Re: [PATCH] Add p4-clockmod driver in x86-64

[Rutger Nijlunsing]

On Wed, Oct 27, 2004 at 03:59:06PM +0100, Paulo Marques wrote:
> Pallipadi, Venkatesh wrote:
> >>....
> >Yes. Clock modulation is not as useful compared to enhanced speedstep.
> >But, 
> >I feel, it should be OK to have the driver, though it is not really
> >useful 
> >in common case. It may be useful in some exceptional cases. 
> 
> I think I have one of such cases.
> 
> I am one of the members of the robotic soccer team from the University 
> of Oporto, and a couple of months ago we were looking for new 
> motherboards for our robots, because we are starting to need new 
> hardware (on-board lan, usb2.0, etc.).
> 
> We really don't need excepcional performance, but we really, really need 
> low power consumption, so lowering the clock on a standard mainboard 
> seemed to be the best cost/performance scenario.
> 
> Could this driver be used to keep a standard p4 processor at say 25% 
> clock speed at all times?

Nope, p4-clockmod is completely useles. It doesn't slow down the CPU
frequency, it only executes 7000 tick some kind of 'hlt' / 'halt'
instruction out of 8000 ticks (for example, to get 12.5%) just like
Linux's idle routine.

So you've got the _disadvantages_ of a slow clock (programs run
slower), and not the _advantages_ (power consumption is same as idle
CPU and not lower, temperature is same as idle CPU and not lower).

But why does the P4 have such a mode? It uses this mode during thermal
throttling to get to the 'idle' temperature.

Therefore, p4-clockmod is completely misnamed: it's _not_ a cpufreq
driver in the sense that it does not change the frequency. The
documentation should be updated to reflect this (eventually).

In short: p4-clockmod can be emulated in software.

-- 
Rutger Nijlunsing ---------------------------- rutger ed tux tmfweb nl
never attribute to a conspiracy which can be explained by incompetence
----------------------------------------------------------------------
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/