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SubjectRe: Hibernation considerations
Date
david@lang.hm wrote:
> On Mon, 16 Jul 2007, Al Boldi wrote:
> > 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

It's a given that kexec works in the absence of ACPI; what we have to handle
is the ACPI states across kernel invocations, to ensure wakeup functions
succeed. If you don't need this, then just power off.

> >> 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.

If you mean by kernel 'the normal kernel', then this won't work, because it
would imply a change of state after saving its image.

If you mean by kernel 'the kexec'd hibernation kernel', then you wouldn't
need to do #2, but rather do #3 right after dumping the image in #1.

[...insert from another post...]
> > BTW, it would be really helpful if people would actually try the kexec
> > hibernation patches, as this may yield a much more constructive
> > discussion.
>
> I would love to, but so far I don't see the nessasary pieces
>
> once I kexec to the new kernel, how can it find out what pages of memory
> (and swap) need to be saved?

No need to save the swap, all you need to do is to dump /dev/oldmem onto
storage, and if that dump image is compatible with swsusp, then a normal
kernel should be able to resume from this image via /dev/snapshot.


Thanks!

--
Al

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