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-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/