[patch 2.6.19-rc6] Documentation/rtc.txt updates (for rtc class)

[David Brownell <david-b@pacbell.net>]

This updates the RTC documentation to summarize the two APIs now available:
the old PC/AT one, and the new RTC class drivers.  It also updates the
included "rtctest.c" file to better meet Linux style guidelines, and to
work with the new RTC drivers.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

Index: g26/Documentation/rtc.txt
===================================================================
--- g26.orig/Documentation/rtc.txt	2006-11-16 18:16:17.000000000 -0800
+++ g26/Documentation/rtc.txt	2006-11-16 23:07:35.000000000 -0800
@@ -1,12 +1,49 @@
 
-	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
@@ -63,223 +100,331 @@ Rather than write 50 pages describing th
 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;
+/*
+ * 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";
 
-fd = open ("/dev/rtc", O_RDONLY);
 
-if (fd ==  -1) {
-	perror("/dev/rtc");
-	exit(errno);
-}
+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;
+	}
 
-fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
+	fd = open(rtc, O_RDONLY);
 
-/* Turn on update interrupts (one per second) */
-retval = ioctl(fd, RTC_UIE_ON, 0);
-if (retval == -1) {
-	perror("ioctl");
-	exit(errno);
-}
+	if (fd ==  -1) {
+		perror(rtc);
+		exit(errno);
+	}
 
-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));
+	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) {
-		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;
+
+		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++;
+	}
 
-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);
+	/* 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, "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);
-}
+	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;
 
-/* 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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