linux-kernel - Re: Hibernation considerations

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Date:	Mon, 16 Jul 2007 14:06:27 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	david@...g.hm
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Huang, Ying" <ying.huang@...el.com>,
	Jeremy Maitin-Shepard <jbms@....edu>,
	Kyle Moffett <mrmacman_g4@....com>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Pavel Machek <pavel@....cz>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Al Boldi <a1426z@...ab.com>
Subject: Re: Hibernation considerations

On Monday, 16 July 2007 01:49, david@...g.hm wrote:
> On Mon, 16 Jul 2007, Rafael J. Wysocki wrote:
> 
> > On Monday, 16 July 2007 00:42, david@...g.hm wrote:
> >> On Mon, 16 Jul 2007, Rafael J. Wysocki wrote:
> >>
> >>> On Sunday, 15 July 2007 22:13, david@...g.hm wrote:
> >>>> On Sun, 15 Jul 2007, Rafael J. Wysocki wrote:
> >>>>
> >>>>>    The ACPI specification requires us to invoke some global ACPI methods
> >>>>>    during the hibernation and during the restore.  Moreover, the ordering of
> >>>>>    code related to these ACPI methods may not be arbitrary (eg. some of
> >>>>>    them have to be executed after devices are put into low power states etc.).
> >>>>
> >>>> for a pure hibernate mode, you will be powering off the box after saving
> >>>> the suspend image. why are there any special ACPI modes involved?
> >>>
> >>> Because, for example, on my machine the status of power supply (present
> >>> vs not present) is not updated correctly after the restore if ACPI callbacks
> >>> aren't used during the hibernation.  That's just experience and it's in line
> >>> with the ACPI spec.
> >>
> >> so if a machine is actually powered off the /dev/suspend process won't
> >> work?
> >
> > No, it sort of works as usual, but after the restore the platform is not in the
> > correct state.
> 
> this is not hibernate as I and many others are thinking of it.
> 
> hibernate as we are thinking would work on basicly any hardware, including 
> things with no ACPI or power savings support. and the system could be in 
> hibernate mode for any time period.
> 
> for that matter, after a system is put into hibernate mode the system 
> could be completely disassembled and any components replaced and the 
> system would work after a resume (assuming you still have access to the 
> suspend image)

Well, this is not how ACPI defines the S4 sleep state.  If the system is in
S4, that corresponds to our hibernation, you are _not_ allowed to disassemble
it.

I've just done an experiment on my test desktop.  I had enabled suspend support
in the CMOS setup and afterwards I made Linux hibernate in the "platform" mode.
Then, when the system was powred on, the BIOS showed me a nice "Resume from
hibernation" screen that is not normally displayed during boot.  This clearly
means that some information has been preserved by the platform across the
hibernate/restore cycle.  We are supposed to handle that.

Greetngs,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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