On Sunday, May 11, 2014 12:46:10 PM Alan Stern wrote: > On Sat, 10 May 2014, Rafael J. Wysocki wrote: > > > On Thursday, May 08, 2014 09:52:18 PM Alan Stern wrote: > > > On Thu, 8 May 2014, Rafael J. Wysocki wrote: > > > > > > > Well, no. > > > > > > > > The reason why that doesn't work is because ->prepare() callbacks are > > > > executed in the reverse order, so the perent's ones will be run before > > > > the ->prepare() of the children. Thus if ->prepare() sets the flag > > > > with the expectation that ->suspend() (and the subsequent callbacks) > > > > won't be executed, that expectation may not be met actually. > > > > > > That's true also if the flag gets set in ->suspend(), isn't it? A > > > driver may set direct_resume in its ->suspend() callback, expecting > > > that the subsequent callbacks won't be executed. But if a descendant > > > hasn't also set its flag then the callbacks _will_ be executed. > > > > No, that's not possible with the current patch, because __device_suspend() is > > executed for descendants first and then for ancestors and it clears > > direct_suspend for the parents of devices that don't have it set. This means > > that the ancestor's ->suspend() will see the flag clear if it is unset for > > any of its descendants. > > > > IOW, the only case in which the ancestor's ->suspend() sees the flag set is > > when it has been set for all of its descendants. Thus, if it leaves the > > flag set, the late/early and noirq callbacks won't be executed for it. > > > > Now, there is a reason for concern in that, because ->suspend() may set the > > flag as a result of an error and that may lead to unexpected consequences. > > Ah, my mistake; I should have read the patch more carefully. I didn't > realize that your plan was for subsystems/drivers to set the flag > during ->prepare() and then clear it (or leave it set) during > ->suspend(). > > > Then the parent will have direct_resume unset. That is not a concern. > > The only concern to me is possible errors in ->suspend() setting the > > flag when it shouldn't. > > So now one question is: Why would a subsystem or driver want to clear a > flag that it had set earlier? I can't think of any good reasons. The > only obvious possibility would be if the wakeup requirements got > changed between ->prepare() and ->suspend(), but that should never > happen because wakeup settings are changed by userspace and userspace > will be frozen. > > Another question is: Does a subsystem or driver need to know if the > original flag setting couldn't be honored? Again, I don't think it is > necessary to call ->suspend() just for this reason. More precisely, I > think it will be good enough to call ->suspend() when the flag is clear > (either because a descendant device didn't set its flag or because the > device is no longer in runtime suspend); if the flag is set then there > is no reason to call ->suspend(). The subsystem can assume that > ->suspend() won't be called; then if it does get called, the subsystem > will realize something has changed. > > Thus, a suitable algorithm now appears to be: > > Have subsystems/drivers set the flag during ->prepare(). They > don't even have to check if the device is runtime-suspended; > if it isn't then the PM core will turn off the flag later. > > In __device_suspend(), before invoking the ->supend() callback, > check the flag. If it is still set and if the device is > runtime-suspended (a barrier may be necessary here), skip > ->suspend() and the following callbacks. Otherwise clear the > parent's flag and proceed as usual. > > > > Several of these questions are a lot easier to answer if the flag gets > > > set during ->prepare() rather than ->suspend(). I actually decided to go that way with one difference. I think it's better to make the PM core own the new flag, so that bus types/drivers don't have to set/clear it, so I got back to my very first idea about possibly returning positive values from ->prepare(). The idea is this: - If ->prepare() returns a positive number, that means "this device is runtime-suspended and you can leave it like that if you do the same thing for all of its descendants". - If that happens, the PM core sets the new flag for the device in question *if* the device is indeed runtime-suspended *and* *if* the transition is a suspend (and not hibernation, for example). Otherwise, it clears the flag for the device. All of that happens in device_prepare(). - In __device_suspend() the PM core clears the flag for the device's parent if it is clear for the device to ensure that the flag will only be set for a device if it is also set for all of its descendants. - PM core skips ->suspend/late/noirq and ->resume/early/noirq for all devices having the flag set - so the flag can be called "direct_complete" as it causes the PM core to go directy for the ->complete() callback when set. - The ->complete() callback has to check direct_complete if ->prepare() returned a positive number previously and is responsible for further handling of the device. > > I agree with that, but I have one concern about this approach. Namely, > > in that case the PM core has to use pm_runtime_resume() or equivalent to > > resume devices with the flag set during the device resume stage. Now, > > in the next step we may want to leave certain devices suspended at that > > point and the PM core has no way to tell which ones. Also subsystems > > don't really have a chance to tell it about that (they would need to > > know in advance during ->prepare(), which is kind of unrealistic, or > > perhaps it isn't). > > > > However, if ->resume() is called for devices with the flag set, like in > > my most recent patch, the subsystem may decide not to resume the device > > if it knows enough about it. > > > > This pretty much is my only concern here, so I'm open to ideas how to deal > > with leaving devices suspended (if possible) during the device resume stage. :-) > > This is a good question. I'm not sure of the best answer at the > moment. > > > For one, postponing the resume to ->complete() is an option, but it will have > > to be done with care, because the ->complete() callbacks are executed > > sequentially, so calling pm_runtime_resume() from there is rather out of the > > question. > > Calling pm_request_resume() would be okay, though. > > There's another aspect to this we need to consider: hibernation. I'm > quite sure we don't want to come out of hibernation thinking that > devices are still in their runtime-suspended states. Skipping the > callbacks for freeze and thaw would be all right in principle, and > maybe even for poweroff, but not for restore. And of course, during > the restore stages, the last thing subsystems and drivers will remember > happening is freeze -- which might mean you shouldn't skip the freeze > callbacks either. I agree. I think the outline above should address this too. I'm going to send a new version of the patches implementing the idea above. 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
