On Mon, 03 May 2010 10:01:50 -0700 Kevin Hilman <khilman@xxxxxxxxxxxxxxxxxxx> wrote: > > Well, shallow could mean that the state lacks the CPUIDLE_FLAG_DEEP > > flag; that should be relatively portable. In any case, it seems > > more so than "if I put in a 55us latency requirement, I'll stay out > > of C3". > > I guess it depends on your goal. Do you just want to stay out of C3 > on your current platform? or do you want to stay out of any low-power > state (on any platform) where you'll have a latency of > 55 usecs? My case is a camera driver; as long as latencies are on the scale of the frame rate, things won't go too wrong. 1ms latencies would not be a huge problem. What happens is that the hardware corrupts the data if it goes into C3 while video capture is happening. Comments in drivers/net/e1000e/netdev.c suggest that they were dealing with similar issues. > The former is not portable (as C-states don't have the same meanings > across arches/SoCs) where as using a real-world number in usecs will > have meaning on any platform. Understood. But the end result is that I've ended up specifying a latency requirement that is far tighter than the situation really needs in order to achieve the real goal. > My main concern is that drivers not be written with latency > constraints that assume an Intel-centric set of power states. There > are other SoCs out there with different sets of states and > corresponding latencies, so keeping things in real-world numbers > (latency, throughput, etc.) seems to me to be the only portable way. Understood. Perhaps the pm_qos interface isn't the best way to achieve the real goal here, but it's all we have at the moment. Thanks, jon _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
