Messages in this thread | | | Date | Wed, 20 Sep 2017 18:27:31 +0200 | From | Johannes Stezenbach <> | Subject | Re: [PATCH] PM: Document rules on using pm_runtime_resume() in system suspend callbacks |
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On Wed, Sep 20, 2017 at 04:01:32PM +0200, Rafael J. Wysocki wrote: > On Wed, Sep 20, 2017 at 2:28 PM, Ulf Hansson <ulf.hansson@linaro.org> wrote: > > On 20 September 2017 at 02:26, Rafael J. Wysocki <rjw@rjwysocki.net> wrote: > >> > >> Second, leaving devices in runtime suspend in the "suspend" phase of system > >> suspend is fishy even when their runtime PM is disabled, because that doesn't > >> guarantee anything regarding their children or possible consumers. Runtime > >> PM may still be enabled for those devices at that time and runtime resume may > >> be triggered for them later, in which case it all quickly falls apart. > > > > This is true, although to me this is a about a different problem and > > has very little to do with pm_runtime_force_suspend(). > > > > More precisely, whether runtime PM becomes disabled in the suspend > > phase or suspend_late phase, really doesn't matter. Because in the end > > this is about suspending/resuming devices in the correct order. > > Yes, it is, but this is not my point (I didn't make it clear enough I guess). > > At the time you make the decision to disable runtime PM for a parent > (say) and leave it in runtime suspend, all of its children are > suspended just fine (otherwise the parent wouldn't have been suspended > too). However, you *also* need to make sure that there will be no > attempts to resume any of them *after* that point, which practically > means that either runtime PM has to have been disabled already for all > of them at the time it is disabled for the parent, or there has to be > another guarantee in place. > > That's why the core tries to enforce the "runtime PM disabled for the > entire hierarchy below" guarantee for the devices with direct_complete > set, but that may just be overkill in many cases. I guess it may be > better to use WARN_ON() to catch the cases in which things may really > go wrong.
I read this half a dozen times and I'm still confused. Moreover, Documentation/driver-api/pm/devices.rst says:
Runtime Power Management model:
Devices may also be put into low-power states while the system is running, independently of other power management activity in principle. However, devices are not generally independent of each other (for example, a parent device cannot be suspended unless all of its child devices have been suspended). ...
However, isn't this a fundamental difference of runtime suspend vs. system suspend that parent devices *can* be runtime suspended before their children? E.g. an audio codec could keep running while the i2c bus used to program its registers can be runtime suspended. If this is correct I think it would be useful to spell it out explicitly in the documentation.
During system suspend, pm core will suspend children first, and if the child's ->suspend hook uses the i2c bus to access registers, it will implicitly runtime resume the i2c bus (e.g. due to pm_runtime_get_sync() in i2c_dw_xfer()). Later pm core will ->suspend the i2c bus.
I have a hunch the root of the problem is that ->prepare walks the tree in top-down order, and its return value is used to decide about direct-complete. Why does it do that? Shouldn't pm core check the direct_complete flag during ->suspend if the device is in runtime suspend, to decide whether to skip runtime resume + ->suspend for *this* device?
Johannes
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