Re: Re: [PATCH] swsusp: do not use pm_ops (was: Re: suspend2 merge (was: Re: CFS and suspend2: hang in atomic copy))

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Saturday, 5 May 2007 18:19, Alan Stern wrote:
> On Fri, 4 May 2007, Rafael J. Wysocki wrote:
> 
> > > > Entering hibernation basically involves these steps:
> > > > 
> > > > 	1. Freeze tasks
> > > > 
> > > > 	2. Quiesce devices and drivers
> > > > 
> > > > 	3. Create snapshot
> > > > 
> > > > 	4. Reactivate devices and drivers
> > > > 
> > > > 	5. Save snapshot to disk
> > > > 
> > > > 	6. Prepare devices for wakeup
> > > > 
> > > > 	7. Power down (ACPI S4 on systems which support it)
> > > > 
> > > > Leaving hibernation involves a similar sequence which I won't discuss.
> > > > 
> > > > Notice that steps 1-5 above are _completely_ independent of all issues 
> > > > concerning wakeup devices and S4 vs. S5 vs. whatever.  They have to
> > > > be
> > > 
> > > No, they are not. You probably should tell ACPI at step 2 that you are
> > > suspending,
> 
> At step 2 you don't _know_ that you are suspending!  Step 5 might fail.  
> You should tell ACPI during step 6 or 7.
> 
> > You can, but even if you don't, the BIOS shouldn't have problems.  What might
> > have problems is our ACPI code during the resume, if it cannot get appropriate
> > information from the BIOS.
> > 
> > > and you definitely need to tell ACPI that you have resumed 
> > > (so it can re-scan AC adapters, for example).
> > 
> > Yes, but that can be done in two different ways:
> > 
> > 1) "We have restored the hibernation image, but the BIOS state corresponds to
> > a fresh reboot, so please initialize everything from scratch."
> > 
> > 2) "We have restored the hibernation image and the ACPI S4 was used for
> > powering off (hint: you may try not to initialize everything from scratch)."
> > 
> > Of course, in the case 2) we are responsible for ensuring that the contents of
> > the hibernation image are consistent with the information preserved by the
> > BIOS.
> 
> Keep in mind also that before you can do either 1) or 2), the boot kernel 
> has already communicated with the BIOS, possibly changing some of the ACPI 
> state.

That's correct, but it follows from the ACPI spec that there is a way for the
boot kernel to distinguish 'normal' boot from 'S4 resume' boot.  If this
mechanism is used and the boot kernel states that it's doing a 'S4 resume',
it will be able to leave ACPI alone and restore the hibernation image.

Greetings,
Rafael