Re: 500 ms offset in i386 Real Time Clock setting

[Paul Gortmaker]

[e-mail] wrote:
> 
> I still haven't looked at things, but two points:
> (i) is the behaviour constant on all architectures?

As it is a property of the mc146818, it should be constant across all 
arch that use drivers/char/rtc.c

Sparc uses drivers/sbus/char/rtc.c which is for Mostek 4802.  No comment
mentions a 500ms delay there - or in the file arch/sparc/kernel/time.c 
*However* the test for the 500ms is still in the latter (in set_rtc_mmss).

> (ii) instead of waiting, isn't it much easier to redefine
> what it means to access rtc?

Yes, and possibly what I had in mind some 5 years ago (as I'm sure I would
have looked at set_rtc_mmss at the time...)

> (If you read a certain value then on average you are halfway
> that second; if you write a certain value you are precisely
> halfway that second. Maybe no delays are needed or desired.)

Calling it a "feature" is clearly easier - no code patched, no flag day
for new behaviour, and no need for user space utils to have to do a 
uname() to see if a 500ms delay is implemented.  The more I think about
it, the better I like this option.

Paul.

--- drivers/char/rtc.c~	Sat Jan  6 05:40:24 2001
+++ drivers/char/rtc.c	Mon Jan  8 04:57:59 2001
@@ -20,6 +20,14 @@
  *	interrupts since the last read in the remaining high bytes. The 
  *	/dev/rtc interface can also be used with the select(2) call.
  *
+ *	The driver also supports ioctls for reading and setting the
+ *	date/time stored in the RTC in a SMP safe fashion (used by
+ *	the [hw]clock program).  Note that for the mc146818 RTC, the
+ *	second for which the RTC is set is half over, by definition.
+ *	Thus your application may require a 0.5 second delay before
+ *	calling this driver to set the RTC time if exact synchronization
+ *	is desired.
+ *
  *	This program is free software; you can redistribute it and/or
  *	modify it under the terms of the GNU General Public License
  *	as published by the Free Software Foundation; either version

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