Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 EFI Real Time Clock driver 2 ------------------------------- 3 S. Eranian <eranian@hpl.hp.com> 4 March 2000 5 6 I/ Introduction 7 8 This document describes the efirtc.c driver has provided for 9 the IA-64 platform. 10 11 The purpose of this driver is to supply an API for kernel and user applications 12 to get access to the Time Service offered by EFI version 0.92. 13 14 EFI provides 4 calls one can make once the OS is booted: GetTime(), 15 SetTime(), GetWakeupTime(), SetWakeupTime() which are all supported by this 16 driver. We describe those calls as well the design of the driver in the 17 following sections. 18 19 II/ Design Decisions 20 21 The original ideas was to provide a very simple driver to get access to, 22 at first, the time of day service. This is required in order to access, in a 23 portable way, the CMOS clock. A program like /sbin/hwclock uses such a clock 24 to initialize the system view of the time during boot. 25 26 Because we wanted to minimize the impact on existing user-level apps using 27 the CMOS clock, we decided to expose an API that was very similar to the one 28 used today with the legacy RTC driver (driver/char/rtc.c). However, because 29 EFI provides a simpler services, not all ioctl() are available. Also 30 new ioctl()s have been introduced for things that EFI provides but not the 31 legacy. 32 33 EFI uses a slightly different way of representing the time, noticeably 34 the reference date is different. Year is the using the full 4-digit format. 35 The Epoch is January 1st 1998. For backward compatibility reasons we don't 36 expose this new way of representing time. Instead we use something very 37 similar to the struct tm, i.e. struct rtc_time, as used by hwclock. 38 One of the reasons for doing it this way is to allow for EFI to still evolve 39 without necessarily impacting any of the user applications. The decoupling 40 enables flexibility and permits writing wrapper code is ncase things change. 41 42 The driver exposes two interfaces, one via the device file and a set of 43 ioctl()s. The other is read-only via the /proc filesystem. 44 45 As of today we don't offer a /proc/sys interface. 46 47 To allow for a uniform interface between the legacy RTC and EFI time service, 48 we 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 50 in include/linux/mc146818rtc.h. 51 52 53 III/ Time of day service 54 55 The part of the driver gives access to the time of day service of EFI. 56 Two 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 62 The rtc is a pointer to a data structure defined in rtc.h which is close 63 to a struct tm: 64 65 struct 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 77 The driver takes care of converting back an forth between the EFI time and 78 this format. 79 80 Those two ioctl()s can be exercised with the hwclock command: 81 82 For reading: 83 # /sbin/hwclock --show 84 Mon Mar 6 15:32:32 2000 -0.910248 seconds 85 86 For setting: 87 # /sbin/hwclock --systohc 88 89 Root privileges are required to be able to set the time of day. 90 91 IV/ Wakeup Alarm service 92 93 EFI provides an API by which one can program when a machine should wakeup, 94 i.e. reboot. This is very different from the alarm provided by the legacy 95 RTC which is some kind of interval timer alarm. For this reason we don't use 96 the same ioctl()s to get access to the service. Instead we have 97 introduced 2 news ioctl()s to the interface of an RTC. 98 99 We 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) 106 107 The wkt structure encapsulates a struct rtc_time + 2 extra fields to get 108 status information: 109 110 struct 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 } 117 118 As of today, none of the existing user-level apps supports this feature. 119 However writing such a program should be hard by simply using those two 120 ioctl(). 121 122 Root privileges are required to be able to set the alarm. 123 124 V/ References. 125 126 Checkout the following Web site for more information on EFI: 127 128 http://developer.intel.com/technology/efi/