283 lines
8.1 KiB
Plaintext
283 lines
8.1 KiB
Plaintext
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Real Time Clock Driver for Linux
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================================
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All PCs (even Alpha machines) have a Real Time Clock built into them.
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Usually they are built into the chipset of the computer, but some may
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actually have a Motorola MC146818 (or clone) on the board. This is the
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clock that keeps the date and time while your computer is turned off.
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However it can also be used to generate signals from a slow 2Hz to a
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relatively fast 8192Hz, in increments of powers of two. These signals
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are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
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for...) It can also function as a 24hr alarm, raising IRQ 8 when the
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alarm goes off. The alarm can also be programmed to only check any
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subset of the three programmable values, meaning that it could be set to
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ring on the 30th second of the 30th minute of every hour, for example.
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The clock can also be set to generate an interrupt upon every clock
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update, thus generating a 1Hz signal.
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The interrupts are reported via /dev/rtc (major 10, minor 135, read only
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character device) in the form of an unsigned long. The low byte contains
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the type of interrupt (update-done, alarm-rang, or periodic) that was
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raised, and the remaining bytes contain the number of interrupts since
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the last read. Status information is reported through the pseudo-file
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/proc/driver/rtc if the /proc filesystem was enabled. The driver has
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built in locking so that only one process is allowed to have the /dev/rtc
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interface open at a time.
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A user process can monitor these interrupts by doing a read(2) or a
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select(2) on /dev/rtc -- either will block/stop the user process until
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the next interrupt is received. This is useful for things like
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reasonably high frequency data acquisition where one doesn't want to
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burn up 100% CPU by polling gettimeofday etc. etc.
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At high frequencies, or under high loads, the user process should check
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the number of interrupts received since the last read to determine if
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there has been any interrupt "pileup" so to speak. Just for reference, a
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typical 486-33 running a tight read loop on /dev/rtc will start to suffer
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occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
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frequencies above 1024Hz. So you really should check the high bytes
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of the value you read, especially at frequencies above that of the
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normal timer interrupt, which is 100Hz.
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Programming and/or enabling interrupt frequencies greater than 64Hz is
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only allowed by root. This is perhaps a bit conservative, but we don't want
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an evil user generating lots of IRQs on a slow 386sx-16, where it might have
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a negative impact on performance. Note that the interrupt handler is only
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a few lines of code to minimize any possibility of this effect.
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Also, if the kernel time is synchronized with an external source, the
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kernel will write the time back to the CMOS clock every 11 minutes. In
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the process of doing this, the kernel briefly turns off RTC periodic
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interrupts, so be aware of this if you are doing serious work. If you
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don't synchronize the kernel time with an external source (via ntp or
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whatever) then the kernel will keep its hands off the RTC, allowing you
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exclusive access to the device for your applications.
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The alarm and/or interrupt frequency are programmed into the RTC via
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various ioctl(2) calls as listed in ./include/linux/rtc.h
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Rather than write 50 pages describing the ioctl() and so on, it is
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perhaps more useful to include a small test program that demonstrates
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how to use them, and demonstrates the features of the driver. This is
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probably a lot more useful to people interested in writing applications
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that will be using this driver.
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Paul Gortmaker
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-------------------- 8< ---------------- 8< -----------------------------
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/*
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* Real Time Clock Driver Test/Example Program
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*
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* Compile with:
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* gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
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*
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* Copyright (C) 1996, Paul Gortmaker.
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*
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* Released under the GNU General Public License, version 2,
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* included herein by reference.
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*
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*/
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#include <stdio.h>
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#include <linux/rtc.h>
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#include <sys/ioctl.h>
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#include <sys/time.h>
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#include <sys/types.h>
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#include <fcntl.h>
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#include <unistd.h>
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#include <errno.h>
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int main(void) {
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int i, fd, retval, irqcount = 0;
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unsigned long tmp, data;
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struct rtc_time rtc_tm;
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fd = open ("/dev/rtc", O_RDONLY);
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if (fd == -1) {
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perror("/dev/rtc");
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exit(errno);
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}
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fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
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/* Turn on update interrupts (one per second) */
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retval = ioctl(fd, RTC_UIE_ON, 0);
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if (retval == -1) {
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perror("ioctl");
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exit(errno);
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}
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fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading /dev/rtc:");
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fflush(stderr);
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for (i=1; i<6; i++) {
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/* This read will block */
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retval = read(fd, &data, sizeof(unsigned long));
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if (retval == -1) {
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perror("read");
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exit(errno);
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}
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fprintf(stderr, " %d",i);
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fflush(stderr);
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irqcount++;
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}
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fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
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fflush(stderr);
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for (i=1; i<6; i++) {
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struct timeval tv = {5, 0}; /* 5 second timeout on select */
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fd_set readfds;
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FD_ZERO(&readfds);
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FD_SET(fd, &readfds);
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/* The select will wait until an RTC interrupt happens. */
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retval = select(fd+1, &readfds, NULL, NULL, &tv);
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if (retval == -1) {
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perror("select");
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exit(errno);
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}
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/* This read won't block unlike the select-less case above. */
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retval = read(fd, &data, sizeof(unsigned long));
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if (retval == -1) {
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perror("read");
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exit(errno);
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}
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fprintf(stderr, " %d",i);
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fflush(stderr);
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irqcount++;
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}
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/* Turn off update interrupts */
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retval = ioctl(fd, RTC_UIE_OFF, 0);
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if (retval == -1) {
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perror("ioctl");
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exit(errno);
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}
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/* Read the RTC time/date */
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retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
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if (retval == -1) {
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perror("ioctl");
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exit(errno);
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}
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fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
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rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
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rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
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/* Set the alarm to 5 sec in the future, and check for rollover */
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rtc_tm.tm_sec += 5;
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if (rtc_tm.tm_sec >= 60) {
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rtc_tm.tm_sec %= 60;
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rtc_tm.tm_min++;
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}
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if (rtc_tm.tm_min == 60) {
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rtc_tm.tm_min = 0;
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rtc_tm.tm_hour++;
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}
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if (rtc_tm.tm_hour == 24)
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rtc_tm.tm_hour = 0;
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retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
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if (retval == -1) {
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perror("ioctl");
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exit(errno);
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}
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/* Read the current alarm settings */
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retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
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if (retval == -1) {
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perror("ioctl");
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exit(errno);
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}
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fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
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rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
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/* Enable alarm interrupts */
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retval = ioctl(fd, RTC_AIE_ON, 0);
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if (retval == -1) {
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perror("ioctl");
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exit(errno);
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}
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fprintf(stderr, "Waiting 5 seconds for alarm...");
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fflush(stderr);
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/* This blocks until the alarm ring causes an interrupt */
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retval = read(fd, &data, sizeof(unsigned long));
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if (retval == -1) {
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perror("read");
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exit(errno);
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}
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irqcount++;
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fprintf(stderr, " okay. Alarm rang.\n");
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/* Disable alarm interrupts */
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retval = ioctl(fd, RTC_AIE_OFF, 0);
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if (retval == -1) {
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perror("ioctl");
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exit(errno);
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}
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/* Read periodic IRQ rate */
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retval = ioctl(fd, RTC_IRQP_READ, &tmp);
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if (retval == -1) {
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perror("ioctl");
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exit(errno);
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}
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fprintf(stderr, "\nPeriodic IRQ rate was %ldHz.\n", tmp);
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fprintf(stderr, "Counting 20 interrupts at:");
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fflush(stderr);
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/* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
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for (tmp=2; tmp<=64; tmp*=2) {
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retval = ioctl(fd, RTC_IRQP_SET, tmp);
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if (retval == -1) {
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perror("ioctl");
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exit(errno);
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}
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fprintf(stderr, "\n%ldHz:\t", tmp);
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fflush(stderr);
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/* Enable periodic interrupts */
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retval = ioctl(fd, RTC_PIE_ON, 0);
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if (retval == -1) {
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perror("ioctl");
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exit(errno);
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}
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for (i=1; i<21; i++) {
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/* This blocks */
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retval = read(fd, &data, sizeof(unsigned long));
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if (retval == -1) {
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perror("read");
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exit(errno);
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}
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fprintf(stderr, " %d",i);
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fflush(stderr);
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irqcount++;
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}
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/* Disable periodic interrupts */
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retval = ioctl(fd, RTC_PIE_OFF, 0);
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if (retval == -1) {
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perror("ioctl");
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exit(errno);
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}
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}
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fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
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fprintf(stderr, "\nTyping \"cat /proc/interrupts\" will show %d more events on IRQ 8.\n\n",
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irqcount);
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close(fd);
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return 0;
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} /* end main */
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