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SubjectRe: Hibernation considerations
On Tue, 17 Jul 2007, Alan Stern wrote:

> On Tue, 17 Jul 2007, Rafael J. Wysocki wrote:
>> I'm afraid of one thing, though.
>> If we create a framework without ACPI (well, ACPI needs to be enabled in the
>> kernel anyway for other reasons, like the ability to suspend to RAM) and then
>> it turns out that we have to add some ACPI hooks to it, that might be difficult
>> to do cleanly.
>> Thus, it seems reasonable to think of the ACPI handling in advance.
> Absolutely. This needs to be done in such a way that it will work:
> On platforms without ACPI;
> On platforms with ACPI where we do a non-ACPI type of shutdown
> to whatever extent it is possible (or perhaps an ACPI-aware
> shutdown rather than change to S4);
> On platforms with ACPI where we do an ACPI-aware transition
> to S4.
> Rafael, for those of us who aren't thoroughly familiar with all the ins
> and outs of the ACPI spec, could you please summarize a list of the
> ACPI calls needed in the second and third cases above? Indicate which
> ones need to be done from within the original kernel and which should
> be done from within a kexec'd hibernation kernel.

there was just a link on slashdot toa primer on the subject of power

> 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? 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.

this is what I'm thinking, but the issue here is that the original kernel
needs to go into suspend-to-ram mode instead of resuming operation. per
the e-mail I got from Ying last night this should not be hard to

> 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.

no, user tasks just don't get scheduled during shutdown.

the big problem with the freezer isn't stopping anything from happening,
it's _selectivly_ stopping things.

with kexec you don't need to let any portion of the origional kernel or
userspace operate so you don't have a problem.

David Lang
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