Messages in this thread | | | Date | Fri, 18 Sep 2015 02:13:39 +0200 | Subject | Re: [PATCH] driver core: Ensure proper suspend/resume ordering | From | "Rafael J. Wysocki" <> |
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On Thu, Sep 17, 2015 at 11:06 PM, Alan Stern <stern@rowland.harvard.edu> wrote: > On Thu, 17 Sep 2015, Rafael J. Wysocki wrote: > >> Note: Problems also may happen if device A depends on device B and its >> driver to be present and functional and then the B's driver module is >> unloaded. The core doesn't prevent that from happening AFAICS. > > It also doesn't prevent B's driver from being unbound from the B > device. > > To some extent the kernel _does_ prevent driver modules from being > unloaded. If A's driver uses code resources provided by B's driver > then the module's refcount would be larger than 0.
Right.
>> I'd like to go back to my initial hunch that the driver knowing about >> a dependency on another one should tell the core about that, so the >> core can make the right things happen at various times (like system >> suspend/resume etc). >> >> What if we introduce a mechanism allowing drivers to say "I depend on >> device X and its driver to be present and functional from now on" and >> store that information somewhere for the core to use? >> >> Some time ago (a few years ago actually IIRC) I proposed something >> called "PM links". The idea was to have objects representing such >> dependencies, although I was not taking the "the driver of the device >> I depend on should be present and functional going forward" condition. >> >> Say, if a driver wants to check the presence of the device+driver it >> needs to be functional, it will do something like >> >> ret = create_pm_link(dev, producer); >> >> and that will return -EPROBE_DEFER if the producer device is not >> functional. If success is returned, the link has been created and now >> the core will take it into account. >> >> On driver removal the core may just delete the links where the device >> is the "consumer". Also there may be a delete_pm_link(dev, producer) >> operation if needed. >> >> The creation of a link may then include the reordering of dpm_list as >> appropriate so all "producers" are now followed by all of their >> "consumers". Going forward, though, the core may use the links to >> make all "producers" wait for the PM callbacks of their "consumers" to >> complete during system suspend etc. It also may use them to prevent >> drivers being depended on from being unloaded and/or to force the >> removal of drivers that depend on something being removed. In >> principle it may also use those links to coordinate runtime PM >> transitions, but I guess that's not going to be useful in all cases, >> so there needs to be an opt-in mechanism for that. >> >> Please tell me what you think. > > Sounds familiar. I recall this basic approach from a Plumbers > conference some years ago -- maybe that was when you first proposed it! > > You might want to categorize the dependencies into different types. I > can think of three types offhand: > > The target device must be present before the current device > can be probed (hard to imagine how that could be stored as a PM > link if the target device isn't present, though);
Right, but there is a tricky part here. The presence of the device object need not imply that the device is physically present. :-)
> The target device must be bound to a driver before the current > device can be probed; > > The target device must be at full power whenever the current > device is.
Or even before attempting to put the current device at full power.
> Maybe you can think of others. > > [Oddly enough, the USB subsystem has some dependencies that don't fall > into any of these categories. They have to do with the peculiar way in > which a low- or full-speed device is handed off from a high-speed > controller to its companion low/full-speed controller, and they apply > only to system resume, not to normal operation. (That is, device A > requires device B to be at full power when A is being resumed from a > system sleep, but not when A is operating normally or when A is being > runtime-resumed.) For such things, we should keep the existing > device_pm_wait_for_dev() API.]
Absolutely. The idea is to use the existing APIs for that where it makes sense.
> This sounds like a big change, but it might be worthwhile.
Well, the more I think about that the more it seems to me that some redesign is needed.
Thanks, Rafael
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