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144 lines
4.4 KiB
144 lines
4.4 KiB
========================== |
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EFI Real Time Clock driver |
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========================== |
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S. Eranian <[email protected]> |
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March 2000 |
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1. Introduction |
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=============== |
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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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2. Design Decisions |
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=================== |
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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 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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3. Time of day service |
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====================== |
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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:: |
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ioctl(d, RTC_RD_TIME, &rtc); |
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Write the CMOS clock:: |
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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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4. Wakeup Alarm service |
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======================= |
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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_WKALM_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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5. References |
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============= |
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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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