rtc.txt 15 KB

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  1. Real Time Clock (RTC) Drivers for Linux
  2. =======================================
  3. When Linux developers talk about a "Real Time Clock", they usually mean
  4. something that tracks wall clock time and is battery backed so that it
  5. works even with system power off. Such clocks will normally not track
  6. the local time zone or daylight savings time -- unless they dual boot
  7. with MS-Windows -- but will instead be set to Coordinated Universal Time
  8. (UTC, formerly "Greenwich Mean Time").
  9. The newest non-PC hardware tends to just count seconds, like the time(2)
  10. system call reports, but RTCs also very commonly represent time using
  11. the Gregorian calendar and 24 hour time, as reported by gmtime(3).
  12. Linux has two largely-compatible userspace RTC API families you may
  13. need to know about:
  14. * /dev/rtc ... is the RTC provided by PC compatible systems,
  15. so it's not very portable to non-x86 systems.
  16. * /dev/rtc0, /dev/rtc1 ... are part of a framework that's
  17. supported by a wide variety of RTC chips on all systems.
  18. Programmers need to understand that the PC/AT functionality is not
  19. always available, and some systems can do much more. That is, the
  20. RTCs use the same API to make requests in both RTC frameworks (using
  21. different filenames of course), but the hardware may not offer the
  22. same functionality. For example, not every RTC is hooked up to an
  23. IRQ, so they can't all issue alarms; and where standard PC RTCs can
  24. only issue an alarm up to 24 hours in the future, other hardware may
  25. be able to schedule one any time in the upcoming century.
  26. Old PC/AT-Compatible driver: /dev/rtc
  27. --------------------------------------
  28. All PCs (even Alpha machines) have a Real Time Clock built into them.
  29. Usually they are built into the chipset of the computer, but some may
  30. actually have a Motorola MC146818 (or clone) on the board. This is the
  31. clock that keeps the date and time while your computer is turned off.
  32. ACPI has standardized that MC146818 functionality, and extended it in
  33. a few ways (enabling longer alarm periods, and wake-from-hibernate).
  34. That functionality is NOT exposed in the old driver.
  35. However it can also be used to generate signals from a slow 2Hz to a
  36. relatively fast 8192Hz, in increments of powers of two. These signals
  37. are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
  38. for...) It can also function as a 24hr alarm, raising IRQ 8 when the
  39. alarm goes off. The alarm can also be programmed to only check any
  40. subset of the three programmable values, meaning that it could be set to
  41. ring on the 30th second of the 30th minute of every hour, for example.
  42. The clock can also be set to generate an interrupt upon every clock
  43. update, thus generating a 1Hz signal.
  44. The interrupts are reported via /dev/rtc (major 10, minor 135, read only
  45. character device) in the form of an unsigned long. The low byte contains
  46. the type of interrupt (update-done, alarm-rang, or periodic) that was
  47. raised, and the remaining bytes contain the number of interrupts since
  48. the last read. Status information is reported through the pseudo-file
  49. /proc/driver/rtc if the /proc filesystem was enabled. The driver has
  50. built in locking so that only one process is allowed to have the /dev/rtc
  51. interface open at a time.
  52. A user process can monitor these interrupts by doing a read(2) or a
  53. select(2) on /dev/rtc -- either will block/stop the user process until
  54. the next interrupt is received. This is useful for things like
  55. reasonably high frequency data acquisition where one doesn't want to
  56. burn up 100% CPU by polling gettimeofday etc. etc.
  57. At high frequencies, or under high loads, the user process should check
  58. the number of interrupts received since the last read to determine if
  59. there has been any interrupt "pileup" so to speak. Just for reference, a
  60. typical 486-33 running a tight read loop on /dev/rtc will start to suffer
  61. occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
  62. frequencies above 1024Hz. So you really should check the high bytes
  63. of the value you read, especially at frequencies above that of the
  64. normal timer interrupt, which is 100Hz.
  65. Programming and/or enabling interrupt frequencies greater than 64Hz is
  66. only allowed by root. This is perhaps a bit conservative, but we don't want
  67. an evil user generating lots of IRQs on a slow 386sx-16, where it might have
  68. a negative impact on performance. This 64Hz limit can be changed by writing
  69. a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
  70. interrupt handler is only a few lines of code to minimize any possibility
  71. of this effect.
  72. Also, if the kernel time is synchronized with an external source, the
  73. kernel will write the time back to the CMOS clock every 11 minutes. In
  74. the process of doing this, the kernel briefly turns off RTC periodic
  75. interrupts, so be aware of this if you are doing serious work. If you
  76. don't synchronize the kernel time with an external source (via ntp or
  77. whatever) then the kernel will keep its hands off the RTC, allowing you
  78. exclusive access to the device for your applications.
  79. The alarm and/or interrupt frequency are programmed into the RTC via
  80. various ioctl(2) calls as listed in ./include/linux/rtc.h
  81. Rather than write 50 pages describing the ioctl() and so on, it is
  82. perhaps more useful to include a small test program that demonstrates
  83. how to use them, and demonstrates the features of the driver. This is
  84. probably a lot more useful to people interested in writing applications
  85. that will be using this driver. See the code at the end of this document.
  86. (The original /dev/rtc driver was written by Paul Gortmaker.)
  87. New portable "RTC Class" drivers: /dev/rtcN
  88. --------------------------------------------
  89. Because Linux supports many non-ACPI and non-PC platforms, some of which
  90. have more than one RTC style clock, it needed a more portable solution
  91. than expecting a single battery-backed MC146818 clone on every system.
  92. Accordingly, a new "RTC Class" framework has been defined. It offers
  93. three different userspace interfaces:
  94. * /dev/rtcN ... much the same as the older /dev/rtc interface
  95. * /sys/class/rtc/rtcN ... sysfs attributes support readonly
  96. access to some RTC attributes.
  97. * /proc/driver/rtc ... the first RTC (rtc0) may expose itself
  98. using a procfs interface. More information is (currently) shown
  99. here than through sysfs.
  100. The RTC Class framework supports a wide variety of RTCs, ranging from those
  101. integrated into embeddable system-on-chip (SOC) processors to discrete chips
  102. using I2C, SPI, or some other bus to communicate with the host CPU. There's
  103. even support for PC-style RTCs ... including the features exposed on newer PCs
  104. through ACPI.
  105. The new framework also removes the "one RTC per system" restriction. For
  106. example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
  107. a high functionality RTC is integrated into the SOC. That system might read
  108. the system clock from the discrete RTC, but use the integrated one for all
  109. other tasks, because of its greater functionality.
  110. The ioctl() calls supported by /dev/rtc are also supported by the RTC class
  111. framework. However, because the chips and systems are not standardized,
  112. some PC/AT functionality might not be provided. And in the same way, some
  113. newer features -- including those enabled by ACPI -- are exposed by the
  114. RTC class framework, but can't be supported by the older driver.
  115. * RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading
  116. time, returning the result as a Gregorian calendar date and 24 hour
  117. wall clock time. To be most useful, this time may also be updated.
  118. * RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
  119. is connected to an IRQ line, it can often issue an alarm IRQ up to
  120. 24 hours in the future.
  121. * RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
  122. the next 24 hours use a slightly more powerful API, which supports
  123. setting the longer alarm time and enabling its IRQ using a single
  124. request (using the same model as EFI firmware).
  125. * RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, it probably
  126. also offers update IRQs whenever the "seconds" counter changes.
  127. If needed, the RTC framework can emulate this mechanism.
  128. * RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... another
  129. feature often accessible with an IRQ line is a periodic IRQ, issued
  130. at settable frequencies (usually 2^N Hz).
  131. In many cases, the RTC alarm can be a system wake event, used to force
  132. Linux out of a low power sleep state (or hibernation) back to a fully
  133. operational state. For example, a system could enter a deep power saving
  134. state until it's time to execute some scheduled tasks.
  135. Note that many of these ioctls need not actually be implemented by your
  136. driver. The common rtc-dev interface handles many of these nicely if your
  137. driver returns ENOIOCTLCMD. Some common examples:
  138. * RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
  139. called with appropriate values.
  140. * RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: the
  141. set_alarm/read_alarm functions will be called. To differentiate
  142. between the ALM and WKALM, check the larger fields of the rtc_wkalrm
  143. struct (like tm_year). These will be set to -1 when using ALM and
  144. will be set to proper values when using WKALM.
  145. * RTC_IRQP_SET, RTC_IRQP_READ: the irq_set_freq function will be called
  146. to set the frequency while the framework will handle the read for you
  147. since the frequency is stored in the irq_freq member of the rtc_device
  148. structure. Also make sure you set the max_user_freq member in your
  149. initialization routines so the framework can sanity check the user
  150. input for you.
  151. If all else fails, check out the rtc-test.c driver!
  152. -------------------- 8< ---------------- 8< -----------------------------
  153. /*
  154. * Real Time Clock Driver Test/Example Program
  155. *
  156. * Compile with:
  157. * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
  158. *
  159. * Copyright (C) 1996, Paul Gortmaker.
  160. *
  161. * Released under the GNU General Public License, version 2,
  162. * included herein by reference.
  163. *
  164. */
  165. #include <stdio.h>
  166. #include <linux/rtc.h>
  167. #include <sys/ioctl.h>
  168. #include <sys/time.h>
  169. #include <sys/types.h>
  170. #include <fcntl.h>
  171. #include <unistd.h>
  172. #include <stdlib.h>
  173. #include <errno.h>
  174. /*
  175. * This expects the new RTC class driver framework, working with
  176. * clocks that will often not be clones of what the PC-AT had.
  177. * Use the command line to specify another RTC if you need one.
  178. */
  179. static const char default_rtc[] = "/dev/rtc0";
  180. int main(int argc, char **argv)
  181. {
  182. int i, fd, retval, irqcount = 0;
  183. unsigned long tmp, data;
  184. struct rtc_time rtc_tm;
  185. const char *rtc = default_rtc;
  186. switch (argc) {
  187. case 2:
  188. rtc = argv[1];
  189. /* FALLTHROUGH */
  190. case 1:
  191. break;
  192. default:
  193. fprintf(stderr, "usage: rtctest [rtcdev]\n");
  194. return 1;
  195. }
  196. fd = open(rtc, O_RDONLY);
  197. if (fd == -1) {
  198. perror(rtc);
  199. exit(errno);
  200. }
  201. fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
  202. /* Turn on update interrupts (one per second) */
  203. retval = ioctl(fd, RTC_UIE_ON, 0);
  204. if (retval == -1) {
  205. if (errno == ENOTTY) {
  206. fprintf(stderr,
  207. "\n...Update IRQs not supported.\n");
  208. goto test_READ;
  209. }
  210. perror("RTC_UIE_ON ioctl");
  211. exit(errno);
  212. }
  213. fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
  214. rtc);
  215. fflush(stderr);
  216. for (i=1; i<6; i++) {
  217. /* This read will block */
  218. retval = read(fd, &data, sizeof(unsigned long));
  219. if (retval == -1) {
  220. perror("read");
  221. exit(errno);
  222. }
  223. fprintf(stderr, " %d",i);
  224. fflush(stderr);
  225. irqcount++;
  226. }
  227. fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
  228. fflush(stderr);
  229. for (i=1; i<6; i++) {
  230. struct timeval tv = {5, 0}; /* 5 second timeout on select */
  231. fd_set readfds;
  232. FD_ZERO(&readfds);
  233. FD_SET(fd, &readfds);
  234. /* The select will wait until an RTC interrupt happens. */
  235. retval = select(fd+1, &readfds, NULL, NULL, &tv);
  236. if (retval == -1) {
  237. perror("select");
  238. exit(errno);
  239. }
  240. /* This read won't block unlike the select-less case above. */
  241. retval = read(fd, &data, sizeof(unsigned long));
  242. if (retval == -1) {
  243. perror("read");
  244. exit(errno);
  245. }
  246. fprintf(stderr, " %d",i);
  247. fflush(stderr);
  248. irqcount++;
  249. }
  250. /* Turn off update interrupts */
  251. retval = ioctl(fd, RTC_UIE_OFF, 0);
  252. if (retval == -1) {
  253. perror("RTC_UIE_OFF ioctl");
  254. exit(errno);
  255. }
  256. test_READ:
  257. /* Read the RTC time/date */
  258. retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
  259. if (retval == -1) {
  260. perror("RTC_RD_TIME ioctl");
  261. exit(errno);
  262. }
  263. fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
  264. rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
  265. rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
  266. /* Set the alarm to 5 sec in the future, and check for rollover */
  267. rtc_tm.tm_sec += 5;
  268. if (rtc_tm.tm_sec >= 60) {
  269. rtc_tm.tm_sec %= 60;
  270. rtc_tm.tm_min++;
  271. }
  272. if (rtc_tm.tm_min == 60) {
  273. rtc_tm.tm_min = 0;
  274. rtc_tm.tm_hour++;
  275. }
  276. if (rtc_tm.tm_hour == 24)
  277. rtc_tm.tm_hour = 0;
  278. retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
  279. if (retval == -1) {
  280. if (errno == ENOTTY) {
  281. fprintf(stderr,
  282. "\n...Alarm IRQs not supported.\n");
  283. goto test_PIE;
  284. }
  285. perror("RTC_ALM_SET ioctl");
  286. exit(errno);
  287. }
  288. /* Read the current alarm settings */
  289. retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
  290. if (retval == -1) {
  291. perror("RTC_ALM_READ ioctl");
  292. exit(errno);
  293. }
  294. fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
  295. rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
  296. /* Enable alarm interrupts */
  297. retval = ioctl(fd, RTC_AIE_ON, 0);
  298. if (retval == -1) {
  299. perror("RTC_AIE_ON ioctl");
  300. exit(errno);
  301. }
  302. fprintf(stderr, "Waiting 5 seconds for alarm...");
  303. fflush(stderr);
  304. /* This blocks until the alarm ring causes an interrupt */
  305. retval = read(fd, &data, sizeof(unsigned long));
  306. if (retval == -1) {
  307. perror("read");
  308. exit(errno);
  309. }
  310. irqcount++;
  311. fprintf(stderr, " okay. Alarm rang.\n");
  312. /* Disable alarm interrupts */
  313. retval = ioctl(fd, RTC_AIE_OFF, 0);
  314. if (retval == -1) {
  315. perror("RTC_AIE_OFF ioctl");
  316. exit(errno);
  317. }
  318. test_PIE:
  319. /* Read periodic IRQ rate */
  320. retval = ioctl(fd, RTC_IRQP_READ, &tmp);
  321. if (retval == -1) {
  322. /* not all RTCs support periodic IRQs */
  323. if (errno == ENOTTY) {
  324. fprintf(stderr, "\nNo periodic IRQ support\n");
  325. return 0;
  326. }
  327. perror("RTC_IRQP_READ ioctl");
  328. exit(errno);
  329. }
  330. fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
  331. fprintf(stderr, "Counting 20 interrupts at:");
  332. fflush(stderr);
  333. /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
  334. for (tmp=2; tmp<=64; tmp*=2) {
  335. retval = ioctl(fd, RTC_IRQP_SET, tmp);
  336. if (retval == -1) {
  337. /* not all RTCs can change their periodic IRQ rate */
  338. if (errno == ENOTTY) {
  339. fprintf(stderr,
  340. "\n...Periodic IRQ rate is fixed\n");
  341. goto done;
  342. }
  343. perror("RTC_IRQP_SET ioctl");
  344. exit(errno);
  345. }
  346. fprintf(stderr, "\n%ldHz:\t", tmp);
  347. fflush(stderr);
  348. /* Enable periodic interrupts */
  349. retval = ioctl(fd, RTC_PIE_ON, 0);
  350. if (retval == -1) {
  351. perror("RTC_PIE_ON ioctl");
  352. exit(errno);
  353. }
  354. for (i=1; i<21; i++) {
  355. /* This blocks */
  356. retval = read(fd, &data, sizeof(unsigned long));
  357. if (retval == -1) {
  358. perror("read");
  359. exit(errno);
  360. }
  361. fprintf(stderr, " %d",i);
  362. fflush(stderr);
  363. irqcount++;
  364. }
  365. /* Disable periodic interrupts */
  366. retval = ioctl(fd, RTC_PIE_OFF, 0);
  367. if (retval == -1) {
  368. perror("RTC_PIE_OFF ioctl");
  369. exit(errno);
  370. }
  371. }
  372. done:
  373. fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
  374. close(fd);
  375. return 0;
  376. }