Re: Re: Possible problem with device_move()

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Wednesday, 1 August 2007 19:35, Alan Stern wrote:
> On Wed, 1 Aug 2007, Cornelia Huck wrote:
> 
> > On Wed, 1 Aug 2007 11:22:12 -0400 (EDT),
> > Alan Stern <stern@rowland.harvard.edu> wrote:
> > 
> > > As it stands right now, every place device_move() gets called is 
> > > already special!
> > 
> > The "special cases" I was thinking about are those where the order to
> > suspend/resume is not covered by a parent/child relationship, but by
> > (possibly random) order of registration. I'd have thought the rule "the
> > child must be suspended before the parent" was pretty straightforward,
> > but...
> 
> So you mean additional requirements, like what I encountered with EHCI.  
> It's peculiar in that the controller contains a hardware switch which
> can literally connect a USB device to a different (companion)
> controller, and setting the switch before the companion controller has
> been resumed won't work.  The correct way to handle this would be to
> set the switch later when the USB device is resumed, but that would be
> much more awkward.
> 
> I haven't heard of any other cases.

Well, that's really exceptional and I have no idea how to handle things like
that in a generic way.

> > (The whole list based on registration order thing seems a bit fragile
> > to me, but I don't know enough of the PM core and suspend/resume in
> > general to make a better suggestion :/)
> 
> It hasn't been bad in the past.  If A was discovered before B then ipso 
> facto it is safe to suspend B before suspending A.  Likewise, in the 
> absence of device_move, if A was discovered before B then A cannot 
> appear below B in the device tree.  Of course, this assumes devices are 
> registered as they are discovered.

Which is a weak assupmtion ...

Well, we seem to have some examples of possible situations in which the
design might not be adequate and that's alarming.

Perhaps we should create dpm_active right before the suspend, by really
traversing the device tree, when we own all device semaphores and no device
objects can be added/removed?

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth