On Wed, 6 May 2015, Ulf Hansson wrote: > On 30 April 2015 at 16:53, Alan Stern <[email protected]> wrote: > > On Thu, 30 Apr 2015, Ulf Hansson wrote: > > > >> I hesitated to send this reply, since it might add confusion. If > >> that's the case, please ignore it. > >> > >> I have a long term vision to fully enable support for a runtime PM > >> centric configuration for drivers/subsystems. The idea is, that such > >> driver/subsystem should get system PM for "free". > >> > >> The main goal is to simplify PM implementation for these > >> drivers/subsystems. > >> > >> They should need to implement the runtime PM callbacks only and not > >> the system PM ones. During system PM suspend, the requirement is that > >> the corresponding devices should be guaranteed to be "runtime PM > >> suspended". Somehow that then needs to be managed by the PM core. > >> > >> I am not sure it's doable, but I wanted to bring it up within the > >> context of $subject patch, since it proposes yet another optimization > >> path for runtime PM during system PM. > > > > I suspect it is _not_ doable. Consider a reasonable scenario: a driver > > that does pm_runtime_get_sync() in its open routine and > > pm_runtime_put() in its release routine. If a user process holds the > > device file open during a system suspend, it will be impossible for the > > PM core to do a runtime suspend. > > Alan, thanks for your reply. > > There are certainly drivers/subsystems that can't full-fill the > requirements to have the PM core to deal with what I propose. Somehow > drivers/subsystems would have to announce its capability for this. > > Those drivers/subsystems I have been looking at, is dealing with I/O. > Typically platform/amba devices, which drivers has registered > subsystem specific callbacks at ->probe(). One of these callbacks are > invoked when there is an I/O request to serve from the subsystem's > core layer. > > In the beginning of that callback, pm_runtime_get_sync() is invoked. > When the request has been served and the controller can be runtime PM > suspended, the driver call pm_runtime_put() or possibly > pm_runtime_put_autosuspend(). > > These drivers/subsystem may be considered as being "runtime PM > centric", since during system PM suspend they don't have any system PM > specific things to deal with. They only want to make sure their > devices becomes "runtime PM suspended". > > There's no doubt that they can do that by implementing the system PM > ->suspend() callbacks, in one way or the other. > > To simplify PM implementation for these drivers/subsystems, it would > have been nice if the PM core could handle this "automagically", thus > drivers/subsystems wouldn't have to implement the system PM callbacks > at all. Reaching that point, would likely make it easier to understand > how to implement a "runtime PM centric" driver/subsystem.
The drivers/subsystems don't have to implement these things, because you have _already_ implemented them: pm_runtime_force_suspend() and pm_runtime_force_resume(). A driver/subsystem merely has to store pointers to these routines in its dev_pm_ops structure. > > On the other hand, there's nothing to prevent drivers from setting > > their ->suspend and ->runtime_suspend structure members to point at the > > same routine. The routine would need to handle the case where it was > > called for a system suspend while the device was already runtime > > suspended, but that doesn't seem too hard. With the "direct-suspend" > > option, even this wouldn't be necessary. > > That would likely work, but again it would require drivers/subsystems > to assign system PM callbacks. You said just above that the driver/subsystem would have to announce its capability for this somehow. Using suitable callback pointers would be a good way to make that announcement. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
