[maemo-developers] Internet Tablet Power Management presentation from linux-pm summit 2007

From: Frantisek Dufka dufkaf at seznam.cz
Date: Wed Jul 11 15:29:52 EEST 2007
Igor Stoppa wrote:
> I certainly will run my tablet at higher speed and/or lower voltage;
> finland makes it unlikely to incur in heating problems ;-)

CPU temperature sensor might be useful to guess the limit and cut the 
speed down in case one is not in Finland :-) Is there one?

>> Does it mean the arm core in current N800 can run at 
>> 400Mhz?
> Yes, the data is for stock N800 (we have this SpeedSorted OMAP2 that can
> run with ARM @ 400MHz).

Cool :-) Well, maybe 'Hot' actually :-)

>> Do you plan to have user selectable power/speed profiles to let people 
>> choose whether they want slower system or shorter battery life?
> My personal belief is that the user should not have to care about this:
> something is broken if the user has to be involved. The system should
> have all the info (and means) to run at good enough speed when needed.

Well, I'm not sure but maybe when being conservative with power saving 
and when some hints are applied (i.e some API) it could work. I'm mainly 
thinking about CPU spikes when applications are starting. I fear the 
system will not react quickly enough with 'overclocking' when 
application starts since otherwise the device does nothing before and 
after. But this specific problem could be solved with some hints done 
from application launcher or maybe kernel or libc (exec/fork call) itself.

I'm not sure how linux currently does it on x86 (shame on me, using XP 
on laptop and linux only in vmware) but my experience with RMClock on XP 
(http://cpu.rightmark.org/products/rmclock.shtml) is that it is 
hard/impossible to tune it in such way that application startup is not 
slower and you still save some power.

> It's a similar case to sleep while idle vs user-controlled suspend: just
> because old devices were doing suspend that doesn't make it desirable.

Yes, this is old discussion. I still think proper suspend is useful. 
While current implementation is great and is mostly what users expect or 
can tolerate, sometimes you simply want to 'pause' or 'hibernate' the 
device no matter what, throw it in the bag and 8 hours (or 10 days) 
later resume exactly where you left off with minimum energy lost. 
Without stopping applications or thinking about anything. This is what 
proper suspend can give you. This is how notebooks and PDAs work. This 
is what would be sometimes useful even on internet tablets (no matter 
how 'always connected' they are supposed to be). But I agree current 
power management on Nokia tablets is great so this is not critical. 
Still I can't resist when this is brought up :-)

I'm not saying this needs to be definitely implemented by freezing 
everything in user and/or kernel space. I'm saying that apart from 
current power saving when idle there should be easy way of telling the 
device to go to sleep completely (pausing audio, disconecting from 

>> Or do you consider some API so apps can suggest how fast system 
>> they want (i.e. media players, games, emulators vs book readers)?
> You mean QoS. Yes, that seems to be the general understanding.

Yes that's it. Didn't know this term is used also in power management, 


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