Re: [PATCH v2 01/47] kernel: Add support for poweroff handler call chain

[Guenter Roeck]

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) ?

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.
Am I missing something ?

Thanks,
Guenter

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