Lane Margining which was introduced in PCIe 4.0 and has been a very important technology since then. With the doubling of the bandwidth from 8 GT/s to 16 GT/s per Lane in formulating the PCIe 4.0 specifications, there arises the need-to-know overall link health as channels are pushed near operating limits by frequency doubling. By link health I mean -how much signaling margin is available in the design to squeeze out full 16GT/s performance. It’s now very important to determine the link health while running the actual traffic.
To address these challenges, PCIe 4.0 introduces a new feature that takes place when the Link is in L0. While designers have their own ways of calculating the margin information and evaluate the signal quality but there has been no industry standard before.PCIe 4.0 was standardized and mandated it for the ports that support 16GT/s and above.
Lane Margining is a way to measure the electrical margin in each system. This feature measures the signal eye width (time) and height (voltage). The control of Lane margining takes the form of commands by moving the data or error sample location in the PHY for error scanning. The receiver moves its sampling point around (either left or right), within the signal eye, to determine its width and height. The controller, using the margin information from the PHY, identifies where the failure occurs in the system and determines the lane margin.
If all the Lanes have good operating margin, then the process ends else the Lane margin process is repeated until the eye width become acceptable for operating conditions. This all happens while the link is in L0.
This margin information enables the designer to evaluate their design before it starts experiencing errors, hence allowing an efficient design and the system.
PCIe 4.0 specifications defined two Registers per lane to achieve Lane Margining at the Receiver: Lane Margin Control Register and Lane Margin Status Register in each Port.
The command responsible for moving the sampling point can be initiated by using Lane Margin Control Register and then a response is read using Lane Margin Status Register
For the Serial Interface, thecommands will be sent in Control SKP OS (Fig 1 (a). While for Pipe interface instead of Control SKP ordered sets, the commands will be sent on P2M/M2P Message Bus interface (Message Bus Reg in Fig 1 (b)).
PCIe Specification defines the high-level commands for the Lane Margining whereas The PIPE 5.1 spec introduced a table that maps these commands to a series of Message Bus Commands making it easier. 
With the latest addition of PCIe 6.0, Lane Margin at Receiver continues to make an important feature to verify the reliability of the link. Though the changes have been made from NRZ (Gen5) to PAM4 (Gen6) which We can discuss in further posts.
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More Information:
- For more info on how Cadence PCIe Verification IP and TripleCheck enables users to confidently verify these new disruptive changes, see our VIP for PCI Express, VIP for Compute Express Link for and TripleCheck for PCI Express
- For more information on PCIe in general, and on the various PCI standards, see the PCI-SIG website.
- For more information on Pcie Lane Margin at Receiver, please visitPCI-SIG.