USB4 Version 2.0 specification was released by the USB Promoter Group two years back. This specification enables up to 80Gbps link speed per direction in symmetric mode and 120Gbps link speed in asymmetric mode.
Here, we take an overview of the Gen4 link recovery mechanism, which is an autonomous process. It is initiated by a router when it encounters uncorrectable error events. These error events could be a timeout error, de-skew buffer error, or RS-FEC decode error.
Gen4 link recovery uses the newly defined ELT_recovery transaction of the sideband channel. This transaction is used for both Gen4 Link Recovery initiation and completion.
A router sends ELT_recovery transaction if there is an error case that requires Gen4 link recovery, or it receives ELT_recovery transaction, or ‘Initiate Gen 4 Link Recovery’ bit in PORT_CS_19 is set to ‘b1.
Gen4 link recovery preserves the current configuration space and the router states. It would also stop time-sync handshakes and transport layer scheduling, but other than that, it would stop idle packets.
An ELT_recovery transaction is sent on the sideband channel to the other router. When the other router receives this ELT_Recovery transaction, it also sends an ELT_Recovery Transaction back to the initiating router.
Once the last bits of ELT_recovery transactions are sent and received, the router starts the lane initialization from phase 4, but the last set of TxFFE parameters are used prior to Gen4 link recovery without performing the TxFFE negotiation flow.
Once the link is active again, the router with the downstream facing port on the recovered link sends a link recovery Notification Packet.
It is important to note that Gen4 link recovery takes precedence over all other flows except for Disconnect.
Hence, in cases when the conditions to initiate Gen4 link recovery are met but if either the value of the ‘Target Asymmetric Link’ field does not reflect the actual negotiated link width as described in the ‘Negotiated Link Width’ field or ‘Enable Gen4 Link Recovery’ bit in PORT_CS_19 is set to ‘b0, a disconnect is initiated instead by driving Sideband TX to a logical low.
Verification of Gen4 Link Recovery gives rise to a large scope, which should be covered for various combinations. It is applicable whether the link is symmetric or asymmetric. Within an asymmetric link it can happen whether the router is in 3Tx or 3Rx. It can also happen during the CL0s state or during the process for entry and exit for CLx.
One needs to carefully test out all the combinations of symmetric/asymmetric with 3Tx/asymmetric with 3Rx, with all possible FSM flows, along with different ways of error insertion and recovery via both Gen4 link recovery or disconnect due to it not being enabled.
To achieve all of this, the user should be able to generate conditions for injecting the right kind of errors, like RS-FEC error or de-skew buffer error with any type of link. Also, the user should be able to detect such errors and the consequent link recovery flow and get the protocol traffic up and running after it.
Cadence USB4 VIP provides capabilities for all such aspects of Gen4 Link Recovery verification.
Cadence has a mature Verification IP solution for the verification of various aspects of USB4 Version 2.0 and Version 1.0 design, with verification capabilities provided to do a comprehensive verification of these.
You may refer to https://www.cadence.com/en_US/home/tools/system-design-and-verification/verification-ip/simulation-vip.html for more information.