1======================================= 2Real 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 35Old 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 108New 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 139Check out tools/testing/selftests/timers/rtctest.c for an example usage of the 140ioctl interface. 141
