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kIOReturnUnsupportedMode
Mode is part of the contract
kIOReturnUnsupportedMode means that there is no such mode for the operation in its current context. A device or service can expose several configurations, interfaces, alternate settings, or operating modes; a mode value that is valid for one object or state is not automatically valid for another.
This is narrower than kIOReturnUnsupported, which reports an unsupported function. Here the function can exist while the requested mode is unavailable. The appropriate response is to inspect the service’s advertised capabilities and state transitions, not to cycle through mode values without understanding their effects.
How to narrow it down
- Record the requested mode, current active configuration, interface, and service state.
- Obtain supported modes from the family’s documented capability or descriptor mechanism before changing configuration.
- Check whether a required open, claim, power state, or alternate interface selection precedes the mode request.
- Keep unsupported-mode handling separate from permission and device-removal paths.
References
- Apple XNU source: IOReturn.h
- Apple: IOKit architectural overview
- Apple: Introduction to IOKit Fundamentals
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