linux-kernel - Re: [RFC][PATCH 1/3] PM / sleep: Flag to speed up suspend-resume of runtime-suspended devices

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Message-ID: <2356017.WpDFDYKWcr@vostro.rjw.lan>
Date:	Thu, 08 May 2014 23:50:23 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Aaron Lu <aaron.lu@...el.com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 1/3] PM / sleep: Flag to speed up suspend-resume of runtime-suspended devices

On Thursday, May 08, 2014 11:42:01 PM Rafael J. Wysocki wrote:
> On Thursday, May 08, 2014 05:20:43 PM Alan Stern wrote:
> > On Thu, 8 May 2014, Rafael J. Wysocki wrote:
> > 
> > > > > Wait a minute.  Following ->runtime_suspend(), you are going to call 
> > > > > ->suspend() and then ->runtime_resume()?  That doesn't seem like what 
> > > > > you really want; a ->suspend() call should always have a matching 
> > > > > ->resume().
> > > > 
> > > > Yes, it should, but I didn't see any other way to do that.
> > > 
> > > Actually, that's kind of easy to resolve. :-)
> > > 
> > > When ->suspend() leaves power.leave_runtime_suspended set, the PM core can
> > > simply skip the early/late and noirq callbacks and then call ->resume()
> > > that will be responsible for using whatever is necessary to resume the
> > > device.
> > > 
> > > And perhaps the flag should be called something different then, like
> > > direct_resume (meaning go directly for ->resume() without executing
> > > the intermediate callbacks)?
> > 
> > In light of what I wrote earlier, it should be okay for the ->prepare() 
> > callback to be responsible for setting leave_runtime_suspended.  Then 
> > there will be no need to call either ->suspend() or ->resume().
> 
> Hmm.  OK, let's try that.

Well, no.

The reason why that doesn't work is because ->prepare() callbacks are
executed in the reverse order, so the perent's ones will be run before
the ->prepare() of the children.  Thus if ->prepare() sets the flag
with the expectation that ->suspend() (and the subsequent callbacks)
won't be executed, that expectation may not be met actually.

So I'm going to do what I said above.  I prefer it anyway. :-)

Rafael

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