Date:	Tue, 17 Jul 2007 13:44:53 -0700 (PDT)
From:	david@...g.hm
To:	Jeremy Maitin-Shepard <jbms@....edu>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Alan Stern <stern@...land.harvard.edu>,
	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 Tue, 17 Jul 2007, Jeremy Maitin-Shepard wrote:

> "Rafael J. Wysocki" <rjw@...k.pl> writes:
>
> [snip]
>
>> Well, first, the fact is that _some_ systems _will_ be powered while in
>> hibernation (the majority of notebooks, for example) and you should assume
>> that the platform _may_ retain some information accross the hibernation/restore
>> cycle.  In that case you _should_ _not_ trash the information retained by the
>> platform.
>
> I'm not sure the majority of notebook users will want wakeup support in
> exchange for some power consumption while the system is off.  I think
> many people would not consider the trouble of having to press the power
> button instead of merely opening the lid too great.

I think you mean to say that they would be willing to trade wakeup support 
in exchange for lower power consumption...

> Furthermore, S4 mode is of course also not suitable if you intend to
> replace the battery while the system is hibernated.

exactly, and how many users realize that replacing the battery, or 
allowing the battery to die completely will cause the restore to fail?

> It does seem that it is useful to provide S4 as an option, but certainly
> just shutting down should also be an option on all systems.

absolutly.

>> Now, with that in mind, ACPI requires us to make the system enter the S4 sleep
>> state as a result of the hibernation procedure.  In my opinion this may be done
>> after saving the image, but still this means, for example, that the
>> image-saving kernel needs to support ACPI.
>
> It seems that it most certainly must be done AFTER saving the image, as
> the image obviously cannot be saved after entering S4 state, since S4
> state is nearly the same as powering off completely and all memory will
> be lost.

and the image-saving kernel could kexec back to the main kernel if the 
main kernel then knows that it should execute the S4 mode instead of 
restoring.

>> Next, during the restore, we should first check if the image is present (and
>> valid) _without_ turning ACPI on (note that this is not done by the current
>> hibernation code and that leads to strange problems on some systems).  Then,
>> if the image is present (and valid), we should first load it, jump to the
>> hibernated kernel and _then_ turn ACPI on and execute the _BFS and
>> _WAK ACPI global methods (again, this is not done by the current code in that
>> order, which is wrong).  Only after that is the hibernated kernel supposed to
>> continue.
>
> It seems that the implementation of that behavior for Linux cannot be
> quite so simple, since resume from hibernation is driven (in general)
> from an initrd/initramfs rather than directly from the kernel
> initialization sequence, in order to support modular drivers and
> features like DM and LVM.
>
> Thus, there would have to be a new "delay_acpi_initialization" kernel
> command-line option.  Additionally, there would be a sysfs interface to
> tell the kernel to proceed with the ACPI initialization as normal.  This
> would be used by an initrd/initramfs after determining that a resume
> from hibernate will not be done.  If a resume from hibernate is done,
> this hook won't be used, and instead the resumed kernel will call the
> ACPI hibernate resume stuff if S4 state was used; otherwise, the resumed
> kernel will just re-initialize ACPI as normal.  Also, if the in-kernel
> code for checking if a resume can be done does not find a hibernate
> image, it will also invoke the delayed ACPI initialization.

remember, one of the thoughts for a good hibernate implementation is that 
the image may be over the network, not on the local disk. you don't want 
the kernel trying to implment everything nessasary to get the image back.

David Lang
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