PCIe (C5) x8 slot backward compatibility (x1/x2/x4) with Lane Reversal

Robotics & Edge Computing Jetson Systems Jetson AGX Orin
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1

Hi,

We are developing a custom carrier board based on the AGX Orin module. Due to a change in the PCIe connector’s location on our upcoming board revision, we want to implement PCIe Lane Reversal to simplify PCB routing.

Current Status (Working):

  • Routing: Standard order (Orin PHY Lane 0~7 connected to Slot Lane 0~7).

  • Behavior: Works perfectly. x1, x2, x4, and x8 PCIe devices can all successfully link up.

Planned Change (Next Revision):

  • Routing: Lane Reversal (Orin PHY Lane 7~0 connected to Slot Lane 0~7).

  • Note: In this reversed routing, an x1 PCIe card plugged into the slot will physically mate with Orin’s PHY Lane 7.

Questions:
Does the AGX Orin PCIe controller support Lane Reversal on an x8 slot while maintaining backward compatibility for x1, x2, and x4 devices? (Specifically, can it successfully complete link training when an x1 device is connected to PHY Lane 7?)

Thanks

2

— 🤖 This is an automated AI response. I’m here to help, but please verify important details! —

According to the provided context, the Jetson AGX Orin PCIe controller supports downward compatibility. This means that even if a PCIe slot is configured as x8, it can still work with x1, x2, or x4 hardware.

Regarding Lane Reversal, the context states that any combination of lanes can be P/N swapped, and this is handled automatically during the PCIe link training process. Therefore, it is possible to implement Lane Reversal on an x8 slot while maintaining backward compatibility for x1, x2, and x4 devices.

Specifically, when an x1 device is connected to PHY Lane 7 (in the reversed routing configuration), the AGX Orin PCIe controller should be able to successfully complete link training.

Here’s a relevant quote from the context:

“This is handled automatically during the PCIe link training process so you don’t need to do anything in software. Any combination of lanes can be P/N swapped.”

Additionally, the context mentions that Lane Reversal is possible by the PCIe standard, and it’s an option to make PCB routing easier and avoid dealing with crossing traces.

Therefore, based on the provided context, it appears that the AGX Orin PCIe controller supports Lane Reversal on an x8 slot while maintaining backward compatibility for x1, x2, and x4 devices.

— We support team will also look into your issue to have further suggestions soon. please stay tuned. —

5

Hi,

Could a human FAE confirm if the AI’s response is actually correct for our specific scenario?

While the AI indicates lane reversal is supported, does this still apply to down-plugging?

Thank you.

6

I am double checking internally.

7

Yes, this is supported. So these would be the lanes used with reversal:

  • x1: Lane 7
  • x2: Lanes 7:6
  • x4: Lanes 7:4
  • x8: Lanes 7:0