RE: [Xen-devel] [PATCH] Disable Xen PowerNow! support on Opteron 2nd gen and earlier processors

["Langsdorf, Mark" <mark.langsdorf@xxxxxxx>]

> > It seems to happen most reliably when there's a large jump in
> > frequency (2.6 GHz to 800 MHz or vice versa) but I've seen it
> > happen for smaller switches.  Usually there's 1 large backward
> > movement, and then 2-4 smaller ones.  It doesn't always happen,
> > especially if there are multiple shifts in a short period (ie,
> > constant changes on the ondemand governor) and it seems to
> > happen more consistently when increasing frequency.
> 
> Increasing the frequency requires (1) instructing the regulator to
> increasing the voltage, (2) waiting for it to settle, (3) 
> increasing the multipler, (4) wait for the PLL to settle,
> (5) unpause the CPU.

Essentially, except that frequency shifts have to be performed in
steps of 200 MHz, aside from an elavator jump from 800 or 1000 MHz
to 1600 or 1800 MHz.  So steps 3-5 have to happen repeatedly,
and there are steps 6) instruct the regulator to decrease the
voltage
> 
> Which of these steps are under software control on PowerNow?  
 
For this generation of parts, all steps are under active software
control except unpausing the CPU.

> Since we probably don't know how long we had to wait between when the
> multipler was increased and when the CPU was un-paused, it will be
> necessary to recalibrate the CPU to the global time source (PIT/HPET).

How is that performed?

-Mark Langsdorf
Operating System Research Center
AMD



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