[maemo-developers] Frequencies scaling with OS2008

[From: Igor Stoppa]

On Mon, 2007-12-31 at 12:40 +0200, ext Siarhei Siamashka wrote:
> On 30 December 2007, Frantisek Dufka wrote:
> > Krischan Keitsch wrote:
> > > 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?
> >
> > Well, yes it looks a bit simplistic now. Even if you play audio decoded
> > by ARM cpu (ogg, real audio) it seems to lock ARM core to 330 and dsp to
> > 220Mhz. I suppose it is because you need pcm dsp task running for audio
> > output and any active dsp task locks it to 220Mhz (and thus cpu to 330).
> > I wonder if it is just simple implementation that can be tuned in next
> > firmwares or there is some fundamental problem (like changing dsp clock
> > of already running dsp task may break it so it is hardcoded to 220).
> 
> ARM cpu frequency is apparently not locked at 330MHz when using pcm dsp task.
> 
> That's why it is faster to do MP3 decoding on ARM core with the 
> current OS2008 firmware. Extra 70MHz of ARM core frequency are more 
> than enough to handle MP3 and there are even some resources left to 
> speed up video decoding. 
> 
> It is interesting if it is possible to lock ARM/DSP frequency at 400/166
> instead of 330/220 when playing video. That would probably improve 
> built-in player performance on some heavy bitrate/resolution videos.
> 
> Also it is interesting to know what is the difference in power consumption 
> between 400/166 and 330/220 modes?

Sadly, because of DSP sw issues, little power saving is possible when
the DSP is running (ARM can still go to idle and most of the processor
can have its clock gated, but it's not possible to reach OMAP
retention). It would be nice to have the DSP able to sleep between
frames; the time taken to decode an mp3 frame is significantly shorter
than the related playback.
However the voltage used @330MHz is the same used @400MHz and the
associated DSP clock is actually lower, therefore it _might_ be that the
power consumption is the same or lower.

If anybody has time and the means to measure it (*), it would certainly
be interesting to do a comparison.


* a clock is enough if one is sufficiently patient: fully charge the
battery and measure how many hours of playback can be achieved in each
mode before the battery goes flat
this requires _exclusive_ utilization of the system by the player,
therefore one has to give up doing anything while the test is ongoing
Also, note that for the test to be reliable, it should be executed on a
freshly flashed unit with stock sw (apart from the modified kernel) and
no extras whatsoever

-- 
Cheers, Igor

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