Why is the e500v2 core not using cpuidle?

[Scott Wood]

On 02/02/2013 03:41:27 AM, Thomas Waldecker wrote:
> Hi Scott,
> 
> >> Why is there no support for the cpuidle framework?
> > Because nobody implemented it. :-)
> That's the reason I thought before :-)
> 
> > The only reason I can think of to implement it on this chip would  
> be to
> > dynamically choose when to enter nap versus doze, rather than always
> > just using doze.  It's not clear whether the difference in power
> > savings is worth it -- do you have any way of measuring?
> 
> Is the e500 only using doze? There are comments in the file
> arch/powerpc/kernel/idle_e500.S
> which are stating:
> /*  Go to NAP or DOZE now */
> or
> /* Return from NAP/DOZE ...*/
> 
> and because of this comments I thought that both modes are in use.

e500 can use nap instead, but it's statically chosen via sysctl.  The  
default is doze.  Entering nap requires flushing the cache, so you'd  
only use nap if you care more about lowering idle power consumption  
than performance, and you wake infrequently enough that you're not  
burning more power on the cache flushes than you save with the deeper  
idle state.

> I have a way of measuring the power and it is also a small part of my  
> masterthesis,
> but it is not very meaningful because at the measuring point there  
> are other peripheral
> components too.
> 
> According to the comments can I activate the nap mode somehow?

echo 1 > /proc/sys/kernel/powersave-nap

If you're able to measure a meaningful difference between the two, I'd  
be interested in hearing your results.

> >> How can I debug the e500 idle modes?
> >> Are there any statistics?
> > Top reports idle percentage...
> If the e500 and e500v2 are indeed using only the doze mode it
> would be enough statistics.

Whichever mode you have selected, that will be used for all idling.   
Statistics would only be useful if the idle mode were dynamically  
chosen.

> Such statistics would be great for the doze, nap (and sleep for the  
> whole package).

The only way you'll get into sleep mode is through /sys/power/state.

-Scott