Date:	Tue, 17 Jul 2007 14:17:35 -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 22:34, david@...g.hm wrote:
>> 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.
>
> How do you guarantee that no tasks are scheduled when you get back to the
> hibernated kernel?

just don't schedule any userspace tasks. all you need to do is to execute 
the ACPI sleep functions. you normally do that after stopping userspace 
anyway.

>> 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.
>
> Well, not exactly.  If your battery runs out of power while you're suspended,
> but you have the image saved, it's still better to restore from the image, even
> if something may not work correctly after the restore, than to risk a loss of
> data.

if things don't work correctly you are still risking the loss of data, the 
user just doesn't know it.

>> 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.
>
> See above.
>
> Greetings,
> Rafael
>
>
>
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