PCIe knowledge point -008: structure of PCIe switch

1、Switch The basic structure

Switch The structure diagram is shown in the figure 1-3 Shown ,switch Contains a upstream port And a number of downstream port,upstream port and downstream port It's through virtual PCI-PCI bridge Connect .

Here are PCIE 5.0 The original description ：

• Switches appear to configuration software as two or more logical PCI-to-PCI Bridges.

• A Switch forwards transactions using PCI Bridge mechanisms; e.g., address-based routing except when engaged in a Multicast, as defined in Section 6.14 .

• Except as noted in this document, a Switch must forward all types of Transaction Layer Packets (TLPs) between any set of Ports.

• Locked Requests must be supported as specified in Section 6.5 . Switches are not required to support Downstream Ports as initiating Ports for Locked Requests.

• Each enabled Switch Port must comply with the Flow Control specification within this document.

• A Switch is not allowed to split a packet into smaller packets, e.g., a single packet with a 256-byte payload must not be divided into two packets of 128 bytes payload each.

• Arbitration between Ingress Ports (inbound Link) of a Switch may be implemented using round robin or weighted round robin when contention occurs on the same Virtual Channel. This is described in more detail later within the specification.

• Endpoints (represented by Type 00h Configuration Space headers) must not appear to configuration software on the Switch’s internal bus as peers of the virtual PCI-to-PCI Bridges representing the Switch Downstream Ports.

 

2、Switch Medium downstream port Quantity and device number

To not support ARI Structural switch As a case study ：

2.1 Single-function Type of switch

Common design ,Downstream Ports and upstream port All are Single-function , That a single function. All that is ( The quantity is not more than 32)Downstream Ports Share a bus number , Pictured 1-3 The results shown are . Different downstream port only device number（8-bit） Dissimilarity , from 0~31.

​​​​​​​2.1 Multifunction Type of switch

When switch in downstream port More than 32 Time ,Single-function Type of switch The old can not meet the demand . here downstream port perhaps upstream port All need to be implemented as Multifunction The equipment .

Pictured 21-10 Shown , only downstream port Implemented as a Multifunction, be-all downstream port Still share a set of bus numbers ,, Every function Have one type1 Configuration space of type .

Pictured 21-11 Shown ,upstream port and downstream port Are implemented as Multifunction,downstream port Share two groups of buses respectively .

But not non-ARI Of upstream port Only a single... Can be used device（ see PCIE 5.0 agreement P680）

3、 reference

MindShare_PCI_Express_System_Architecture P819 chart 21-10 Sum graph 21-11

PCI Express Base Specification Revision 5.0, Version 1.0  7.3.1 Device Number P680

        With non-ARI Devices, PCI Express components are restricted to implementing a single Device Number on their primary interface (Upstream Port), but are permitted to implement up to eight independent Functions within that Device Number. Each internal Function is selected based on decoded address information that is provided as part of the address portion of Configuration Request packets.