From owner-freebsd-acpi@FreeBSD.ORG  Wed Oct  3 17:40:23 2012
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Date: Thu, 4 Oct 2012 03:40:14 +1000 (EST)
From: Ian Smith <smithi@nimnet.asn.au>
To: Andriy Gapon <avg@freebsd.org>
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Cc: Sean Bruno <seanbru@yahoo-inc.com>,
	"freebsd-acpi@freebsd.org" <freebsd-acpi@freebsd.org>
Subject: Re: notify userland about C-state changes
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On Wed, 3 Oct 2012, Andriy Gapon wrote:
 > on 03/10/2012 19:21 Sean Bruno said the following:
 > > On Wed, 2012-10-03 at 06:08 -0700, Andriy Gapon wrote:
 > >>>
 > >>> So quick question, does this happen a lot on a system with a
 > >> sporadic
 > >>> workload?  Does this introduce overhead to the system to service the
 > >>> notification requests?
 > >>
 > >> I am not sure who can answer this question.  It is up to ACPI platform
 > >> to decide
 > >> when it changes _available C-states_.  OS doesn't have control over
 > >> that.

If you give us the notification :) we could instrument behaviour on 
different systems, and check for correlation with your [?] ..

 > >>
 > > Hrm ... what changes to the machine would make this happen while the
 > > machine is running?  things like the switching from battery to line
 > > power?
 > 
 > Yes.  Or something else [?] of similar nature/effect.

Docking/undocking a laptop maybe?  LAN/Wireless busy/not (re C3 anyway).  
As you say, up to ACPI code by vendor, modulo ASL tweaks.

 > >> P.S.  I hope you haven't confused this notification for a notification
 > >> about
 > >> _current_ C-state changing. 
 > > 
 > > I did have it confused.  Thanks for putting this note in.
 > 
 > OK :-)

Me too .. my first reaction was ooo, that could get really busy :)

So are you talking about a devd notification to trigger activity like 
the AC/battery script, or a sysctl that scripts could poll, or?

cheers, Ian