linux-kernel - Re: Hibernation considerations

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Date:	Tue, 17 Jul 2007 13:34:24 -0700 (PDT)
From:	david@...g.hm
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	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>,
	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 Tue, 17 Jul 2007, Rafael J. Wysocki wrote:

> On Tuesday, 17 July 2007 20:32, Alan Stern wrote:
>
>> I'm still not entirely clear on how "suspend-to-both" ought to be
>> handled.  Presumably it will start off as a normal hibernation.  But
>> instead of shutting down, wouldn't the kexec'd kernel return to the
>> original kernel?
>
> No, I think the image-saving kernel should suspend.  Then, on resume the
> platform will go back to it and it will jump back to the hibernated kernel.
>
>> After all, the original kernel knows about all the devices and can put them
>> into a low-power state, while the kexec'd kernel might not have sufficient
>> information.
>
> That's correct, but ...
>
>> But what about the freezer?  The original reason for using kexec was to
>> avoid the need for the freezer.  With no freezer, while the original
>> kernel is busy powering down its devices, user tasks will be free to
>> carry out I/O -- which will make the memory snapshot inconsistent with
>> the on-disk data structures.
>
> ... we can't return to the hibernated kernel unless we are going to cancel the
> hibernation.

this is where we disagree.

why not? if all that the hibernated kernel does is to suspend-to-ram and 
makes no changes to disks or TCP connections anything that it does do 
would be lost if power were to fail and you instead did a restore from 
disk.

there is only a problem if something takes place that would prevent the 
restore-from-disk from working. if this is done in a non-ACPI way that 
will work across a power cycle you don't have to worry about the hardware 
state not matching anyway.

> That's why I think that for the suspend-to-both the image-saving kernel will
> need to support the same set of devices as the hibernated kernel.

suspend-to-both doesn't really make sense if the suspend-to-disk portion 
is useing the ACPI S4 mode.

if you don't run out of power you will restore-from-ram

if you do run out of power the restore-from-disk won't work either becouse 
devices are not in the right ACPI states.

David Lang
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