RE: [Xen-devel] [PATCH] 1/2: cpufreq/PowerNow! in Xen: Time and platform changes

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Keir Fraser [mailto:Keir.Fraser@xxxxxxxxxxxx]
>Sent: 2007年9月1日 21:58
>
>While I'm hassling you about C states: are there any examples in real
>system
>of available C states changing dynamically? I mean, it makes sense to
>me
>that available power/frequency combinations may change in response to
>something like AC power being removed (this may make higher
>power/freq
>options unavailable). But why would available C states change? I
>thought
>these states were implemented internally on the CPU, and so are either
>universally available or not, and I don't see any physical explanation for
>why available deep-sleep options would be affected by e.g., battery vs.
>AC
>operation.
>
> -- Keir

I have to say that I didn't think it carefully before. Following is the 
description about _CST from ACPI spec (8.4.2.1):

The platform may change the number or type of C States available 
for OSPM use dynamically by issuing a Notify events on the processor 
object with a notification value of 0x81. This will cause OSPM to 
re-evaluate any _CST object residing under the processor object 
notified. For example, the platform might notify OSPM that the number 
of supported C States has changed as a result of an asynchronous AC 
insertion / removal event.

Maybe some hardware people know the tricks inside. :-(

Thanks,
Kevin

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