The previous article described two types of configuration space, Type0 and Type1, corresponding to non-bridge devices (Endpoints) and bridge devices (P2P bridges in the Root and Switch ports).
Type0 or Type1 is determined by the Type Field in the Transaction Layer Packet (TLP) header, and read or write is determined by the Format Field in the TLP header. They are shown in the following two figures:
In previous articles, it was emphasized that only Root can initiate configuration space read and write requests in PCIe, and we know that each port of Root contains a P2P bridge. When Root initiates a configuration space read/write request, the corresponding bridge first checks whether the Bus number in the requested BDF is equal to its own Secondary Bus Number. If it is equal, Type1 is first converted to Type0, and then Send to the next level (ie Endpoint).
If they are not equal but between their own Secondary Bus Number and Subordinate Bus Number, the Type 1 request is sent directly to the next level. If it is still not equal, then the bridge considers the request as having nothing to do with itself and ignores the request.
Note: The configuration request sent by Root first must be Type1. Endpoints directly ignore Type 1 configuration requests.
A simple example is shown below:
Note: The original "PCIe Literacy - Topology Logic of the PCIe Bus" was canceled. If you are interested, you can read the books of PCIe Spec or MindShare yourself.
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