[coreboot] Lenovo X200 running Coreboot drains 3-4W more power than with Vendor BIOS

[Daniel Kulesz]

> > - disabling "cpu power management" makes the idle consumption raise to 12,8W
> Is this 12.8W compared to 7.5W (i.e. with lowest backlight)?
> 

Nope, I am only comparing with highest backlight now, so it is 12.8W versus 10.0W here.

> > - disabling "PCI Bus power management" and "PCI express power management" makes the idle consumption raise to 13,3W
> > - disabling the AMT firmware had no effect
> > - running the stress test still drains only 24,2W
> > - performance is the same as before
> >
> > I still don't understand the whole performance issue. Therefore, I took
> > another X200 with a P8600, CCFL screen and an older vendor BIOS and
> > re-ran the benchmark there --- with almost identical results.
> > 
> > So in the end I'm just confused. This would mean that running Coreboot
> > makes the X200 *much* faster at the expense of battery life, both in
> > idle and under stress conditions.
> > 
> Maybe Lenovo limited the processor clock on purpose to get a better
> battery life. Maybe it's just an unexpected side effect of running
> Linux (not Windows, what Lenovo tested against). Anyway, I wouldn't care
> about the power consumption under load, it might even result in a longer
> battery life: Being faster means shorter periods in higher performance
> states. The idle power consumption is what really matters.
> 
Yes I agree, I am almost never putting high load on the machine anyways, so even if it would make an impact I would not care too much. And then you can still limit the max. frequency or something and be performance-wise probably still comparable to the vendor BIOS.

> > Any ideas which could solve this mystery?
> >
> One more thing you can test, in case your Linux uses the intel_idle
> driver: There is a kernel parameter intel_idle.max_cstate, if you boot
> the vendor BIOS with defaults and Linux with
>   intel_idle.max_cstate=2
> it should use C1/C2 but not C3/C4 and thus behave more like coreboot.
> 
Okay. I was just aware of the generic "processor.max_cstate=2" parameter. I tried with both parameters using the vendor BIOS and here are the results:

intel_idle.max_cstate=2: 10W in idle at full brightness (no effect)
processor.max_cstate=2: 12.6W in idle at full brightness

So it seems plausible that this issue is related to Coreboot not (properly?) supporting C3/C4. So this is a known issue then? If yes, imho it should be *definitely* documented on the wiki page since it could be a "showstopper" for many adopters who need the maximum battery life the X200 is able to deliver only with the vendor BIOS at this point in time ...

Cheers, Daniel