[maemo-developers] Frequencies scaling with OS2008

[From: Igor Stoppa]

Hi,
On Sun, 2007-12-30 at 17:16 +0100, ext Krischan Keitsch wrote:
> Hi,
> 
> I read an interesting thread about overclocking the n800 [1]. Based on that I 
> started experimenting with the n800 running OS2008. The OS scales the cpu 
> frequency nicely from 165MHz up to 400MHz.
> 
> The current cpu scaling can be checked with
> 
> cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq
> 
> So far so good. I noticed that during playback of mp3 files the cpu is scaled 
> to 330MHz. Stressing the cpu during mp3 playback will not change the 
> scaling - the cpu will not scale up to 400MHz. 
> According to a presentation "Nokia Internet Tablet Power Managemen" [2] the 
> dsp runs at 220MHz when the cpu is set to 330MHz.
> (This is the default setting in OS2007, by the way.) 
> 
> I was wondering if the device really needs to run at 300MHz (220MHz dsp) for 
> mp3 playback? Is the max dsp power needed for such a task? Or would 220MHz 
> (177MHz dsp) or 165MHz (85MHz dsp) be sufficient? Would a lower dsp scaling 
> save even more battery?

The issues are:
- doing dvfs while ther is audio playback will generate audible "ticks"
because of problems with the dsp dma
- the dsp is always set to support even the weirdest cases, therefore
there is a lot of spare DSP horsepower that could be cut off if you knew
that your mpu load is not going to change too much and it will be low

Practically speaking: if you remove the constraint of fixed MPU at 330MHz
DSP at 220MHz when there is a dsp task active, you might end up in a
situation where you are listening to MP3 @ low OMAP clock, then start
the browser, therefore causing the OMAP clock to change rapidly and get
lots of garbage on the audio out.


-- 
Cheers, Igor

Igor Stoppa <igor.stoppa at nokia.com>
(Nokia Multimedia - CP - OSSO / Helsinki, Finland)