linux-kernel - Re: Hibernation considerations

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Date:	Mon, 16 Jul 2007 21:33:06 -0700 (PDT)
From:	david@...g.hm
To:	Al Boldi <a1426z@...ab.com>
cc:	Alan Stern <stern@...land.harvard.edu>,
	"Rafael J. Wysocki" <rjw@...k.pl>, 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>
Subject: Re: Hibernation considerations

On Mon, 16 Jul 2007, Al Boldi wrote:

> david@...g.hm wrote:
>> On Mon, 16 Jul 2007, Al Boldi wrote:
>>> david@...g.hm wrote:
>>>> suspend-to-RAM should not involve kexec, the only reason for doing the
>>>> kexec to to get a seperate userspace to use for suspend-to-disk
>>>> operations instead of trying to partially freeze the sustem and keep
>>>> useing it.
>>>
>>> Or you could do suspend-to-disk-and-RAM.  But in the above case, it was
>>> meant to test kexec compatibility with device suspend/resume calls.
>>
>> the point I am trying to make here is that there is no reason that the
>> kexec approach needs to do _any_ suspend/resume calls.
>
> When you go through ACPI, then that implies calling suspend/resume calls.

then don't go through ACPI

>> all that is needed is the ability of the new kernel to initialize the
>> devices it needs.
>
> We have to go through ACPI, for wakeup functions to succeed.  A simple
> power-off won't do.

the kexec switch being posted requires ACPI be disabled, so it's clearly 
possible to switch kernels and initialize devices without ACPI

>> suspend-to-disk-and-ram could be implemented as three
>> seperate steps
>>
>> 1. suspend-to-disk
>>
>> 2. resume-from-disk
>>
>> 3. suspend-to-ram
>>
>> followed by either
>>
>> 4. resume-from-ram
>>
>> or
>>
>> 4. battery dies and loptop powers off completely
>>
>> 5. power-on boot.
>>
>> 6. resume-from-disk
>>
>> all that you need to do is to make sure that the system doesn't run
>> anything that would affect permanent media or the outside world between
>> steps #2 and #3
>
> Exactly, which is why your scheme would break down on #3, and that's why you
> need to call S3 from within the kexec'd hibernation kernel after saving the
> hibernation image.

when a kexec is called, how does the kernel know what to execute? 
something needs to tell it what to do, and I think that something is 
either something in the kexec image, or it's something passed as a 
parameter to that image.

all that would be needed to do #3 safely is to have the kernel that you 
restarted on #2 do a suspend-to-ram before it does anything else.

David Lang
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