PCIe enumeration process

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1. The discovery of equipment and bdf number The determination of

The first process of enumeration is rc End to end bus The configuration space access is initiated to obtain the device information of the whole bus topology .

To put it simply, it's going to traverse each bus, as well as bus Next device The configuration space of . If you read valid device ID value （ Incomplete F） We think we've found something available .

1.1 bdf number The distribution of

bus device function this 3 individual number It's an interesting thing . For each pcie For equipment , There are no registers in the configuration space to configure or modify bdf number. So this 3 How are the values determined .

The classic saying is ,bdf number By pcie Bus hardware topology to determine . This statement is based on pcie There's no problem with the specification , But the precondition is that the entire bus must be enumerated according to the specification . Before enumeration , In fact, for each device bdf number It's uncertain .[ notes 1]

1.1.1 function number The determination of

function number It's for more function The device is used to distinguish which one of the devices is specifically accessed function Of . So for most single function In terms of equipment ,function number It's fixed to 0; For multiple function Of device Or support virtual function Of device,function number Also by device Internal to manage and differentiate , You don't need to enumerate the bus to determine .

1.1.2 device and bus number The determination of

For support ＩＤ route , Every PCIE equipment （ Endpoints and switches ） A register for storing device bus number and device number shall be set in , Reset time , The register is clear ０, Every time a device detects a Type0 Of config transaction  Transaction package , It should be TLP In the header ８～９ byte “ Capture ” Bus number and device As its own bus and device Number , And stored in the above bus number and device number register . So it's receiving at least one Type0 Before configuring access to transaction packages , The device will not respond to any transaction other than the configuration cycle .PCIE Association The configuration space unit for storing bus number and device number information is not defined in the protocol , It's just that it has to do it .

Two docking PCIE equipment , The main device is RC End , The slave device is EP End , After power on and reset ,EP How to get yourself bus number What about ？

In such a PCIE In structure , After power on reset ,RC The end initiates a pair of EP Configuration access transaction ,EP Will resolve the configuration request bus number, And save it , As one's own bus number Number . And then you can do something else TLP Business . in other words , After power on reset ,EP You must first receive a configuration request , And then you can respond to other TLP Business .

@【 notes 1】, Here's a point , If the device is not enumerated according to the specification . about rc Go straight to the next ep device The situation of , In a bare metal environment, in fact rc Of driver It can be the opposite end ep To assign arbitrary bus and device number. because pcie It's a point-to-point connection , The first stroke Type0 Of config transaction  The transaction bus and device number Whatever the value , It's going to be opposite ep Received , And keep it as your own bus and device number. Follow up with this bus and device number You can access the opposite end of ep equipment , until ep Device reset .

1.2 enumeration

The previous content has already explained a device's bdf number The determination of , Then the process of enumeration is rc The system software uses configuration space access to determine and scan the entire bus topology .