Re: [PATCH v4 3/7] acpi-cpufreq: Add support for disabling dynamicoverclocking

From: Matthew Garrett
Date: Sat Jul 23 2011 - 23:33:23 EST

On Sat, Jul 23, 2011 at 11:15:02PM -0400, Andy Lutomirski wrote:

> I was curious and looked it up. Intel SDM volume 3, says:
> Opportunistic processor performance operation can be disabled by
> setting bit 38 of
> IA32_MISC_ENABLE. This mechanism is intended for BIOS only. If
> IA32_MISC_ENABLE[38] is set, CPUID.06H:EAX[1] will return 0.

Hm. Interesting. The observed behaviour is that it works fine if we
toggle this ourselves, and I don't remember the version of the docs I
was looking at adding the BIOS proviso. Having said that...

> System software can temporarily disengage opportunistic processor
> performance
> operation by setting bit 32 of the IA32_PERF_CTL MSR (0199H), using a read-
> modify-write sequence on the MSR.

This complicates things a little, since right now we just write the
firmware's P state value directly into PERF_CTL. We'd need to add code
to acpi_cpufreq_target to make sure that it masked that bit off. It's a
little more awkward, but if we're being told not to do it by just
hitting the bit in MISC_ENABLE it's probably worth it. I'll try to
handle that this week.

Matthew Garrett | mjg59@xxxxxxxxxxxxx
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