It is important to note that PCIe's Spec specifies that only Root has the authority to initiate Originate Configuration Requests. That is, other devices in the PCIe system are not allowed to configure the configuration space of other devices, ie peer-to. -peer configuration request is not allowed. And the routing of configuration requests can only use BDF (Bus, Device, Function).
Processors generally cannot initiate configuration read and write requests directly because they can only generate Memory Requests and IO Requests. This means that the root must translate the processor's relevant requests into configuration read and write requests. For legacy PCI devices, IO indirect accesses are used; for PCIe devices, Memory-Mapped Accesses are used.
The previous article also introduced that each port of the Root and Switch contains a P2P bridge, and it is known that the configuration space header of the bridge is Type1. As shown below:
Each Type 1 Header contains information such as the Subordinate Bus Number, Secondary Bus Number, and Primary Bus Number. When the configuration request for BDF routing, it is to rely on this information to determine the device to find. A simple example is shown below:
Note: The above example shows the case where there is only one Root in the entire PCIe bus system. In fact, the PCIe Spec also allows multiple Roots (ie, Multi-Root) in the bus system. With regard to Multi-Root, it is not described in detail here, and interested individuals can read the Spec of PCIe.
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