1
  2	Real Time Clock (RTC) Drivers for Linux
  3	=======================================
  4
  5When Linux developers talk about a "Real Time Clock", they usually mean
  6something that tracks wall clock time and is battery backed so that it
  7works even with system power off.  Such clocks will normally not track
  8the local time zone or daylight savings time -- unless they dual boot
  9with MS-Windows -- but will instead be set to Coordinated Universal Time
 10(UTC, formerly "Greenwich Mean Time").
 11
 12The newest non-PC hardware tends to just count seconds, like the time(2)
 13system call reports, but RTCs also very commonly represent time using
 14the Gregorian calendar and 24 hour time, as reported by gmtime(3).
 15
 16Linux has two largely-compatible userspace RTC API families you may
 17need to know about:
 18
 19    *	/dev/rtc ... is the RTC provided by PC compatible systems,
 20	so it's not very portable to non-x86 systems.
 21
 22    *	/dev/rtc0, /dev/rtc1 ... are part of a framework that's
 23	supported by a wide variety of RTC chips on all systems.
 24
 25Programmers need to understand that the PC/AT functionality is not
 26always available, and some systems can do much more.  That is, the
 27RTCs use the same API to make requests in both RTC frameworks (using
 28different filenames of course), but the hardware may not offer the
 29same functionality.  For example, not every RTC is hooked up to an
 30IRQ, so they can't all issue alarms; and where standard PC RTCs can
 31only issue an alarm up to 24 hours in the future, other hardware may
 32be able to schedule one any time in the upcoming century.
 33
 34
 35	Old PC/AT-Compatible driver:  /dev/rtc
 36	--------------------------------------
 37
 38All PCs (even Alpha machines) have a Real Time Clock built into them.
 39Usually they are built into the chipset of the computer, but some may
 40actually have a Motorola MC146818 (or clone) on the board. This is the
 41clock that keeps the date and time while your computer is turned off.
 42
 43ACPI has standardized that MC146818 functionality, and extended it in
 44a few ways (enabling longer alarm periods, and wake-from-hibernate).
 45That functionality is NOT exposed in the old driver.
 46
 47However it can also be used to generate signals from a slow 2Hz to a
 48relatively fast 8192Hz, in increments of powers of two. These signals
 49are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
 50for...) It can also function as a 24hr alarm, raising IRQ 8 when the
 51alarm goes off. The alarm can also be programmed to only check any
 52subset of the three programmable values, meaning that it could be set to
 53ring on the 30th second of the 30th minute of every hour, for example.
 54The clock can also be set to generate an interrupt upon every clock
 55update, thus generating a 1Hz signal.
 56
 57The interrupts are reported via /dev/rtc (major 10, minor 135, read only
 58character device) in the form of an unsigned long. The low byte contains
 59the type of interrupt (update-done, alarm-rang, or periodic) that was
 60raised, and the remaining bytes contain the number of interrupts since
 61the last read.  Status information is reported through the pseudo-file
 62/proc/driver/rtc if the /proc filesystem was enabled.  The driver has
 63built in locking so that only one process is allowed to have the /dev/rtc
 64interface open at a time.
 65
 66A user process can monitor these interrupts by doing a read(2) or a
 67select(2) on /dev/rtc -- either will block/stop the user process until
 68the next interrupt is received. This is useful for things like
 69reasonably high frequency data acquisition where one doesn't want to
 70burn up 100% CPU by polling gettimeofday etc. etc.
 71
 72At high frequencies, or under high loads, the user process should check
 73the number of interrupts received since the last read to determine if
 74there has been any interrupt "pileup" so to speak. Just for reference, a
 75typical 486-33 running a tight read loop on /dev/rtc will start to suffer
 76occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
 77frequencies above 1024Hz. So you really should check the high bytes
 78of the value you read, especially at frequencies above that of the
 79normal timer interrupt, which is 100Hz.
 80
 81Programming and/or enabling interrupt frequencies greater than 64Hz is
 82only allowed by root. This is perhaps a bit conservative, but we don't want
 83an evil user generating lots of IRQs on a slow 386sx-16, where it might have
 84a negative impact on performance. This 64Hz limit can be changed by writing
 85a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
 86interrupt handler is only a few lines of code to minimize any possibility
 87of this effect.
 88
 89Also, if the kernel time is synchronized with an external source, the 
 90kernel will write the time back to the CMOS clock every 11 minutes. In 
 91the process of doing this, the kernel briefly turns off RTC periodic 
 92interrupts, so be aware of this if you are doing serious work. If you
 93don't synchronize the kernel time with an external source (via ntp or
 94whatever) then the kernel will keep its hands off the RTC, allowing you
 95exclusive access to the device for your applications.
 96
 97The alarm and/or interrupt frequency are programmed into the RTC via
 98various ioctl(2) calls as listed in ./include/linux/rtc.h
 99Rather than write 50 pages describing the ioctl() and so on, it is
100perhaps more useful to include a small test program that demonstrates
101how to use them, and demonstrates the features of the driver. This is
102probably a lot more useful to people interested in writing applications
103that will be using this driver.  See the code at the end of this document.
104
105(The original /dev/rtc driver was written by Paul Gortmaker.)
106
107
108	New portable "RTC Class" drivers:  /dev/rtcN
109	--------------------------------------------
110
111Because Linux supports many non-ACPI and non-PC platforms, some of which
112have more than one RTC style clock, it needed a more portable solution
113than expecting a single battery-backed MC146818 clone on every system.
114Accordingly, a new "RTC Class" framework has been defined.  It offers
115three different userspace interfaces:
116
117    *	/dev/rtcN ... much the same as the older /dev/rtc interface
118
119    *	/sys/class/rtc/rtcN ... sysfs attributes support readonly
120	access to some RTC attributes.
121
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.
126
127The RTC Class framework supports a wide variety of RTCs, ranging from those
128integrated into embeddable system-on-chip (SOC) processors to discrete chips
129using I2C, SPI, or some other bus to communicate with the host CPU.  There's
130even support for PC-style RTCs ... including the features exposed on newer PCs
131through ACPI.
132
133The new framework also removes the "one RTC per system" restriction.  For
134example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
135a high functionality RTC is integrated into the SOC.  That system might read
136the system clock from the discrete RTC, but use the integrated one for all
137other tasks, because of its greater functionality.
138
139SYSFS INTERFACE
140---------------
141
142The sysfs interface under /sys/class/rtc/rtcN provides access to various
143rtc attributes without requiring the use of ioctls. All dates and times
144are in the RTC's timezone, rather than in system time.
145
146date:  	   	 RTC-provided date
147hctosys:   	 1 if the RTC provided the system time at boot via the
148		 CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
149max_user_freq:	 The maximum interrupt rate an unprivileged user may request
150		 from this RTC.
151name:		 The name of the RTC corresponding to this sysfs directory
152since_epoch:	 The number of seconds since the epoch according to the RTC
153time:		 RTC-provided time
154wakealarm:	 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.
160offset:		 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.
166
167IOCTL INTERFACE
168---------------
169
170The ioctl() calls supported by /dev/rtc are also supported by the RTC class
171framework.  However, because the chips and systems are not standardized,
172some PC/AT functionality might not be provided.  And in the same way, some
173newer features -- including those enabled by ACPI -- are exposed by the
174RTC class framework, but can't be supported by the older driver.
175
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.
179
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.)
183
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).
188
189    *	RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework
190	will emulate this mechanism.
191
192    *	RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls
193	are emulated via a kernel hrtimer.
194
195In many cases, the RTC alarm can be a system wake event, used to force
196Linux out of a low power sleep state (or hibernation) back to a fully
197operational state.  For example, a system could enter a deep power saving
198state until it's time to execute some scheduled tasks.
199
200Note that many of these ioctls are handled by the common rtc-dev interface.
201Some common examples:
202
203    *	RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be
204	called with appropriate values.
205
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.
208
209    *	RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.
210
211    *	RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.
212
213If all else fails, check out the tools/testing/selftests/timers/rtctest.c test!