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

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

[David Brownell]

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;
+}

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

[Oleg Verych]

Hallo, David.

On 2006-11-17, David Brownell wrote:
> 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.

How about documenting  things you've told me back  in September?  I.e.
i386/amd64 have motorola RTC (and  clones) based system clock setup in
boot  sequence "arch/x86_64/kernel/time.c".  Also,  how it  interferes
with time_hpet.c? There seem to  be confusion what RTC timer and
RTC clock is: <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=386226>

Also  util-linux  package  maintaining  was  under  question  in  lkml
recently.  I  think, i should update  rc script i  rewrote for hwclock
with  more info.  I agree  with you,  that on  PC hwclock  is useless,
unless dual-boot users are not in consern.

I'll try to review and test patches. I wonder, why they were not posted
before, but just before 2.6.19?

Thanks!
____

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

[Oleg Verych]

On 2006-11-17, David Brownell wrote:
> Index: g26/Documentation/rtc.txt

Just another emacs visiting...
Will try to build and reboot to test stuff tomorrow.

Index: 2.6_current/Documentation/rtc.txt
===================================================================
--- 2.6_current.orig/Documentation/rtc.txt	2006-11-17 13:33:10.979411250 +0100
+++ 2.6_current/Documentation/rtc.txt	2006-11-17 13:33:40.981286250 +0100
@@ -18,7 +18,7 @@
 
     *	/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.
 
@@ -86,9 +86,9 @@
 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 
+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

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

[Alessandro Zummo]

On Thu, 16 Nov 2006 23:09:31 -0800
David Brownell <david-b@pacbell.net> wrote:

> 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>

 Thanks for your patches, David!

 Acked-by: Alessandro Zummo <a.zummo@towertech.it>

-- 

 Best regards,

 Alessandro Zummo,
  Tower Technologies - Turin, Italy

  http://www.towertech.it