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  2         Real Time Clock (RTC) Drivers for Linux
  3         =======================================
  5 When Linux developers talk about a "Real Time Clock", they usually mean
  6 something that tracks wall clock time and is battery backed so that it
  7 works even with system power off.  Such clocks will normally not track
  8 the local time zone or daylight savings time -- unless they dual boot
  9 with MS-Windows -- but will instead be set to Coordinated Universal Time
 10 (UTC, formerly "Greenwich Mean Time").
 12 The newest non-PC hardware tends to just count seconds, like the time(2)
 13 system call reports, but RTCs also very commonly represent time using
 14 the Gregorian calendar and 24 hour time, as reported by gmtime(3).
 16 Linux has two largely-compatible userspace RTC API families you may
 17 need to know about:
 19     *   /dev/rtc ... is the RTC provided by PC compatible systems,
 20         so it's not very portable to non-x86 systems.
 22     *   /dev/rtc0, /dev/rtc1 ... are part of a framework that's
 23         supported by a wide variety of RTC chips on all systems.
 25 Programmers need to understand that the PC/AT functionality is not
 26 always available, and some systems can do much more.  That is, the
 27 RTCs use the same API to make requests in both RTC frameworks (using
 28 different filenames of course), but the hardware may not offer the
 29 same functionality.  For example, not every RTC is hooked up to an
 30 IRQ, so they can't all issue alarms; and where standard PC RTCs can
 31 only issue an alarm up to 24 hours in the future, other hardware may
 32 be able to schedule one any time in the upcoming century.
 35         Old PC/AT-Compatible driver:  /dev/rtc
 36         --------------------------------------
 38 All PCs (even Alpha machines) have a Real Time Clock built into them.
 39 Usually they are built into the chipset of the computer, but some may
 40 actually have a Motorola MC146818 (or clone) on the board. This is the
 41 clock that keeps the date and time while your computer is turned off.
 43 ACPI has standardized that MC146818 functionality, and extended it in
 44 a few ways (enabling longer alarm periods, and wake-from-hibernate).
 45 That functionality is NOT exposed in the old driver.
 47 However it can also be used to generate signals from a slow 2Hz to a
 48 relatively fast 8192Hz, in increments of powers of two. These signals
 49 are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
 50 for...) It can also function as a 24hr alarm, raising IRQ 8 when the
 51 alarm goes off. The alarm can also be programmed to only check any
 52 subset of the three programmable values, meaning that it could be set to
 53 ring on the 30th second of the 30th minute of every hour, for example.
 54 The clock can also be set to generate an interrupt upon every clock
 55 update, thus generating a 1Hz signal.
 57 The interrupts are reported via /dev/rtc (major 10, minor 135, read only
 58 character device) in the form of an unsigned long. The low byte contains
 59 the type of interrupt (update-done, alarm-rang, or periodic) that was
 60 raised, and the remaining bytes contain the number of interrupts since
 61 the last read.  Status information is reported through the pseudo-file
 62 /proc/driver/rtc if the /proc filesystem was enabled.  The driver has
 63 built in locking so that only one process is allowed to have the /dev/rtc
 64 interface open at a time.
 66 A user process can monitor these interrupts by doing a read(2) or a
 67 select(2) on /dev/rtc -- either will block/stop the user process until
 68 the next interrupt is received. This is useful for things like
 69 reasonably high frequency data acquisition where one doesn't want to
 70 burn up 100% CPU by polling gettimeofday etc. etc.
 72 At high frequencies, or under high loads, the user process should check
 73 the number of interrupts received since the last read to determine if
 74 there has been any interrupt "pileup" so to speak. Just for reference, a
 75 typical 486-33 running a tight read loop on /dev/rtc will start to suffer
 76 occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
 77 frequencies above 1024Hz. So you really should check the high bytes
 78 of the value you read, especially at frequencies above that of the
 79 normal timer interrupt, which is 100Hz.
 81 Programming and/or enabling interrupt frequencies greater than 64Hz is
 82 only allowed by root. This is perhaps a bit conservative, but we don't want
 83 an evil user generating lots of IRQs on a slow 386sx-16, where it might have
 84 a negative impact on performance. This 64Hz limit can be changed by writing
 85 a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
 86 interrupt handler is only a few lines of code to minimize any possibility
 87 of this effect.
 89 Also, if the kernel time is synchronized with an external source, the 
 90 kernel will write the time back to the CMOS clock every 11 minutes. In 
 91 the process of doing this, the kernel briefly turns off RTC periodic 
 92 interrupts, so be aware of this if you are doing serious work. If you
 93 don't synchronize the kernel time with an external source (via ntp or
 94 whatever) then the kernel will keep its hands off the RTC, allowing you
 95 exclusive access to the device for your applications.
 97 The alarm and/or interrupt frequency are programmed into the RTC via
 98 various ioctl(2) calls as listed in ./include/linux/rtc.h
 99 Rather than write 50 pages describing the ioctl() and so on, it is
100 perhaps more useful to include a small test program that demonstrates
101 how to use them, and demonstrates the features of the driver. This is
102 probably a lot more useful to people interested in writing applications
103 that will be using this driver.  See the code at the end of this document.
105 (The original /dev/rtc driver was written by Paul Gortmaker.)
108         New portable "RTC Class" drivers:  /dev/rtcN
109         --------------------------------------------
111 Because Linux supports many non-ACPI and non-PC platforms, some of which
112 have more than one RTC style clock, it needed a more portable solution
113 than expecting a single battery-backed MC146818 clone on every system.
114 Accordingly, a new "RTC Class" framework has been defined.  It offers
115 three different userspace interfaces:
117     *   /dev/rtcN ... much the same as the older /dev/rtc interface
119     *   /sys/class/rtc/rtcN ... sysfs attributes support readonly
120         access to some RTC attributes.
122     *   /proc/driver/rtc ... the system clock RTC may expose itself
123         using a procfs interface. If there is no RTC for the system clock,
124         rtc0 is used by default. More information is (currently) shown
125         here than through sysfs.
127 The RTC Class framework supports a wide variety of RTCs, ranging from those
128 integrated into embeddable system-on-chip (SOC) processors to discrete chips
129 using I2C, SPI, or some other bus to communicate with the host CPU.  There's
130 even support for PC-style RTCs ... including the features exposed on newer PCs
131 through ACPI.
133 The new framework also removes the "one RTC per system" restriction.  For
134 example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
135 a high functionality RTC is integrated into the SOC.  That system might read
136 the system clock from the discrete RTC, but use the integrated one for all
137 other tasks, because of its greater functionality.
140 ---------------
142 The sysfs interface under /sys/class/rtc/rtcN provides access to various
143 rtc attributes without requiring the use of ioctls. All dates and times
144 are in the RTC's timezone, rather than in system time.
146 date:            RTC-provided date
147 hctosys:         1 if the RTC provided the system time at boot via the
148                  CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
149 max_user_freq:   The maximum interrupt rate an unprivileged user may request
150                  from this RTC.
151 name:            The name of the RTC corresponding to this sysfs directory
152 since_epoch:     The number of seconds since the epoch according to the RTC
153 time:            RTC-provided time
154 wakealarm:       The time at which the clock will generate a system wakeup
155                  event. This is a one shot wakeup event, so must be reset
156                  after wake if a daily wakeup is required. Format is seconds since
157                  the epoch by default, or if there's a leading +, seconds in the
158                  future, or if there is a leading +=, seconds ahead of the current
159                  alarm.
160 offset:          The amount which the rtc clock has been adjusted in firmware.
161                  Visible only if the driver supports clock offset adjustment.
162                  The unit is parts per billion, i.e. The number of clock ticks
163                  which are added to or removed from the rtc's base clock per
164                  billion ticks. A positive value makes a day pass more slowly,
165                  longer, and a negative value makes a day pass more quickly.
168 ---------------
170 The ioctl() calls supported by /dev/rtc are also supported by the RTC class
171 framework.  However, because the chips and systems are not standardized,
172 some PC/AT functionality might not be provided.  And in the same way, some
173 newer features -- including those enabled by ACPI -- are exposed by the
174 RTC class framework, but can't be supported by the older driver.
176     *   RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading
177         time, returning the result as a Gregorian calendar date and 24 hour
178         wall clock time.  To be most useful, this time may also be updated.
180     *   RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
181         is connected to an IRQ line, it can often issue an alarm IRQ up to
182         24 hours in the future.  (Use RTC_WKALM_* by preference.)
184     *   RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
185         the next 24 hours use a slightly more powerful API, which supports
186         setting the longer alarm time and enabling its IRQ using a single
187         request (using the same model as EFI firmware).
189     *   RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework
190         will emulate this mechanism.
192     *   RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
193         are emulated via a kernel hrtimer.
195 In many cases, the RTC alarm can be a system wake event, used to force
196 Linux out of a low power sleep state (or hibernation) back to a fully
197 operational state.  For example, a system could enter a deep power saving
198 state until it's time to execute some scheduled tasks.
200 Note that many of these ioctls are handled by the common rtc-dev interface.
201 Some common examples:
203     *   RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
204         called with appropriate values.
206     *   RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets
207         the alarm rtc_timer. May call the set_alarm driver function.
209     *   RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
211     *   RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
213 If all else fails, check out the tools/testing/selftests/timers/rtctest.c test!

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