In (UniSim) functional simulation or (SimPrim) timing simulation, if TXPOWERDOWN#_IN is 'X', this causes GTP outputs TXN/TXP to always be 'X'. If TXPOWERDOWN#_IN never goes to 'X', I do not see the problem.
If TXPOWERDOWN#_IN is tied low and not used by user logic, there is not a problem.
If TXPOWERDOWN#_IN is controlled in the FPGA logic:
- For functional simulation, this can be avoided by never allowing TXPOWERDOWN#_IN to be 'X'. If this cannot be easily avoided, the simulation will also recover if TXPOWERDOWN#_IN is toggled to '1' and then a '0'.
- For timing simulation, routing delays cause it to be unavoidable for TXPOWERDOWN#_IN to not be 'X' for a very short time at the start of simulation. The simulation will recover if TXPOWERDOWN#_IN is toggled to '1' and then a '0'.