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kIOUSBDeviceCountExceeded
Enumeration reached a host-controller limit
kIOUSBDeviceCountExceeded means that the USB controller could not allocate the resources needed to enumerate another device. In an xHCI design, device contexts and device slots are controller-managed resources. This differs from a malformed device descriptor: the limiting condition is the controller and its current topology, not necessarily the newly attached peripheral.
A hub does not make this limit disappear. It expands the physical tree, while the host controller still has to represent each enumerated device. The practical question is therefore which controller owns the entire tree and how many devices are already attached to it, rather than whether one visible port appears unused.
Evidence before changing anything
- Capture the USB tree from System Information or the I/O Registry, including hubs, dock controllers, internal devices, and the controller that owns the failing port.
- Preserve controller and kernel log messages from the enumeration attempt. They distinguish a capacity failure from power, descriptor, or link failures that can look similar at the application level.
- Move the peripheral to a port backed by another host controller, or reduce the device tree attached to the saturated controller, then compare the result.
- Do not rewrite the device configuration or endpoint descriptors to address this specific status; those are relevant to different failures such as endpoint-capacity exhaustion.
References
- Apple: kIOUSBDeviceCountExceeded
- Intel: xHCI specification
- Linux kernel documentation: USB host-side API model
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