linux-kernel - Re: [PATCH 5] PM: Update comments describing device power management callbacks

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Message-Id: <201111212333.07799.rjw@sisk.pl>
Date:	Mon, 21 Nov 2011 23:33:07 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Randy Dunlap <rdunlap@...otime.net>
Subject: Re: [PATCH 5] PM: Update comments describing device power management callbacks

On Monday, November 21, 2011, Alan Stern wrote:
> On Mon, 21 Nov 2011, Rafael J. Wysocki wrote:
> 
> > > > I said "Analogous to @suspend()" instead.  I'm not sure why this is not
> > > > sufficient?
> > > 
> > > Because @suspend() is very different!  Its description basically says 
> > > to do three things:
> > > 
> > > 	Quiesce the device,
> > > 
> > > 	Put it into a low-power state,
> > > 
> > > 	And enable wakeup events.
> > 
> > No, it doesn't any more.  It's being changed by the proposed patch too. :-)
> 
> I must have missed reading that part.  Okay...  but it seems weird that
> none of the new descriptions says anything about changing the power
> state.  Shouldn't the description of @suspend say something like "For
> many platforms and subsystems, the device should be put in a low-power
> state"?

Hmm.  I'm not really sure, actually, because I'd recommend that subsystems
rather than drivers change power states of devices and this description
is targeted at driver writers mostly.

> > > @freeze() is supposed to do the first but not the second or third.  
> > > This makes it only 33% similar to @suspend().  :-)
> > > 
> > > Also, the description of @suspend() says nothing about having a
> > > consistent memory image.
> > 
> > Because that is irrelevant.  The state of the device after the resume
> > has to be consistent, regardless of whether the resume is from RAM or
> > from an on-disk image.
> 
> Sure, the device's state will be consistent.  But will the contents of
> memory image be consistent?  Not if the device was doing DMA writes
> during the time when the image was created.

Well, since .suspend() is also expected to stop DMA, that's rather moot.

Thanks,
Rafael
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