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0001 
0002         Real Time Clock (RTC) Drivers for Linux
0003         =======================================
0004 
0005 When Linux developers talk about a "Real Time Clock", they usually mean
0006 something that tracks wall clock time and is battery backed so that it
0007 works even with system power off.  Such clocks will normally not track
0008 the local time zone or daylight savings time -- unless they dual boot
0009 with MS-Windows -- but will instead be set to Coordinated Universal Time
0010 (UTC, formerly "Greenwich Mean Time").
0011 
0012 The newest non-PC hardware tends to just count seconds, like the time(2)
0013 system call reports, but RTCs also very commonly represent time using
0014 the Gregorian calendar and 24 hour time, as reported by gmtime(3).
0015 
0016 Linux has two largely-compatible userspace RTC API families you may
0017 need to know about:
0018 
0019     *   /dev/rtc ... is the RTC provided by PC compatible systems,
0020         so it's not very portable to non-x86 systems.
0021 
0022     *   /dev/rtc0, /dev/rtc1 ... are part of a framework that's
0023         supported by a wide variety of RTC chips on all systems.
0024 
0025 Programmers need to understand that the PC/AT functionality is not
0026 always available, and some systems can do much more.  That is, the
0027 RTCs use the same API to make requests in both RTC frameworks (using
0028 different filenames of course), but the hardware may not offer the
0029 same functionality.  For example, not every RTC is hooked up to an
0030 IRQ, so they can't all issue alarms; and where standard PC RTCs can
0031 only issue an alarm up to 24 hours in the future, other hardware may
0032 be able to schedule one any time in the upcoming century.
0033 
0034 
0035         Old PC/AT-Compatible driver:  /dev/rtc
0036         --------------------------------------
0037 
0038 All PCs (even Alpha machines) have a Real Time Clock built into them.
0039 Usually they are built into the chipset of the computer, but some may
0040 actually have a Motorola MC146818 (or clone) on the board. This is the
0041 clock that keeps the date and time while your computer is turned off.
0042 
0043 ACPI has standardized that MC146818 functionality, and extended it in
0044 a few ways (enabling longer alarm periods, and wake-from-hibernate).
0045 That functionality is NOT exposed in the old driver.
0046 
0047 However it can also be used to generate signals from a slow 2Hz to a
0048 relatively fast 8192Hz, in increments of powers of two. These signals
0049 are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
0050 for...) It can also function as a 24hr alarm, raising IRQ 8 when the
0051 alarm goes off. The alarm can also be programmed to only check any
0052 subset of the three programmable values, meaning that it could be set to
0053 ring on the 30th second of the 30th minute of every hour, for example.
0054 The clock can also be set to generate an interrupt upon every clock
0055 update, thus generating a 1Hz signal.
0056 
0057 The interrupts are reported via /dev/rtc (major 10, minor 135, read only
0058 character device) in the form of an unsigned long. The low byte contains
0059 the type of interrupt (update-done, alarm-rang, or periodic) that was
0060 raised, and the remaining bytes contain the number of interrupts since
0061 the last read.  Status information is reported through the pseudo-file
0062 /proc/driver/rtc if the /proc filesystem was enabled.  The driver has
0063 built in locking so that only one process is allowed to have the /dev/rtc
0064 interface open at a time.
0065 
0066 A user process can monitor these interrupts by doing a read(2) or a
0067 select(2) on /dev/rtc -- either will block/stop the user process until
0068 the next interrupt is received. This is useful for things like
0069 reasonably high frequency data acquisition where one doesn't want to
0070 burn up 100% CPU by polling gettimeofday etc. etc.
0071 
0072 At high frequencies, or under high loads, the user process should check
0073 the number of interrupts received since the last read to determine if
0074 there has been any interrupt "pileup" so to speak. Just for reference, a
0075 typical 486-33 running a tight read loop on /dev/rtc will start to suffer
0076 occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
0077 frequencies above 1024Hz. So you really should check the high bytes
0078 of the value you read, especially at frequencies above that of the
0079 normal timer interrupt, which is 100Hz.
0080 
0081 Programming and/or enabling interrupt frequencies greater than 64Hz is
0082 only allowed by root. This is perhaps a bit conservative, but we don't want
0083 an evil user generating lots of IRQs on a slow 386sx-16, where it might have
0084 a negative impact on performance. This 64Hz limit can be changed by writing
0085 a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
0086 interrupt handler is only a few lines of code to minimize any possibility
0087 of this effect.
0088 
0089 Also, if the kernel time is synchronized with an external source, the 
0090 kernel will write the time back to the CMOS clock every 11 minutes. In 
0091 the process of doing this, the kernel briefly turns off RTC periodic 
0092 interrupts, so be aware of this if you are doing serious work. If you
0093 don't synchronize the kernel time with an external source (via ntp or
0094 whatever) then the kernel will keep its hands off the RTC, allowing you
0095 exclusive access to the device for your applications.
0096 
0097 The alarm and/or interrupt frequency are programmed into the RTC via
0098 various ioctl(2) calls as listed in ./include/linux/rtc.h
0099 Rather than write 50 pages describing the ioctl() and so on, it is
0100 perhaps more useful to include a small test program that demonstrates
0101 how to use them, and demonstrates the features of the driver. This is
0102 probably a lot more useful to people interested in writing applications
0103 that will be using this driver.  See the code at the end of this document.
0104 
0105 (The original /dev/rtc driver was written by Paul Gortmaker.)
0106 
0107 
0108         New portable "RTC Class" drivers:  /dev/rtcN
0109         --------------------------------------------
0110 
0111 Because Linux supports many non-ACPI and non-PC platforms, some of which
0112 have more than one RTC style clock, it needed a more portable solution
0113 than expecting a single battery-backed MC146818 clone on every system.
0114 Accordingly, a new "RTC Class" framework has been defined.  It offers
0115 three different userspace interfaces:
0116 
0117     *   /dev/rtcN ... much the same as the older /dev/rtc interface
0118 
0119     *   /sys/class/rtc/rtcN ... sysfs attributes support readonly
0120         access to some RTC attributes.
0121 
0122     *   /proc/driver/rtc ... the system clock RTC may expose itself
0123         using a procfs interface. If there is no RTC for the system clock,
0124         rtc0 is used by default. More information is (currently) shown
0125         here than through sysfs.
0126 
0127 The RTC Class framework supports a wide variety of RTCs, ranging from those
0128 integrated into embeddable system-on-chip (SOC) processors to discrete chips
0129 using I2C, SPI, or some other bus to communicate with the host CPU.  There's
0130 even support for PC-style RTCs ... including the features exposed on newer PCs
0131 through ACPI.
0132 
0133 The new framework also removes the "one RTC per system" restriction.  For
0134 example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
0135 a high functionality RTC is integrated into the SOC.  That system might read
0136 the system clock from the discrete RTC, but use the integrated one for all
0137 other tasks, because of its greater functionality.
0138 
0139 SYSFS INTERFACE
0140 ---------------
0141 
0142 The sysfs interface under /sys/class/rtc/rtcN provides access to various
0143 rtc attributes without requiring the use of ioctls. All dates and times
0144 are in the RTC's timezone, rather than in system time.
0145 
0146 date:            RTC-provided date
0147 hctosys:         1 if the RTC provided the system time at boot via the
0148                  CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
0149 max_user_freq:   The maximum interrupt rate an unprivileged user may request
0150                  from this RTC.
0151 name:            The name of the RTC corresponding to this sysfs directory
0152 since_epoch:     The number of seconds since the epoch according to the RTC
0153 time:            RTC-provided time
0154 wakealarm:       The time at which the clock will generate a system wakeup
0155                  event. This is a one shot wakeup event, so must be reset
0156                  after wake if a daily wakeup is required. Format is seconds since
0157                  the epoch by default, or if there's a leading +, seconds in the
0158                  future, or if there is a leading +=, seconds ahead of the current
0159                  alarm.
0160 offset:          The amount which the rtc clock has been adjusted in firmware.
0161                  Visible only if the driver supports clock offset adjustment.
0162                  The unit is parts per billion, i.e. The number of clock ticks
0163                  which are added to or removed from the rtc's base clock per
0164                  billion ticks. A positive value makes a day pass more slowly,
0165                  longer, and a negative value makes a day pass more quickly.
0166 
0167 IOCTL INTERFACE
0168 ---------------
0169 
0170 The ioctl() calls supported by /dev/rtc are also supported by the RTC class
0171 framework.  However, because the chips and systems are not standardized,
0172 some PC/AT functionality might not be provided.  And in the same way, some
0173 newer features -- including those enabled by ACPI -- are exposed by the
0174 RTC class framework, but can't be supported by the older driver.
0175 
0176     *   RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading
0177         time, returning the result as a Gregorian calendar date and 24 hour
0178         wall clock time.  To be most useful, this time may also be updated.
0179 
0180     *   RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
0181         is connected to an IRQ line, it can often issue an alarm IRQ up to
0182         24 hours in the future.  (Use RTC_WKALM_* by preference.)
0183 
0184     *   RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
0185         the next 24 hours use a slightly more powerful API, which supports
0186         setting the longer alarm time and enabling its IRQ using a single
0187         request (using the same model as EFI firmware).
0188 
0189     *   RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework
0190         will emulate this mechanism.
0191 
0192     *   RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
0193         are emulated via a kernel hrtimer.
0194 
0195 In many cases, the RTC alarm can be a system wake event, used to force
0196 Linux out of a low power sleep state (or hibernation) back to a fully
0197 operational state.  For example, a system could enter a deep power saving
0198 state until it's time to execute some scheduled tasks.
0199 
0200 Note that many of these ioctls are handled by the common rtc-dev interface.
0201 Some common examples:
0202 
0203     *   RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
0204         called with appropriate values.
0205 
0206     *   RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets
0207         the alarm rtc_timer. May call the set_alarm driver function.
0208 
0209     *   RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
0210 
0211     *   RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
0212 
0213 If all else fails, check out the tools/testing/selftests/timers/rtctest.c test!