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Diffstat (limited to 'kernel/Documentation/ia64/efirtc.txt')
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diff --git a/kernel/Documentation/ia64/efirtc.txt b/kernel/Documentation/ia64/efirtc.txt new file mode 100644 index 000000000..057e6bebd --- /dev/null +++ b/kernel/Documentation/ia64/efirtc.txt @@ -0,0 +1,128 @@ +EFI Real Time Clock driver +------------------------------- +S. Eranian <eranian@hpl.hp.com> +March 2000 + +I/ Introduction + +This document describes the efirtc.c driver has provided for +the IA-64 platform. + +The purpose of this driver is to supply an API for kernel and user applications +to get access to the Time Service offered by EFI version 0.92. + +EFI provides 4 calls one can make once the OS is booted: GetTime(), +SetTime(), GetWakeupTime(), SetWakeupTime() which are all supported by this +driver. We describe those calls as well the design of the driver in the +following sections. + +II/ Design Decisions + +The original ideas was to provide a very simple driver to get access to, +at first, the time of day service. This is required in order to access, in a +portable way, the CMOS clock. A program like /sbin/hwclock uses such a clock +to initialize the system view of the time during boot. + +Because we wanted to minimize the impact on existing user-level apps using +the CMOS clock, we decided to expose an API that was very similar to the one +used today with the legacy RTC driver (driver/char/rtc.c). However, because +EFI provides a simpler services, not all ioctl() are available. Also +new ioctl()s have been introduced for things that EFI provides but not the +legacy. + +EFI uses a slightly different way of representing the time, noticeably +the reference date is different. Year is the using the full 4-digit format. +The Epoch is January 1st 1998. For backward compatibility reasons we don't +expose this new way of representing time. Instead we use something very +similar to the struct tm, i.e. struct rtc_time, as used by hwclock. +One of the reasons for doing it this way is to allow for EFI to still evolve +without necessarily impacting any of the user applications. The decoupling +enables flexibility and permits writing wrapper code is ncase things change. + +The driver exposes two interfaces, one via the device file and a set of +ioctl()s. The other is read-only via the /proc filesystem. + +As of today we don't offer a /proc/sys interface. + +To allow for a uniform interface between the legacy RTC and EFI time service, +we have created the include/linux/rtc.h header file to contain only the +"public" API of the two drivers. The specifics of the legacy RTC are still +in include/linux/mc146818rtc.h. + + +III/ Time of day service + +The part of the driver gives access to the time of day service of EFI. +Two ioctl()s, compatible with the legacy RTC calls: + + Read the CMOS clock: ioctl(d, RTC_RD_TIME, &rtc); + + Write the CMOS clock: ioctl(d, RTC_SET_TIME, &rtc); + +The rtc is a pointer to a data structure defined in rtc.h which is close +to a struct tm: + +struct rtc_time { + int tm_sec; + int tm_min; + int tm_hour; + int tm_mday; + int tm_mon; + int tm_year; + int tm_wday; + int tm_yday; + int tm_isdst; +}; + +The driver takes care of converting back an forth between the EFI time and +this format. + +Those two ioctl()s can be exercised with the hwclock command: + +For reading: +# /sbin/hwclock --show +Mon Mar 6 15:32:32 2000 -0.910248 seconds + +For setting: +# /sbin/hwclock --systohc + +Root privileges are required to be able to set the time of day. + +IV/ Wakeup Alarm service + +EFI provides an API by which one can program when a machine should wakeup, +i.e. reboot. This is very different from the alarm provided by the legacy +RTC which is some kind of interval timer alarm. For this reason we don't use +the same ioctl()s to get access to the service. Instead we have +introduced 2 news ioctl()s to the interface of an RTC. + +We have added 2 new ioctl()s that are specific to the EFI driver: + + Read the current state of the alarm + ioctl(d, RTC_WKLAM_RD, &wkt) + + Set the alarm or change its status + ioctl(d, RTC_WKALM_SET, &wkt) + +The wkt structure encapsulates a struct rtc_time + 2 extra fields to get +status information: + +struct rtc_wkalrm { + + unsigned char enabled; /* =1 if alarm is enabled */ + unsigned char pending; /* =1 if alarm is pending */ + + struct rtc_time time; +} + +As of today, none of the existing user-level apps supports this feature. +However writing such a program should be hard by simply using those two +ioctl(). + +Root privileges are required to be able to set the alarm. + +V/ References. + +Checkout the following Web site for more information on EFI: + +http://developer.intel.com/technology/efi/ |