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, it probably
 182        also offers update IRQs whenever the "seconds" counter changes.
 183        If needed, the RTC framework can emulate this mechanism.
 184
 185    *   RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... another
 186        feature often accessible with an IRQ line is a periodic IRQ, issued
 187        at settable frequencies (usually 2^N Hz).
 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 need not actually be implemented by your
 195driver.  The common rtc-dev interface handles many of these nicely if your
 196driver returns ENOIOCTLCMD.  Some common examples:
 197
 198    *   RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
 199        called with appropriate values.
 200
 201    *   RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: the
 202        set_alarm/read_alarm functions will be called.
 203
 204    *   RTC_IRQP_SET, RTC_IRQP_READ: the irq_set_freq function will be called
 205        to set the frequency while the framework will handle the read for you
 206        since the frequency is stored in the irq_freq member of the rtc_device
 207        structure.  Your driver needs to initialize the irq_freq member during
 208        init.  Make sure you check the requested frequency is in range of your
 209        hardware in the irq_set_freq function.  If it isn't, return -EINVAL.  If
 210        you cannot actually change the frequency, do not define irq_set_freq.
 211
 212    *   RTC_PIE_ON, RTC_PIE_OFF: the irq_set_state function will be called.
 213
 214If all else fails, check out the rtc-test.c driver!
 215
 216
 217-------------------- 8< ---------------- 8< -----------------------------
 218
 219/*
 220 *      Real Time Clock Driver Test/Example Program
 221 *
 222 *      Compile with:
 223 *                   gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
 224 *
 225 *      Copyright (C) 1996, Paul Gortmaker.
 226 *
 227 *      Released under the GNU General Public License, version 2,
 228 *      included herein by reference.
 229 *
 230 */
 231
 232#include <stdio.h>
 233#include <linux/rtc.h>
 234#include <sys/ioctl.h>
 235#include <sys/time.h>
 236#include <sys/types.h>
 237#include <fcntl.h>
 238#include <unistd.h>
 239#include <stdlib.h>
 240#include <errno.h>
 241
 242
 243/*
 244 * This expects the new RTC class driver framework, working with
 245 * clocks that will often not be clones of what the PC-AT had.
 246 * Use the command line to specify another RTC if you need one.
 247 */
 248static const char default_rtc[] = "/dev/rtc0";
 249
 250
 251int main(int argc, char **argv)
 252{
 253        int i, fd, retval, irqcount = 0;
 254        unsigned long tmp, data;
 255        struct rtc_time rtc_tm;
 256        const char *rtc = default_rtc;
 257
 258        switch (argc) {
 259        case 2:
 260                rtc = argv[1];
 261                /* FALLTHROUGH */
 262        case 1:
 263                break;
 264        default:
 265                fprintf(stderr, "usage:  rtctest [rtcdev]\n");
 266                return 1;
 267        }
 268
 269        fd = open(rtc, O_RDONLY);
 270
 271        if (fd ==  -1) {
 272                perror(rtc);
 273                exit(errno);
 274        }
 275
 276        fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
 277
 278        /* Turn on update interrupts (one per second) */
 279        retval = ioctl(fd, RTC_UIE_ON, 0);
 280        if (retval == -1) {
 281                if (errno == ENOTTY) {
 282                        fprintf(stderr,
 283                                "\n...Update IRQs not supported.\n");
 284                        goto test_READ;
 285                }
 286                perror("RTC_UIE_ON ioctl");
 287                exit(errno);
 288        }
 289
 290        fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
 291                        rtc);
 292        fflush(stderr);
 293        for (i=1; i<6; i++) {
 294                /* This read will block */
 295                retval = read(fd, &data, sizeof(unsigned long));
 296                if (retval == -1) {
 297                        perror("read");
 298                        exit(errno);
 299                }
 300                fprintf(stderr, " %d",i);
 301                fflush(stderr);
 302                irqcount++;
 303        }
 304
 305        fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
 306        fflush(stderr);
 307        for (i=1; i<6; i++) {
 308                struct timeval tv = {5, 0};     /* 5 second timeout on select */
 309                fd_set readfds;
 310
 311                FD_ZERO(&readfds);
 312                FD_SET(fd, &readfds);
 313                /* The select will wait until an RTC interrupt happens. */
 314                retval = select(fd+1, &readfds, NULL, NULL, &tv);
 315                if (retval == -1) {
 316                        perror("select");
 317                        exit(errno);
 318                }
 319                /* This read won't block unlike the select-less case above. */
 320                retval = read(fd, &data, sizeof(unsigned long));
 321                if (retval == -1) {
 322                        perror("read");
 323                        exit(errno);
 324                }
 325                fprintf(stderr, " %d",i);
 326                fflush(stderr);
 327                irqcount++;
 328        }
 329
 330        /* Turn off update interrupts */
 331        retval = ioctl(fd, RTC_UIE_OFF, 0);
 332        if (retval == -1) {
 333                perror("RTC_UIE_OFF ioctl");
 334                exit(errno);
 335        }
 336
 337test_READ:
 338        /* Read the RTC time/date */
 339        retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
 340        if (retval == -1) {
 341                perror("RTC_RD_TIME ioctl");
 342                exit(errno);
 343        }
 344
 345        fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
 346                rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
 347                rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 348
 349        /* Set the alarm to 5 sec in the future, and check for rollover */
 350        rtc_tm.tm_sec += 5;
 351        if (rtc_tm.tm_sec >= 60) {
 352                rtc_tm.tm_sec %= 60;
 353                rtc_tm.tm_min++;
 354        }
 355        if (rtc_tm.tm_min == 60) {
 356                rtc_tm.tm_min = 0;
 357                rtc_tm.tm_hour++;
 358        }
 359        if (rtc_tm.tm_hour == 24)
 360                rtc_tm.tm_hour = 0;
 361
 362        retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
 363        if (retval == -1) {
 364                if (errno == ENOTTY) {
 365                        fprintf(stderr,
 366                                "\n...Alarm IRQs not supported.\n");
 367                        goto test_PIE;
 368                }
 369                perror("RTC_ALM_SET ioctl");
 370                exit(errno);
 371        }
 372
 373        /* Read the current alarm settings */
 374        retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
 375        if (retval == -1) {
 376                perror("RTC_ALM_READ ioctl");
 377                exit(errno);
 378        }
 379
 380        fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
 381                rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 382
 383        /* Enable alarm interrupts */
 384        retval = ioctl(fd, RTC_AIE_ON, 0);
 385        if (retval == -1) {
 386                perror("RTC_AIE_ON ioctl");
 387                exit(errno);
 388        }
 389
 390        fprintf(stderr, "Waiting 5 seconds for alarm...");
 391        fflush(stderr);
 392        /* This blocks until the alarm ring causes an interrupt */
 393        retval = read(fd, &data, sizeof(unsigned long));
 394        if (retval == -1) {
 395                perror("read");
 396                exit(errno);
 397        }
 398        irqcount++;
 399        fprintf(stderr, " okay. Alarm rang.\n");
 400
 401        /* Disable alarm interrupts */
 402        retval = ioctl(fd, RTC_AIE_OFF, 0);
 403        if (retval == -1) {
 404                perror("RTC_AIE_OFF ioctl");
 405                exit(errno);
 406        }
 407
 408test_PIE:
 409        /* Read periodic IRQ rate */
 410        retval = ioctl(fd, RTC_IRQP_READ, &tmp);
 411        if (retval == -1) {
 412                /* not all RTCs support periodic IRQs */
 413                if (errno == ENOTTY) {
 414                        fprintf(stderr, "\nNo periodic IRQ support\n");
 415                        goto done;
 416                }
 417                perror("RTC_IRQP_READ ioctl");
 418                exit(errno);
 419        }
 420        fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
 421
 422        fprintf(stderr, "Counting 20 interrupts at:");
 423        fflush(stderr);
 424
 425        /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
 426        for (tmp=2; tmp<=64; tmp*=2) {
 427
 428                retval = ioctl(fd, RTC_IRQP_SET, tmp);
 429                if (retval == -1) {
 430                        /* not all RTCs can change their periodic IRQ rate */
 431                        if (errno == ENOTTY) {
 432                                fprintf(stderr,
 433                                        "\n...Periodic IRQ rate is fixed\n");
 434                                goto done;
 435                        }
 436                        perror("RTC_IRQP_SET ioctl");
 437                        exit(errno);
 438                }
 439
 440                fprintf(stderr, "\n%ldHz:\t", tmp);
 441                fflush(stderr);
 442
 443                /* Enable periodic interrupts */
 444                retval = ioctl(fd, RTC_PIE_ON, 0);
 445                if (retval == -1) {
 446                        perror("RTC_PIE_ON ioctl");
 447                        exit(errno);
 448                }
 449
 450                for (i=1; i<21; i++) {
 451                        /* This blocks */
 452                        retval = read(fd, &data, sizeof(unsigned long));
 453                        if (retval == -1) {
 454                                perror("read");
 455                                exit(errno);
 456                        }
 457                        fprintf(stderr, " %d",i);
 458                        fflush(stderr);
 459                        irqcount++;
 460                }
 461
 462                /* Disable periodic interrupts */
 463                retval = ioctl(fd, RTC_PIE_OFF, 0);
 464                if (retval == -1) {
 465                        perror("RTC_PIE_OFF ioctl");
 466                        exit(errno);
 467                }
 468        }
 469
 470done:
 471        fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
 472
 473        close(fd);
 474
 475        return 0;
 476}
 477
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