linux-kernel - Re: [RFC][PATCH] PM: Introduce new top level suspend and hibernation callbacks (rev. 3)

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Message-ID: <Pine.LNX.4.44L0.0803251024120.4838-100000@iolanthe.rowland.org>
Date:	Tue, 25 Mar 2008 10:29:36 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Oliver Neukum <oliver@...kum.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Greg KH <greg@...ah.com>, Len Brown <lenb@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexey Starikovskiy <astarikovskiy@...e.de>,
	David Brownell <david-b@...bell.net>,
	Pavel Machek <pavel@....cz>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC][PATCH] PM: Introduce new top level suspend and hibernation
 callbacks (rev. 3)

On Tue, 25 Mar 2008, Rafael J. Wysocki wrote:

> On Tuesday, 25 of March 2008, Oliver Neukum wrote:
> > Am Montag 24 März 2008 schrieb Rafael J. Wysocki:
> > > -/*
> > > +/**
> > > + * struct pm_ops - device PM callbacks
> > > + *
> > >   * Several driver power state transitions are externally visible, affecting
> > >   * the state of pending I/O queues and (for drivers that touch hardware)
> > >   * interrupts, wakeups, DMA, and other hardware state.  There may also be
> > > @@ -122,6 +124,254 @@ typedef struct pm_message {
> > >   * to the rest of the driver stack (such as a driver that's ON gating off
> > >   * clocks which are not in active use).
> > >   *
> > > + * The externally visible transitions are handled with the help of the following
> > > + * callbacks included in this structure:
> > > + *
> > > + * @prepare: Prepare the device for the upcoming transition, but do NOT change
> > > + *     its hardware state.  Prevent new children of the device from being
> > > + *     registered and prevent new calls to the probe method from being made
> > 
> > How is a driver supposed to prevent calls to probe()? You can block in probe()
> > or you can fail it, but how do you prevent it from being called? User space
> > can trigger probe via sysfs.
> 
> I overlooked that.

The driver isn't supposed to prevent calls to its own probe().  The
comment means that the subsystem -- or the rest of the kernel generally
-- is supposed to avoid binding a driver to the device (thereby calling
the probe routine), assuming the device isn't already bound.

I don't expect this sort of thing to be very common.  Mostly it happens
when new kernel modules are loaded and new drivers are registered; we
will have to block module loading during a sleep transition.  It also
happens when the user writes to a driver's "bind" attribute in sysfs;  
the code there will have to block during a sleep transition.

Alan Stern

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