On Fri, 2007-03-23 at 10:52 -0400, ext tony@xxxxxxxxxxx wrote: > * Igor Stoppa <igor.stoppa@xxxxxxxxx> [070323 09:37]: > > On Fri, 2007-03-23 at 09:17 -0400, ext > > linux-pm-bounces@xxxxxxxxxxxxxxxxxxxxxxxxxx wrote: > > > * Matthew Locke <matt@xxxxxxxxxxx> [070322 21:15]: > > > > > > > > On Mar 22, 2007, at 4:55 PM, David Brownell wrote: > > > > > > > > > On Thursday 22 March 2007 4:21 pm, Rafael J. Wysocki wrote: > > > > > > > > > >>> My answer: there is NO value to such an arbitrary restriction. > > > > >> > > > > >> I'm not talking on restrictions. > > > > > > > > > > You most certainly did talk about them. You said that if the > > > > > hardware doesn't support a "turn CPU off" mode, then you'd > > > > > define that as being incapable of implementing suspend-to-RAM. > > > > > That's a restriction ... a very arbitrary one. > > > > > > > > > > > > > > >> I'm talking on being able to define > > > > >> _anything_ more precisely then just a low-power system-wide state. > > > > > > > > > > Me too. And I'm trying to convey to you the results of the > > > > > investigations I did on that topic. You don't seem to like > > > > > those results though ... > > > > > > > > > > > > > > >> And let's start from just something, please. Like STR and > > > > >> "standby" to begin > > > > >> with? At least on ACPI systems we can distinguish one from the > > > > >> other quite > > > > >> clearly, so why can't we start from that and _then_ generalize? > > > > > > > > > > That's exactly what I did. Looked also at APM, and several > > > > > different SOC designs (AT91, OMAP1, PXA25x, SA1100, more). > > > > > > > > > > The generalization I came up with is what I've described. > > > > > Namely, that coming up with one definition of those states > > > > > that can usefully be mapped all platforms is impractical. > > > > > They're just labels. The platform implementor can choose > > > > > two states to implement, but non-x86 hardware states rarely > > > > > match the expectations of ACPI. > > > > > > > > > > So the fundamental definition needs to be in relative terms, > > > > > because platform-specific differences otherwise make trouble. > > > > > > > > The problem is that a 1:1 mapping between system low power state and > > > > a processor low power state is trying to be forced on every > > > > platform. As Dave pointed out, embedded SoC's provide multiple low > > > > power states that qualify for the suspend-to-ram definition. The > > > > only reasonable platform independent definition is that in STR memory > > > > is powered and contents preserved. The rest is platform specific. > > > > > > > > I think the right answer is that a mechanism for mapping platform > > > > specific states to the system states is needed. Platforms define > > > > their low power states and define the default for each system > > > > state . On x86 platforms, the default just works and is probably > > > > never changed. On embedded platforms, a policy manager can change > > > > the other low power states according to its latency and operational > > > > requirements. > > > > > > Plus the states should be distributed. Trying to force the whole > > > system into certain state turns things messy. > > > > > > Some devices may be active while some are in retention or suspend. > > > > > > Basically everything should idle itself automatically whenever > > > possible based on a idle timer or some other policy, such as > > > suspending a device from user space via sysfs. > > > > The timer sound like a reasonable idea, as long as there is one timer > > for each shared resource, not user. > > > > Example: > > > > Devices A & B share the same voltage domain. > > > > Device A has timeout period Timeout(A) > > Device B has timeout period Timeout(B) > > > > One timer is associated to the voltage regulator/switch and will expire > > at t=TIM > > > > Every time the device d (either A or B) performs some activity, then > > > > TIM = max(TIM, now + Timeout(d)) > > > > When t=TIM (timer expired), then the suspend() function for each device > > is called. > > What problem do you see with with device specific idle timers? That the number of idle timers grows linearly with the number of resources consumers rather than _providers_ See also my comment below. > For example, what's wrong with the following: > > When the device specific idle timer expires, the driver's suspend > function would get called, and the device would release it's clock > and voltage. We might end up doing extra useless activity by saving the state of a device that is re-enabled without even going off. Of course the restoring can be optimised so that it doesn't happen unless the voltage has actually been removed (this implies that the state saving happens in such a way that doesn't compromise the current settings of the device). > Then when a shared voltage domain has 0 users, that voltage domain > can be shut off. Both your and my approaches have drawbacks: in your case the system will probably end up doing extra state saving, but will be ready to perform immediately the transition to off; in my case there will be the overhead of saving the state of the peripherals. However saving state in a preemptive way is decoupled by having idle timers associated to resources providers rather than consumers. > Same thing with clock domains. Clocks is fine, since no saving/restoring is needed, albeit we might consider PLL relock time to fall in this "costy" class of activities. -- Cheers, Igor Igor Stoppa <igor.stoppa@xxxxxxxxx> (Nokia Multimedia - CP - OSSO / Helsinki, Finland) _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
