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1
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, the RTC framework
182 will emulate this mechanism.
183
184 * RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
185 are emulated via a kernel hrtimer.
186
187In many cases, the RTC alarm can be a system wake event, used to force
188Linux out of a low power sleep state (or hibernation) back to a fully
189operational state. For example, a system could enter a deep power saving
190state until it's time to execute some scheduled tasks.
191
192Note that many of these ioctls are handled by the common rtc-dev interface.
193Some common examples:
194
195 * RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
196 called with appropriate values.
197
198 * RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets
199 the alarm rtc_timer. May call the set_alarm driver function.
200
201 * RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
202
203 * RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
204
205If all else fails, check out the rtc-test.c driver!
206
207
208-------------------- 8< ---------------- 8< -----------------------------
209
210/*
211 * Real Time Clock Driver Test/Example Program
212 *
213 * Compile with:
214 * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
215 *
216 * Copyright (C) 1996, Paul Gortmaker.
217 *
218 * Released under the GNU General Public License, version 2,
219 * included herein by reference.
220 *
221 */
222
223#include <stdio.h>
224#include <linux/rtc.h>
225#include <sys/ioctl.h>
226#include <sys/time.h>
227#include <sys/types.h>
228#include <fcntl.h>
229#include <unistd.h>
230#include <stdlib.h>
231#include <errno.h>
232
233
234/*
235 * This expects the new RTC class driver framework, working with
236 * clocks that will often not be clones of what the PC-AT had.
237 * Use the command line to specify another RTC if you need one.
238 */
239static const char default_rtc[] = "/dev/rtc0";
240
241
242int main(int argc, char **argv)
243{
244 int i, fd, retval, irqcount = 0;
245 unsigned long tmp, data;
246 struct rtc_time rtc_tm;
247 const char *rtc = default_rtc;
248
249 switch (argc) {
250 case 2:
251 rtc = argv[1];
252 /* FALLTHROUGH */
253 case 1:
254 break;
255 default:
256 fprintf(stderr, "usage: rtctest [rtcdev]\n");
257 return 1;
258 }
259
260 fd = open(rtc, O_RDONLY);
261
262 if (fd == -1) {
263 perror(rtc);
264 exit(errno);
265 }
266
267 fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
268
269 /* Turn on update interrupts (one per second) */
270 retval = ioctl(fd, RTC_UIE_ON, 0);
271 if (retval == -1) {
272 if (errno == ENOTTY) {
273 fprintf(stderr,
274 "\n...Update IRQs not supported.\n");
275 goto test_READ;
276 }
277 perror("RTC_UIE_ON ioctl");
278 exit(errno);
279 }
280
281 fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
282 rtc);
283 fflush(stderr);
284 for (i=1; i<6; i++) {
285 /* This read will block */
286 retval = read(fd, &data, sizeof(unsigned long));
287 if (retval == -1) {
288 perror("read");
289 exit(errno);
290 }
291 fprintf(stderr, " %d",i);
292 fflush(stderr);
293 irqcount++;
294 }
295
296 fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
297 fflush(stderr);
298 for (i=1; i<6; i++) {
299 struct timeval tv = {5, 0}; /* 5 second timeout on select */
300 fd_set readfds;
301
302 FD_ZERO(&readfds);
303 FD_SET(fd, &readfds);
304 /* The select will wait until an RTC interrupt happens. */
305 retval = select(fd+1, &readfds, NULL, NULL, &tv);
306 if (retval == -1) {
307 perror("select");
308 exit(errno);
309 }
310 /* This read won't block unlike the select-less case above. */
311 retval = read(fd, &data, sizeof(unsigned long));
312 if (retval == -1) {
313 perror("read");
314 exit(errno);
315 }
316 fprintf(stderr, " %d",i);
317 fflush(stderr);
318 irqcount++;
319 }
320
321 /* Turn off update interrupts */
322 retval = ioctl(fd, RTC_UIE_OFF, 0);
323 if (retval == -1) {
324 perror("RTC_UIE_OFF ioctl");
325 exit(errno);
326 }
327
328test_READ:
329 /* Read the RTC time/date */
330 retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
331 if (retval == -1) {
332 perror("RTC_RD_TIME ioctl");
333 exit(errno);
334 }
335
336 fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
337 rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
338 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
339
340 /* Set the alarm to 5 sec in the future, and check for rollover */
341 rtc_tm.tm_sec += 5;
342 if (rtc_tm.tm_sec >= 60) {
343 rtc_tm.tm_sec %= 60;
344 rtc_tm.tm_min++;
345 }
346 if (rtc_tm.tm_min == 60) {
347 rtc_tm.tm_min = 0;
348 rtc_tm.tm_hour++;
349 }
350 if (rtc_tm.tm_hour == 24)
351 rtc_tm.tm_hour = 0;
352
353 retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
354 if (retval == -1) {
355 if (errno == ENOTTY) {
356 fprintf(stderr,
357 "\n...Alarm IRQs not supported.\n");
358 goto test_PIE;
359 }
360 perror("RTC_ALM_SET ioctl");
361 exit(errno);
362 }
363
364 /* Read the current alarm settings */
365 retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
366 if (retval == -1) {
367 perror("RTC_ALM_READ ioctl");
368 exit(errno);
369 }
370
371 fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
372 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
373
374 /* Enable alarm interrupts */
375 retval = ioctl(fd, RTC_AIE_ON, 0);
376 if (retval == -1) {
377 perror("RTC_AIE_ON ioctl");
378 exit(errno);
379 }
380
381 fprintf(stderr, "Waiting 5 seconds for alarm...");
382 fflush(stderr);
383 /* This blocks until the alarm ring causes an interrupt */
384 retval = read(fd, &data, sizeof(unsigned long));
385 if (retval == -1) {
386 perror("read");
387 exit(errno);
388 }
389 irqcount++;
390 fprintf(stderr, " okay. Alarm rang.\n");
391
392 /* Disable alarm interrupts */
393 retval = ioctl(fd, RTC_AIE_OFF, 0);
394 if (retval == -1) {
395 perror("RTC_AIE_OFF ioctl");
396 exit(errno);
397 }
398
399test_PIE:
400 /* Read periodic IRQ rate */
401 retval = ioctl(fd, RTC_IRQP_READ, &tmp);
402 if (retval == -1) {
403 /* not all RTCs support periodic IRQs */
404 if (errno == ENOTTY) {
405 fprintf(stderr, "\nNo periodic IRQ support\n");
406 goto done;
407 }
408 perror("RTC_IRQP_READ ioctl");
409 exit(errno);
410 }
411 fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
412
413 fprintf(stderr, "Counting 20 interrupts at:");
414 fflush(stderr);
415
416 /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
417 for (tmp=2; tmp<=64; tmp*=2) {
418
419 retval = ioctl(fd, RTC_IRQP_SET, tmp);
420 if (retval == -1) {
421 /* not all RTCs can change their periodic IRQ rate */
422 if (errno == ENOTTY) {
423 fprintf(stderr,
424 "\n...Periodic IRQ rate is fixed\n");
425 goto done;
426 }
427 perror("RTC_IRQP_SET ioctl");
428 exit(errno);
429 }
430
431 fprintf(stderr, "\n%ldHz:\t", tmp);
432 fflush(stderr);
433
434 /* Enable periodic interrupts */
435 retval = ioctl(fd, RTC_PIE_ON, 0);
436 if (retval == -1) {
437 perror("RTC_PIE_ON ioctl");
438 exit(errno);
439 }
440
441 for (i=1; i<21; i++) {
442 /* This blocks */
443 retval = read(fd, &data, sizeof(unsigned long));
444 if (retval == -1) {
445 perror("read");
446 exit(errno);
447 }
448 fprintf(stderr, " %d",i);
449 fflush(stderr);
450 irqcount++;
451 }
452
453 /* Disable periodic interrupts */
454 retval = ioctl(fd, RTC_PIE_OFF, 0);
455 if (retval == -1) {
456 perror("RTC_PIE_OFF ioctl");
457 exit(errno);
458 }
459 }
460
461done:
462 fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
463
464 close(fd);
465
466 return 0;
467}