Configuration Accesses: Reaching All Devices

Configuration Accesses : Reaching All Devices

The process of HyperTransport device configuration depends on software being able to access the 256 byte configuration space of each function in each device on each bus in the system. Configuration cycles originate at the CPU that executes the configuration software; the cycles then move in the direction of the target. This section compares the PCI and HyperTransport methods used to reach the configuration space of a device which may reside on a bus many levels deep in the topology.

Implied in plug-and-play address assignment on buses such as PCI and HyperTransport is the fact that until it is discovered and assigned an address range by low-level software, a device can't claim normal memory or I/O transactions. Furthermore, whenever a bus reset occurs, each device "forgets" its address ranges and other information programmed in configuration space and can no longer be targeted with transactions which depend on assigned addresses. So, how can a device's configuration space be set up if it doesn't know its target address?

In addition to the problem of simple devices recognizing their own configuration cycles in an uninitialized system, the complex topologies permitted in PCI, PCI-X, and HyperTransport require that bridges be programmed to forward configuration transactions to the proper bus before a device can even consider claiming it.

Before looking at how HyperTransport differs from PCI in its handling of system-wide configuration accesses, here is a quick review of how PCI handles them.