Re: Hibernation considerations

[david]

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 
management

http://www.techarp.com/showarticle.aspx?artno=420

> 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 
implement.

> 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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