On Sat, 25 Aug 2012, Kevin Cernekee wrote:

I am hoping that these invalid SET_CONFIGURATION / SET_INTERFACE
requests are uncommon.  In what sorts of situations will a host
request a configuration that isn't advertised in the device's
descriptors?  I had trouble just convincing usb_set_interface() /
usb_driver_set_configuration() to send such a request because they
honor bInterfaceNumber / bConfigurationValue from the descriptors.

A request doesn't have to be invalid to fail.  Valid requests can fail
for all sorts of reasons.  -ENOMEM is the prototypical example.

Alan Stern