On Monday 07 May 2007, Alan Stern wrote:
> On Sun, 6 May 2007, David Brownell wrote:
>
> > On Saturday 05 May 2007, Alan Stern wrote:
> >
> > > But who says that hibernate has to use "Non-Volatile Sleep" and normal
> > > shutdown has to use software-controlled "poweroff"? Why shouldn't the
> > > user be able to do it the other way 'round?
> >
> > Well, the definition of NVS matches hibernation, and
> > the definition of soft-off matches poweroff.
>
> Okay, I read sections 2.2 and 2.4 of the ACPI 3.0 spec. Here's the story
> in a nutshell:
>
> G3 = "mechanical off" = no wakeup devices are enabled,
> safe to disassemble
> G2/S5 = "soft off" = wakeup may be enabled, not safe to
> disassemble
> S4 = "non-volatile sleep" = hibernation, memory image is saved
> S5 = "soft off" = almost the same as S4 except there is no
> memory image
This summary suggests there are two S5 states, which I believe
is incorrect. G2 is just another name for S5. See Fig 3-1;
the ACPI 2.0 spec has the same figure.
Also, section 2.2 highlights that after S5 the OS restarts,
which it doesn't do from S4 (table 2-1) ... although when it
describes S4/NVS it fuzzes that issue by saying the key issue
is whether an NVS state file is found and used, not the level
of power available.
> The spec does not explicitly associate S4 with either G2 or G3, and in
> fact it contains language suggesting very strongly that the system could
> be in either one. The spec also uses the same name for G2 and for S5, no
> doubt leading to extra levels of confusion.
Figure 3-1 seemed quite explicit to me ... S4 is one of the G1
states, S5 is the only G2 state, and G3 is is a different beast.
Text elsewhere agrees with that.
What's confusing is how it describes NVS/hibernate. It's very
explicitly a G1 state. But leaving G2 or G3 can also trigger
a resume-from-NVS ... according to the text in 2.2 but not the
state diagrams, which don't show entering G3 even cleanly, much
less uncleanly (like a neighborhood power failure). Bleech.
I think the implication is that going to either G2 or G3 "off"
states discards something that a G1 state preserves. But I'd
have to search more deeply to see if that's clearly defined.
It's suggestive that there are no "_S5D" or "_S5W" methods;
such wake events would evidently be managed by BIOS not OSPM.
> So there's no question that S4 = NVS = hibernation. But hibernation
> can involve either G2 or G3.
I suspect there's a reason this part of ACPI is so vague;
it may relate to the desire to allow direct BIOS handling
of the NVS state.
> And there's no question (in my mind at least) that normal shutdown should
> be able to involve either G2/S5 or G3.
G2/S5, yes ... that can be entered under software control.
But by definition, not G3 since it requires a mechanical/manual
power switch update. ("Mechanical OFF", or in the spec's example
"movement of a large red switch".)
> So although the spec doesn't put
> things quite this way, we could say:
>
> hibernation = S4 = G2/S4 or G3/S4,
>
> shutdown = S5 = G2/S5 or G3/S5.
No, you're missing the key "mechanical" red-switch-ish step in G3.
G3 *can't* be entered under software control. By definition. It's
there for among other things regulatory reasons ... the only power
consumed in G3 is from the on-board RTC battery.
> > > > That's a different suggestion, yes. I'm not sure I see any
> > > > benefit of that flexibility for "soft off" states though,
> > > > especially if it made "off" consume more power.
> > >
> > > The benefit is that it allows more devices to function as wakeup sources,
> > > right?
> >
> > With downsides of "more power consumed during 'off' states"
> > and "invalidating documentation, training, and expectations".
>
> Okay, let's clear up the confusion. The additional flexibility I'm
> suggesting for "soft off" = G2 states is that we should allow both G2/S4
> and G2/S5. They would consume the same amount of power since they are
> both G2 states; the difference is that G2/S4 involves saving and restoring
> a memory image and G2/S5 does not.
There is no G2/S4 state; it's G1/S4 or G2/S5. And S5 does not
involve an NVS file, or it'd be S4. The ACPI spec is sadly
vague in those areas, however.
- Dave
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