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 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, it probably
182    also offers update IRQs whenever the "seconds" counter changes.
183    If needed, the RTC framework can emulate this mechanism.
184
185    * RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... another
186    feature often accessible with an IRQ line is a periodic IRQ, issued
187    at settable frequencies (usually 2^N Hz).
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 need not actually be implemented by your
195driver. The common rtc-dev interface handles many of these nicely if your
196driver returns ENOIOCTLCMD. Some common examples:
197
198    * RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
199    called with appropriate values.
200
201    * RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: the
202    set_alarm/read_alarm functions will be called.
203
204    * RTC_IRQP_SET, RTC_IRQP_READ: the irq_set_freq function will be called
205    to set the frequency while the framework will handle the read for you
206    since the frequency is stored in the irq_freq member of the rtc_device
207    structure. Your driver needs to initialize the irq_freq member during
208    init. Make sure you check the requested frequency is in range of your
209    hardware in the irq_set_freq function. If it isn't, return -EINVAL. If
210    you cannot actually change the frequency, do not define irq_set_freq.
211
212    * RTC_PIE_ON, RTC_PIE_OFF: the irq_set_state function will be called.
213
214If all else fails, check out the rtc-test.c driver!
215
216
217-------------------- 8< ---------------- 8< -----------------------------
218
219/*
220 * Real Time Clock Driver Test/Example Program
221 *
222 * Compile with:
223 * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
224 *
225 * Copyright (C) 1996, Paul Gortmaker.
226 *
227 * Released under the GNU General Public License, version 2,
228 * included herein by reference.
229 *
230 */
231
232#include <stdio.h>
233#include <linux/rtc.h>
234#include <sys/ioctl.h>
235#include <sys/time.h>
236#include <sys/types.h>
237#include <fcntl.h>
238#include <unistd.h>
239#include <stdlib.h>
240#include <errno.h>
241
242
243/*
244 * This expects the new RTC class driver framework, working with
245 * clocks that will often not be clones of what the PC-AT had.
246 * Use the command line to specify another RTC if you need one.
247 */
248static const char default_rtc[] = "/dev/rtc0";
249
250
251int main(int argc, char **argv)
252{
253    int i, fd, retval, irqcount = 0;
254    unsigned long tmp, data;
255    struct rtc_time rtc_tm;
256    const char *rtc = default_rtc;
257
258    switch (argc) {
259    case 2:
260        rtc = argv[1];
261        /* FALLTHROUGH */
262    case 1:
263        break;
264    default:
265        fprintf(stderr, "usage: rtctest [rtcdev]\n");
266        return 1;
267    }
268
269    fd = open(rtc, O_RDONLY);
270
271    if (fd == -1) {
272        perror(rtc);
273        exit(errno);
274    }
275
276    fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
277
278    /* Turn on update interrupts (one per second) */
279    retval = ioctl(fd, RTC_UIE_ON, 0);
280    if (retval == -1) {
281        if (errno == ENOTTY) {
282            fprintf(stderr,
283                "\n...Update IRQs not supported.\n");
284            goto test_READ;
285        }
286        perror("RTC_UIE_ON ioctl");
287        exit(errno);
288    }
289
290    fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
291            rtc);
292    fflush(stderr);
293    for (i=1; i<6; i++) {
294        /* This read will block */
295        retval = read(fd, &data, sizeof(unsigned long));
296        if (retval == -1) {
297            perror("read");
298            exit(errno);
299        }
300        fprintf(stderr, " %d",i);
301        fflush(stderr);
302        irqcount++;
303    }
304
305    fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
306    fflush(stderr);
307    for (i=1; i<6; i++) {
308        struct timeval tv = {5, 0}; /* 5 second timeout on select */
309        fd_set readfds;
310
311        FD_ZERO(&readfds);
312        FD_SET(fd, &readfds);
313        /* The select will wait until an RTC interrupt happens. */
314        retval = select(fd+1, &readfds, NULL, NULL, &tv);
315        if (retval == -1) {
316                perror("select");
317                exit(errno);
318        }
319        /* This read won't block unlike the select-less case above. */
320        retval = read(fd, &data, sizeof(unsigned long));
321        if (retval == -1) {
322                perror("read");
323                exit(errno);
324        }
325        fprintf(stderr, " %d",i);
326        fflush(stderr);
327        irqcount++;
328    }
329
330    /* Turn off update interrupts */
331    retval = ioctl(fd, RTC_UIE_OFF, 0);
332    if (retval == -1) {
333        perror("RTC_UIE_OFF ioctl");
334        exit(errno);
335    }
336
337test_READ:
338    /* Read the RTC time/date */
339    retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
340    if (retval == -1) {
341        perror("RTC_RD_TIME ioctl");
342        exit(errno);
343    }
344
345    fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
346        rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
347        rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
348
349    /* Set the alarm to 5 sec in the future, and check for rollover */
350    rtc_tm.tm_sec += 5;
351    if (rtc_tm.tm_sec >= 60) {
352        rtc_tm.tm_sec %= 60;
353        rtc_tm.tm_min++;
354    }
355    if (rtc_tm.tm_min == 60) {
356        rtc_tm.tm_min = 0;
357        rtc_tm.tm_hour++;
358    }
359    if (rtc_tm.tm_hour == 24)
360        rtc_tm.tm_hour = 0;
361
362    retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
363    if (retval == -1) {
364        if (errno == ENOTTY) {
365            fprintf(stderr,
366                "\n...Alarm IRQs not supported.\n");
367            goto test_PIE;
368        }
369        perror("RTC_ALM_SET ioctl");
370        exit(errno);
371    }
372
373    /* Read the current alarm settings */
374    retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
375    if (retval == -1) {
376        perror("RTC_ALM_READ ioctl");
377        exit(errno);
378    }
379
380    fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
381        rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
382
383    /* Enable alarm interrupts */
384    retval = ioctl(fd, RTC_AIE_ON, 0);
385    if (retval == -1) {
386        perror("RTC_AIE_ON ioctl");
387        exit(errno);
388    }
389
390    fprintf(stderr, "Waiting 5 seconds for alarm...");
391    fflush(stderr);
392    /* This blocks until the alarm ring causes an interrupt */
393    retval = read(fd, &data, sizeof(unsigned long));
394    if (retval == -1) {
395        perror("read");
396        exit(errno);
397    }
398    irqcount++;
399    fprintf(stderr, " okay. Alarm rang.\n");
400
401    /* Disable alarm interrupts */
402    retval = ioctl(fd, RTC_AIE_OFF, 0);
403    if (retval == -1) {
404        perror("RTC_AIE_OFF ioctl");
405        exit(errno);
406    }
407
408test_PIE:
409    /* Read periodic IRQ rate */
410    retval = ioctl(fd, RTC_IRQP_READ, &tmp);
411    if (retval == -1) {
412        /* not all RTCs support periodic IRQs */
413        if (errno == ENOTTY) {
414            fprintf(stderr, "\nNo periodic IRQ support\n");
415            goto done;
416        }
417        perror("RTC_IRQP_READ ioctl");
418        exit(errno);
419    }
420    fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
421
422    fprintf(stderr, "Counting 20 interrupts at:");
423    fflush(stderr);
424
425    /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
426    for (tmp=2; tmp<=64; tmp*=2) {
427
428        retval = ioctl(fd, RTC_IRQP_SET, tmp);
429        if (retval == -1) {
430            /* not all RTCs can change their periodic IRQ rate */
431            if (errno == ENOTTY) {
432                fprintf(stderr,
433                    "\n...Periodic IRQ rate is fixed\n");
434                goto done;
435            }
436            perror("RTC_IRQP_SET ioctl");
437            exit(errno);
438        }
439
440        fprintf(stderr, "\n%ldHz:\t", tmp);
441        fflush(stderr);
442
443        /* Enable periodic interrupts */
444        retval = ioctl(fd, RTC_PIE_ON, 0);
445        if (retval == -1) {
446            perror("RTC_PIE_ON ioctl");
447            exit(errno);
448        }
449
450        for (i=1; i<21; i++) {
451            /* This blocks */
452            retval = read(fd, &data, sizeof(unsigned long));
453            if (retval == -1) {
454                perror("read");
455                exit(errno);
456            }
457            fprintf(stderr, " %d",i);
458            fflush(stderr);
459            irqcount++;
460        }
461
462        /* Disable periodic interrupts */
463        retval = ioctl(fd, RTC_PIE_OFF, 0);
464        if (retval == -1) {
465            perror("RTC_PIE_OFF ioctl");
466            exit(errno);
467        }
468    }
469
470done:
471    fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
472
473    close(fd);
474
475    return 0;
476}
477

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