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Re: enhanced speed step on AMD64

On Mon, May 24, 2010 at 12:12:45AM +0000, wrote:
> Can anybody describe where "we" are at for supporting speedstep on CPUs
> like the i7, then? Will it require an entire new framework to support? Can
> somebody describe where the "problem" exists between this hardware and
> the current NetBSD support for Speedstep on it? I'm interested both for
> the practical matter of getting this CPU supported, and it's something
> I just don't know anything about, so a "teachable moment" :)

Basically, frequency changes in Intel-land done in one of three ways:

(1) The original Speed Step. The frequency changes are done by magic
calls into the BIOS and (AFAIK) a function of the chipset.

(2) EST until ~Core 2. Frequency and voltage are tabularised (older
CPUs) or derived algorithmically from certain MSR (machine status
registers). Change of the performance settings is done via MSR under
application control.

(3) EST since late Core2. The MSRs are mostly discontinued and/or
provide junk data. The kernel is supposed to use the ACPI Processor
object and its method for this purpose. Doesn't work for all older model
as it was often left out.

At the moment, (2) works best for the CPUs it can work on. (1) might or
might not work. (3) is not supported.


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