Root/Documentation/rtc.txt

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 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
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
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
147hctosys: 1 if the RTC provided the system time at boot via the
148         CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
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
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.
160
161IOCTL INTERFACE
162---------------
163
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
176    24 hours in the future. (Use RTC_WKALM_* by preference.)
177
178    * RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
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
183    * RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework
184    will emulate this mechanism.
185
186    * RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
187    are emulated via a kernel hrtimer.
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.
193
194Note that many of these ioctls are handled by the common rtc-dev interface.
195Some common examples:
196
197    * RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
198    called with appropriate values.
199
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.
202
203    * RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
204
205    * RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
206
207If all else fails, check out the rtc-test.c driver!
208
209
210-------------------- 8< ---------------- 8< -----------------------------
211
212/*
213 * Real Time Clock Driver Test/Example Program
214 *
215 * Compile with:
216 * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
217 *
218 * Copyright (C) 1996, Paul Gortmaker.
219 *
220 * Released under the GNU General Public License, version 2,
221 * included herein by reference.
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>
232#include <stdlib.h>
233#include <errno.h>
234
235
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    }
261
262    fd = open(rtc, O_RDONLY);
263
264    if (fd == -1) {
265        perror(rtc);
266        exit(errno);
267    }
268
269    fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
270
271    /* Turn on update interrupts (one per second) */
272    retval = ioctl(fd, RTC_UIE_ON, 0);
273    if (retval == -1) {
274        if (errno == ENOTTY) {
275            fprintf(stderr,
276                "\n...Update IRQs not supported.\n");
277            goto test_READ;
278        }
279        perror("RTC_UIE_ON ioctl");
280        exit(errno);
281    }
282
283    fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
284            rtc);
285    fflush(stderr);
286    for (i=1; i<6; i++) {
287        /* This read will block */
288        retval = read(fd, &data, sizeof(unsigned long));
289        if (retval == -1) {
290            perror("read");
291            exit(errno);
292        }
293        fprintf(stderr, " %d",i);
294        fflush(stderr);
295        irqcount++;
296    }
297
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    }
322
323    /* Turn off update interrupts */
324    retval = ioctl(fd, RTC_UIE_OFF, 0);
325    if (retval == -1) {
326        perror("RTC_UIE_OFF ioctl");
327        exit(errno);
328    }
329
330test_READ:
331    /* Read the RTC time/date */
332    retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
333    if (retval == -1) {
334        perror("RTC_RD_TIME ioctl");
335        exit(errno);
336    }
337
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);
341
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    }
348    if (rtc_tm.tm_min == 60) {
349        rtc_tm.tm_min = 0;
350        rtc_tm.tm_hour++;
351    }
352    if (rtc_tm.tm_hour == 24)
353        rtc_tm.tm_hour = 0;
354
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        }
362        perror("RTC_ALM_SET ioctl");
363        exit(errno);
364    }
365
366    /* Read the current alarm settings */
367    retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
368    if (retval == -1) {
369        perror("RTC_ALM_READ ioctl");
370        exit(errno);
371    }
372
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);
375
376    /* Enable alarm interrupts */
377    retval = ioctl(fd, RTC_AIE_ON, 0);
378    if (retval == -1) {
379        perror("RTC_AIE_ON ioctl");
380        exit(errno);
381    }
382
383    fprintf(stderr, "Waiting 5 seconds for alarm...");
384    fflush(stderr);
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");
393
394    /* Disable alarm interrupts */
395    retval = ioctl(fd, RTC_AIE_OFF, 0);
396    if (retval == -1) {
397        perror("RTC_AIE_OFF ioctl");
398        exit(errno);
399    }
400
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");
408            goto done;
409        }
410        perror("RTC_IRQP_READ ioctl");
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);
422        if (retval == -1) {
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            }
429            perror("RTC_IRQP_SET ioctl");
430            exit(errno);
431        }
432
433        fprintf(stderr, "\n%ldHz:\t", tmp);
434        fflush(stderr);
435
436        /* Enable periodic interrupts */
437        retval = ioctl(fd, RTC_PIE_ON, 0);
438        if (retval == -1) {
439            perror("RTC_PIE_ON ioctl");
440            exit(errno);
441        }
442
443        for (i=1; i<21; i++) {
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++;
453        }
454
455        /* Disable periodic interrupts */
456        retval = ioctl(fd, RTC_PIE_OFF, 0);
457        if (retval == -1) {
458            perror("RTC_PIE_OFF ioctl");
459            exit(errno);
460        }
461    }
462
463done:
464    fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
465
466    close(fd);
467
468    return 0;
469}
470

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