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kIOFireWireChannelActive
An active channel is not a blank reservation
kIOFireWireChannelActive is a channel-lifecycle status. It means that the requested operation conflicts with a FireWire isochronous channel which the stack already treats as active. An active channel may have a talker, listeners, queued stream work, or a reservation that cannot safely be replaced in place.
Do not classify this as a transport error or as proof that a cable, camera, or audio interface failed. It is distinct from kIOFireWireMultipleTalkers, where the stream topology violates the one-talker rule, and from kIOFireWireChannelNotAvailable, where a requested channel resource cannot be allocated. Here the important fact is the lifecycle of an existing channel object.
Safe sequence to investigate
- Correlate the returned status with the channel number, bus generation, stream direction, and the operation that attempted to start, reconfigure, or dispose of the channel.
- Drain or stop the application’s isochronous work through the API that owns the channel before changing its ports, allocation, or configuration. Do not destroy bookkeeping while callbacks or completions can still refer to it.
- Keep state changes serialized per channel. Concurrent start, stop, reconnect, and bus-reset recovery paths can otherwise race and attempt incompatible transitions.
- When a bus reset occurred, treat the old generation and the new generation separately. The resource manager may reallocate reservations, but the application still needs a coherent channel state before resuming the stream.
References
- Apple: IOFireWireIsochChannelInterface
- Apple: FireWire Device Interface Guide — bus resets and isochronous communication
- Linux kernel documentation: stopping isochronous contexts and managing resources
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