Temporarily disabling SNB package C-states?

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I wrote a driver for the SNB Xeon iMC SMBUS adapter, but it doesn't
work right due to erratum BT109 (I think).  To work around it, I need
to keep a particular cpu package awake for a few ms when my driver is
in use.  Is there a good way to do this?  So far I've thought of:

 - pm_qos.  This seems hacky and it will keep a lot more of the system
awake than I need.

 - Hook into cpuidle to keep a single core awake.

 - Actually run on the target package, possibly in a kthread or
workqueue, and stay busy.  Ugh.

 - Fiddle with the package c-state limit (in
MSR_PKG_CST_CONFIG_CONTROL) directly.  This seems like it's asking for
trouble.  I'd have to coordinate with intel_idle, presumably.

Any ideas/suggestions?

--Andy
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