On Fri, 12 Aug 2016, Lukas Wunner wrote:

Thank you for explaining the motivation to carry out ->prepare top-down.
However my problem is really that ->complete is carried out bottom-up.
What's the motivation for that? Merely to mirror the behaviour of
->prepare? Would it be possible to change it to top-down? Note that
re-enablement of device addition is already allowed in ->resume,
which is called top-down.

I'm not aware of any particular reason why making ->complete run
top-down wouldn't work.  Of course, if you did then the environment at
the start of the ->complete callback wouldn't be the same as it was at
the end of the ->prepare callback.

I think originally the idea was just to mirror ->prepare.  Perhaps
Rafael will remember something that has escaped me.

By the way, neither the PCI nor USB bus-level ->prepare callbacks perform
any action that would stop device addition. Same for the pciehp driver
(we don't even have a ->prepare callback defined for PCIe port services.
So it *is* possible to hotplug PCI devices after ->prepare.

I don't know about PCI (although what you describe sounds like a bug).  

USB relies on a freezable workqueue for adding child devices, so it
stops adding children even before the prepare phase begins.

Alan Stern