Messages in this thread |  | | Date | Tue, 21 Oct 2014 09:11:04 -0700 | From | Guenter Roeck <> | Subject | Re: [PATCH v2 01/47] kernel: Add support for poweroff handler call chain |
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On Tue, Oct 21, 2014 at 04:15:11PM +0200, Rafael J. Wysocki wrote: > 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. > It is the same machanism we are using for the newly introduced restart handler, with the same logic. Notifiers come handy, because the infrastructure is already there, but I consider that to be more of an implementation detail. It is useful in many cases, though, since the notifier_block can be part of a local data structure which can be referenced from the callback using container_of.
If I don't use notifiers, and the callback function doesn't get a reference to its control block, I don't get that reference, and another means to pass context data into the notifier function would be necessary - either a static variable, as widely used so far, or another parameter. While the current code of course permits the use of a static variable, I very much like that it is possible to avoid that by using notifiers.
> I would probably use something along the lines of syscore_ops, but with added > execution priority.
But wouldn't that mean to, for all practical purposes, re-implement a notifier call chain in syscore just for the purpose of naming it differently ?
From a practical side, it would also be a bit awkward since syscore_ops are typically not installed from the same file as the poweroff handler. So I would either have to rearrange code significantly, or install two sysore_ops handlers for the same architecture / platform. Hope it would be ok to do the latter; if the first approach is asked for, I'd rather only implement the core code and not do the full conversion, as it would add more risk than I think it adds value.
Guenter
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