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kIOUSBBitstufErr
This is a serial-encoding error, not an application payload error
kIOUSBBitstufErr reports that the USB receiver could not accept the bit-stuffing pattern in a transaction. USB serial-interface hardware performs NRZI decoding and bit stuffing or unstuffing before higher layers see packet fields. The result therefore points below the device class protocol: changing JSON, HID report bytes, a storage command, or another application payload does not directly correct a malformed serial sequence.
Keep this distinct from kIOUSBCRCErr, where a decoded field fails its integrity check, and from kIOUSBDataToggleErr, where the host and endpoint are out of synchronization on a non-isochronous data sequence. These are different failure stages and should not be collapsed into one generic USB error bucket.
What to compare
- Capture the endpoint address, transfer direction and type, USB speed, cable, connector orientation where relevant, and every hub or dock between the device and the host controller.
- Run the same reproducible transfer on a direct host port and through a different hub path. A result that moves with the path is more useful than a result attached only to a product name.
- Check whether the failure appears only under sustained traffic, only after sleep or resume, or only at a particular negotiated speed. Those observations narrow the boundary between device behavior, power or signal conditions, and controller-specific handling.
- For a device implementation, inspect the USB serial-interface engine and protocol analyzer trace, not merely the driver callback status. Bit stuffing is handled at the link encoding stage.
Do not infer too much from the label alone
Apple documents that some host-controller paths can surface less specific errors than the device-side event. Treat kIOUSBBitstufErr as a concrete transport-integrity symptom, but corroborate it with the physical path, controller diagnostics, and a trace before assigning a single root cause.
References
- Apple: IOUSBFamily hardware error codes
- Apple: USB transfer classes and error reporting
- USB-IF: USB serial interface engine, NRZI, and bit stuffing
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