linux-kernel - Re: Reworking suspend-resume sequence (was: Re: PCI PM: Restore standard config registers of all devices early)

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Date:	Wed, 4 Feb 2009 00:25:27 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jesse Barnes <jesse.barnes@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andreas Schwab <schwab@...e.de>, Len Brown <lenb@...nel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Reworking suspend-resume sequence (was: Re: PCI PM: Restore standard config registers of all devices early)

On Wednesday 04 February 2009, Benjamin Herrenschmidt wrote:
> 
> > (Newer) ACPI says that devices should be put into low power states (presumably
> > with the help of appropriate ACPI AML routines) before the _PTS method is
> > called.  In turn, we're supposed to disable nonboot CPUs after calling _PTS.
> > There is analogous requirement for the _WAK method during resume.
> > 
> > Currently, the suspend code ordering follows these rules, but if we move
> > the putting of devices into low power states into the suspend_late part, they
> > will have to be done after _PTS and that is likely to break things (we've
> > already had this problem once and I have really bad memories related to it).
> 
> Wait wait wait ... the -whole- point of the exercise, wether using
> local_irq_save or disable_irq, -is- to put the ACPI bit -after- setting
> the device in low power state and before the restore on wakeup...
> 
> So basically, that isn't changing.
> 
> The -one- thing that indeed conflicts here is that we disable nonboot
> CPUs earlier. Right ?
> 
> Now, I doubt that would be a big issue, ie, we are supposedly capable of
> dynamically disabling/enabling CPUs anyway, but if it is, then indeed I
> see how the using of higher level PIC irq disabling would allow to move
> the whole suspend_late() over to before disabling non-boot CPUs.
> 
> That does introduce a significant change in semantics for drivers in the
> sense that now, suspend_late will be called with timers running, things
> scheduling, requests coming in, etc... One of the big reasons for doing
> suspend_late with IRQs off was precisely that drivers wouldn't have to
> synchronize with all these things. They now do.

I realize the problem, but IMO only a few drivers will be affected, since only
a few of them implement suspend_late/resume_early.

> Thus I tend to think that keeping the disabling of nonboot CPUs earlier
> than the suspending of devices is the least of two evils. _BUT_ As I
> said, I'm no ACPI expert and not -that- familiar with x86 land, it might
> indeed be a can of worms.

It is. :-)

Thanks,
Rafael
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