- are exposed by the
 166RTC 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
 187In many cases, the RTC alarm can be a system wake event, used to force
 188Linux out of a low power sleep state (or hibernation) back to a fully
 189operational state.  For example, a system could enter a deep power saving
 190state until it's time to execute some scheduled tasks.
 191
 192Note that many of these ioctls are handled by the common rtc-dev interface.
 193Some 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
 205If 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 */
 239static const char default_rtc[] = "/dev/rtc0";
 240
 241
 242int 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
 328test_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
 399test_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
 461done:
 462        fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
 463
 464        close(fd);
 465
 466        return 0;
 467}
 468
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