linux-kernel - Re: suspend2 merge (was Re: [Suspend2-devel] Re: CFS and suspend2: hang in atomic copy)

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Message-ID: <20070425224216.GI17387@elf.ucw.cz>
Date:	Thu, 26 Apr 2007 00:42:16 +0200
From:	Pavel Machek <pavel@....cz>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Kenneth Crudup <kenny@...ix.com>, Nick Piggin <npiggin@...e.de>,
	Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Con Kolivas <kernel@...ivas.org>,
	suspend2-devel@...ts.suspend2.net, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: suspend2 merge (was Re: [Suspend2-devel] Re: CFS and suspend2: hang in atomic copy)

Hi!

> > Not the same... but they are still related. "freeze" (for atomic
> > snapshot) is actually subset of "suspend"... freeze needs DMAs off and
> > saved state, and you need DMAs off and saved state for "suspend".
> 
> THEY HAVE ABSOLUTELY NOTHING IN COMMON!
> 
> Nobody in their right mind thinks that "disable DMA" and "suspend" are 
> similar operations. 
> 
> > So it is actually correct to do "suspend" when you want "freeze"; it
> > is just slow. That's why they only differ in parameter these days.
> 
> It is *not* correct to "suspend" when you want "freeze".

Example?

> I don't understand how you can even *claim* something like that.
> 
> Here's a trivial example:
>  - SCSI disk
> 
> Tell me, what does "suspend" do, and what does "freeze" (snapshot) do?

Suspend syncs caches/spins down. Freeze does not do anything.

That's okay, I keep claiming "freeze" is subset of "suspend". Can you
name device where that is not true?

Remember we do

suspend(PMSG_FREEZE)
atomic snapshot
resume()
write snapshot.

So if we do spin the scsi disk down, nothing really bad happens, we'll
just spin it up. (So scsi disk is not example I want. Spining down
scsi disk on freeze is slow and stupid, but it is not incorrect).

Yes, If I'd knew what I know now, drivers would have
suspend/freeze/thaw/resume methods. We probably still can do that
change. Unfortunately, it needs driver authors to understand 4 hooks
(not 2) and do the right thing.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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