If an HCD needs mutual exclusion for any purpose, it must handle that itself.
Does the PCI layer at least guarantee that it won't call through any of the other routines while either probe or remove is running?
Nope. It's common to start getting IRQs during probe, for example. Or in the case of the HCD glue, "start". Why would you want such a guarantee? With a lock in your code, it should work either way.
And I've been thinking that such "restart this controller" policy choices probably should be triggered from userspace/sysfs. (Along with "shut down the controller"; in general, device management should be explicit, not just a side-effect of "modprobe". Among other things, not everyone uses modular builds...)
One approach might be to have a "running" sysfs attribute (0 or 1), plus a max_restarts attribute (max number of times the controller will be automatically restarted within a 1-minute interval).
Are you sure "running" is just two values? Seems to me that suspend/resume adds at least two more states...
Such a "max_restarts" parameter could easily be managed in the shared part of HCD infrastructure, with a timer. But it could also be done entirely in userspace, given some on/off mechanism and something monitoring root hub removal through hotplug.
Automatic restarts don't appeal to me though. For OHCI and EHCI, the "controller died" cases have been almost exclusively software bugs. (Which have been vanishing over time...)
On the other hand, an even simpler and more general-purpose mechanism here would be if sysfs were to support binding and unbinding of particular drivers to particular devices. (There'd need to be a notion of a "null" binding, maybe a NOP driver, so hotplugging wouldn't automagically re-bind.)
When root hub is removed, nullify the binding of the HCD to that hardware ... then re-bind would do the complete driver initialization, whether or not it's a modular build.
That mechanism could be used for other things too.
- Dave
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