Polling

The goal is to find out lanes that are at lease capable of exchanging training sequences, by repeatedly exchanging TS1 and then TS2, and see response from the other side.

The Polling state only makes sure each Lane is functional, but doesn't group them into Links. At the end of Polling state, Lanes still haven't been bonded into Links. Grouping Lanes into Links happens dynamically in the Configuration state of the LTSSM (which is the next state after Polling if things goes normally).

Polling.Active

Polling begins from each Lane sending 1024 TS1s, in the meanwhile receiving TS1 from the other side of a Lane, basically trying to confirm that the link partner has recovered the clock and then achieve symbol lock.

Once all Lanes receives enough of TS1, or start seeing the link partner sending TS2, the LTSSM is ready to enter the Polling.Configuration state.

Polling.Configuration

in Polling.Configuration state, both sides of the Lanes start sending TS2 instead. Again, once certain amount of TS2s are successfully sent and received, the link training moves on to the Polling state.

References

Videos

• PCIE Protocol - LTSSM Detailed discussion (00:08:27)

Links

• PCIe Deep Dive, Part 4: LTSSM

Specification

• 4.2.4.13 Lane vs. Link Training
• 4.2.6.2 Polling