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authorFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
committerFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
commit3b4bd47f8f4ed3aaf7c81c9b5d2d37ad79fadf4a (patch)
treeb9996006addfd7ae70a39672b76843b49aebc189 /Documentation/rtc.txt
downloadlinux-linaro-highbank-upstream/3.9.0.tar.gz
Imported Upstream version 3.9.0HEADupstream/3.9.0upstreammaster
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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 system clock RTC may expose itself
+ using a procfs interface. If there is no RTC for the system clock,
+ rtc0 is used by default. 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;
+}