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7.0 KiB
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141 lines
7.0 KiB
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=======================================
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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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Check out tools/testing/selftests/rtc/rtctest.c for an example usage of the
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ioctl interface.
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