Re: Processor state C6 only used in battery mode, workaround possible?

[garkein@mailueberfall.de]

Am Mittwoch, den 18.03.2009, 14:08 -0400 schrieb Len Brown:
> On Sun, 29 Jun 2008, garkein@mailueberfall.de wrote:
> 
> > Hello,
> > with recent powertop, I could confirm that my T61 uses the C6 processor
> > state only in battery mode:
> > 
> > (Kernel 2.6.25)
> > 
> > AC:
> > Your CPU supports the following C-states : C1 C2 C3 C4 C5 C6 
> > Your BIOS reports the following C-states : C1 C2 C3
> > 
> > Battery:
> > Your CPU supports the following C-states : C1 C2 C3 C4 C5 C6 
> > Your BIOS reports the following C-states : C1 C2 C6
> > 
> > Since the notebook runs quite a bit cooler, and thus less noisy (fan) in
> > battery mode, I would like to have it use that C6 state also in AC mode.
> > Is it possible, maybe with some slight tweaks, to convice the kernel (or
> > BIOS?) to also use that C6 state when in AC mode?
> 
> Lenovo decided when they designed the platform to do extra work to
> switch what ACPI C3 means on AC vs DC.  They chose to have lower latency
> on AC, and save energy on DC.  (you will notice lower latency reported
> for C3 when on DC if you look in /proc/acpi/processor/*/power or
> in /sys/devices/system/cpu/cpu*/cpuidle/*/latency
> eg. it may switch between 57usec and 17usec
> 
> if you boot with "processor.nocst=1"
> then you'll use the legach FADT method of specifying C-states,
> and will lose the AC/DC switching capability, and also lose
> the MWAIT capability -- though you'll unlikely notice.
> So you'll get stuck with a single C3 state -- dunno if it
> is the one you prefer or the one you don't prefer...
> 
> If this doesn't help and you want to hack your driver,
> then just go to acpi_processor_notify() in processor_core.c
> and disable the call to acpi_processor_cst_has_changed()
> and boot on DC...
> 
> The long term answer to your question is that we are hoping
> to get permission to ship a native intel_idle driver which
> will give Linux the option to ignore the ACPI BIOS in this 
> area.
> 
> cheers,
> -Len

Thank you Len for your explanation. The long term way would be my
preference as I don't want to be "stuck" with a single state. (I want
them all! ;-)).

Regards,
Daniel