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-
- Real Time Clock (RTC) Drivers for Linux
- =======================================
-
-When Linux developers talk about a "Real Time Clock", they usually mean
-something that tracks wall clock time and is battery backed so that it
-works even with system power off. Such clocks will normally not track
-the local time zone or daylight savings time -- unless they dual boot
-with MS-Windows -- but will instead be set to Coordinated Universal Time
-(UTC, formerly "Greenwich Mean Time").
-
-The newest non-PC hardware tends to just count seconds, like the time(2)
-system call reports, but RTCs also very commonly represent time using
-the Gregorian calendar and 24 hour time, as reported by gmtime(3).
-
-Linux has two largely-compatible userspace RTC API families you may
-need to know about:
-
- * /dev/rtc ... is the RTC provided by PC compatible systems,
- so it's not very portable to non-x86 systems.
-
- * /dev/rtc0, /dev/rtc1 ... are part of a framework that's
- supported by a wide variety of RTC chips on all systems.
-
-Programmers need to understand that the PC/AT functionality is not
-always available, and some systems can do much more. That is, the
-RTCs use the same API to make requests in both RTC frameworks (using
-different filenames of course), but the hardware may not offer the
-same functionality. For example, not every RTC is hooked up to an
-IRQ, so they can't all issue alarms; and where standard PC RTCs can
-only issue an alarm up to 24 hours in the future, other hardware may
-be able to schedule one any time in the upcoming century.
-
-
- Old PC/AT-Compatible driver: /dev/rtc
- --------------------------------------
-
-All PCs (even Alpha machines) have a Real Time Clock built into them.
-Usually they are built into the chipset of the computer, but some may
-actually have a Motorola MC146818 (or clone) on the board. This is the
-clock that keeps the date and time while your computer is turned off.
-
-ACPI has standardized that MC146818 functionality, and extended it in
-a few ways (enabling longer alarm periods, and wake-from-hibernate).
-That functionality is NOT exposed in the old driver.
-
-However it can also be used to generate signals from a slow 2Hz to a
-relatively fast 8192Hz, in increments of powers of two. These signals
-are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
-for...) It can also function as a 24hr alarm, raising IRQ 8 when the
-alarm goes off. The alarm can also be programmed to only check any
-subset of the three programmable values, meaning that it could be set to
-ring on the 30th second of the 30th minute of every hour, for example.
-The clock can also be set to generate an interrupt upon every clock
-update, thus generating a 1Hz signal.
-
-The interrupts are reported via /dev/rtc (major 10, minor 135, read only
-character device) in the form of an unsigned long. The low byte contains
-the type of interrupt (update-done, alarm-rang, or periodic) that was
-raised, and the remaining bytes contain the number of interrupts since
-the last read. Status information is reported through the pseudo-file
-/proc/driver/rtc if the /proc filesystem was enabled. The driver has
-built in locking so that only one process is allowed to have the /dev/rtc
-interface open at a time.
-
-A user process can monitor these interrupts by doing a read(2) or a
-select(2) on /dev/rtc -- either will block/stop the user process until
-the next interrupt is received. This is useful for things like
-reasonably high frequency data acquisition where one doesn't want to
-burn up 100% CPU by polling gettimeofday etc. etc.
-
-At high frequencies, or under high loads, the user process should check
-the number of interrupts received since the last read to determine if
-there has been any interrupt "pileup" so to speak. Just for reference, a
-typical 486-33 running a tight read loop on /dev/rtc will start to suffer
-occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
-frequencies above 1024Hz. So you really should check the high bytes
-of the value you read, especially at frequencies above that of the
-normal timer interrupt, which is 100Hz.
-
-Programming and/or enabling interrupt frequencies greater than 64Hz is
-only allowed by root. This is perhaps a bit conservative, but we don't want
-an evil user generating lots of IRQs on a slow 386sx-16, where it might have
-a negative impact on performance. This 64Hz limit can be changed by writing
-a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
-interrupt handler is only a few lines of code to minimize any possibility
-of this effect.
-
-Also, if the kernel time is synchronized with an external source, the
-kernel will write the time back to the CMOS clock every 11 minutes. In
-the process of doing this, the kernel briefly turns off RTC periodic
-interrupts, so be aware of this if you are doing serious work. If you
-don't synchronize the kernel time with an external source (via ntp or
-whatever) then the kernel will keep its hands off the RTC, allowing you
-exclusive access to the device for your applications.
-
-The alarm and/or interrupt frequency are programmed into the RTC via
-various ioctl(2) calls as listed in ./include/linux/rtc.h
-Rather than write 50 pages describing the ioctl() and so on, it is
-perhaps more useful to include a small test program that demonstrates
-how to use them, and demonstrates the features of the driver. This is
-probably a lot more useful to people interested in writing applications
-that will be using this driver. See the code at the end of this document.
-
-(The original /dev/rtc driver was written by Paul Gortmaker.)
-
-
- New portable "RTC Class" drivers: /dev/rtcN
- --------------------------------------------
-
-Because Linux supports many non-ACPI and non-PC platforms, some of which
-have more than one RTC style clock, it needed a more portable solution
-than expecting a single battery-backed MC146818 clone on every system.
-Accordingly, a new "RTC Class" framework has been defined. It offers
-three different userspace interfaces:
-
- * /dev/rtcN ... much the same as the older /dev/rtc interface
-
- * /sys/class/rtc/rtcN ... sysfs attributes support readonly
- access to some RTC attributes.
-
- * /proc/driver/rtc ... the first RTC (rtc0) may expose itself
- using a procfs interface. More information is (currently) shown
- here than through sysfs.
-
-The RTC Class framework supports a wide variety of RTCs, ranging from those
-integrated into embeddable system-on-chip (SOC) processors to discrete chips
-using I2C, SPI, or some other bus to communicate with the host CPU. There's
-even support for PC-style RTCs ... including the features exposed on newer PCs
-through ACPI.
-
-The new framework also removes the "one RTC per system" restriction. For
-example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
-a high functionality RTC is integrated into the SOC. That system might read
-the system clock from the discrete RTC, but use the integrated one for all
-other tasks, because of its greater functionality.
-
-SYSFS INTERFACE
----------------
-
-The sysfs interface under /sys/class/rtc/rtcN provides access to various
-rtc attributes without requiring the use of ioctls. All dates and times
-are in the RTC's timezone, rather than in system time.
-
-date: RTC-provided date
-hctosys: 1 if the RTC provided the system time at boot via the
- CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
-max_user_freq: The maximum interrupt rate an unprivileged user may request
- from this RTC.
-name: The name of the RTC corresponding to this sysfs directory
-since_epoch: The number of seconds since the epoch according to the RTC
-time: RTC-provided time
-wakealarm: The time at which the clock will generate a system wakeup
- event. This is a one shot wakeup event, so must be reset
- after wake if a daily wakeup is required. Format is either
- seconds since the epoch or, if there's a leading +, seconds
- in the future.
-
-IOCTL INTERFACE
----------------
-
-The ioctl() calls supported by /dev/rtc are also supported by the RTC class
-framework. However, because the chips and systems are not standardized,
-some PC/AT functionality might not be provided. And in the same way, some
-newer features -- including those enabled by ACPI -- are exposed by the
-RTC class framework, but can't be supported by the older driver.
-
- * RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading
- time, returning the result as a Gregorian calendar date and 24 hour
- wall clock time. To be most useful, this time may also be updated.
-
- * RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
- is connected to an IRQ line, it can often issue an alarm IRQ up to
- 24 hours in the future. (Use RTC_WKALM_* by preference.)
-
- * RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
- the next 24 hours use a slightly more powerful API, which supports
- setting the longer alarm time and enabling its IRQ using a single
- request (using the same model as EFI firmware).
-
- * RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework
- will emulate this mechanism.
-
- * RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
- are emulated via a kernel hrtimer.
-
-In many cases, the RTC alarm can be a system wake event, used to force
-Linux out of a low power sleep state (or hibernation) back to a fully
-operational state. For example, a system could enter a deep power saving
-state until it's time to execute some scheduled tasks.
-
-Note that many of these ioctls are handled by the common rtc-dev interface.
-Some common examples:
-
- * RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
- called with appropriate values.
-
- * RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets
- the alarm rtc_timer. May call the set_alarm driver function.
-
- * RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
-
- * RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
-
-If all else fails, check out the rtc-test.c driver!
-
-
--------------------- 8< ---------------- 8< -----------------------------
-
-/*
- * Real Time Clock Driver Test/Example Program
- *
- * Compile with:
- * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
- *
- * Copyright (C) 1996, Paul Gortmaker.
- *
- * Released under the GNU General Public License, version 2,
- * included herein by reference.
- *
- */
-
-#include <stdio.h>
-#include <linux/rtc.h>
-#include <sys/ioctl.h>
-#include <sys/time.h>
-#include <sys/types.h>
-#include <fcntl.h>
-#include <unistd.h>
-#include <stdlib.h>
-#include <errno.h>
-
-
-/*
- * This expects the new RTC class driver framework, working with
- * clocks that will often not be clones of what the PC-AT had.
- * Use the command line to specify another RTC if you need one.
- */
-static const char default_rtc[] = "/dev/rtc0";
-
-
-int main(int argc, char **argv)
-{
- int i, fd, retval, irqcount = 0;
- unsigned long tmp, data;
- struct rtc_time rtc_tm;
- const char *rtc = default_rtc;
-
- switch (argc) {
- case 2:
- rtc = argv[1];
- /* FALLTHROUGH */
- case 1:
- break;
- default:
- fprintf(stderr, "usage: rtctest [rtcdev]\n");
- return 1;
- }
-
- fd = open(rtc, O_RDONLY);
-
- if (fd == -1) {
- perror(rtc);
- exit(errno);
- }
-
- fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
-
- /* Turn on update interrupts (one per second) */
- retval = ioctl(fd, RTC_UIE_ON, 0);
- if (retval == -1) {
- if (errno == ENOTTY) {
- fprintf(stderr,
- "\n...Update IRQs not supported.\n");
- goto test_READ;
- }
- perror("RTC_UIE_ON ioctl");
- exit(errno);
- }
-
- fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
- rtc);
- fflush(stderr);
- for (i=1; i<6; i++) {
- /* This read will block */
- retval = read(fd, &data, sizeof(unsigned long));
- if (retval == -1) {
- perror("read");
- exit(errno);
- }
- fprintf(stderr, " %d",i);
- fflush(stderr);
- irqcount++;
- }
-
- fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
- fflush(stderr);
- for (i=1; i<6; i++) {
- struct timeval tv = {5, 0}; /* 5 second timeout on select */
- fd_set readfds;
-
- FD_ZERO(&readfds);
- FD_SET(fd, &readfds);
- /* The select will wait until an RTC interrupt happens. */
- retval = select(fd+1, &readfds, NULL, NULL, &tv);
- if (retval == -1) {
- perror("select");
- exit(errno);
- }
- /* This read won't block unlike the select-less case above. */
- retval = read(fd, &data, sizeof(unsigned long));
- if (retval == -1) {
- perror("read");
- exit(errno);
- }
- fprintf(stderr, " %d",i);
- fflush(stderr);
- irqcount++;
- }
-
- /* Turn off update interrupts */
- retval = ioctl(fd, RTC_UIE_OFF, 0);
- if (retval == -1) {
- perror("RTC_UIE_OFF ioctl");
- exit(errno);
- }
-
-test_READ:
- /* Read the RTC time/date */
- retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
- if (retval == -1) {
- perror("RTC_RD_TIME ioctl");
- exit(errno);
- }
-
- fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
- rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
- rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
-
- /* Set the alarm to 5 sec in the future, and check for rollover */
- rtc_tm.tm_sec += 5;
- if (rtc_tm.tm_sec >= 60) {
- rtc_tm.tm_sec %= 60;
- rtc_tm.tm_min++;
- }
- if (rtc_tm.tm_min == 60) {
- rtc_tm.tm_min = 0;
- rtc_tm.tm_hour++;
- }
- if (rtc_tm.tm_hour == 24)
- rtc_tm.tm_hour = 0;
-
- retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
- if (retval == -1) {
- if (errno == ENOTTY) {
- fprintf(stderr,
- "\n...Alarm IRQs not supported.\n");
- goto test_PIE;
- }
- perror("RTC_ALM_SET ioctl");
- exit(errno);
- }
-
- /* Read the current alarm settings */
- retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
- if (retval == -1) {
- perror("RTC_ALM_READ ioctl");
- exit(errno);
- }
-
- fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
- rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
-
- /* Enable alarm interrupts */
- retval = ioctl(fd, RTC_AIE_ON, 0);
- if (retval == -1) {
- perror("RTC_AIE_ON ioctl");
- exit(errno);
- }
-
- fprintf(stderr, "Waiting 5 seconds for alarm...");
- fflush(stderr);
- /* This blocks until the alarm ring causes an interrupt */
- retval = read(fd, &data, sizeof(unsigned long));
- if (retval == -1) {
- perror("read");
- exit(errno);
- }
- irqcount++;
- fprintf(stderr, " okay. Alarm rang.\n");
-
- /* Disable alarm interrupts */
- retval = ioctl(fd, RTC_AIE_OFF, 0);
- if (retval == -1) {
- perror("RTC_AIE_OFF ioctl");
- exit(errno);
- }
-
-test_PIE:
- /* Read periodic IRQ rate */
- retval = ioctl(fd, RTC_IRQP_READ, &tmp);
- if (retval == -1) {
- /* not all RTCs support periodic IRQs */
- if (errno == ENOTTY) {
- fprintf(stderr, "\nNo periodic IRQ support\n");
- goto done;
- }
- perror("RTC_IRQP_READ ioctl");
- exit(errno);
- }
- fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp);
-
- fprintf(stderr, "Counting 20 interrupts at:");
- fflush(stderr);
-
- /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
- for (tmp=2; tmp<=64; tmp*=2) {
-
- retval = ioctl(fd, RTC_IRQP_SET, tmp);
- if (retval == -1) {
- /* not all RTCs can change their periodic IRQ rate */
- if (errno == ENOTTY) {
- fprintf(stderr,
- "\n...Periodic IRQ rate is fixed\n");
- goto done;
- }
- perror("RTC_IRQP_SET ioctl");
- exit(errno);
- }
-
- fprintf(stderr, "\n%ldHz:\t", tmp);
- fflush(stderr);
-
- /* Enable periodic interrupts */
- retval = ioctl(fd, RTC_PIE_ON, 0);
- if (retval == -1) {
- perror("RTC_PIE_ON ioctl");
- exit(errno);
- }
-
- for (i=1; i<21; i++) {
- /* This blocks */
- retval = read(fd, &data, sizeof(unsigned long));
- if (retval == -1) {
- perror("read");
- exit(errno);
- }
- fprintf(stderr, " %d",i);
- fflush(stderr);
- irqcount++;
- }
-
- /* Disable periodic interrupts */
- retval = ioctl(fd, RTC_PIE_OFF, 0);
- if (retval == -1) {
- perror("RTC_PIE_OFF ioctl");
- exit(errno);
- }
- }
-
-done:
- fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
-
- close(fd);
-
- return 0;
-}