linux/Documentation/ia64/efirtc.txt
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   1EFI Real Time Clock driver
   2-------------------------------
   3S. Eranian <eranian@hpl.hp.com>
   4March 2000
   5
   6I/ Introduction
   7
   8This document describes the efirtc.c driver has provided for
   9the IA-64 platform. 
  10
  11The purpose of this driver is to supply an API for kernel and user applications
  12to get access to the Time Service offered by EFI version 0.92.
  13
  14EFI provides 4 calls one can make once the OS is booted: GetTime(),
  15SetTime(), GetWakeupTime(), SetWakeupTime() which are all supported by this
  16driver. We describe those calls as well the design of the driver in the
  17following sections.
  18
  19II/ Design Decisions
  20
  21The original ideas was to provide a very simple driver to get access to, 
  22at first, the time of day service. This is required in order to access, in a 
  23portable way, the CMOS clock. A program like /sbin/hwclock uses such a clock 
  24to initialize the system view of the time during boot.
  25
  26Because we wanted to minimize the impact on existing user-level apps using
  27the CMOS clock, we decided to expose an API that was very similar to the one
  28used today with the legacy RTC driver (driver/char/rtc.c). However, because 
  29EFI provides a simpler services, not all ioctl() are available. Also
  30new ioctl()s have been introduced for things that EFI provides but not the 
  31legacy.
  32
  33EFI uses a slightly different way of representing the time, noticeably
  34the reference date is different. Year is the using the full 4-digit format.
  35The Epoch is January 1st 1998. For backward compatibility reasons we don't
  36expose this new way of representing time. Instead we use something very 
  37similar to the struct tm, i.e. struct rtc_time, as used by hwclock.
  38One of the reasons for doing it this way is to allow for EFI to still evolve
  39without necessarily impacting any of the user applications. The decoupling
  40enables flexibility and permits writing wrapper code is ncase things change.
  41
  42The driver exposes two interfaces, one via the device file and a set of
  43ioctl()s. The other is read-only via the /proc filesystem. 
  44
  45As of today we don't offer a /proc/sys interface.
  46
  47To allow for a uniform interface between the legacy RTC and EFI time service,
  48we have created the include/linux/rtc.h header file to contain only the 
  49"public" API of the two drivers.  The specifics of the legacy RTC are still 
  50in include/linux/mc146818rtc.h.
  51
  52 
  53III/ Time of day service
  54
  55The part of the driver gives access to the time of day service of EFI.
  56Two ioctl()s, compatible with the legacy RTC calls:
  57
  58        Read the CMOS clock: ioctl(d, RTC_RD_TIME, &rtc);
  59
  60        Write the CMOS clock: ioctl(d, RTC_SET_TIME, &rtc);
  61
  62The rtc is a pointer to a data structure defined in rtc.h which is close
  63to a struct tm:
  64
  65struct rtc_time {
  66        int tm_sec;
  67        int tm_min;
  68        int tm_hour;
  69        int tm_mday;
  70        int tm_mon;
  71        int tm_year;
  72        int tm_wday;
  73        int tm_yday;
  74        int tm_isdst;
  75};
  76
  77The driver takes care of converting back an forth between the EFI time and
  78this format.
  79
  80Those two ioctl()s can be exercised with the hwclock command:
  81
  82For reading:
  83# /sbin/hwclock --show
  84Mon Mar  6 15:32:32 2000  -0.910248 seconds
  85
  86For setting:
  87# /sbin/hwclock --systohc
  88
  89Root privileges are required to be able to set the time of day.
  90
  91IV/ Wakeup Alarm service
  92
  93EFI provides an API by which one can program when a machine should wakeup,
  94i.e. reboot. This is very different from the alarm provided by the legacy
  95RTC which is some kind of interval timer alarm. For this reason we don't use
  96the same ioctl()s to get access to the service. Instead we have
  97introduced 2 news ioctl()s to the interface of an RTC. 
  98
  99We have added 2 new ioctl()s that are specific to the EFI driver:
 100
 101        Read the current state of the alarm
 102        ioctl(d, RTC_WKLAM_RD, &wkt)
 103
 104        Set the alarm or change its status
 105        ioctl(d, RTC_WKALM_SET, &wkt)
 106
 107The wkt structure encapsulates a struct rtc_time + 2 extra fields to get 
 108status information:
 109        
 110struct rtc_wkalrm {
 111
 112        unsigned char enabled; /* =1 if alarm is enabled */
 113        unsigned char pending; /* =1 if alarm is pending  */
 114
 115        struct rtc_time time;
 116} 
 117
 118As of today, none of the existing user-level apps supports this feature.
 119However writing such a program should be hard by simply using those two 
 120ioctl(). 
 121
 122Root privileges are required to be able to set the alarm.
 123
 124V/ References.
 125
 126Checkout the following Web site for more information on EFI:
 127
 128http://developer.intel.com/technology/efi/
 129
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