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db9a0975 | 1 | ========================== |
1da177e4 | 2 | EFI Real Time Clock driver |
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3 | ========================== |
4 | ||
1da177e4 | 5 | S. Eranian <eranian@hpl.hp.com> |
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7 | March 2000 |
8 | ||
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9 | 1. Introduction |
10 | =============== | |
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11 | |
12 | This document describes the efirtc.c driver has provided for | |
db9a0975 | 13 | the IA-64 platform. |
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14 | |
15 | The purpose of this driver is to supply an API for kernel and user applications | |
16 | to get access to the Time Service offered by EFI version 0.92. | |
17 | ||
18 | EFI provides 4 calls one can make once the OS is booted: GetTime(), | |
19 | SetTime(), GetWakeupTime(), SetWakeupTime() which are all supported by this | |
20 | driver. We describe those calls as well the design of the driver in the | |
21 | following sections. | |
22 | ||
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23 | 2. Design Decisions |
24 | =================== | |
1da177e4 | 25 | |
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26 | The original ideas was to provide a very simple driver to get access to, |
27 | at first, the time of day service. This is required in order to access, in a | |
28 | portable way, the CMOS clock. A program like /sbin/hwclock uses such a clock | |
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29 | to initialize the system view of the time during boot. |
30 | ||
31 | Because we wanted to minimize the impact on existing user-level apps using | |
32 | the CMOS clock, we decided to expose an API that was very similar to the one | |
db9a0975 | 33 | used today with the legacy RTC driver (driver/char/rtc.c). However, because |
670e9f34 | 34 | EFI provides a simpler services, not all ioctl() are available. Also |
db9a0975 | 35 | new ioctl()s have been introduced for things that EFI provides but not the |
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36 | legacy. |
37 | ||
38 | EFI uses a slightly different way of representing the time, noticeably | |
39 | the reference date is different. Year is the using the full 4-digit format. | |
40 | The Epoch is January 1st 1998. For backward compatibility reasons we don't | |
db9a0975 | 41 | expose this new way of representing time. Instead we use something very |
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42 | similar to the struct tm, i.e. struct rtc_time, as used by hwclock. |
43 | One of the reasons for doing it this way is to allow for EFI to still evolve | |
44 | without necessarily impacting any of the user applications. The decoupling | |
45 | enables flexibility and permits writing wrapper code is ncase things change. | |
46 | ||
47 | The driver exposes two interfaces, one via the device file and a set of | |
db9a0975 | 48 | ioctl()s. The other is read-only via the /proc filesystem. |
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49 | |
50 | As of today we don't offer a /proc/sys interface. | |
51 | ||
52 | To allow for a uniform interface between the legacy RTC and EFI time service, | |
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53 | we have created the include/linux/rtc.h header file to contain only the |
54 | "public" API of the two drivers. The specifics of the legacy RTC are still | |
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55 | in include/linux/mc146818rtc.h. |
56 | ||
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57 | |
58 | 3. Time of day service | |
59 | ====================== | |
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60 | |
61 | The part of the driver gives access to the time of day service of EFI. | |
62 | Two ioctl()s, compatible with the legacy RTC calls: | |
63 | ||
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64 | Read the CMOS clock:: |
65 | ||
66 | ioctl(d, RTC_RD_TIME, &rtc); | |
67 | ||
68 | Write the CMOS clock:: | |
1da177e4 | 69 | |
db9a0975 | 70 | ioctl(d, RTC_SET_TIME, &rtc); |
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71 | |
72 | The rtc is a pointer to a data structure defined in rtc.h which is close | |
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73 | to a struct tm:: |
74 | ||
75 | struct rtc_time { | |
76 | int tm_sec; | |
77 | int tm_min; | |
78 | int tm_hour; | |
79 | int tm_mday; | |
80 | int tm_mon; | |
81 | int tm_year; | |
82 | int tm_wday; | |
83 | int tm_yday; | |
84 | int tm_isdst; | |
85 | }; | |
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86 | |
87 | The driver takes care of converting back an forth between the EFI time and | |
88 | this format. | |
89 | ||
90 | Those two ioctl()s can be exercised with the hwclock command: | |
91 | ||
db9a0975 | 92 | For reading:: |
1da177e4 | 93 | |
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94 | # /sbin/hwclock --show |
95 | Mon Mar 6 15:32:32 2000 -0.910248 seconds | |
96 | ||
97 | For setting:: | |
98 | ||
99 | # /sbin/hwclock --systohc | |
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100 | |
101 | Root privileges are required to be able to set the time of day. | |
102 | ||
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103 | 4. Wakeup Alarm service |
104 | ======================= | |
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105 | |
106 | EFI provides an API by which one can program when a machine should wakeup, | |
107 | i.e. reboot. This is very different from the alarm provided by the legacy | |
108 | RTC which is some kind of interval timer alarm. For this reason we don't use | |
109 | the same ioctl()s to get access to the service. Instead we have | |
db9a0975 | 110 | introduced 2 news ioctl()s to the interface of an RTC. |
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111 | |
112 | We have added 2 new ioctl()s that are specific to the EFI driver: | |
113 | ||
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114 | Read the current state of the alarm:: |
115 | ||
d4210f70 | 116 | ioctl(d, RTC_WKALM_RD, &wkt) |
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117 | |
118 | Set the alarm or change its status:: | |
119 | ||
120 | ioctl(d, RTC_WKALM_SET, &wkt) | |
1da177e4 | 121 | |
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122 | The wkt structure encapsulates a struct rtc_time + 2 extra fields to get |
123 | status information:: | |
1da177e4 | 124 | |
db9a0975 | 125 | struct rtc_wkalrm { |
1da177e4 | 126 | |
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127 | unsigned char enabled; /* =1 if alarm is enabled */ |
128 | unsigned char pending; /* =1 if alarm is pending */ | |
1da177e4 | 129 | |
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130 | struct rtc_time time; |
131 | } | |
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132 | |
133 | As of today, none of the existing user-level apps supports this feature. | |
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134 | However writing such a program should be hard by simply using those two |
135 | ioctl(). | |
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136 | |
137 | Root privileges are required to be able to set the alarm. | |
138 | ||
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139 | 5. References |
140 | ============= | |
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141 | |
142 | Checkout the following Web site for more information on EFI: | |
143 | ||
144 | http://developer.intel.com/technology/efi/ |