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vishnu.k
Contributor
Contributor
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Registered: ‎07-24-2018

PCIe Lane reversal

What happens if i enabled the bit DISABLE_LANE_REVERSAL. This will harm the FPGA or not?

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garethc
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Registered: ‎06-29-2011

Hi @vishnu.k 

Yes for End Point configuration, you can enable this option through a parameter DISABLE_LANE_REVERSAL that is set to a default value in the IP generated files, but I must stress this is not tested or supported.

If you do a search for DISABLE_LANE_REVERSAL in 7-Series generated files, you will see that the parameter is set to TRUE in the core_top.v file and is set to TRUE in pcie_7x and pcie_top files. Since the hierarchy of the files is in the following order core_top -> pcie_top -> pcie_7x, this would mean that the hard block does have support for lane reversal however the IP does not allow for this option. To do a lane reversal you will have to modify the DISABLE_LANE_REVERSAL parameter to FALSE in the core_top.v however as I stated this would not be a tested/supported configuration and I would not recommend it as we cannot confirm what behavior you will see and I would expect you may see problems with the link.

Thanks,

Gareth


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vishnu.k
Contributor
Contributor
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Registered: ‎07-24-2018

Thanks garethc,

I forgot to mention the FPGA series. its ultrascale plus series (I think there is no importance for that).

What i did is,  i have changed the DISABLE_LANE_REVERSAL bit in the files you have mentioned through a tcl script given by xilinx SR team.

The lane got reversed in Vivado. Before testing what i want to know is, does this harm the FPGA permanently?.

Unexpected behaviour for that corresponding bit file is ok fine.

Any one tested this?

 

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garethc
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Registered: ‎06-29-2011

Hi @vishnu.k 

This just a protocol thing and it should not damage the FPGA. But as I stated this is not tested and functionally it may not work from a PCIe protocol level when link training. Advise as per PG213 is that if the link partner has lane reversal enabled you should not enable this.

Thanks,

Gareth


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vishnu.k
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Contributor
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Registered: ‎07-24-2018

Hi Garethc,

It is very difficult to identify whether the link partner (PC) have that capability enabled or not. 

Any I'm gonna try this. Hope my PC dont have the lane reversal capability.

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