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Linus Torvalds1da177e2005-04-16 15:20:36 -07001
David Brownell7531d8f2006-11-25 11:09:26 -08002 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 --------------------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -070037
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
David Brownell7531d8f2006-11-25 11:09:26 -080043ACPI 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
Linus Torvalds1da177e2005-04-16 15:20:36 -070047However 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
Jean Delvare9be05b52006-06-25 05:48:23 -070084a 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070088
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
David Brownell7531d8f2006-11-25 11:09:26 -0800103that will be using this driver. See the code at the end of this document.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700104
David Brownell7531d8f2006-11-25 11:09:26 -0800105(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
David Brownellf8245c22007-05-08 00:34:07 -0700150 24 hours in the future. (Use RTC_WKALM_* by preference.)
David Brownell7531d8f2006-11-25 11:09:26 -0800151
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800152 * RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
David Brownell7531d8f2006-11-25 11:09:26 -0800153 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
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800170Note 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
David Brownellf8245c22007-05-08 00:34:07 -0700178 set_alarm/read_alarm functions will be called.
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800179
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. Also make sure you set the max_user_freq member in your
184 initialization routines so the framework can sanity check the user
185 input for you.
186
187If all else fails, check out the rtc-test.c driver!
188
Linus Torvalds1da177e2005-04-16 15:20:36 -0700189
190-------------------- 8< ---------------- 8< -----------------------------
191
192/*
David Brownell7531d8f2006-11-25 11:09:26 -0800193 * Real Time Clock Driver Test/Example Program
Linus Torvalds1da177e2005-04-16 15:20:36 -0700194 *
David Brownell7531d8f2006-11-25 11:09:26 -0800195 * Compile with:
196 * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
Linus Torvalds1da177e2005-04-16 15:20:36 -0700197 *
David Brownell7531d8f2006-11-25 11:09:26 -0800198 * Copyright (C) 1996, Paul Gortmaker.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700199 *
David Brownell7531d8f2006-11-25 11:09:26 -0800200 * Released under the GNU General Public License, version 2,
201 * included herein by reference.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700202 *
203 */
204
205#include <stdio.h>
Linus Torvalds1da177e2005-04-16 15:20:36 -0700206#include <linux/rtc.h>
207#include <sys/ioctl.h>
208#include <sys/time.h>
209#include <sys/types.h>
210#include <fcntl.h>
211#include <unistd.h>
David Brownell7531d8f2006-11-25 11:09:26 -0800212#include <stdlib.h>
Linus Torvalds1da177e2005-04-16 15:20:36 -0700213#include <errno.h>
214
Linus Torvalds1da177e2005-04-16 15:20:36 -0700215
David Brownell7531d8f2006-11-25 11:09:26 -0800216/*
217 * This expects the new RTC class driver framework, working with
218 * clocks that will often not be clones of what the PC-AT had.
219 * Use the command line to specify another RTC if you need one.
220 */
221static const char default_rtc[] = "/dev/rtc0";
Linus Torvalds1da177e2005-04-16 15:20:36 -0700222
Linus Torvalds1da177e2005-04-16 15:20:36 -0700223
David Brownell7531d8f2006-11-25 11:09:26 -0800224int main(int argc, char **argv)
225{
226 int i, fd, retval, irqcount = 0;
227 unsigned long tmp, data;
228 struct rtc_time rtc_tm;
229 const char *rtc = default_rtc;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700230
David Brownell7531d8f2006-11-25 11:09:26 -0800231 switch (argc) {
232 case 2:
233 rtc = argv[1];
234 /* FALLTHROUGH */
235 case 1:
236 break;
237 default:
238 fprintf(stderr, "usage: rtctest [rtcdev]\n");
239 return 1;
240 }
Linus Torvalds1da177e2005-04-16 15:20:36 -0700241
David Brownell7531d8f2006-11-25 11:09:26 -0800242 fd = open(rtc, O_RDONLY);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700243
David Brownell7531d8f2006-11-25 11:09:26 -0800244 if (fd == -1) {
245 perror(rtc);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700246 exit(errno);
247 }
Linus Torvalds1da177e2005-04-16 15:20:36 -0700248
David Brownell7531d8f2006-11-25 11:09:26 -0800249 fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700250
David Brownell7531d8f2006-11-25 11:09:26 -0800251 /* Turn on update interrupts (one per second) */
252 retval = ioctl(fd, RTC_UIE_ON, 0);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700253 if (retval == -1) {
David Brownell7531d8f2006-11-25 11:09:26 -0800254 if (errno == ENOTTY) {
255 fprintf(stderr,
256 "\n...Update IRQs not supported.\n");
257 goto test_READ;
258 }
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800259 perror("RTC_UIE_ON ioctl");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700260 exit(errno);
261 }
262
David Brownell7531d8f2006-11-25 11:09:26 -0800263 fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
264 rtc);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700265 fflush(stderr);
David Brownell7531d8f2006-11-25 11:09:26 -0800266 for (i=1; i<6; i++) {
267 /* This read will block */
Linus Torvalds1da177e2005-04-16 15:20:36 -0700268 retval = read(fd, &data, sizeof(unsigned long));
269 if (retval == -1) {
David Brownell7531d8f2006-11-25 11:09:26 -0800270 perror("read");
271 exit(errno);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700272 }
273 fprintf(stderr, " %d",i);
274 fflush(stderr);
275 irqcount++;
276 }
277
David Brownell7531d8f2006-11-25 11:09:26 -0800278 fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
279 fflush(stderr);
280 for (i=1; i<6; i++) {
281 struct timeval tv = {5, 0}; /* 5 second timeout on select */
282 fd_set readfds;
283
284 FD_ZERO(&readfds);
285 FD_SET(fd, &readfds);
286 /* The select will wait until an RTC interrupt happens. */
287 retval = select(fd+1, &readfds, NULL, NULL, &tv);
288 if (retval == -1) {
289 perror("select");
290 exit(errno);
291 }
292 /* This read won't block unlike the select-less case above. */
293 retval = read(fd, &data, sizeof(unsigned long));
294 if (retval == -1) {
295 perror("read");
296 exit(errno);
297 }
298 fprintf(stderr, " %d",i);
299 fflush(stderr);
300 irqcount++;
301 }
302
303 /* Turn off update interrupts */
304 retval = ioctl(fd, RTC_UIE_OFF, 0);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700305 if (retval == -1) {
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800306 perror("RTC_UIE_OFF ioctl");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700307 exit(errno);
308 }
David Brownell7531d8f2006-11-25 11:09:26 -0800309
310test_READ:
311 /* Read the RTC time/date */
312 retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
313 if (retval == -1) {
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800314 perror("RTC_RD_TIME ioctl");
David Brownell7531d8f2006-11-25 11:09:26 -0800315 exit(errno);
316 }
317
318 fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
319 rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
320 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
321
322 /* Set the alarm to 5 sec in the future, and check for rollover */
323 rtc_tm.tm_sec += 5;
324 if (rtc_tm.tm_sec >= 60) {
325 rtc_tm.tm_sec %= 60;
326 rtc_tm.tm_min++;
327 }
328 if (rtc_tm.tm_min == 60) {
329 rtc_tm.tm_min = 0;
330 rtc_tm.tm_hour++;
331 }
332 if (rtc_tm.tm_hour == 24)
333 rtc_tm.tm_hour = 0;
334
335 retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
336 if (retval == -1) {
337 if (errno == ENOTTY) {
338 fprintf(stderr,
339 "\n...Alarm IRQs not supported.\n");
340 goto test_PIE;
341 }
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800342 perror("RTC_ALM_SET ioctl");
David Brownell7531d8f2006-11-25 11:09:26 -0800343 exit(errno);
344 }
345
346 /* Read the current alarm settings */
347 retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
348 if (retval == -1) {
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800349 perror("RTC_ALM_READ ioctl");
David Brownell7531d8f2006-11-25 11:09:26 -0800350 exit(errno);
351 }
352
353 fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
354 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
355
356 /* Enable alarm interrupts */
357 retval = ioctl(fd, RTC_AIE_ON, 0);
358 if (retval == -1) {
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800359 perror("RTC_AIE_ON ioctl");
David Brownell7531d8f2006-11-25 11:09:26 -0800360 exit(errno);
361 }
362
363 fprintf(stderr, "Waiting 5 seconds for alarm...");
364 fflush(stderr);
365 /* This blocks until the alarm ring causes an interrupt */
366 retval = read(fd, &data, sizeof(unsigned long));
367 if (retval == -1) {
368 perror("read");
369 exit(errno);
370 }
371 irqcount++;
372 fprintf(stderr, " okay. Alarm rang.\n");
373
374 /* Disable alarm interrupts */
375 retval = ioctl(fd, RTC_AIE_OFF, 0);
376 if (retval == -1) {
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800377 perror("RTC_AIE_OFF ioctl");
David Brownell7531d8f2006-11-25 11:09:26 -0800378 exit(errno);
379 }
380
381test_PIE:
382 /* Read periodic IRQ rate */
383 retval = ioctl(fd, RTC_IRQP_READ, &tmp);
384 if (retval == -1) {
385 /* not all RTCs support periodic IRQs */
386 if (errno == ENOTTY) {
387 fprintf(stderr, "\nNo periodic IRQ support\n");
Hans-Christian Egtvedt7a39a492007-07-17 04:04:59 -0700388 goto done;
David Brownell7531d8f2006-11-25 11:09:26 -0800389 }
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800390 perror("RTC_IRQP_READ ioctl");
David Brownell7531d8f2006-11-25 11:09:26 -0800391 exit(errno);
392 }
393 fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
394
395 fprintf(stderr, "Counting 20 interrupts at:");
396 fflush(stderr);
397
398 /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
399 for (tmp=2; tmp<=64; tmp*=2) {
400
401 retval = ioctl(fd, RTC_IRQP_SET, tmp);
402 if (retval == -1) {
403 /* not all RTCs can change their periodic IRQ rate */
404 if (errno == ENOTTY) {
405 fprintf(stderr,
406 "\n...Periodic IRQ rate is fixed\n");
407 goto done;
408 }
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800409 perror("RTC_IRQP_SET ioctl");
David Brownell7531d8f2006-11-25 11:09:26 -0800410 exit(errno);
411 }
412
413 fprintf(stderr, "\n%ldHz:\t", tmp);
414 fflush(stderr);
415
416 /* Enable periodic interrupts */
417 retval = ioctl(fd, RTC_PIE_ON, 0);
418 if (retval == -1) {
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800419 perror("RTC_PIE_ON ioctl");
David Brownell7531d8f2006-11-25 11:09:26 -0800420 exit(errno);
421 }
422
423 for (i=1; i<21; i++) {
424 /* This blocks */
425 retval = read(fd, &data, sizeof(unsigned long));
426 if (retval == -1) {
427 perror("read");
428 exit(errno);
429 }
430 fprintf(stderr, " %d",i);
431 fflush(stderr);
432 irqcount++;
433 }
434
435 /* Disable periodic interrupts */
436 retval = ioctl(fd, RTC_PIE_OFF, 0);
437 if (retval == -1) {
Mike Frysinger2b1cd4c2007-02-10 01:46:30 -0800438 perror("RTC_PIE_OFF ioctl");
David Brownell7531d8f2006-11-25 11:09:26 -0800439 exit(errno);
440 }
441 }
442
443done:
444 fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
445
446 close(fd);
447
448 return 0;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700449}