€•ÄXŒsphinx.addnodes”Œdocument”“”)”}”(Œ rawsource”Œ”Œchildren”]”(Œ translations”Œ LanguagesNode”“”)”}”(hhh]”(hŒ pending_xref”“”)”}”(hhh]”Œdocutils.nodes”ŒText”“”ŒChinese (Simplified)”…””}”(hhŒparent”hubaŒ attributes”}”(Œids”]”Œclasses”]”Œnames”]”Œdupnames”]”Œbackrefs”]”Œ refdomain”Œstd”Œreftype”Œdoc”Œ reftarget”Œ#/translations/zh_CN/admin-guide/rtc”Œmodname”NŒ classname”NŒ refexplicit”ˆuŒtagname”hhh ubh)”}”(hhh]”hŒChinese (Traditional)”…””}”(hhhh2ubah}”(h]”h ]”h"]”h$]”h&]”Œ refdomain”h)Œreftype”h+Œ reftarget”Œ#/translations/zh_TW/admin-guide/rtc”Œmodname”NŒ classname”NŒ refexplicit”ˆuh1hhh ubh)”}”(hhh]”hŒItalian”…””}”(hhhhFubah}”(h]”h ]”h"]”h$]”h&]”Œ refdomain”h)Œreftype”h+Œ reftarget”Œ#/translations/it_IT/admin-guide/rtc”Œmodname”NŒ classname”NŒ refexplicit”ˆuh1hhh ubh)”}”(hhh]”hŒJapanese”…””}”(hhhhZubah}”(h]”h ]”h"]”h$]”h&]”Œ refdomain”h)Œreftype”h+Œ reftarget”Œ#/translations/ja_JP/admin-guide/rtc”Œmodname”NŒ classname”NŒ refexplicit”ˆuh1hhh ubh)”}”(hhh]”hŒKorean”…””}”(hhhhnubah}”(h]”h ]”h"]”h$]”h&]”Œ refdomain”h)Œreftype”h+Œ reftarget”Œ#/translations/ko_KR/admin-guide/rtc”Œmodname”NŒ classname”NŒ refexplicit”ˆuh1hhh ubh)”}”(hhh]”hŒSpanish”…””}”(hhhh‚ubah}”(h]”h ]”h"]”h$]”h&]”Œ refdomain”h)Œreftype”h+Œ reftarget”Œ#/translations/sp_SP/admin-guide/rtc”Œmodname”NŒ classname”NŒ refexplicit”ˆuh1hhh ubeh}”(h]”h ]”h"]”h$]”h&]”Œcurrent_language”ŒEnglish”uh1h hhŒ _document”hŒsource”NŒline”NubhŒsection”“”)”}”(hhh]”(hŒtitle”“”)”}”(hŒ'Real Time Clock (RTC) Drivers for Linux”h]”hŒ'Real Time Clock (RTC) Drivers for Linux”…””}”(hhªhh¨hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hh£hžhhŸŒ=/var/lib/git/docbuild/linux/Documentation/admin-guide/rtc.rst”h KubhŒ paragraph”“”)”}”(hX‹When Linux developers talk about a "Real Time Clock", they usually mean something that tracks wall clock time and is battery backed so that it works even with system power off. Such clocks will normally not track the local time zone or daylight savings time -- unless they dual boot with MS-Windows -- but will instead be set to Coordinated Universal Time (UTC, formerly "Greenwich Mean Time").”h]”hX“When Linux developers talk about a “Real Time Clockâ€, they usually mean something that tracks wall clock time and is battery backed so that it works even with system power off. Such clocks will normally not track the local time zone or daylight savings time -- unless they dual boot with MS-Windows -- but will instead be set to Coordinated Universal Time (UTC, formerly “Greenwich Mean Timeâ€).”…””}”(hh»hh¹hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khh£hžhubh¸)”}”(hŒÑThe newest non-PC hardware tends to just count seconds, like the time(2) system call reports, but RTCs also very commonly represent time using the Gregorian calendar and 24 hour time, as reported by gmtime(3).”h]”hŒÑThe newest non-PC hardware tends to just count seconds, like the time(2) system call reports, but RTCs also very commonly represent time using the Gregorian calendar and 24 hour time, as reported by gmtime(3).”…””}”(hhÉhhÇhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K hh£hžhubh¸)”}”(hŒWLinux has two largely-compatible userspace RTC API families you may need to know about:”h]”hŒWLinux has two largely-compatible userspace RTC API families you may need to know about:”…””}”(hh×hhÕhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khh£hžhubhŒ block_quote”“”)”}”(hhh]”hŒ bullet_list”“”)”}”(hhh]”(hŒ list_item”“”)”}”(hŒi/dev/rtc ... is the RTC provided by PC compatible systems, so it's not very portable to non-x86 systems. ”h]”h¸)”}”(hŒh/dev/rtc ... is the RTC provided by PC compatible systems, so it's not very portable to non-x86 systems.”h]”hŒj/dev/rtc ... is the RTC provided by PC compatible systems, so it’s not very portable to non-x86 systems.”…””}”(hhõhhóhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khhïubah}”(h]”h ]”h"]”h$]”h&]”uh1híhhêubhî)”}”(hŒq/dev/rtc0, /dev/rtc1 ... are part of a framework that's supported by a wide variety of RTC chips on all systems. ”h]”h¸)”}”(hŒp/dev/rtc0, /dev/rtc1 ... are part of a framework that's supported by a wide variety of RTC chips on all systems.”h]”hŒr/dev/rtc0, /dev/rtc1 ... are part of a framework that’s supported by a wide variety of RTC chips on all systems.”…””}”(hj hj hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khjubah}”(h]”h ]”h"]”h$]”h&]”uh1híhhêubeh}”(h]”h ]”h"]”h$]”h&]”Œbullet”Œ*”uh1hèhŸh¶h Khhåubah}”(h]”h ]”h"]”h$]”h&]”uh1hãhh£hžhhŸNh Nubh¸)”}”(hXProgrammers need to understand that the PC/AT functionality is not always available, and some systems can do much more. That is, the RTCs use the same API to make requests in both RTC frameworks (using different filenames of course), but the hardware may not offer the same functionality. For example, not every RTC is hooked up to an IRQ, so they can't all issue alarms; and where standard PC RTCs can only issue an alarm up to 24 hours in the future, other hardware may be able to schedule one any time in the upcoming century.”h]”hXProgrammers need to understand that the PC/AT functionality is not always available, and some systems can do much more. That is, the RTCs use the same API to make requests in both RTC frameworks (using different filenames of course), but the hardware may not offer the same functionality. For example, not every RTC is hooked up to an IRQ, so they can’t all issue alarms; and where standard PC RTCs can only issue an alarm up to 24 hours in the future, other hardware may be able to schedule one any time in the upcoming century.”…””}”(hj/hj-hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khh£hžhubh¢)”}”(hhh]”(h§)”}”(hŒ&Old PC/AT-Compatible driver: /dev/rtc”h]”hŒ&Old PC/AT-Compatible driver: /dev/rtc”…””}”(hj@hj>hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hj;hžhhŸh¶h K$ubh¸)”}”(hXAll PCs (even Alpha machines) have a Real Time Clock built into them. Usually they are built into the chipset of the computer, but some may actually have a Motorola MC146818 (or clone) on the board. This is the clock that keeps the date and time while your computer is turned off.”h]”hXAll PCs (even Alpha machines) have a Real Time Clock built into them. Usually they are built into the chipset of the computer, but some may actually have a Motorola MC146818 (or clone) on the board. This is the clock that keeps the date and time while your computer is turned off.”…””}”(hjNhjLhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K&hj;hžhubh¸)”}”(hŒ¿ACPI has standardized that MC146818 functionality, and extended it in a few ways (enabling longer alarm periods, and wake-from-hibernate). That functionality is NOT exposed in the old driver.”h]”hŒ¿ACPI has standardized that MC146818 functionality, and extended it in a few ways (enabling longer alarm periods, and wake-from-hibernate). That functionality is NOT exposed in the old driver.”…””}”(hj\hjZhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K+hj;hžhubh¸)”}”(hXPHowever it can also be used to generate signals from a slow 2Hz to a relatively fast 8192Hz, in increments of powers of two. These signals are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is for...) It can also function as a 24hr alarm, raising IRQ 8 when the alarm goes off. The alarm can also be programmed to only check any subset of the three programmable values, meaning that it could be set to ring on the 30th second of the 30th minute of every hour, for example. The clock can also be set to generate an interrupt upon every clock update, thus generating a 1Hz signal.”h]”(hŒ·However it can also be used to generate signals from a slow 2Hz to a relatively fast 8192Hz, in increments of powers of two. These signals are reported by interrupt number 8. (Oh! So ”…””}”(hŒ·However it can also be used to generate signals from a slow 2Hz to a relatively fast 8192Hz, in increments of powers of two. These signals are reported by interrupt number 8. (Oh! So ”hjhhžhhŸNh NubhŒemphasis”“”)”}”(hŒ*that*”h]”hŒthat”…””}”(hhhjshžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jqhjhubhX“ is what IRQ 8 is for...) It can also function as a 24hr alarm, raising IRQ 8 when the alarm goes off. The alarm can also be programmed to only check any subset of the three programmable values, meaning that it could be set to ring on the 30th second of the 30th minute of every hour, for example. The clock can also be set to generate an interrupt upon every clock update, thus generating a 1Hz signal.”…””}”(hX“ is what IRQ 8 is for...) It can also function as a 24hr alarm, raising IRQ 8 when the alarm goes off. The alarm can also be programmed to only check any subset of the three programmable values, meaning that it could be set to ring on the 30th second of the 30th minute of every hour, for example. The clock can also be set to generate an interrupt upon every clock update, thus generating a 1Hz signal.”hjhhžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K/hj;hžhubh¸)”}”(hXThe interrupts are reported via /dev/rtc (major 10, minor 135, read only character device) in the form of an unsigned long. The low byte contains the type of interrupt (update-done, alarm-rang, or periodic) that was raised, and the remaining bytes contain the number of interrupts since the last read. Status information is reported through the pseudo-file /proc/driver/rtc if the /proc filesystem was enabled. The driver has built in locking so that only one process is allowed to have the /dev/rtc interface open at a time.”h]”hXThe interrupts are reported via /dev/rtc (major 10, minor 135, read only character device) in the form of an unsigned long. The low byte contains the type of interrupt (update-done, alarm-rang, or periodic) that was raised, and the remaining bytes contain the number of interrupts since the last read. Status information is reported through the pseudo-file /proc/driver/rtc if the /proc filesystem was enabled. The driver has built in locking so that only one process is allowed to have the /dev/rtc interface open at a time.”…””}”(hjŽhjŒhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K9hj;hžhubh¸)”}”(hXAA user process can monitor these interrupts by doing a read(2) or a select(2) on /dev/rtc -- either will block/stop the user process until the next interrupt is received. This is useful for things like reasonably high frequency data acquisition where one doesn't want to burn up 100% CPU by polling gettimeofday etc. etc.”h]”hXCA user process can monitor these interrupts by doing a read(2) or a select(2) on /dev/rtc -- either will block/stop the user process until the next interrupt is received. This is useful for things like reasonably high frequency data acquisition where one doesn’t want to burn up 100% CPU by polling gettimeofday etc. etc.”…””}”(hjœhjšhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KBhj;hžhubh¸)”}”(hXAt high frequencies, or under high loads, the user process should check the number of interrupts received since the last read to determine if there has been any interrupt "pileup" so to speak. Just for reference, a typical 486-33 running a tight read loop on /dev/rtc will start to suffer occasional interrupt pileup (i.e. > 1 IRQ event since last read) for frequencies above 1024Hz. So you really should check the high bytes of the value you read, especially at frequencies above that of the normal timer interrupt, which is 100Hz.”h]”hXAt high frequencies, or under high loads, the user process should check the number of interrupts received since the last read to determine if there has been any interrupt “pileup†so to speak. Just for reference, a typical 486-33 running a tight read loop on /dev/rtc will start to suffer occasional interrupt pileup (i.e. > 1 IRQ event since last read) for frequencies above 1024Hz. So you really should check the high bytes of the value you read, especially at frequencies above that of the normal timer interrupt, which is 100Hz.”…””}”(hjªhj¨hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KHhj;hžhubh¸)”}”(hXÉProgramming and/or enabling interrupt frequencies greater than 64Hz is only allowed by root. This is perhaps a bit conservative, but we don't want an evil user generating lots of IRQs on a slow 386sx-16, where it might have a negative impact on performance. This 64Hz limit can be changed by writing a different value to /proc/sys/dev/rtc/max-user-freq. Note that the interrupt handler is only a few lines of code to minimize any possibility of this effect.”h]”hXËProgramming and/or enabling interrupt frequencies greater than 64Hz is only allowed by root. This is perhaps a bit conservative, but we don’t want an evil user generating lots of IRQs on a slow 386sx-16, where it might have a negative impact on performance. This 64Hz limit can be changed by writing a different value to /proc/sys/dev/rtc/max-user-freq. Note that the interrupt handler is only a few lines of code to minimize any possibility of this effect.”…””}”(hj¸hj¶hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KQhj;hžhubh¸)”}”(hXÛAlso, if the kernel time is synchronized with an external source, the kernel will write the time back to the CMOS clock every 11 minutes. In the process of doing this, the kernel briefly turns off RTC periodic interrupts, so be aware of this if you are doing serious work. If you don't synchronize the kernel time with an external source (via ntp or whatever) then the kernel will keep its hands off the RTC, allowing you exclusive access to the device for your applications.”h]”hXÝAlso, if the kernel time is synchronized with an external source, the kernel will write the time back to the CMOS clock every 11 minutes. In the process of doing this, the kernel briefly turns off RTC periodic interrupts, so be aware of this if you are doing serious work. If you don’t synchronize the kernel time with an external source (via ntp or whatever) then the kernel will keep its hands off the RTC, allowing you exclusive access to the device for your applications.”…””}”(hjÆhjÄhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KYhj;hžhubh¸)”}”(hXàThe alarm and/or interrupt frequency are programmed into the RTC via various ioctl(2) calls as listed in ./include/linux/rtc.h Rather than write 50 pages describing the ioctl() and so on, it is perhaps more useful to include a small test program that demonstrates how to use them, and demonstrates the features of the driver. This is probably a lot more useful to people interested in writing applications that will be using this driver. See the code at the end of this document.”h]”hXàThe alarm and/or interrupt frequency are programmed into the RTC via various ioctl(2) calls as listed in ./include/linux/rtc.h Rather than write 50 pages describing the ioctl() and so on, it is perhaps more useful to include a small test program that demonstrates how to use them, and demonstrates the features of the driver. This is probably a lot more useful to people interested in writing applications that will be using this driver. See the code at the end of this document.”…””}”(hjÔhjÒhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Kahj;hžhubh¸)”}”(hŒ=(The original /dev/rtc driver was written by Paul Gortmaker.)”h]”hŒ=(The original /dev/rtc driver was written by Paul Gortmaker.)”…””}”(hjâhjàhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Kihj;hžhubeh}”(h]”Œ#old-pc-at-compatible-driver-dev-rtc”ah ]”h"]”Œ%old pc/at-compatible driver: /dev/rtc”ah$]”h&]”uh1h¡hh£hžhhŸh¶h K$ubh¢)”}”(hhh]”(h§)”}”(hŒ,New portable "RTC Class" drivers: /dev/rtcN”h]”hŒ0New portable “RTC Class†drivers: /dev/rtcN”…””}”(hjûhjùhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hjöhžhhŸh¶h Kmubh¸)”}”(hXBBecause Linux supports many non-ACPI and non-PC platforms, some of which have more than one RTC style clock, it needed a more portable solution than expecting a single battery-backed MC146818 clone on every system. Accordingly, a new "RTC Class" framework has been defined. It offers three different userspace interfaces:”h]”hXFBecause Linux supports many non-ACPI and non-PC platforms, some of which have more than one RTC style clock, it needed a more portable solution than expecting a single battery-backed MC146818 clone on every system. Accordingly, a new “RTC Class†framework has been defined. It offers three different userspace interfaces:”…””}”(hj hjhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Kohjöhžhubhä)”}”(hhh]”hé)”}”(hhh]”(hî)”}”(hŒ