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

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Date:	Thu, 5 Jul 2007 10:28:24 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Alan Stern <stern@...land.harvard.edu>
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

Alan Stern writes:

> 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.

OK; I can accept that provided there is a way to change the "what to
do with an I/O request" policy from auto-resume to something else
while we're suspending the system (and presumably restore the old
policy on system resume if the device was runtime-suspended at the
point where the system was suspended).

> > 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?

No; the late suspend call isn't appropriate for all drivers.  It is a
simple and safe way to do the suspend for some drivers, mostly the
simpler ones.  Things that are complex enough to have a subsystem
(e.g. USB) would want to use the early suspend call.

> 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.

Then late-suspend is not appropriate for that driver, and it needs to
use the early-suspend call, and do something such as setting a flag
that the I/O request function tests.

Paul.
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