I am using external feedback, or I do not have a complete feedback loop to the DCM CLKFB input, and I have received the following warning:
"WARNING:Timing:2804 - Feedback on DCM %s forms an incomplete loop. The Tdcmino calculation will be invalid. If the DCM uses external feedback, please apply the FEEDBACK constraint to indicate the external board delay. Consult the Constraints Guide for further information on how to apply the FEEDBACK constraint.\n"
This warning indicates that the feedback loop does not make a complete loop.
A DCM can use a feedback loop to deskew clock signals to help meet timing. This is done by connecting the output of the DCM CLK0 or CLK2X to the input CLKFB pin of the DCM (you should use a BUFG in this feedback loop).
If you are using external feedback, you must define the external feedback delay with the FEEDBACK constraint. This is specified in the Constraints Guide.