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"No possible path" Routing issue #1226

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MohamedElgammal opened this issue Dec 13, 2019 · 1 comment

MohamedElgammal commented Dec 13, 2019

while adding one of the VPR7.0 benchmarks to Symbiflow setup, the design failed in routing with the following error:

Cannot route from SYN-GND.GND[0] (RR node: 444787 type: SOURCE location: (36,27) class: 0 capacity: 1) to SYN-OUTPAD.outpad[0] (RR node: 125935 type: SINK location: (10,29) class: 0 capacity: 1) -- no possible path

The added design was stereovision3 with some edits. The edits were serializing the multibit input ports, ORing and ANDing the multibit output port.

I think having an error of "no possible path" with a small design like that might mean that there something broken in the routing resource graph

The edited version of the benchmark and the pin assignment file are attached.
rr_issue.tar.gz

Contributor

litghost commented Dec 13, 2019

This is an artifact of the ROI and how the 7-series routing graph is designed. Possible solutions:

Remove the output pin completely, as an output pin tied to false is relatively uninteresting from a P&R perspective
Use the ROI-less routing graph (e.g. basys3-full instead of basys3)
Have the synthesis emit a false LUT to generate the signal

Example synthesis for a false LUT:

(* KEEP *)
LUT1 #(.INIT(2'b00)) (
  .O(signal_to_outpad)
);

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