=======================================
  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
  for...) It can also function as a 24hr alarm, raising IRQ 8 when the
  alarm goes off. The alarm can also be programmed to only check any
  subset of the three programmable values, meaning that it could be set to
  ring on the 30th second of the 30th minute of every hour, for example.
  The clock can also be set to generate an interrupt upon every clock
  update, thus generating a 1Hz signal.
  
  The interrupts are reported via /dev/rtc (major 10, minor 135, read only
  character device) in the form of an unsigned long. The low byte contains
  the type of interrupt (update-done, alarm-rang, or periodic) that was
  raised, and the remaining bytes contain the number of interrupts since
  the last read.  Status information is reported through the pseudo-file
  /proc/driver/rtc if the /proc filesystem was enabled.  The driver has
  built in locking so that only one process is allowed to have the /dev/rtc
  interface open at a time.
  
  A user process can monitor these interrupts by doing a read(2) or a
  select(2) on /dev/rtc -- either will block/stop the user process until
  the next interrupt is received. This is useful for things like
  reasonably high frequency data acquisition where one doesn't want to
  burn up 100% CPU by polling gettimeofday etc. etc.
  
  At high frequencies, or under high loads, the user process should check
  the number of interrupts received since the last read to determine if
  there has been any interrupt "pileup" so to speak. Just for reference, a
  typical 486-33 running a tight read loop on /dev/rtc will start to suffer
  occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
  frequencies above 1024Hz. So you really should check the high bytes
  of the value you read, especially at frequencies above that of the
  normal timer interrupt, which is 100Hz.
  
  Programming and/or enabling interrupt frequencies greater than 64Hz is
  only allowed by root. This is perhaps a bit conservative, but we don't want
  an evil user generating lots of IRQs on a slow 386sx-16, where it might have

  a negative impact on performance. This 64Hz limit can be changed by writing
  a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
  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 
  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
  exclusive access to the device for your applications.
  
  The alarm and/or interrupt frequency are programmed into the RTC via
  various ioctl(2) calls as listed in ./include/linux/rtc.h
  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.  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 system clock RTC may expose itself
  	using a procfs interface. If there is no RTC for the system clock,
  	rtc0 is used by default. 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.

  SYSFS interface

  ---------------
  
  The sysfs interface under /sys/class/rtc/rtcN provides access to various
  rtc attributes without requiring the use of ioctls. All dates and times
  are in the RTC's timezone, rather than in system time.

  ================ ==============================================================
  date  	   	 RTC-provided date
  hctosys   	 1 if the RTC provided the system time at boot via the

  		 CONFIG_RTC_HCTOSYS kernel option, 0 otherwise

  max_user_freq	 The maximum interrupt rate an unprivileged user may request

  		 from this RTC.

  name		 The name of the RTC corresponding to this sysfs directory
  since_epoch	 The number of seconds since the epoch according to the RTC
  time		 RTC-provided time
  wakealarm	 The time at which the clock will generate a system wakeup

  		 event. This is a one shot wakeup event, so must be reset

  		 after wake if a daily wakeup is required. Format is seconds
  		 since the epoch by default, or if there's a leading +, seconds
  		 in the future, or if there is a leading +=, seconds ahead of
  		 the current alarm.
  offset		 The amount which the rtc clock has been adjusted in firmware.

  		 Visible only if the driver supports clock offset adjustment.
  		 The unit is parts per billion, i.e. The number of clock ticks
  		 which are added to or removed from the rtc's base clock per
  		 billion ticks. A positive value makes a day pass more slowly,
  		 longer, and a negative value makes a day pass more quickly.

  */nvmem		 The non volatile storage exported as a raw file, as described
  		 in Documentation/nvmem/nvmem.txt

  ================ ==============================================================

  IOCTL interface

  ---------------

  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.  (Use RTC_WKALM_* by preference.)

      *	RTC_WKALM_SET, RTC_WKALM_RD ... 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, the RTC framework
  	will emulate this mechanism.

      *	RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
  	are emulated via a kernel hrtimer.

  
  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.

  Note that many of these ioctls are handled by the common rtc-dev interface.
  Some common examples:

  
      *	RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
  	called with appropriate values.

      *	RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets
  	the alarm rtc_timer. May call the set_alarm driver function.

      *	RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.

      *	RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.

  If all else fails, check out the tools/testing/selftests/timers/rtctest.c test!