linux-kernel - Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway

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Date:	Wed, 4 Jul 2007 11:04:04 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Paul Mackerras <paulus@...ba.org>
cc:	Oliver Neukum <oliver@...kum.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	<linux-pm@...ts.linux-foundation.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [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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