clock problem

[M. Warner Losh]

In message: <1178900585.1231.63.camel at zoot.mintel.co.uk>
            Tom Evans <tevans.uk at googlemail.com> writes:
: On Fri, 2007-05-11 at 08:53 -0600, M. Warner Losh wrote:
: > In message: <20070511110759.L700 at thinkpad.dieringer.dyndns.org>
: >             Martin Dieringer <martin.dieringer at gmx.de> writes:
: > : This is NOT a hardware problem. 1. I have this on 2 machines, 2. the
: > : problem is solved by switching to ACPI instead of APM
: > 
: > It is a hardware problem.  APM + powerd changes the frequency of the
: > TSC.  If the TSC is used as the time source, then you'll get bad
: > timekeeping.  ACPI uses its own frequency source that is much more
: > stable and independent of the TSC, so switching to it fixes the
: > problem because you are switching the hardware from using a really bad
: > frequency source with ugly steps to using a good frequency source w/o
: > steps.
: > 
: > Warner
: 
: Surely that would imply that it is a software misconfiguration issue. If
: the TSC is unreliable under fairly standard duties, and there exists an
: alternate source that is reliable, surely that indicates the
: manufacturer has identified a problem, and solved it with alternate
: hardware.
: 
: The failure then to use the correct hardware is a software
: misconfiguration.

TSC is very accurate if you don't have the clock frequency slammed
around, which is why its quality is listed as 800 and the i8254 is
listed as 0.  If you do anything that slams the TSC frequency, then
you need to reconfigure the timecounter used.

It is hard for the timekeeping part of the software to know if you are
on a sane system (TSC-wise) or an insane one.

Warner