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SubjectRe: 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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