Re: p4-clockmod doesn't seem to allow the highest possible speed

From: Dominik Brodowski <linux@dominikbrodowski.de>
To: Wouter Verhelst <wouter@grep.be>
Cc: Ducrot Bruno <ducrot@poupinou.org>, cpufreq@www.linux.org.uk
Subject: Re: p4-clockmod doesn't seem to allow the highest possible speed
Date: Sun, 11 Jan 2004 21:06:18 +0100	[thread overview]
Message-ID: <20040111200617.GA25713@dominikbrodowski.de> (raw)
In-Reply-To: <20040111195619.GA25048@grep.be>

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On Sun, Jan 11, 2004 at 08:56:19PM +0100, Wouter Verhelst wrote:
> On Sun, Jan 11, 2004 at 06:35:47PM +0100, Dominik Brodowski wrote:
> > On Sat, Jan 10, 2004 at 08:35:26PM +0100, Wouter Verhelst wrote:
> > > That happens both when it's in P0 or P1.
> > 
> > So ACPI works to set the processor in P0 and P1?
> 
> Yes.

Excellent.
> 
> > If so, you really really should use the ACPI cpufreq driver instead of
> > the p4-clockmod driver. The former will offer you true "SpeedStep"
> > support, the latter only some clock modulation which won't save you
> > much energy as ACPI supports the C2 idle state anyway. 
> 
> Ah, so what p4-clockmod does is not SpeedStep?
> 
> Hm. That sucks.

No. p4-clockmod stops the CPU for certain short periods of time, it
"modulates" the CPU frequency. 

------------------> time  axis

x x x x x x x x x   normal clock ticks
x x x x - - - - x   50 % frequency modulation [ACPI T-States or P4-clockmod]
x  x  x  x  x  x    frequency scaling     [ACPI P-States]

Only if the frequency is scaled, the voltage can be scaled also. So only
that offers the best reduction of energy usage. For details, check the
cpufreq archives for a lengthy thread with many calculations...

> > > Not that it matters -- p4-clockmod seems to do it right :)
> > 
> > It's not as noisy, indeed. But it's not right -- as long as acpi.ko works.
> 
> It mostly does, although I've had lockups when using the /proc interface
> to switch CPU frequencies -- yes, should've reported that, but I only
> now remember :)

The /proc/ interface is deprecated, and the acpi module is full of errors
[I'm working on it, though...]. You should use the /sys-fs interface with
the acpi.ko module as well.

> That does not change.

Very strange... I'm out of ideas for the moment being.

	Dominik

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