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
 138The ioctl() calls supported by /dev/rtc are also supported by the RTC class
 139framework.  However, because the chips and systems are not standardized,
 140some PC/AT functionality might not be provided.  And in the same way, some
 141newer features -- including those enabled by ACPI -- are exposed by the
 142RTC class framework, but can't be supported by the older driver.
 143
 144    *   RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading
 145        time, returning the result as a Gregorian calendar date and 24 hour
 146        wall clock time.  To be most useful, this time may also be updated.
 147
 148    *   RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
 149        is connected to an IRQ line, it can often issue an alarm IRQ up to
 150        24 hours in the future.  (Use RTC_WKALM_* by preference.)
 151
 152    *   RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
 153        the next 24 hours use a slightly more powerful API, which supports
 154        setting the longer alarm time and enabling its IRQ using a single
 155        request (using the same model as EFI firmware).
 156
 157    *   RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, it probably
 158        also offers update IRQs whenever the "seconds" counter changes.
 159        If needed, the RTC framework can emulate this mechanism.
 160
 161    *   RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... another
 162        feature often accessible with an IRQ line is a periodic IRQ, issued
 163        at settable frequencies (usually 2^N Hz).
 164
 165In many cases, the RTC alarm can be a system wake event, used to force
 166Linux out of a low power sleep state (or hibernation) back to a fully
 167operational state.  For example, a system could enter a deep power saving
 168state until it's time to execute some scheduled tasks.
 169
 170Note that many of these ioctls need not actually be implemented by your
 171driver.  The common rtc-dev interface handles many of these nicely if your
 172driver returns ENOIOCTLCMD.  Some common examples:
 173
 174    *   RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
 175        called with appropriate values.
 176
 177    *   RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: the
 178        set_alarm/read_alarm functions will be called.
 179
 180    *   RTC_IRQP_SET, RTC_IRQP_READ: the irq_set_freq function will be called
 181        to set the frequency while the framework will handle the read for you
 182        since the frequency is stored in the irq_freq member of the rtc_device
 183        structure.  Your driver needs to initialize the irq_freq member during
 184        init.  Make sure you check the requested frequency is in range of your
 185        hardware in the irq_set_freq function.  If it isn't, return -EINVAL.  If
 186        you cannot actually change the frequency, do not define irq_set_freq.
 187
 188If all else fails, check out the rtc-test.c driver!
 189
 190
 191-------------------- 8< ---------------- 8< -----------------------------
 192
 193/*
 194 *      Real Time Clock Driver Test/Example Program
 195 *
 196 *      Compile with:
 197 *                   gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
 198 *
 199 *      Copyright (C) 1996, Paul Gortmaker.
 200 *
 201 *      Released under the GNU General Public License, version 2,
 202 *      included herein by reference.
 203 *
 204 */
 205
 206#include <stdio.h>
 207#include <linux/rtc.h>
 208#include <sys/ioctl.h>
 209#include <sys/time.h>
 210#include <sys/types.h>
 211#include <fcntl.h>
 212#include <unistd.h>
 213#include <stdlib.h>
 214#include <errno.h>
 215
 216
 217/*
 218 * This expects the new RTC class driver framework, working with
 219 * clocks that will often not be clones of what the PC-AT had.
 220 * Use the command line to specify another RTC if you need one.
 221 */
 222static const char default_rtc[] = "/dev/rtc0";
 223
 224
 225int main(int argc, char **argv)
 226{
 227        int i, fd, retval, irqcount = 0;
 228        unsigned long tmp, data;
 229        struct rtc_time rtc_tm;
 230        const char *rtc = default_rtc;
 231
 232        switch (argc) {
 233        case 2:
 234                rtc = argv[1];
 235                /* FALLTHROUGH */
 236        case 1:
 237                break;
 238        default:
 239                fprintf(stderr, "usage:  rtctest [rtcdev]\n");
 240                return 1;
 241        }
 242
 243        fd = open(rtc, O_RDONLY);
 244
 245        if (fd ==  -1) {
 246                perror(rtc);
 247                exit(errno);
 248        }
 249
 250        fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
 251
 252        /* Turn on update interrupts (one per second) */
 253        retval = ioctl(fd, RTC_UIE_ON, 0);
 254        if (retval == -1) {
 255                if (errno == ENOTTY) {
 256                        fprintf(stderr,
 257                                "\n...Update IRQs not supported.\n");
 258                        goto test_READ;
 259                }
 260                perror("RTC_UIE_ON ioctl");
 261                exit(errno);
 262        }
 263
 264        fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
 265                        rtc);
 266        fflush(stderr);
 267        for (i=1; i<6; i++) {
 268                /* This read will block */
 269                retval = read(fd, &data, sizeof(unsigned long));
 270                if (retval == -1) {
 271                        perror("read");
 272                        exit(errno);
 273                }
 274                fprintf(stderr, " %d",i);
 275                fflush(stderr);
 276                irqcount++;
 277        }
 278
 279        fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
 280        fflush(stderr);
 281        for (i=1; i<6; i++) {
 282                struct timeval tv = {5, 0};     /* 5 second timeout on select */
 283                fd_set readfds;
 284
 285                FD_ZERO(&readfds);
 286                FD_SET(fd, &readfds);
 287                /* The select will wait until an RTC interrupt happens. */
 288                retval = select(fd+1, &readfds, NULL, NULL, &tv);
 289                if (retval == -1) {
 290                        perror("select");
 291                        exit(errno);
 292                }
 293                /* This read won't block unlike the select-less case above. */
 294                retval = read(fd, &data, sizeof(unsigned long));
 295                if (retval == -1) {
 296                        perror("read");
 297                        exit(errno);
 298                }
 299                fprintf(stderr, " %d",i);
 300                fflush(stderr);
 301                irqcount++;
 302        }
 303
 304        /* Turn off update interrupts */
 305        retval = ioctl(fd, RTC_UIE_OFF, 0);
 306        if (retval == -1) {
 307                perror("RTC_UIE_OFF ioctl");
 308                exit(errno);
 309        }
 310
 311test_READ:
 312        /* Read the RTC time/date */
 313        retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
 314        if (retval == -1) {
 315                perror("RTC_RD_TIME ioctl");
 316                exit(errno);
 317        }
 318
 319        fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
 320                rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
 321                rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 322
 323        /* Set the alarm to 5 sec in the future, and check for rollover */
 324        rtc_tm.tm_sec += 5;
 325        if (rtc_tm.tm_sec >= 60) {
 326                rtc_tm.tm_sec %= 60;
 327                rtc_tm.tm_min++;
 328        }
 329        if (rtc_tm.tm_min == 60) {
 330                rtc_tm.tm_min = 0;
 331                rtc_tm.tm_hour++;
 332        }
 333        if (rtc_tm.tm_hour == 24)
 334                rtc_tm.tm_hour = 0;
 335
 336        retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
 337        if (retval == -1) {
 338                if (errno == ENOTTY) {
 339                        fprintf(stderr,
 340                                "\n...Alarm IRQs not supported.\n");
 341                        goto test_PIE;
 342                }
 343                perror("RTC_ALM_SET ioctl");
 344                exit(errno);
 345        }
 346
 347        /* Read the current alarm settings */
 348        retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
 349        if (retval == -1) {
 350                perror("RTC_ALM_READ ioctl");
 351                exit(errno);
 352        }
 353
 354        fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
 355                rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 356
 357        /* Enable alarm interrupts */
 358        retval = ioctl(fd, RTC_AIE_ON, 0);
 359        if (retval == -1) {
 360                perror("RTC_AIE_ON ioctl");
 361                exit(errno);
 362        }
 363
 364        fprintf(stderr, "Waiting 5 seconds for alarm...");
 365        fflush(stderr);
 366        /* This blocks until the alarm ring causes an interrupt */
 367        retval = read(fd, &data, sizeof(unsigned long));
 368        if (retval == -1) {
 369                perror("read");
 370                exit(errno);
 371        }
 372        irqcount++;
 373        fprintf(stderr, " okay. Alarm rang.\n");
 374
 375        /* Disable alarm interrupts */
 376        retval = ioctl(fd, RTC_AIE_OFF, 0);
 377        if (retval == -1) {
 378                perror("RTC_AIE_OFF ioctl");
 379                exit(errno);
 380        }
 381
 382test_PIE:
 383        /* Read periodic IRQ rate */
 384        retval = ioctl(fd, RTC_IRQP_READ, &tmp);
 385        if (retval == -1) {
 386                /* not all RTCs support periodic IRQs */
 387                if (errno == ENOTTY) {
 388                        fprintf(stderr, "\nNo periodic IRQ support\n");
 389                        goto done;
 390                }
 391                perror("RTC_IRQP_READ ioctl");
 392                exit(errno);
 393        }
 394        fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
 395
 396        fprintf(stderr, "Counting 20 interrupts at:");
 397        fflush(stderr);
 398
 399        /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
 400        for (tmp=2; tmp<=64; tmp*=2) {
 401
 402                retval = ioctl(fd, RTC_IRQP_SET, tmp);
 403                if (retval == -1) {
 404                        /* not all RTCs can change their periodic IRQ rate */
 405                        if (errno == ENOTTY) {
 406                                fprintf(stderr,
 407                                        "\n...Periodic IRQ rate is fixed\n");
 408                                goto done;
 409                        }
 410                        perror("RTC_IRQP_SET ioctl");
 411                        exit(errno);
 412                }
 413
 414                fprintf(stderr, "\n%ldHz:\t", tmp);
 415                fflush(stderr);
 416
 417                /* Enable periodic interrupts */
 418                retval = ioctl(fd, RTC_PIE_ON, 0);
 419                if (retval == -1) {
 420                        perror("RTC_PIE_ON ioctl");
 421                        exit(errno);
 422                }
 423
 424                for (i=1; i<21; i++) {
 425                        /* This blocks */
 426                        retval = read(fd, &data, sizeof(unsigned long));
 427                        if (retval == -1) {
 428                                perror("read");
 429                                exit(errno);
 430                        }
 431                        fprintf(stderr, " %d",i);
 432                        fflush(stderr);
 433                        irqcount++;
 434                }
 435
 436                /* Disable periodic interrupts */
 437                retval = ioctl(fd, RTC_PIE_OFF, 0);
 438                if (retval == -1) {
 439                        perror("RTC_PIE_OFF ioctl");
 440                        exit(errno);
 441                }
 442        }
 443
 444done:
 445        fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
 446
 447        close(fd);
 448
 449        return 0;
 450}
 451
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