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SubjectRe: [PATCH] * RTC problem ???
Rafael Reilova writes:
>
>
> On Wed, 26 May 1999, Richard Gooch wrote:
>
> > Rafael Reilova writes:
> > > Lastly, the problem we are trying to solve seems to be mostly a non-issue,
> > > at least at boot, since hwclock can set the system clock properly before
> > > anything important happens. The more interesting problem is the one
> > > Richard brought up with APM. I think it goes like this:
> > >
> > > 1) DST changeover is about to happen
> > >
> > > 2) the machine goes into APM, before doing so the kernel in apm.c
> > > computes a timezone offset to be used when coming out of APM.
> > >
> > > 3) timezone changeover occurs due to DST going on or off.
> > >
> > > 4) machine comes out of APM, read CMOS and applies wrong offset, sytem
> > > time is now warped.
> >
> > No, this isn't what is happening. The problem is that some machines
> > slow down the RTC measurment to the point where the measurement may be
> > inaccurate by several seconds. Once the lid is closed, it's too late
> > to do an accurate RTC measurement.
>
> OK understood, but is the above scenario possible? I took a quick look at
> apm.c and it seemed to fall for such a situation so I thought that was
> what you meant. Happens for posting without checking the old mail...grr

Yes, I guess that's possible.

> About the slowdown, I guess the situation could be detected by doing
> two successive reads and checking if they differ significantly. Now
> what to do about it is another matter. Maybe we could use a
> previously computed offset calculated last time the system time was
> changed?

The simplest approach would be to compute the offset whenever the
system time is changed (as with netdate or rdate). Stephen is working
on a better algorithm than what we have now.

Regards,

Richard....

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