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

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Date:	Tue, 21 Oct 2014 06:17:09 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Alan Cox <gnomes@...rguk.ukuu.org.uk>,
	Alexander Graf <agraf@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Heiko Stuebner <heiko@...ech.de>,
	Lee Jones <lee.jones@...aro.org>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Philippe Rétornaz <philippe.retornaz@...il.com>,
	Romain Perier <romain.perier@...il.com>
Subject: Re: [PATCH v2 01/47] kernel: Add support for poweroff handler call
 chain

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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