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SubjectRe: [GIT PULL] PM updates for 2.6.33
On Sun, 2009-12-06 at 23:23 +0800, Alan Stern wrote:
> On Sat, 5 Dec 2009, Linus Torvalds wrote:
> > Think of a situation that we already handle pretty poorly: USB mass
> > storage devices over a suspend/resume.
> >
> > > The device tree represents a good deal of the dependences
> > > between devices and the other dependences may be represented as PM links
> > > enforcing specific ordering of the PM callbacks.
> >
> > The device tree means nothing at all, because it may need to be entirely
> > rebuilt at resume time.
> Nonsense.
> > Optimally, what we _should_ be doing (and aren't) for suspend/resume of
> > USB is to just tear down the whole topology and rebuild it and re-connect
> > the things like mass storage devices. IOW, there would be no device tree
> > to describe the topology, because we're finding it anew. And it's one of
> > the things we _would_ want to do asynchronously with other things.
> That's ridiculous. Having gone to all the trouble of building a device
> tree, one which is presumably still almost entirely correct, why go to
> all the trouble of tearing it down only to rebuild it again? (Note:
> I'm talking about resume-from-RAM here, not resume-from-hibernation.)
> Instead what we do is verify that the devices we remember from before
> the suspend are still there, and then asynchronously handle new devices
> which have been plugged in during the meantime. Doing this involves
> relatively little extra or new code; most of the routines are shared
> with the runtime PM and device reset paths.
> As for asynchronicity... At init time, USB device discovery truly is
> asynchronous. It can happen long after you log in (especially if you
> don't plug in the device until after you log in!). But at resume time
> we are more highly constrained. User processes cannot be unfrozen
> until all the devices have been resumed; otherwise they would encounter
> errors when trying to do I/O to a suspended device. (With the runtime
> PM framework this is much less of a problem, but plenty of drivers
> don't support runtime PM yet.)
> > We don't want to build up some irrelevant PM links and callbacks. We don't
> > want to have some completely made-up new infrastructure for something that
> > we _already_ want to handle totally differently for init time.
> >
> > IOW, I argue very strongly against making up something PM-specific, when
> > there really doesn't seem to be much of an advantage. We're much better
> > off trying to share the init code than making up something new.
> If I understand correctly, what you're suggesting is impractical. You
> would have each driver responsible for resuming the devices it
> registers. If it registered some children synchronously (during the
> parent's probe) then it would resume them synchronously (during the
> parent's resume); if it registered them asynchronously then it would
> resume them asynchronously. In essence, every single device_add() or
> device_register() call would have to be paired with a resume call.
> To make such significant changes in every driver would be prohibitively
> difficult. What we need is a compromise which gives drivers control
> over the resume process without making them responsible for actually
> carrying it out.
> So consider this suggestion: Let's define PM groups. Each device
> belongs to a group, and each group (except group 0, the initial group)
> has an owner device. By default a device is added to its parent's
> group during registration, but the driver can request that it be
> assigned to a different group, which must be owned by that parent.
> During resume, each PM group would correspond to an async task. The
> devices in each group would be resumed sequentially, in order of
> registration, but asynchronously with respect to other groups. The
> async thread to resume a group would be launched after the group's
> owner device was resumed.
yes, we've talked about something similar to this before. :)

Hi, Linus,
can you please look at this patch set and see if the idea is right?

If yes, I'll pick them up again and rework a patch set, including some
good thoughts from Rafael.


> So for example, the sibling functions on a PCI card could all be
> assigned to the same group, but different cards could belong to
> different groups. Likewise for ATA and PCMCIA controllers. Extra
> cross-group constraints could be added if needed, but there should be
> relatively few of them.
> This way drivers can decide which of their devices will be resumed in
> sequence or concurrently, but they won't have to do any of the
> necessary work.

> Alan Stern
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