Documentation/rtc.txt

   1
   2         Real Time Clock (RTC) Drivers for Linux
   3         =======================================
   4
   5 When Linux developers talk about a "Real Time Clock", they usually mean
   6 something that tracks wall clock time and is battery backed so that it
   7 works even with system power off.  Such clocks will normally not track
   8 the local time zone or daylight savings time -- unless they dual boot
   9 with MS-Windows -- but will instead be set to Coordinated Universal Time
  10 (UTC, formerly "Greenwich Mean Time").
  11
  12 The newest non-PC hardware tends to just count seconds, like the time(2)
  13 system call reports, but RTCs also very commonly represent time using
  14 the Gregorian calendar and 24 hour time, as reported by gmtime(3).
  15
  16 Linux has two largely-compatible userspace RTC API families you may
  17 need 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
  25 Programmers need to understand that the PC/AT functionality is not
  26 always available, and some systems can do much more.  That is, the
  27 RTCs use the same API to make requests in both RTC frameworks (using
  28 different filenames of course), but the hardware may not offer the
  29 same functionality.  For example, not every RTC is hooked up to an
  30 IRQ, so they can't all issue alarms; and where standard PC RTCs can
  31 only issue an alarm up to 24 hours in the future, other hardware may
  32 be able to schedule one any time in the upcoming century.
  33
  34
  35         Old PC/AT-Compatible driver:  /dev/rtc
  36         --------------------------------------
  37
  38 All PCs (even Alpha machines) have a Real Time Clock built into them.
  39 Usually they are built into the chipset of the computer, but some may
  40 actually have a Motorola MC146818 (or clone) on the board. This is the
  41 clock that keeps the date and time while your computer is turned off.
  42
  43 ACPI has standardized that MC146818 functionality, and extended it in
  44 a few ways (enabling longer alarm periods, and wake-from-hibernate).
  45 That functionality is NOT exposed in the old driver.
  46
  47 However it can also be used to generate signals from a slow 2Hz to a
  48 relatively fast 8192Hz, in increments of powers of two. These signals
  49 are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
  50 for...) It can also function as a 24hr alarm, raising IRQ 8 when the
  51 alarm goes off. The alarm can also be programmed to only check any
  52 subset of the three programmable values, meaning that it could be set to
  53 ring on the 30th second of the 30th minute of every hour, for example.
  54 The clock can also be set to generate an interrupt upon every clock
  55 update, thus generating a 1Hz signal.
  56
  57 The interrupts are reported via /dev/rtc (major 10, minor 135, read only
  58 character device) in the form of an unsigned long. The low byte contains
  59 the type of interrupt (update-done, alarm-rang, or periodic) that was
  60 raised, and the remaining bytes contain the number of interrupts since
  61 the last read.  Status information is reported through the pseudo-file
  62 /proc/driver/rtc if the /proc filesystem was enabled.  The driver has
  63 built in locking so that only one process is allowed to have the /dev/rtc
  64 interface open at a time.
  65
  66 A user process can monitor these interrupts by doing a read(2) or a
  67 select(2) on /dev/rtc -- either will block/stop the user process until
  68 the next interrupt is received. This is useful for things like
  69 reasonably high frequency data acquisition where one doesn't want to
  70 burn up 100% CPU by polling gettimeofday etc. etc.
  71
  72 At high frequencies, or under high loads, the user process should check
  73 the number of interrupts received since the last read to determine if
  74 there has been any interrupt "pileup" so to speak. Just for reference, a
  75 typical 486-33 running a tight read loop on /dev/rtc will start to suffer
  76 occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
  77 frequencies above 1024Hz. So you really should check the high bytes
  78 of the value you read, especially at frequencies above that of the
  79 normal timer interrupt, which is 100Hz.
  80
  81 Programming and/or enabling interrupt frequencies greater than 64Hz is
  82 only allowed by root. This is perhaps a bit conservative, but we don't want
  83 an evil user generating lots of IRQs on a slow 386sx-16, where it might have
  84 a negative impact on performance. This 64Hz limit can be changed by writing
  85 a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
  86 interrupt handler is only a few lines of code to minimize any possibility
  87 of this effect.
  88
  89 Also, if the kernel time is synchronized with an external source, the
  90 kernel will write the time back to the CMOS clock every 11 minutes. In
  91 the process of doing this, the kernel briefly turns off RTC periodic
  92 interrupts, so be aware of this if you are doing serious work. If you
  93 don't synchronize the kernel time with an external source (via ntp or
  94 whatever) then the kernel will keep its hands off the RTC, allowing you
  95 exclusive access to the device for your applications.
  96
  97 The alarm and/or interrupt frequency are programmed into the RTC via
  98 various ioctl(2) calls as listed in ./include/linux/rtc.h
  99 Rather than write 50 pages describing the ioctl() and so on, it is
 100 perhaps more useful to include a small test program that demonstrates
 101 how to use them, and demonstrates the features of the driver. This is
 102 probably a lot more useful to people interested in writing applications
 103 that 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
 111 Because Linux supports many non-ACPI and non-PC platforms, some of which
 112 have more than one RTC style clock, it needed a more portable solution
 113 than expecting a single battery-backed MC146818 clone on every system.
 114 Accordingly, a new "RTC Class" framework has been defined.  It offers
 115 three 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
 126 The RTC Class framework supports a wide variety of RTCs, ranging from those
 127 integrated into embeddable system-on-chip (SOC) processors to discrete chips
 128 using I2C, SPI, or some other bus to communicate with the host CPU.  There's
 129 even support for PC-style RTCs ... including the features exposed on newer PCs
 130 through ACPI.
 131
 132 The new framework also removes the "one RTC per system" restriction.  For
 133 example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
 134 a high functionality RTC is integrated into the SOC.  That system might read
 135 the system clock from the discrete RTC, but use the integrated one for all
 136 other tasks, because of its greater functionality.
 137
 138 The ioctl() calls supported by /dev/rtc are also supported by the RTC class
 139 framework.  However, because the chips and systems are not standardized,
 140 some PC/AT functionality might not be provided.  And in the same way, some
 141 newer features -- including those enabled by ACPI -- are exposed by the
 142 RTC 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
 165 In many cases, the RTC alarm can be a system wake event, used to force
 166 Linux out of a low power sleep state (or hibernation) back to a fully
 167 operational state.  For example, a system could enter a deep power saving
 168 state until it's time to execute some scheduled tasks.
 169
 170 Note that many of these ioctls need not actually be implemented by your
 171 driver.  The common rtc-dev interface handles many of these nicely if your
 172 driver 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
 188 If 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  */
 222 static const char default_rtc[] = "/dev/rtc0";
 223
 224
 225 int 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
 311 test_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
 382 test_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
 444 done:
 445         fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
 446
 447         close(fd);
 448
 449         return 0;
 450 }