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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	=======================================
2	Real Time Clock (RTC) Drivers for Linux
3	=======================================
4	
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").
11	
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).
15	
16	Linux has two largely-compatible userspace RTC API families you may
17	need 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	
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.
33	
34	
35	Old PC/AT-Compatible driver:  /dev/rtc
36	--------------------------------------
37	
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.
42	
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.
46	
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.
56	
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.
65	
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.
71	
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.
80	
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.
88	
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.
96	
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.
104	
105	(The original /dev/rtc driver was written by Paul Gortmaker.)
106	
107	
108	New portable "RTC Class" drivers:  /dev/rtcN
109	--------------------------------------------
110	
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:
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	
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.
132	
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.
138	
139	Check out tools/testing/selftests/timers/rtctest.c for an example usage of the
140	ioctl interface.
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