C4 non-strangeness [was: C4 strangeness [was Re: [PATCH] i386 No Idle HZ aka dynticks 051221]

[Dominik Brodowski <linux@dominikbrodowski.net>]

Hi,

On Tue, Dec 27, 2005 at 03:22:38PM +0100, Pavel Machek wrote:
> So... I guess I found out what is going on.
> 
> When power is unplugged, X32 adds C4 state. When power is plugged, X32
> removes C4 state (behaviour Ted seen). When I load ipw2200, this
> behaviour stops, and I see everything up-to C4. Strange. I remember
> ipw had some problems with C3 and C4, perhaps this is related?

Nothing strange at all. The C-States are exported by the BIOS to the OS
using the _CST method/object/whatever. This can change on runtime. When the
BIOS recognizes it is on battery power, the _CST contains the C4 state, if
it is on AC power, the _CST doesn't contain it. The ACPI code follows what
it is told by the BIOS, for it has no chance to know about this additional
C-State if on AC power, and it wouldn't be wise to second-guess the BIOS.

Ipw does limit the max_cstate setting dynamically if it recognizes problems;
however I haven't seen _any_ such things lately on my own system. Might be
related to dyntick being _enabled_, though ;-)

	Dominik
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