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 either
 157                 seconds since the epoch or, if there's a leading +, seconds
 158                 in the future.
 159
 160IOCTL INTERFACE
 161---------------
 162
 163The ioctl() calls supported by /dev/rtc are also supported by the RTC class
 164framework.  However, because the chips and systems are not standardized,
 165some PC/AT functionality might not be provided.  And in the same way, some
 166newer features -- including those enabled by ACPI -- are exposed by the
 167RTC 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
 188In many cases, the RTC alarm can be a system wake event, used to force
 189Linux out of a low power sleep state (or hibernation) back to a fully
 190operational state.  For example, a system could enter a deep power saving
 191state until it's time to execute some scheduled tasks.
 192
 193Note that many of these ioctls are handled by the common rtc-dev interface.
 194Some 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
 206If 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 */
 240static const char default_rtc[] = "/dev/rtc0";
 241
 242
 243int 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
 329test_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
 400test_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
 462done:
 463        fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
 464
 465        close(fd);
 466
 467        return 0;
 468}
 469
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