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