On Monday, 25 June 2007 02:01, David Brownell wrote: > On Sunday 24 June 2007, Rafael J. Wysocki wrote: > > On Sunday, 24 June 2007 02:28, Alan Stern wrote: > > > On Sun, 24 Jun 2007, Rafael J. Wysocki wrote: > > > > > > > > Right now the states we have are On, Standby, and Suspend, and the CPU > > > > > runs only in the On state. But on some platforms there could be > > > > > multiple states in which the CPU is able to run, albeit with degraded > > > > > performance. > > That starts to get into those cpufreq/"operating point" discussions". > > > > > > I wouldn't call those system sleep states. For example, ACPI defines system > > > > sleep states as the states in which no instructions are executed by any CPUs > > > > and I think that's reasonable. > > Its Embedded Controller surely executes instructions! Oh, I don't think that they regard the EC as a CPU, but never mind. :-) > > > > Moreover, the ACPI spec insists that transitions between different sleep > > > > states should be made through the On state. > > Not that the world should feel compelled to restrict itself to what > ACPI envisions of course... Sure, but if ACPI does something in a reasonable way, then why not? > > > Okay. But on non-ACPI systems, do we want to restrict the > > > /sys/power/state interface to sleep states? How then should the user > > > tell the system to go to a low-performance run state? Or should that > > > be handled automatically within the kernel? > > > > I think that the /sys/power/state interface should be restricted to system > > sleep states only and we should introduce another one for handling non-sleep > > low-power states. > > Then /sys/power/state would more accurately be named something > like /sys/power/sleep_state, right? Yes, but there's user land that uses this interface. Let's not confuse it. > The core reason it would make sense to have such a bifurcation is > IMO that those lower power operating states mostly shouldn't involve > walking the device tree and suspending everything. They involve some > set of hardware (possibly including CPUs) entering low power states. > But not *every* bit of hardware. > > Another aspect here is what idle state is used. Sometimes that > can be all but indistinguishable from a "sleep" state. Yes. I think we can define a 'system sleep state' as a state that requires the main code path in kernel/power/main.c (starting from enter_state()) to be executed in order for the system to enter it. > > IMHO, the kernel can automatically transition to non-sleep low-power states, > > but the users should be able to define the conditions for that. Also, the user > > should be able to force the transition if necessary/desired. > > There are several ways those lowpower run states can be entered. > But I'm not keen on userspace being able to "force" transitions > like that. > > IMO it's strongly preferable if the platform code -- perhaps its > custom idle loop -- inventories the system regularly to see if > it's eligible to enter one of the states. > > As a rule, if certain resources are in use that means related > power states can't be entered. If they're in use, then userspace > trying to force a transition means breaking something -- else > those resources would stay used! > > The similarity with cpufreq is obvious: cpufreq inventories the > CPU usage regularly to see if the CPU should be revved up or down. > > Now, inputs to that inventory can come from userspace ... there's > information about upcoming usage that the kernel could otherwise > only guess at. User exiting the video playback app, vs just pausing > it, might suggest more aggressive savings are appropriate; etc. > Userspace can also provide tuning parameters. I have no problems > with that model. OK Greetings, Rafael -- "Premature optimization is the root of all evil." - Donald Knuth _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
