On Thu, 21 Sep 2017, Johannes Stezenbach wrote: > On Thu, Sep 21, 2017 at 02:39:30AM +0200, Rafael J. Wysocki wrote: > > On Wed, Sep 20, 2017 at 6:27 PM, Johannes Stezenbach <j...@sig21.net> wrote: > > > > > > 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. > > > > That's because the i2c bus uses the ignore_children flag that allows > > it to override the general rules. :-) > > Ah! I was looking at Documentation/driver-api/pm only (which is > changed by your patch), but this is documented in Documentation/power > (and obviously I hadn't checked the code, shame on me). > > > direct_complete has nothing to do with this. > > Oh? Reading again, do I get this right: > > 1. simple method: always call pm_runtime_resume() in ->suspend(), > then suspend the driver again > 2. optimization: if pm_runtime_suspended(), the driver's ->suspend() > can possibly do nothing if conditions permit, otherwise it calls > pm_runtime_resume() and then suspends > 3. optimization: tell pm core to skip ->suspend() via return value > from ->prepare() which sets direct_complete > > ...and your patch only deals with 1 and 2. > > Sorry to hijack your thread for side discussion, it was > inadvertant due to my lack of understanding. > > > > First off, the PM core does check the direct_complete flag in > > __device_suspend() and does more-or-less what you are saying. > > > > However, that flag is initialized in device_prepare() with the help of > > the ->suspend() return value, because whether or not it makes sense to > > you mean ->prepare(), right? > > > set that flag depends on some conditions that may change between > > consecutive system suspend-resume cycles in general and need to be > > checked in advance before setting it. > > > > HTH > > It does, however the question remains *why* it needs to check > it in ->prepare() and not right before calling ->suspend(). > Using ->prepare() for the purpose seems wrong since it traverses > the hierarchy in the "wrong" order.
No, it is the _right_ order. If a device's ->prepare() says that direct_complete is okay, but one of its descendants disallows direct_complete, we then want to clear the direct_complete flag in the original device structure. We couldn't do this if we checked the descendant's driver first. > Only right before > calling ->suspend() the driver knows if its current state > allows it to skip any further actions for suspend, because > suspending children or other users may cause pm_runtime_resume() > for it. If the device gets runtime-resumed before ->suspend() would be called then the direct_complete setting doesn't matter. The PM core follows the direct_complete path only if the device is already in runtime suspend when the ->suspend() callback would normally be invoked. And if the device does get runtime-resumed like this, it can't be runtime-suspended again. The PM core makes sure of that. Alan Stern > (In the back of my head I have the scenario of > bug #196861, some completely different driver uses > i2c via ACPI OpRegion during its suspend.) > > > Thanks, > Johannes
