Message-Id: <200707172257.22959.rjw@sisk.pl>
Date:	Tue, 17 Jul 2007 22:57:21 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	david@...g.hm
Cc:	Alan Stern <stern@...land.harvard.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Huang, Ying" <ying.huang@...el.com>,
	Jeremy Maitin-Shepard <jbms@....edu>,
	Kyle Moffett <mrmacman_g4@....com>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Pavel Machek <pavel@....cz>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Al Boldi <a1426z@...ab.com>
Subject: Re: Hibernation considerations

On Tuesday, 17 July 2007 22:18, david@...g.hm wrote:
> On Tue, 17 Jul 2007, Rafael J. Wysocki wrote:
> 
> > On Tuesday, 17 July 2007 19:06, david@...g.hm wrote:
> >> On Tue, 17 Jul 2007, Rafael J. Wysocki wrote:
> >>
> >>> On Tuesday, 17 July 2007 17:29, david@...g.hm wrote:
> >>>> On Tue, 17 Jul 2007, Rafael J. Wysocki wrote:
> >>>>
> >>>>> On Tuesday, 17 July 2007 16:15, Alan Stern wrote:
> >>>>>> On Mon, 16 Jul 2007 david@...g.hm wrote:
> >>>>>>
> >>>>>>>> I agree, it would be good to have a non-ACPI-specific hibernation mode,
> >>>>>>>> something which would look to ACPI like a normal shutdown.  But I'm not
> >>>>>>>> so sure this is possible.
> >>>>>>>
> >>>>>>> why would it not be possible?
> >>>>>>
> >>>>>>> I can't think of anything much more frustrating then thinking that I
> >>>>>>> suspended a system and then discovering that becouse the battery went dead
> >>>>>>> (a complete power loss) that the system wouldn't boot up properly. to me
> >>>>>>> this would be a fairly common condition (when I'm mobile I use the machine
> >>>>>>> until I am out of battery, then stop and it may be a long time (days)
> >>>>>>> before I can charge the thing up again) this would not be a reliable
> >>>>>>> suspend as far as I'm concerned.
> >>>>>>>
> >>>>>>> for suspend-to-ram you have to worry about ACPI states and what you are
> >>>>>>> doing with them, for suspend-to-disk you can ignore them and completely
> >>>>>>> power the system off instead.
> >>>>>>
> >>>>>> If the only problem with doing this would be lack of wakeup support
> >>>>>> then I'm all for it.  There must be a lot of people who would like
> >>>>>> their computers to hibernate with power drain as close to 0 as possible
> >>>>>> and who don't care about remote wakeup.  In fact they might even prefer
> >>>>>> not to have wakeup support, so the computer doesn't resume at
> >>>>>> unexpected times.
> >>>>>
> >>>>> 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.
> >>>>
> >>>> doing suspend-to-ram should be orthoginal to doing hibernate-to-disk. some
> >>>> people will want both, some won't.
> >>>>
> >>>> at the moment kexec doesn't work with ACPI, that is a limitation that
> >>>> should be fixed, but makeing it able to work with ACPI enabled doesn't
> >>>> mean that it needs to be changed to depend on ACPI and it especially
> >>>> doesn't mean that it should pick up the limitations of the existing ACPI
> >>>> based hibernation approaches.
> >>>>
> >>>> if there is no ACPI on the system it should work, if ther is ACPI on the
> >>>> system it should still work.
> >>>>
> >>>>> Thus, it seems reasonable to think of the ACPI handling in advance.
> >>>>
> >>>> but don't become dependant on ACPI.
> >>>
> >>> Not dependent, but with the possibility of ACPI support taken into account.
> >>>
> >>> Arguably you can create a framework that, for example, will not allow the user
> >>> to adjust the size of the image, but then adding such a functionality may
> >>> require you to change the entire design.  Same thing with ACPI.
> >>>
> >>> I would rather avoid such pitfalls, if I could.
> >>
> >> Ok, what is it that you think ACPI fundamentally changes in this process?
> >>
> >> keep in mind that we are not makeing the assumption that the hardware
> >> will remain powered (even a little bit), or the assumption that nothing
> >> else will run on the hardware (eliminating any possibility that the
> >> hardware is in a known ACPI state)
> >
> > Well, first, the fact is that _some_ systems _will_ be powered while in
> > hibernation (the majority of notebooks, for example) and you should assume
> > that the platform _may_ retain some information accross the hibernation/restore
> > cycle.  In that case you _should_ _not_ trash the information retained by the
> > platform.
> 
> no, systems that remain powered while asleep are a different type of 
> suspend then ones that don't.
> 
> > Now, with that in mind, ACPI requires us to make the system enter the S4 sleep
> > state as a result of the hibernation procedure.  In my opinion this may be done
> > after saving the image, but still this means, for example, that the
> > image-saving kernel needs to support ACPI.
> >
> > Next, during the restore, we should first check if the image is present (and
> > valid) _without_ turning ACPI on (note that this is not done by the current
> > hibernation code and that leads to strange problems on some systems).  Then,
> > if the image is present (and valid), we should first load it, jump to the
> > hibernated kernel and _then_ turn ACPI on and execute the _BFS and
> > _WAK ACPI global methods (again, this is not done by the current code in that
> > order, which is wrong).  Only after that is the hibernated kernel supposed to
> > continue.
> >
> > [Please refer to section 15.3 of the 3.0b ACPI spec for details.]
> 
> you are starting from the assumption that ACPI S4 mode should be used.
> 
> I'm saying that a suspend that uses ACPI S4 mode is fundamentally 
> different from one that does a power off instead.

It is different, but not fundamentally.

> from my point of view the ACPI S4 sleep mode has far more in common with 
> suspend-to-ram then with the suspend-to-disk that I'm talking about
> 
> non-ACPI hibernate
> 
>    since the box powers off
>      it uses zero power while suspended
>      another OS could be run before a resume
>      hardware can be swapped, suspend image could be sent around the world to be restored on another system.
>      restore makes no assumptions about the state of the hardware when it is restored
>      restore is slower (full BIOS boot is required)
>    should be able to work on just about any hardware (the limit is the ability to initialize the devices)
> 
> 
> ACPI suspends
> 
>    since the box never completely powers off:
>      a complete power failure breaks the suspend
>      the OS must remain in control so other uses must be prevented.
>      hardware must remain in the ACPI state from suspend until restore.
>      restore can be faster (some initialization may be able to be skipped)
>    requires ACPI hardware support
> 
> under the catagory of ACPI suspends you have
> 
>    fast suspend-to-ram (stop scheduling, put the CPU to sleep, as long as 
> the memory keeps getting refreshed)
>    slow suspend-to-ram (stop scheduling, put as much of the hardware as 
> possible to sleep, including spinning down disks and other things that 
> take a while to undo)
>    suspend-to-disk (stop scheduleing, copy the ram somewhere so that it 
> doesn't need to be refreshed, put everything into low-power mode)
> 
>    and there are probably quite a few others as well. but they are all in 
> the same family in that you have to worry about ACPI states, and they all 
> have the same restrictions on what can happen between suspend and resume
> 
> the non-ACPI hibernate behaves very differently, and for some people (and 
> I think I am one of them) it will meet their needs better then _any_ of 
> the ACPI suspends.

OTOH, there are many people who would want the ACPI suspends to be handled
and they don't really care for the power-off-only hibernation.

If you aren't going to support the ACPI hibernation, your framework will be
incomplete and therefore not generally useful.

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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