[PATCH] Documentation/rtc.txt updates (for rtc class)

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>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 304 insertions, 159 deletions

diff --git a/Documentation/rtc.txt b/Documentation/rtc.txt
index 2a58f985795a..7cf1ec5bcdd3 100644
--- a/Documentation/rtc.txt
+++ b/Documentation/rtc.txt
@@ -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 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.
+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;
+}