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 either
157         seconds since the epoch or, if there's a leading +, seconds
158         in the future.
159
160IOCTL INTERFACE
161---------------
162
163The ioctl() calls supported by /dev/rtc are also supported by the RTC class
164framework. However, because the chips and systems are not standardized,
165some PC/AT functionality might not be provided. And in the same way, some
166newer features -- including those enabled by ACPI -- are exposed by the
167RTC class framework, but can't be supported by the older driver.
168
169    * RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading
170    time, returning the result as a Gregorian calendar date and 24 hour
171    wall clock time. To be most useful, this time may also be updated.
172
173    * RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
174    is connected to an IRQ line, it can often issue an alarm IRQ up to
175    24 hours in the future. (Use RTC_WKALM_* by preference.)
176
177    * RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
178    the next 24 hours use a slightly more powerful API, which supports
179    setting the longer alarm time and enabling its IRQ using a single
180    request (using the same model as EFI firmware).
181
182    * RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework
183    will emulate this mechanism.
184
185    * RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
186    are emulated via a kernel hrtimer.
187
188In many cases, the RTC alarm can be a system wake event, used to force
189Linux out of a low power sleep state (or hibernation) back to a fully
190operational state. For example, a system could enter a deep power saving
191state until it's time to execute some scheduled tasks.
192
193Note that many of these ioctls are handled by the common rtc-dev interface.
194Some common examples:
195
196    * RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
197    called with appropriate values.
198
199    * RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets
200    the alarm rtc_timer. May call the set_alarm driver function.
201
202    * RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
203
204    * RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
205
206If all else fails, check out the rtc-test.c driver!
207
208
209-------------------- 8< ---------------- 8< -----------------------------
210
211/*
212 * Real Time Clock Driver Test/Example Program
213 *
214 * Compile with:
215 * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
216 *
217 * Copyright (C) 1996, Paul Gortmaker.
218 *
219 * Released under the GNU General Public License, version 2,
220 * included herein by reference.
221 *
222 */
223
224#include <stdio.h>
225#include <linux/rtc.h>
226#include <sys/ioctl.h>
227#include <sys/time.h>
228#include <sys/types.h>
229#include <fcntl.h>
230#include <unistd.h>
231#include <stdlib.h>
232#include <errno.h>
233
234
235/*
236 * This expects the new RTC class driver framework, working with
237 * clocks that will often not be clones of what the PC-AT had.
238 * Use the command line to specify another RTC if you need one.
239 */
240static const char default_rtc[] = "/dev/rtc0";
241
242
243int main(int argc, char **argv)
244{
245    int i, fd, retval, irqcount = 0;
246    unsigned long tmp, data;
247    struct rtc_time rtc_tm;
248    const char *rtc = default_rtc;
249
250    switch (argc) {
251    case 2:
252        rtc = argv[1];
253        /* FALLTHROUGH */
254    case 1:
255        break;
256    default:
257        fprintf(stderr, "usage: rtctest [rtcdev]\n");
258        return 1;
259    }
260
261    fd = open(rtc, O_RDONLY);
262
263    if (fd == -1) {
264        perror(rtc);
265        exit(errno);
266    }
267
268    fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
269
270    /* Turn on update interrupts (one per second) */
271    retval = ioctl(fd, RTC_UIE_ON, 0);
272    if (retval == -1) {
273        if (errno == ENOTTY) {
274            fprintf(stderr,
275                "\n...Update IRQs not supported.\n");
276            goto test_READ;
277        }
278        perror("RTC_UIE_ON ioctl");
279        exit(errno);
280    }
281
282    fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
283            rtc);
284    fflush(stderr);
285    for (i=1; i<6; i++) {
286        /* This read will block */
287        retval = read(fd, &data, sizeof(unsigned long));
288        if (retval == -1) {
289            perror("read");
290            exit(errno);
291        }
292        fprintf(stderr, " %d",i);
293        fflush(stderr);
294        irqcount++;
295    }
296
297    fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
298    fflush(stderr);
299    for (i=1; i<6; i++) {
300        struct timeval tv = {5, 0}; /* 5 second timeout on select */
301        fd_set readfds;
302
303        FD_ZERO(&readfds);
304        FD_SET(fd, &readfds);
305        /* The select will wait until an RTC interrupt happens. */
306        retval = select(fd+1, &readfds, NULL, NULL, &tv);
307        if (retval == -1) {
308                perror("select");
309                exit(errno);
310        }
311        /* This read won't block unlike the select-less case above. */
312        retval = read(fd, &data, sizeof(unsigned long));
313        if (retval == -1) {
314                perror("read");
315                exit(errno);
316        }
317        fprintf(stderr, " %d",i);
318        fflush(stderr);
319        irqcount++;
320    }
321
322    /* Turn off update interrupts */
323    retval = ioctl(fd, RTC_UIE_OFF, 0);
324    if (retval == -1) {
325        perror("RTC_UIE_OFF ioctl");
326        exit(errno);
327    }
328
329test_READ:
330    /* Read the RTC time/date */
331    retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
332    if (retval == -1) {
333        perror("RTC_RD_TIME ioctl");
334        exit(errno);
335    }
336
337    fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
338        rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
339        rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
340
341    /* Set the alarm to 5 sec in the future, and check for rollover */
342    rtc_tm.tm_sec += 5;
343    if (rtc_tm.tm_sec >= 60) {
344        rtc_tm.tm_sec %= 60;
345        rtc_tm.tm_min++;
346    }
347    if (rtc_tm.tm_min == 60) {
348        rtc_tm.tm_min = 0;
349        rtc_tm.tm_hour++;
350    }
351    if (rtc_tm.tm_hour == 24)
352        rtc_tm.tm_hour = 0;
353
354    retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
355    if (retval == -1) {
356        if (errno == ENOTTY) {
357            fprintf(stderr,
358                "\n...Alarm IRQs not supported.\n");
359            goto test_PIE;
360        }
361        perror("RTC_ALM_SET ioctl");
362        exit(errno);
363    }
364
365    /* Read the current alarm settings */
366    retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
367    if (retval == -1) {
368        perror("RTC_ALM_READ ioctl");
369        exit(errno);
370    }
371
372    fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
373        rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
374
375    /* Enable alarm interrupts */
376    retval = ioctl(fd, RTC_AIE_ON, 0);
377    if (retval == -1) {
378        perror("RTC_AIE_ON ioctl");
379        exit(errno);
380    }
381
382    fprintf(stderr, "Waiting 5 seconds for alarm...");
383    fflush(stderr);
384    /* This blocks until the alarm ring causes an interrupt */
385    retval = read(fd, &data, sizeof(unsigned long));
386    if (retval == -1) {
387        perror("read");
388        exit(errno);
389    }
390    irqcount++;
391    fprintf(stderr, " okay. Alarm rang.\n");
392
393    /* Disable alarm interrupts */
394    retval = ioctl(fd, RTC_AIE_OFF, 0);
395    if (retval == -1) {
396        perror("RTC_AIE_OFF ioctl");
397        exit(errno);
398    }
399
400test_PIE:
401    /* Read periodic IRQ rate */
402    retval = ioctl(fd, RTC_IRQP_READ, &tmp);
403    if (retval == -1) {
404        /* not all RTCs support periodic IRQs */
405        if (errno == ENOTTY) {
406            fprintf(stderr, "\nNo periodic IRQ support\n");
407            goto done;
408        }
409        perror("RTC_IRQP_READ ioctl");
410        exit(errno);
411    }
412    fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
413
414    fprintf(stderr, "Counting 20 interrupts at:");
415    fflush(stderr);
416
417    /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
418    for (tmp=2; tmp<=64; tmp*=2) {
419
420        retval = ioctl(fd, RTC_IRQP_SET, tmp);
421        if (retval == -1) {
422            /* not all RTCs can change their periodic IRQ rate */
423            if (errno == ENOTTY) {
424                fprintf(stderr,
425                    "\n...Periodic IRQ rate is fixed\n");
426                goto done;
427            }
428            perror("RTC_IRQP_SET ioctl");
429            exit(errno);
430        }
431
432        fprintf(stderr, "\n%ldHz:\t", tmp);
433        fflush(stderr);
434
435        /* Enable periodic interrupts */
436        retval = ioctl(fd, RTC_PIE_ON, 0);
437        if (retval == -1) {
438            perror("RTC_PIE_ON ioctl");
439            exit(errno);
440        }
441
442        for (i=1; i<21; i++) {
443            /* This blocks */
444            retval = read(fd, &data, sizeof(unsigned long));
445            if (retval == -1) {
446                perror("read");
447                exit(errno);
448            }
449            fprintf(stderr, " %d",i);
450            fflush(stderr);
451            irqcount++;
452        }
453
454        /* Disable periodic interrupts */
455        retval = ioctl(fd, RTC_PIE_OFF, 0);
456        if (retval == -1) {
457            perror("RTC_PIE_OFF ioctl");
458            exit(errno);
459        }
460    }
461
462done:
463    fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
464
465    close(fd);
466
467    return 0;
468}
469

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