Message-Id: <200707161729.16440.rjw@sisk.pl>
Date:	Mon, 16 Jul 2007 17:29:15 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jim Crilly <jim@....dont.jablowme.net>
Cc:	david@...g.hm, 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 14:38, Jim Crilly wrote:
> On 07/16/07 02:06:27PM +0200, Rafael J. Wysocki wrote:
> > 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.
> > 
> 
> What I believe he's getting at is that Linux hibernation shouldn't be tied
> to any ACPI states. Yes, when available and working most people will want
> to enter ACPI S4 but we should still have the option of doing a normal
> poweroff. With the latter method it would look just like regular power off/on
> cycle to the firmware. And that would definitely be useful for things like
> working around buggy ACPI implementations or supporting platforms that don't
> do ACPI at all. That is the difference between the platform and shutdown
> options in /sys/power/disk, isn't it?

Yes, but this is not my point.

The point is that there are systems that _require_ the ACPI handling to work
correctly after the restore and we need to that _that_ into consideration.

IOW, there are poeple for whom the non-ACPI framework won't work as expected.

Greetings,
Rafael


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