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SubjectRe: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway
On Wed, 4 Jul 2007, Paul Mackerras wrote:

> Alan Stern writes:
>
> > > Most drivers suspended their hardware in the second call. If they are
> > > in the middle of a conversation with their device that *has* to be
> > > completed, they can do that by polling.
> >
> > Ugh. That will cause problems when you try to integrate runtime
> > suspend. In fact this whole approach is unsuitable for runtime PM and
> > it obscures the similarities between runtime PM and STR.
>
> Yes there are similarities, but it would be a big mistake to say that
> a requirement for STR is that all drivers do runtime PM.

That's not what I'm saying. What I'm saying is that it would be a big
mistake to force all drivers which implement runtime PM to do it using
a separate code path from system PM.

> The main attraction of the late-suspend call is that it really does,
> reliably, guarantee that the driver's I/O request methods won't get
> called between the late-suspend call and the early-resume call.

For some drivers (like USB), carrying out an actual suspend requires a
delay. Right now we implement those delays using wait_event(),
wait_for_completion(), and so on. Would you have us check at runtime
whether or not a system suspend is underway and in each case use a
busy-loop instead if it is?

What happens if, in order to carry out the late-suspend, a driver needs
to acquire a mutex which happens to be held by some other task? That
other task won't be able to run and release the mutex, so you will
deadlock.

Alan Stern

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