- Real Time Clock Driver for Linux
- ================================
+ 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
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.
+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.
+
+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.
+
+ * RTC_WKALM_SET, RTC_WKALM_READ ... 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, it probably
+ also offers update IRQs whenever the "seconds" counter changes.
+ If needed, the RTC framework can emulate this mechanism.
+
+ * RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... another
+ feature often accessible with an IRQ line is a periodic IRQ, issued
+ at settable frequencies (usually 2^N Hz).
+
+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.
- Paul Gortmaker
-------------------- 8< ---------------- 8< -----------------------------
/*
- * Real Time Clock Driver Test/Example Program
+ * Real Time Clock Driver Test/Example Program
*
- * Compile with:
- * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
+ * Compile with:
+ * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
*
- * Copyright (C) 1996, Paul Gortmaker.
+ * Copyright (C) 1996, Paul Gortmaker.
*
- * Released under the GNU General Public License, version 2,
- * included herein by reference.
+ * Released under the GNU General Public License, version 2,
+ * included herein by reference.
*
*/
#include <stdio.h>
-#include <stdlib.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>
-int main(void) {
-
-int i, fd, retval, irqcount = 0;
-unsigned long tmp, data;
-struct rtc_time rtc_tm;
-fd = open ("/dev/rtc", O_RDONLY);
+/*
+ * 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;
+ }
-if (fd == -1) {
- perror("/dev/rtc");
- exit(errno);
-}
+ fd = open(rtc, O_RDONLY);
-fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
+ if (fd == -1) {
+ perror(rtc);
+ exit(errno);
+ }
-/* Turn on update interrupts (one per second) */
-retval = ioctl(fd, RTC_UIE_ON, 0);
-if (retval == -1) {
- perror("ioctl");
- exit(errno);
-}
+ fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
-fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading /dev/rtc:");
-fflush(stderr);
-for (i=1; i<6; i++) {
- /* This read will block */
- retval = read(fd, &data, sizeof(unsigned long));
+ /* Turn on update interrupts (one per second) */
+ retval = ioctl(fd, RTC_UIE_ON, 0);
if (retval == -1) {
- perror("read");
+ if (errno == ENOTTY) {
+ fprintf(stderr,
+ "\n...Update IRQs not supported.\n");
+ goto test_READ;
+ }
+ perror("ioctl");
exit(errno);
}
- fprintf(stderr, " %d",i);
+
+ fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
+ rtc);
fflush(stderr);
- irqcount++;
-}
+ 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;
+ 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++;
+ }
- FD_ZERO(&readfds);
- FD_SET(fd, &readfds);
- /* The select will wait until an RTC interrupt happens. */
- retval = select(fd+1, &readfds, NULL, NULL, &tv);
+ /* Turn off update interrupts */
+ retval = ioctl(fd, RTC_UIE_OFF, 0);
if (retval == -1) {
- perror("select");
+ perror("ioctl");
exit(errno);
}
- /* This read won't block unlike the select-less case above. */
- retval = read(fd, &data, sizeof(unsigned long));
+
+test_READ:
+ /* Read the RTC time/date */
+ retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
if (retval == -1) {
- perror("read");
+ perror("ioctl");
exit(errno);
}
- fprintf(stderr, " %d",i);
- fflush(stderr);
- irqcount++;
-}
-
-/* Turn off update interrupts */
-retval = ioctl(fd, RTC_UIE_OFF, 0);
-if (retval == -1) {
- perror("ioctl");
- exit(errno);
-}
-
-/* Read the RTC time/date */
-retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
-if (retval == -1) {
- perror("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) {
- perror("ioctl");
- exit(errno);
-}
-
-/* Read the current alarm settings */
-retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
-if (retval == -1) {
- perror("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("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);
-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("ioctl");
- exit(errno);
-}
+ /* 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;
-/* Read periodic IRQ rate */
-retval = ioctl(fd, RTC_IRQP_READ, &tmp);
-if (retval == -1) {
- perror("ioctl");
- exit(errno);
-}
-fprintf(stderr, "\nPeriodic IRQ rate was %ldHz.\n", tmp);
+ 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("ioctl");
+ exit(errno);
+ }
-fprintf(stderr, "Counting 20 interrupts at:");
-fflush(stderr);
+ /* Read the current alarm settings */
+ retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
+ if (retval == -1) {
+ perror("ioctl");
+ exit(errno);
+ }
-/* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
-for (tmp=2; tmp<=64; tmp*=2) {
+ fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
+ rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
- retval = ioctl(fd, RTC_IRQP_SET, tmp);
+ /* Enable alarm interrupts */
+ retval = ioctl(fd, RTC_AIE_ON, 0);
if (retval == -1) {
perror("ioctl");
exit(errno);
}
- fprintf(stderr, "\n%ldHz:\t", tmp);
+ 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");
- /* Enable periodic interrupts */
- retval = ioctl(fd, RTC_PIE_ON, 0);
+ /* Disable alarm interrupts */
+ retval = ioctl(fd, RTC_AIE_OFF, 0);
if (retval == -1) {
perror("ioctl");
exit(errno);
}
- for (i=1; i<21; i++) {
- /* This blocks */
- retval = read(fd, &data, sizeof(unsigned long));
+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");
+ return 0;
+ }
+ perror("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) {
- perror("read");
- exit(errno);
+ /* not all RTCs can change their periodic IRQ rate */
+ if (errno == ENOTTY) {
+ fprintf(stderr,
+ "\n...Periodic IRQ rate is fixed\n");
+ goto done;
+ }
+ perror("ioctl");
+ exit(errno);
}
- fprintf(stderr, " %d",i);
+
+ fprintf(stderr, "\n%ldHz:\t", tmp);
fflush(stderr);
- irqcount++;
- }
- /* Disable periodic interrupts */
- retval = ioctl(fd, RTC_PIE_OFF, 0);
- if (retval == -1) {
- perror("ioctl");
- exit(errno);
+ /* Enable periodic interrupts */
+ retval = ioctl(fd, RTC_PIE_ON, 0);
+ if (retval == -1) {
+ perror("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("ioctl");
+ exit(errno);
+ }
}
-}
-fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
-fprintf(stderr, "\nTyping \"cat /proc/interrupts\" will show %d more events on IRQ 8.\n\n",
- irqcount);
+done:
+ fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
-close(fd);
-return 0;
+ close(fd);
-} /* end main */
+ return 0;
+}
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