On Saturday, September 25, 2010, Kevin Hilman wrote: > Now that we have runtime PM for devices, I'm exploring ways of how to > couple the runtime PM of certain devices with CPUidle transitions. > Ideally, CPUidle should only manage CPU idle states, and device idle > states would be managed separately using runtime PM. However, there are > cases where the device idle transistions need to be coordinated with CPU > idle transistions. This is already a proposed topic for the PM > mini-conf at Plumbers'[1], so this RFC is to get the discussion started. OK > In the wild west (before runtime PM), we managed these special cases on > OMAP by having some special hacks^Whooks for certain drivers that were > called during idle. When these devices are converted to using runtime > PM, ideally we'd like initiate device runtime PM transitions for these > devices somehow coordinated with CPU idle transitions. > > So, I started to explore how to coordinate device runtime PM transitions > with CPU idle transitions. > > One of the fundamental problems is that by the time CPUidle is entered, > interrupts are already disabled, and runtime PM cannot be used from > interrupts disabled context (c.f. thread on linux-pm[1].) This issue should be addressed by Alan, by adding the new flag to struct dev_pm_info that will tell the runtime PM framework that to work with the assumption that interrupts are off. > So that led me down the path of exploring whether we really need to have > interrupts disabled during the early part of CPUidle. It seems to me > that during the time when the governor is selecting a state, and when > the platform-specific code is checking for device/bus activity, > interrupts do not really need to be disabled yet. At least, I didn't > come up with a good reason why they need to be disabled so early, hence > the RFC. > > Here's a simplified version how it works today: > > /* arch/arm/kernel/process.c, arch/x86/kernel/process_*.c */ > cpu_idle() > local_irq_disable() > pm_idle() --> cpuidle_idle_call() > > cpuidle_idle_call() > dev->prepare() > target_state = governor->select() /* selects next state */ > target_state->enter() > /* the ->enter hook must enable IRQs before returning */ > > As a quick hack, I just (re)enabled interrupts in our CPUidle > ->prepare() hook (they're later disabled again before the core idle is > run.) This allowed the calling of device-specific idle functions which > then use runtime PM and thus allows device-specific idle to be > coordinated with the CPU idle. > > So back to the main question... do we really need interrupts disabled so > early in the idle path? > > I'm sure I'm missing something obvious about why this can't work, but > it's Friday and my brain prefers to think about beer rather than > CPUidle. > > Or, as another potential option... > > I just discovered that x86_64 has an atomic idle_notifier called just > before idle (c.f. arch/x86/kernel/process_64.c.) However this is also > done with interrupts disabled, so using this has the same problems with > interrupts disabled. But, what about adding an additional notifier > chain that happens with interrupts still enabled.... hmm, will > ponder that over that beer... I must admit I haven't looked very deeply into the cpuidle code, but certainly there are good reasons to make it collaborate with the I/O runtime PM. It would be good to know if we can relax the handling of interrupts in the cpuidle framework a bit, this way or another. [Added a few CCs to people that may be interested.] Thanks, Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
