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From: John Baldwin <jhb@freebsd.org>
To: Adrian Chadd <adrian@freebsd.org>
Subject: Re: svn commit: r264250 - head/sys/dev/acpica
Date: Tue, 8 Apr 2014 09:14:25 -0400
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On Monday, April 07, 2014 10:36:27 pm Adrian Chadd wrote:
> Author: adrian
> Date: Tue Apr  8 02:36:27 2014
> New Revision: 264250
> URL: http://svnweb.freebsd.org/changeset/base/264250
> 
> Log:
>   Add a basic set of data points which count the number of sleep entries
>   that are being done by the OS.
>   
>   For now this'll match up with the "wakeups"; although I'll dig deeper into
>   this to see if we can determine which sleep state the CPU managed to get
>   into.  Most things I've seen these days only expose up to C2 or C3 via
>   ACPI even though the CPU goes all the way down to C6 or C7.

No, those are actually the same thing.  ACPI and Intel both use C-states for
the same thing, but the numbers don't line up.  That is, Intel's C6/C7 gets
exposed to the OS as C2/C3 via ACPI.  The 6/7 does matter, (I think) if you
are using monitor/mwait as I believe the value you configure for an mwait
sleep has to use Intel's number (6/7) whereas the ACPI number (2/3) is
assigned by the results of _CST or whichever object it is ACPI queries.

All that to say that ACPI is already using Intel's C6/C7 if you have 
configured your BIOS to expose it.

-- 
John Baldwin