linux-kernel - Re: Hibernation considerations

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Date:	Tue, 17 Jul 2007 21:54:06 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jeremy Maitin-Shepard <jbms@....edu>
Cc:	Alan Stern <stern@...land.harvard.edu>, david@...g.hm,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Huang\, Ying" <ying.huang@...el.com>,
	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 Tuesday, 17 July 2007 18:09, Jeremy Maitin-Shepard wrote:
> "Rafael J. Wysocki" <rjw@...k.pl> writes:
> 
> [snip]
> 
> > I'm afraid of one thing, though.
> 
> > If we create a framework without ACPI (well, ACPI needs to be enabled in the
> > kernel anyway for other reasons, like the ability to suspend to RAM) and then
> > it turns out that we have to add some ACPI hooks to it, that might be difficult
> > to do cleanly.
> 
> > Thus, it seems reasonable to think of the ACPI handling in advance.
> 
> As far as I understand, ACPI support is only useful for hibernate to the
> extent that it allows some or all of the following features:
> 
>  - possibly shows a nice looking "hibernate" LED
>  - possibly allows the BIOS to show something about hibernate
>  - possibly allows the lid or keyboard to "wake up" (turn on) the system
> 
> Note that properly restoring device state (or even properly determining
> whether on external/mains power vs. battery) on resume is not something
> that should require special hibernate ACPI support, since it should be
> possible to make hibernate (and in general it will be the case that
> hibernate will) look exactly like a reboot to the BIOS/ACPI/devices.
> The problem that you mentioned on your system regarding power source
> information would seem to just be a problem with how ACPI is
> reinitialized after resuming from hibernation, which is not at all
> surprising since we know it (the use of driver calls for hibernate) is
> currently broken in many ways.
> 
> It seems that enabling S4 mode should just be treated as a special
> shutdown mode, independent of hibernate.  In practice, it may likely
> only be useful in conjunction with hibernate, but there doesn't seem to
> be any reason it needs to be coupled.
> 
> It would be useful to determine whether it is necessary to initialize
> ACPI specially after "resuming" from S4 mode, though, or whether they
> can be initialized normally (i.e. by a normal kernel for instance,
> completely unaware of hibernate).  If they can be initialized normally,
> then it seems that it is unnecessary to have any ACPI S4 mode support in
> the resume path, and it can merely exist as a special shutdown mode.
> Note that it seems a bit odd if ACPI can't be initialized normally after
> resume from S4 (and still work), since the "load image" kernel
> initializes everything normally before attempting to resume the
> hibernated system.

Unfortunately, this is more complicated (please see my recent reply to David).

Greetings,
Rafael


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