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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 --------------------------------------
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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
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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
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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
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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.
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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
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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
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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
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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
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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
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147hctosys: 1 if the RTC provided the system time at boot via the
148 CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
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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
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156 after wake if a daily wakeup is required. Format is seconds since
157 the epoch by default, or if there's a leading +, seconds in the
158 future, or if there is a leading +=, seconds ahead of the current
159 alarm.
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160
161IOCTL INTERFACE
162---------------
163
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164The ioctl() calls supported by /dev/rtc are also supported by the RTC class
165framework. However, because the chips and systems are not standardized,
166some PC/AT functionality might not be provided. And in the same way, some
167newer features -- including those enabled by ACPI -- are exposed by the
168RTC class framework, but can't be supported by the older driver.
169
170 * RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading
171 time, returning the result as a Gregorian calendar date and 24 hour
172 wall clock time. To be most useful, this time may also be updated.
173
174 * RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
175 is connected to an IRQ line, it can often issue an alarm IRQ up to
f8245c26 176 24 hours in the future. (Use RTC_WKALM_* by preference.)
7531d8fa 177
2b1cd4c4 178 * RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
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179 the next 24 hours use a slightly more powerful API, which supports
180 setting the longer alarm time and enabling its IRQ using a single
181 request (using the same model as EFI firmware).
182
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183 * RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework
184 will emulate this mechanism.
7531d8fa 185
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186 * RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
187 are emulated via a kernel hrtimer.
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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.
1da177e4 193
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194Note that many of these ioctls are handled by the common rtc-dev interface.
195Some common examples:
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196
197 * RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
198 called with appropriate values.
199
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200 * RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets
201 the alarm rtc_timer. May call the set_alarm driver function.
2b1cd4c4 202
ea04683f 203 * RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
2b1cd4c4 204
ea04683f 205 * RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
ad91fd85 206
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207If all else fails, check out the rtc-test.c driver!
208
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209
210-------------------- 8< ---------------- 8< -----------------------------
211
212/*
7531d8fa 213 * Real Time Clock Driver Test/Example Program
1da177e4 214 *
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215 * Compile with:
216 * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
1da177e4 217 *
7531d8fa 218 * Copyright (C) 1996, Paul Gortmaker.
1da177e4 219 *
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220 * Released under the GNU General Public License, version 2,
221 * included herein by reference.
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222 *
223 */
224
225#include <stdio.h>
226#include <linux/rtc.h>
227#include <sys/ioctl.h>
228#include <sys/time.h>
229#include <sys/types.h>
230#include <fcntl.h>
231#include <unistd.h>
7531d8fa 232#include <stdlib.h>
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233#include <errno.h>
234
1da177e4 235
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236/*
237 * This expects the new RTC class driver framework, working with
238 * clocks that will often not be clones of what the PC-AT had.
239 * Use the command line to specify another RTC if you need one.
240 */
241static const char default_rtc[] = "/dev/rtc0";
242
243
244int main(int argc, char **argv)
245{
246 int i, fd, retval, irqcount = 0;
247 unsigned long tmp, data;
248 struct rtc_time rtc_tm;
249 const char *rtc = default_rtc;
250
251 switch (argc) {
252 case 2:
253 rtc = argv[1];
254 /* FALLTHROUGH */
255 case 1:
256 break;
257 default:
258 fprintf(stderr, "usage: rtctest [rtcdev]\n");
259 return 1;
260 }
1da177e4 261
7531d8fa 262 fd = open(rtc, O_RDONLY);
1da177e4 263
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264 if (fd == -1) {
265 perror(rtc);
266 exit(errno);
267 }
1da177e4 268
7531d8fa 269 fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
1da177e4 270
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271 /* Turn on update interrupts (one per second) */
272 retval = ioctl(fd, RTC_UIE_ON, 0);
1da177e4 273 if (retval == -1) {
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274 if (errno == ENOTTY) {
275 fprintf(stderr,
276 "\n...Update IRQs not supported.\n");
277 goto test_READ;
278 }
2b1cd4c4 279 perror("RTC_UIE_ON ioctl");
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280 exit(errno);
281 }
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282
283 fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
284 rtc);
1da177e4 285 fflush(stderr);
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286 for (i=1; i<6; i++) {
287 /* This read will block */
288 retval = read(fd, &data, sizeof(unsigned long));
289 if (retval == -1) {
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290 perror("read");
291 exit(errno);
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292 }
293 fprintf(stderr, " %d",i);
294 fflush(stderr);
295 irqcount++;
296 }
1da177e4 297
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298 fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
299 fflush(stderr);
300 for (i=1; i<6; i++) {
301 struct timeval tv = {5, 0}; /* 5 second timeout on select */
302 fd_set readfds;
303
304 FD_ZERO(&readfds);
305 FD_SET(fd, &readfds);
306 /* The select will wait until an RTC interrupt happens. */
307 retval = select(fd+1, &readfds, NULL, NULL, &tv);
308 if (retval == -1) {
309 perror("select");
310 exit(errno);
311 }
312 /* This read won't block unlike the select-less case above. */
313 retval = read(fd, &data, sizeof(unsigned long));
314 if (retval == -1) {
315 perror("read");
316 exit(errno);
317 }
318 fprintf(stderr, " %d",i);
319 fflush(stderr);
320 irqcount++;
321 }
1da177e4 322
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323 /* Turn off update interrupts */
324 retval = ioctl(fd, RTC_UIE_OFF, 0);
1da177e4 325 if (retval == -1) {
2b1cd4c4 326 perror("RTC_UIE_OFF ioctl");
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327 exit(errno);
328 }
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329
330test_READ:
331 /* Read the RTC time/date */
332 retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
1da177e4 333 if (retval == -1) {
2b1cd4c4 334 perror("RTC_RD_TIME ioctl");
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335 exit(errno);
336 }
1da177e4 337
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338 fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
339 rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
340 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
1da177e4 341
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342 /* Set the alarm to 5 sec in the future, and check for rollover */
343 rtc_tm.tm_sec += 5;
344 if (rtc_tm.tm_sec >= 60) {
345 rtc_tm.tm_sec %= 60;
346 rtc_tm.tm_min++;
347 }
8696e702 348 if (rtc_tm.tm_min == 60) {
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349 rtc_tm.tm_min = 0;
350 rtc_tm.tm_hour++;
351 }
8696e702 352 if (rtc_tm.tm_hour == 24)
7531d8fa 353 rtc_tm.tm_hour = 0;
1da177e4 354
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355 retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
356 if (retval == -1) {
357 if (errno == ENOTTY) {
358 fprintf(stderr,
359 "\n...Alarm IRQs not supported.\n");
360 goto test_PIE;
361 }
2b1cd4c4 362 perror("RTC_ALM_SET ioctl");
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363 exit(errno);
364 }
1da177e4 365
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366 /* Read the current alarm settings */
367 retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
368 if (retval == -1) {
2b1cd4c4 369 perror("RTC_ALM_READ ioctl");
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370 exit(errno);
371 }
1da177e4 372
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373 fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
374 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
1da177e4 375
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376 /* Enable alarm interrupts */
377 retval = ioctl(fd, RTC_AIE_ON, 0);
1da177e4 378 if (retval == -1) {
2b1cd4c4 379 perror("RTC_AIE_ON ioctl");
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380 exit(errno);
381 }
382
7531d8fa 383 fprintf(stderr, "Waiting 5 seconds for alarm...");
1da177e4 384 fflush(stderr);
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385 /* This blocks until the alarm ring causes an interrupt */
386 retval = read(fd, &data, sizeof(unsigned long));
387 if (retval == -1) {
388 perror("read");
389 exit(errno);
390 }
391 irqcount++;
392 fprintf(stderr, " okay. Alarm rang.\n");
1da177e4 393
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394 /* Disable alarm interrupts */
395 retval = ioctl(fd, RTC_AIE_OFF, 0);
1da177e4 396 if (retval == -1) {
2b1cd4c4 397 perror("RTC_AIE_OFF ioctl");
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398 exit(errno);
399 }
400
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401test_PIE:
402 /* Read periodic IRQ rate */
403 retval = ioctl(fd, RTC_IRQP_READ, &tmp);
404 if (retval == -1) {
405 /* not all RTCs support periodic IRQs */
406 if (errno == ENOTTY) {
407 fprintf(stderr, "\nNo periodic IRQ support\n");
7a39a49c 408 goto done;
7531d8fa 409 }
2b1cd4c4 410 perror("RTC_IRQP_READ ioctl");
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411 exit(errno);
412 }
413 fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
414
415 fprintf(stderr, "Counting 20 interrupts at:");
416 fflush(stderr);
417
418 /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
419 for (tmp=2; tmp<=64; tmp*=2) {
420
421 retval = ioctl(fd, RTC_IRQP_SET, tmp);
1da177e4 422 if (retval == -1) {
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423 /* not all RTCs can change their periodic IRQ rate */
424 if (errno == ENOTTY) {
425 fprintf(stderr,
426 "\n...Periodic IRQ rate is fixed\n");
427 goto done;
428 }
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MF
429 perror("RTC_IRQP_SET ioctl");
430 exit(errno);
1da177e4 431 }
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432
433 fprintf(stderr, "\n%ldHz:\t", tmp);
1da177e4 434 fflush(stderr);
1da177e4 435
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436 /* Enable periodic interrupts */
437 retval = ioctl(fd, RTC_PIE_ON, 0);
438 if (retval == -1) {
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439 perror("RTC_PIE_ON ioctl");
440 exit(errno);
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441 }
442
443 for (i=1; i<21; i++) {
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MF
444 /* This blocks */
445 retval = read(fd, &data, sizeof(unsigned long));
446 if (retval == -1) {
447 perror("read");
448 exit(errno);
449 }
450 fprintf(stderr, " %d",i);
451 fflush(stderr);
452 irqcount++;
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453 }
454
455 /* Disable periodic interrupts */
456 retval = ioctl(fd, RTC_PIE_OFF, 0);
457 if (retval == -1) {
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MF
458 perror("RTC_PIE_OFF ioctl");
459 exit(errno);
7531d8fa 460 }
1da177e4 461 }
1da177e4 462
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463done:
464 fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
1da177e4 465
7531d8fa 466 close(fd);
1da177e4 467
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468 return 0;
469}