Messages in this thread |  | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH v2 01/47] kernel: Add support for poweroff handler call chain | Date | Tue, 21 Oct 2014 16:15:11 +0200 |
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On Tuesday, October 21, 2014 06:17:09 AM Guenter Roeck wrote: > On 10/21/2014 05:26 AM, Rafael J. Wysocki wrote: > > On Monday, October 20, 2014 09:12:17 PM Guenter Roeck wrote: > >> Various drivers implement architecture and/or device specific means to > >> remove power from the system. For the most part, those drivers set the > >> global variable pm_power_off to point to a function within the driver. > >> > >> This mechanism has a number of drawbacks. Typically only one scheme > >> to remove power is supported (at least if pm_power_off is used). > >> At least in theory there can be multiple means remove power, some of > >> which may be less desirable. For example, some mechanisms may only > >> power off the CPU or the CPU card, while another may power off the > >> entire system. Others may really just execute a restart sequence > >> or drop into the ROM monitor. Using pm_power_off can also be racy > >> if the function pointer is set from a driver built as module, as the > >> driver may be in the process of being unloaded when pm_power_off is > >> called. If there are multiple poweroff handlers in the system, removing > >> a module with such a handler may inadvertently reset the pointer to > >> pm_power_off to NULL, leaving the system with no means to remove power. > >> > >> Introduce a system poweroff handler call chain to solve the described > >> problems. This call chain is expected to be executed from the > >> architecture specific machine_power_off() function. Drivers providing > >> system poweroff functionality are expected to register with this call chain. > >> By using the priority field in the notifier block, callers can control > >> poweroff handler execution sequence and thus ensure that the poweroff > >> handler with the optimal capabilities to remove power for a given system > >> is called first. > > > > Well, I must admit to having second thoughts regarding this particular > > mechanism. Namely, notifiers don't seem to be the best way of expressing > > what's needed from the design standpoint. > > > > It looks like we need a list of power off methods and a way to select one > > of them, so it seems that using a plist would be a natural choice here? > > > Isn't a notifier call chain nothing but a list of methods, with its priority > the means to select which one to use (first) ?
Traditionally, the idea behind notifier call chains has been to call all of the supplied methods (meaning whoever supplied them wants to be notified of events) where the higher-priority ones are called first.
In this particular case, though, we call them until one succeeds to power off the system it seems.
> The only difference I can see is that you would only select one of them, > meaning the one with the highest priority, and not try the others.
Yes, this was my thought.
But if you want a fallback mechanism, then I agree that using notifiers makes sense, although it is not exactly about notifications this time.
I would probably use something along the lines of syscore_ops, but with added execution priority.
-- I speak only for myself. Rafael J. Wysocki, Intel Open Source Technology Center.
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