PCIe link not established in gen4

Autonomous Machines Jetson & Embedded Systems Jetson AGX Xavier
1

Hi,
Currently we are having a setup like one jetson xavier acts as RC and another jetson xavior acts as Endpoint.

I could able to see End point is detected in RC and also accessed the end point memory from RC using devmem commands.

But when i checked the link status from register PCIE_X4_RC_PF0_PCIE_CAP_LINK_CONTROL_LINK_STATUS_REG_0 (0x3a000080), i observed link is not established at GEN4. It was established in gen1, Even though in device tree we have the following entry
nvidia,max-speed = <0x00000004>;

Further when i try to change the speed by updating target_speed and apply_speed_change fields in debugfs interface, link speed changes successfully for only Gen1 and Gen2. It fails for Gen3 and Gen4.

3

How exactly are the root port and endpoint systems connected? I mean what kind of cable is used and what is the length of the cable? If the cable is long enough or it has bad electrical characteristics, the link can’t scale up to Gen-3/4 speeds and settles at Gen-1/2 speeds.

4

Im using ADT-Link PCIe Tx-Rx adapter cable(R33NS) and it is a 60 cm cable

5

60cm cable is quite lengthy to have PCIe link operating at Gen-3/4 speeds. Please refer to the design guide and PCIe spec for the max length of the cable to get it running at Gen-3/4 speeds.