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

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Message-ID: <2369971.tIcht5GPeB@vostro.rjw.lan>
Date:	Tue, 21 Oct 2014 16:15:11 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Guenter Roeck <linux@...ck-us.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 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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