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    SubjectRe: suspend2 merge (was Re: [Suspend2-devel] Re: CFS and suspend2: hang in atomic copy)
    Hi!

    > > > Why? Becuase there is no _room_ for inconsistency. There's nothing to be
    > > > "inconsistent with", since any changes to memory (by things like DMA or
    > > > other setup that happens while the suspend process is going on) is by
    > > > _definition_ consistent with the resume image (becasue there is no
    > > > separate image).
    > >
    > > Do you propose to keep DMAs running while suspending-to-RAM?
    >
    > What part of "suspend a chip" do you have trouble with?
    >
    > DMA obviously does *not* happen with a suspended device. There's no need
    > to turn DMA even off - it just doesn't happen!

    Ok, I guess I'll have nightmares of DMA controllers doing DMAs from
    chips that are no longer there tonight.

    > > > For example, the whole myth that "freeze" needs to shut off DMA is a total
    > > > and utter *myth*. It needs nothing of the sort at all. Rather than shut
    > > > off DMA and try to make the hardware be wevy wevy quiet while it's hunting
    > > > wabbits, it's a lot easier to just do nothing at all on "freeze",
    > >
    > > No. Sorry, you are wrong here.
    > >
    > > Remember that during resume we run
    > >
    > > freeze()
    > > copy old data into memory
    > > thaw()
    > >
    > > Now, if the old kernel left DMAs running, it could be overwriting
    > > the data we are copying in.
    >
    > The *thaw* needs to happen with devices quiescent.
    >
    > But that sure doesn't have anythign to do with the "snapshot()" path. In
    > fact, you'll have rebooted the machine in between.

    Only the fact that we are currently using same device call during
    snapshot() and during restore(). We obviously could do _5_ device
    calls

    (suspend/resume/freeze/quiesce_disable_dma/thaw)

    ...but that looks like too many calls to me.

    > So what does that have to do with "snapshotting"?

    I'm not comfortable with memory I'm copying changing under my hands
    because of some DMA. It just looks like asking for trouble. I _think_
    we can get away with DMA running during snapshot, because driver may
    not assume anything about the DMA result before it got completion
    interrupt, but...

    Pavel
    --
    (english) http://www.livejournal.com/~pavelmachek
    (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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