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+
+ 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.
+
+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!