Date:	Tue, 22 Nov 2011 21:47:32 +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 Tuesday, November 22, 2011, Alan Stern wrote:
> On Mon, 21 Nov 2011, Rafael J. Wysocki wrote:
> 
> > 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.
> 
> The same data structure (dev_pm_ops) is used for both drivers and
> subsystems.  Therefore the comments should be directed toward both
> driver writers and subsystem writers.

I don't quite agree.  Trying to do so would turn that comment into a lengthy
document and there's no room for anything like that in the header file.

Perhaps I'll just modify the comment to say when the callbacks are executed
without specifying what the specific callbacks are supposed to do (because
that may vary from one subsystem to another and even between different
drivers belonging to the same subsystem).  If PM domains are taken into
account, it gets even more complicated.

> You could say: "For many platforms, the subsystem @suspend() or
> @suspend_noirq() callback should put the device in a low-power state.  
> Some subsystems may require the driver to do this instead."

Well, that information is not really useful for someone wanting to learn
what the callbacks are supposed to do, because he still has to know what
the subsystem in question expects him to do.

> > > > > @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.
> 
> It won't be moot if you add the sentences I recommend above.  You could
> add an additional sentence: "Either way, the @freeze() and
> @freeze_noirq() callbacks (both the subsystem's and the driver's)  
> should always avoid changing the device's state."

That sounds good.

Thanks,
Rafael
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