When the CLKFB is an external feedback to the DCM, BitGen shows the following warning message:
"WARNING: PhysDesignRules: 1236 - Unknown DCM_ADV feedback loop. The signal .../CLKFB_IBUFG on the CLKFB pin of DCM_ADV comp .../DCM_ADV_INST was driven by an IOB site but the signal loop could not be traced back to the CLK0 pin of comp .../DCM_ADV_INST. The signal out the CLK0 pin must drive an IOB input pin with programming out to the PAD."
What does this message mean, and can it be ignored?
If the CLKFB is coming from an external pin, check the timing analysis to see if there are any issues. If there are no issues with timing, this warning message can be safely ignored.