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Linus Torvalds1da177e2005-04-16 15:20:36 -07001
David Brownell7531d8fa2006-11-25 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 Brownell7531d8fa2006-11-25 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 Brownell7531d8fa2006-11-25 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 Brownell7531d8fa2006-11-25 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
150 24 hours in the future.
151
152 * RTC_WKALM_SET, RTC_WKALM_READ ... 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
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
Linus Torvalds1da177e2005-04-16 15:20:36 -0700170
171-------------------- 8< ---------------- 8< -----------------------------
172
173/*
David Brownell7531d8fa2006-11-25 11:09:26 -0800174 * Real Time Clock Driver Test/Example Program
Linus Torvalds1da177e2005-04-16 15:20:36 -0700175 *
David Brownell7531d8fa2006-11-25 11:09:26 -0800176 * Compile with:
177 * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
Linus Torvalds1da177e2005-04-16 15:20:36 -0700178 *
David Brownell7531d8fa2006-11-25 11:09:26 -0800179 * Copyright (C) 1996, Paul Gortmaker.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700180 *
David Brownell7531d8fa2006-11-25 11:09:26 -0800181 * Released under the GNU General Public License, version 2,
182 * included herein by reference.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700183 *
184 */
185
186#include <stdio.h>
Linus Torvalds1da177e2005-04-16 15:20:36 -0700187#include <linux/rtc.h>
188#include <sys/ioctl.h>
189#include <sys/time.h>
190#include <sys/types.h>
191#include <fcntl.h>
192#include <unistd.h>
David Brownell7531d8fa2006-11-25 11:09:26 -0800193#include <stdlib.h>
Linus Torvalds1da177e2005-04-16 15:20:36 -0700194#include <errno.h>
195
Linus Torvalds1da177e2005-04-16 15:20:36 -0700196
David Brownell7531d8fa2006-11-25 11:09:26 -0800197/*
198 * This expects the new RTC class driver framework, working with
199 * clocks that will often not be clones of what the PC-AT had.
200 * Use the command line to specify another RTC if you need one.
201 */
202static const char default_rtc[] = "/dev/rtc0";
Linus Torvalds1da177e2005-04-16 15:20:36 -0700203
Linus Torvalds1da177e2005-04-16 15:20:36 -0700204
David Brownell7531d8fa2006-11-25 11:09:26 -0800205int main(int argc, char **argv)
206{
207 int i, fd, retval, irqcount = 0;
208 unsigned long tmp, data;
209 struct rtc_time rtc_tm;
210 const char *rtc = default_rtc;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700211
David Brownell7531d8fa2006-11-25 11:09:26 -0800212 switch (argc) {
213 case 2:
214 rtc = argv[1];
215 /* FALLTHROUGH */
216 case 1:
217 break;
218 default:
219 fprintf(stderr, "usage: rtctest [rtcdev]\n");
220 return 1;
221 }
Linus Torvalds1da177e2005-04-16 15:20:36 -0700222
David Brownell7531d8fa2006-11-25 11:09:26 -0800223 fd = open(rtc, O_RDONLY);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700224
David Brownell7531d8fa2006-11-25 11:09:26 -0800225 if (fd == -1) {
226 perror(rtc);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700227 exit(errno);
228 }
Linus Torvalds1da177e2005-04-16 15:20:36 -0700229
David Brownell7531d8fa2006-11-25 11:09:26 -0800230 fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700231
David Brownell7531d8fa2006-11-25 11:09:26 -0800232 /* Turn on update interrupts (one per second) */
233 retval = ioctl(fd, RTC_UIE_ON, 0);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700234 if (retval == -1) {
David Brownell7531d8fa2006-11-25 11:09:26 -0800235 if (errno == ENOTTY) {
236 fprintf(stderr,
237 "\n...Update IRQs not supported.\n");
238 goto test_READ;
239 }
Linus Torvalds1da177e2005-04-16 15:20:36 -0700240 perror("ioctl");
241 exit(errno);
242 }
243
David Brownell7531d8fa2006-11-25 11:09:26 -0800244 fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
245 rtc);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700246 fflush(stderr);
David Brownell7531d8fa2006-11-25 11:09:26 -0800247 for (i=1; i<6; i++) {
248 /* This read will block */
Linus Torvalds1da177e2005-04-16 15:20:36 -0700249 retval = read(fd, &data, sizeof(unsigned long));
250 if (retval == -1) {
David Brownell7531d8fa2006-11-25 11:09:26 -0800251 perror("read");
252 exit(errno);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700253 }
254 fprintf(stderr, " %d",i);
255 fflush(stderr);
256 irqcount++;
257 }
258
David Brownell7531d8fa2006-11-25 11:09:26 -0800259 fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
260 fflush(stderr);
261 for (i=1; i<6; i++) {
262 struct timeval tv = {5, 0}; /* 5 second timeout on select */
263 fd_set readfds;
264
265 FD_ZERO(&readfds);
266 FD_SET(fd, &readfds);
267 /* The select will wait until an RTC interrupt happens. */
268 retval = select(fd+1, &readfds, NULL, NULL, &tv);
269 if (retval == -1) {
270 perror("select");
271 exit(errno);
272 }
273 /* This read won't block unlike the select-less case above. */
274 retval = read(fd, &data, sizeof(unsigned long));
275 if (retval == -1) {
276 perror("read");
277 exit(errno);
278 }
279 fprintf(stderr, " %d",i);
280 fflush(stderr);
281 irqcount++;
282 }
283
284 /* Turn off update interrupts */
285 retval = ioctl(fd, RTC_UIE_OFF, 0);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700286 if (retval == -1) {
287 perror("ioctl");
288 exit(errno);
289 }
David Brownell7531d8fa2006-11-25 11:09:26 -0800290
291test_READ:
292 /* Read the RTC time/date */
293 retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
294 if (retval == -1) {
295 perror("ioctl");
296 exit(errno);
297 }
298
299 fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
300 rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
301 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
302
303 /* Set the alarm to 5 sec in the future, and check for rollover */
304 rtc_tm.tm_sec += 5;
305 if (rtc_tm.tm_sec >= 60) {
306 rtc_tm.tm_sec %= 60;
307 rtc_tm.tm_min++;
308 }
309 if (rtc_tm.tm_min == 60) {
310 rtc_tm.tm_min = 0;
311 rtc_tm.tm_hour++;
312 }
313 if (rtc_tm.tm_hour == 24)
314 rtc_tm.tm_hour = 0;
315
316 retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
317 if (retval == -1) {
318 if (errno == ENOTTY) {
319 fprintf(stderr,
320 "\n...Alarm IRQs not supported.\n");
321 goto test_PIE;
322 }
323 perror("ioctl");
324 exit(errno);
325 }
326
327 /* Read the current alarm settings */
328 retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
329 if (retval == -1) {
330 perror("ioctl");
331 exit(errno);
332 }
333
334 fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
335 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
336
337 /* Enable alarm interrupts */
338 retval = ioctl(fd, RTC_AIE_ON, 0);
339 if (retval == -1) {
340 perror("ioctl");
341 exit(errno);
342 }
343
344 fprintf(stderr, "Waiting 5 seconds for alarm...");
345 fflush(stderr);
346 /* This blocks until the alarm ring causes an interrupt */
347 retval = read(fd, &data, sizeof(unsigned long));
348 if (retval == -1) {
349 perror("read");
350 exit(errno);
351 }
352 irqcount++;
353 fprintf(stderr, " okay. Alarm rang.\n");
354
355 /* Disable alarm interrupts */
356 retval = ioctl(fd, RTC_AIE_OFF, 0);
357 if (retval == -1) {
358 perror("ioctl");
359 exit(errno);
360 }
361
362test_PIE:
363 /* Read periodic IRQ rate */
364 retval = ioctl(fd, RTC_IRQP_READ, &tmp);
365 if (retval == -1) {
366 /* not all RTCs support periodic IRQs */
367 if (errno == ENOTTY) {
368 fprintf(stderr, "\nNo periodic IRQ support\n");
369 return 0;
370 }
371 perror("ioctl");
372 exit(errno);
373 }
374 fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
375
376 fprintf(stderr, "Counting 20 interrupts at:");
377 fflush(stderr);
378
379 /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
380 for (tmp=2; tmp<=64; tmp*=2) {
381
382 retval = ioctl(fd, RTC_IRQP_SET, tmp);
383 if (retval == -1) {
384 /* not all RTCs can change their periodic IRQ rate */
385 if (errno == ENOTTY) {
386 fprintf(stderr,
387 "\n...Periodic IRQ rate is fixed\n");
388 goto done;
389 }
390 perror("ioctl");
391 exit(errno);
392 }
393
394 fprintf(stderr, "\n%ldHz:\t", tmp);
395 fflush(stderr);
396
397 /* Enable periodic interrupts */
398 retval = ioctl(fd, RTC_PIE_ON, 0);
399 if (retval == -1) {
400 perror("ioctl");
401 exit(errno);
402 }
403
404 for (i=1; i<21; i++) {
405 /* This blocks */
406 retval = read(fd, &data, sizeof(unsigned long));
407 if (retval == -1) {
408 perror("read");
409 exit(errno);
410 }
411 fprintf(stderr, " %d",i);
412 fflush(stderr);
413 irqcount++;
414 }
415
416 /* Disable periodic interrupts */
417 retval = ioctl(fd, RTC_PIE_OFF, 0);
418 if (retval == -1) {
419 perror("ioctl");
420 exit(errno);
421 }
422 }
423
424done:
425 fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
426
427 close(fd);
428
429 return 0;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700430}