697e5a47aa
Many RTCs have an on board non volatile storage. It can be battery backed RAM or an EEPROM. Use the nvmem subsystem to export it to both userspace and in-kernel consumers. This stays compatible with the previous (non documented) ABI that was using /sys/class/rtc/rtcx/device/nvram to export that memory. But will warn about the deprecation. Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
217 lines
11 KiB
Text
217 lines
11 KiB
Text
=======================================
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Real Time Clock (RTC) Drivers for Linux
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=======================================
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When Linux developers talk about a "Real Time Clock", they usually mean
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something that tracks wall clock time and is battery backed so that it
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works even with system power off. Such clocks will normally not track
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the local time zone or daylight savings time -- unless they dual boot
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with MS-Windows -- but will instead be set to Coordinated Universal Time
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(UTC, formerly "Greenwich Mean Time").
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The newest non-PC hardware tends to just count seconds, like the time(2)
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system call reports, but RTCs also very commonly represent time using
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the Gregorian calendar and 24 hour time, as reported by gmtime(3).
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Linux has two largely-compatible userspace RTC API families you may
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need to know about:
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* /dev/rtc ... is the RTC provided by PC compatible systems,
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so it's not very portable to non-x86 systems.
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* /dev/rtc0, /dev/rtc1 ... are part of a framework that's
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supported by a wide variety of RTC chips on all systems.
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Programmers need to understand that the PC/AT functionality is not
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always available, and some systems can do much more. That is, the
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RTCs use the same API to make requests in both RTC frameworks (using
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different filenames of course), but the hardware may not offer the
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same functionality. For example, not every RTC is hooked up to an
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IRQ, so they can't all issue alarms; and where standard PC RTCs can
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only issue an alarm up to 24 hours in the future, other hardware may
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be able to schedule one any time in the upcoming century.
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Old PC/AT-Compatible driver: /dev/rtc
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--------------------------------------
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All PCs (even Alpha machines) have a Real Time Clock built into them.
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Usually they are built into the chipset of the computer, but some may
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actually have a Motorola MC146818 (or clone) on the board. This is the
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clock that keeps the date and time while your computer is turned off.
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ACPI has standardized that MC146818 functionality, and extended it in
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a few ways (enabling longer alarm periods, and wake-from-hibernate).
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That functionality is NOT exposed in the old driver.
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However it can also be used to generate signals from a slow 2Hz to a
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relatively fast 8192Hz, in increments of powers of two. These signals
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are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
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for...) It can also function as a 24hr alarm, raising IRQ 8 when the
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alarm goes off. The alarm can also be programmed to only check any
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subset of the three programmable values, meaning that it could be set to
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ring on the 30th second of the 30th minute of every hour, for example.
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The clock can also be set to generate an interrupt upon every clock
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update, thus generating a 1Hz signal.
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The interrupts are reported via /dev/rtc (major 10, minor 135, read only
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character device) in the form of an unsigned long. The low byte contains
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the type of interrupt (update-done, alarm-rang, or periodic) that was
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raised, and the remaining bytes contain the number of interrupts since
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the last read. Status information is reported through the pseudo-file
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/proc/driver/rtc if the /proc filesystem was enabled. The driver has
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built in locking so that only one process is allowed to have the /dev/rtc
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interface open at a time.
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A user process can monitor these interrupts by doing a read(2) or a
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select(2) on /dev/rtc -- either will block/stop the user process until
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the next interrupt is received. This is useful for things like
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reasonably high frequency data acquisition where one doesn't want to
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burn up 100% CPU by polling gettimeofday etc. etc.
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At high frequencies, or under high loads, the user process should check
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the number of interrupts received since the last read to determine if
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there has been any interrupt "pileup" so to speak. Just for reference, a
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typical 486-33 running a tight read loop on /dev/rtc will start to suffer
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occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
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frequencies above 1024Hz. So you really should check the high bytes
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of the value you read, especially at frequencies above that of the
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normal timer interrupt, which is 100Hz.
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Programming and/or enabling interrupt frequencies greater than 64Hz is
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only allowed by root. This is perhaps a bit conservative, but we don't want
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an evil user generating lots of IRQs on a slow 386sx-16, where it might have
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a negative impact on performance. This 64Hz limit can be changed by writing
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a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
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interrupt handler is only a few lines of code to minimize any possibility
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of this effect.
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Also, if the kernel time is synchronized with an external source, the
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kernel will write the time back to the CMOS clock every 11 minutes. In
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the process of doing this, the kernel briefly turns off RTC periodic
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interrupts, so be aware of this if you are doing serious work. If you
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don't synchronize the kernel time with an external source (via ntp or
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whatever) then the kernel will keep its hands off the RTC, allowing you
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exclusive access to the device for your applications.
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The alarm and/or interrupt frequency are programmed into the RTC via
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various ioctl(2) calls as listed in ./include/linux/rtc.h
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Rather than write 50 pages describing the ioctl() and so on, it is
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perhaps more useful to include a small test program that demonstrates
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how to use them, and demonstrates the features of the driver. This is
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probably a lot more useful to people interested in writing applications
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that will be using this driver. See the code at the end of this document.
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(The original /dev/rtc driver was written by Paul Gortmaker.)
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New portable "RTC Class" drivers: /dev/rtcN
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--------------------------------------------
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Because Linux supports many non-ACPI and non-PC platforms, some of which
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have more than one RTC style clock, it needed a more portable solution
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than expecting a single battery-backed MC146818 clone on every system.
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Accordingly, a new "RTC Class" framework has been defined. It offers
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three different userspace interfaces:
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* /dev/rtcN ... much the same as the older /dev/rtc interface
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* /sys/class/rtc/rtcN ... sysfs attributes support readonly
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access to some RTC attributes.
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* /proc/driver/rtc ... the system clock RTC may expose itself
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using a procfs interface. If there is no RTC for the system clock,
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rtc0 is used by default. More information is (currently) shown
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here than through sysfs.
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The RTC Class framework supports a wide variety of RTCs, ranging from those
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integrated into embeddable system-on-chip (SOC) processors to discrete chips
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using I2C, SPI, or some other bus to communicate with the host CPU. There's
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even support for PC-style RTCs ... including the features exposed on newer PCs
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through ACPI.
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The new framework also removes the "one RTC per system" restriction. For
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example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
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a high functionality RTC is integrated into the SOC. That system might read
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the system clock from the discrete RTC, but use the integrated one for all
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other tasks, because of its greater functionality.
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SYSFS interface
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---------------
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The sysfs interface under /sys/class/rtc/rtcN provides access to various
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rtc attributes without requiring the use of ioctls. All dates and times
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are in the RTC's timezone, rather than in system time.
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================ ==============================================================
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date RTC-provided date
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hctosys 1 if the RTC provided the system time at boot via the
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CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
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max_user_freq The maximum interrupt rate an unprivileged user may request
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from this RTC.
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name The name of the RTC corresponding to this sysfs directory
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since_epoch The number of seconds since the epoch according to the RTC
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time RTC-provided time
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wakealarm The time at which the clock will generate a system wakeup
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event. This is a one shot wakeup event, so must be reset
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after wake if a daily wakeup is required. Format is seconds
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since the epoch by default, or if there's a leading +, seconds
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in the future, or if there is a leading +=, seconds ahead of
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the current alarm.
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offset The amount which the rtc clock has been adjusted in firmware.
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Visible only if the driver supports clock offset adjustment.
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The unit is parts per billion, i.e. The number of clock ticks
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which are added to or removed from the rtc's base clock per
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billion ticks. A positive value makes a day pass more slowly,
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longer, and a negative value makes a day pass more quickly.
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*/nvmem The non volatile storage exported as a raw file, as described
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in Documentation/nvmem/nvmem.txt
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================ ==============================================================
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IOCTL interface
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---------------
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The ioctl() calls supported by /dev/rtc are also supported by the RTC class
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framework. However, because the chips and systems are not standardized,
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some PC/AT functionality might not be provided. And in the same way, some
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newer features -- including those enabled by ACPI -- are exposed by the
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RTC class framework, but can't be supported by the older driver.
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* RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading
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time, returning the result as a Gregorian calendar date and 24 hour
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wall clock time. To be most useful, this time may also be updated.
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* RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
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is connected to an IRQ line, it can often issue an alarm IRQ up to
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24 hours in the future. (Use RTC_WKALM_* by preference.)
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* RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
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the next 24 hours use a slightly more powerful API, which supports
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setting the longer alarm time and enabling its IRQ using a single
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request (using the same model as EFI firmware).
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* RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework
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will emulate this mechanism.
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* RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
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are emulated via a kernel hrtimer.
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In many cases, the RTC alarm can be a system wake event, used to force
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Linux out of a low power sleep state (or hibernation) back to a fully
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operational state. For example, a system could enter a deep power saving
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state until it's time to execute some scheduled tasks.
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Note that many of these ioctls are handled by the common rtc-dev interface.
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Some common examples:
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* RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
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called with appropriate values.
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* RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets
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the alarm rtc_timer. May call the set_alarm driver function.
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* RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
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* RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
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If all else fails, check out the tools/testing/selftests/timers/rtctest.c test!
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