When two PCIe devices are connected to each other, they don't instantly start talking PCIe, they start in a complex autonegotiation mode [0], where they find ou

When two PCIe devices are connected to each other, they don't instantly start talking PCIe, they start in a complex autonegotiation mode [0], where they find out the width of the link, the maximum protocol version that each side support, and the maximum speed that the connection between them can support. [0]: PCIe LTSSM. I sadly cannot link the spec here because it requires registration but have the diagram someone kindly ripped for SO: https://i.stack.imgur.com/QYyCM.png https://i.stack.imgur.com/QYyCM.png