On 6 May 2015 at 16:43, Alan Stern <st...@rowland.harvard.edu> wrote: > On Wed, 6 May 2015, Ulf Hansson wrote: > >> On 30 April 2015 at 16:53, Alan Stern <st...@rowland.harvard.edu> 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.
Yes, apparently I have touched this topic earlier. :-) As we are moving towards removing the Kconfig option CONFIG_PM_SLEEP (CONFIG_PM_RUNTIME is already removed) and only have the CONFIG_PM, parts of what the above helper functions addresses becomes redundant. > >> > 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. I was thinking, for simplicity purpose, that we could try to move this a step forward. Having a API like pm_runtime_centric() (or whatever name we can come up with) to announce the capability, could be easier to understand and use. Of course that's my opinion. Kind regards Uffe -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/