On Thursday 13 March 2008, Andi Kleen wrote: > On Thu, Mar 13, 2008 at 09:10:48AM +0100, Pavel Machek wrote: > > Hi! > > > > > > > USB keeps processor out of C3 in many cases. > > > > > > > > I figured that was the case. But I did not see any difference in powertop. > > > > > > Modern Intel mobile processors have a feature called "C2 popup" > > > that allows the processor to retire DMA from C3 without > > > breaking into C0. Instead the processor pops up to C2 > > > where the cache snoop can allow the DMA to retire -- > > > then it returns to C3, all transparent to software. > > > > Does that mean we should go to C3 on modern intels, even with > > busmaster going on, so that cpu can keep going C2..C3..C2 as needed? That decision has already been made for us. BM_STS has been made a no-op on recent processors. It reports bus activity only for a small sub-set of south-bridge devices. Otherwise it tells us there is none and that we should proceed into C3. > C3 is still more expensive power wise to enter, so entering C3 just > to let it immediately go back to C2 for bus mastering would be likely > still a loss over staying at C2. The newer the processor, the less exposed we area to this scenario. cheers, -Len _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
