I want to propose a reorganization of the way the USB core handles device resets, configuration changes, connects, and disconnects. The motivation for this is to fix a number of weaknesses in the way device resets and configuration changes are currently being handled. A good deal of functionality is lacking, and adding it will require either duplicating or sharing code with the connect/disconnect pathways.
Yes, there are problems there. I think the changes you're talking about can now be done with only usbcore changes: no more changes at the HCD level should be needed.
If something goes wrong during a device reset, the device won't be available under either its old incarnation or its new one. This has to be treated as though a disconnect occurred. Furthermore, it's conceivable that some aspect of the device has changed, so that following the reset it appears to be a different, new device. (I suppose a firmware download might have this effect.) This needs to be treated as a disconnect followed by a connect, i.e., unbind the old drivers and reprobe.
So sysfs firmware support might need to hook in there too! ... but it can't realistically do so until some of the cleanups you mention have been achieved.
There are also potential problems involving other connect/disconnect processing on the device during a reset. The best way to solve all these difficulties is to arrange for all device resets to be carried out by the khubd thread.
I'm not sure I'd agree with that. I already find khubd too fragile, mostly because it's a single point-of-failure and things can easily go strange. So I'd rather not create more faults that would wedge all of USB, starting with khubd (wedging rmmod, so reboot is needed).
Much of what khubd does could reasonably be done in the callback for hub status change, or in parallel so state transition bugs affecting a given hub port won't lock up all hubs and ports.
- Dave
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