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=== Scalability ===
The xHCI architecture was designed to be highly scalable, capable of supporting 1 to 255 USB devices and 1 to 255 root hub ports. Since each USB device is allowed to define up to 31 endpoints, an xHCI that supported 255 devices would have to support 7,906 separate total endpoints. Classically, each memory buffer associated with an endpoint is described by a queue of physical memory blocks, where the queue requires a head pointer, tail pointer, length and other registers to define its state. There are many ways to define queue state, however if one were to assume 32 bytes of register space for each queue, then almost a 256KB of register space would be required to support 7,906 queues. Typically only a small number of USB devices are attached to a system at one time, and on the average a USB device supports 3-4 endpoints, of which only a subset of the endpoints are active at the same time.
The xHCI maintains queue state in system memory as Endpoint Context data structures. The contexts are designed so that they can be cached by the xHCI, and “paged” in and out as a function of endpoint activity. Thus a vendor can scale their internal xHCI Endpoint Context cache space and resources to match the practical usage models expected for their products, rather than the architectural limits that they support. Ideally the internal cache space is selected so that under normal usage conditions, there is no context paging by the xHCI.
Also USB endpoint activity tends to be bursty. That is, at any point in time a large number of endpoints may be ready to move data, however only a subset are actively moving data. For instance, the interrupt IN endpoint of a mouse may not transfer data for hours if the user is away from their desk. xHCI vendor specific algorithms could detect this condition and make that endpoint a candidate for paging out if other endpoints become busy.
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