Have You used recommendations of app note AN12444
(PCIe Certification Guide for i.MX8 Series) ?
In particular, section 2.3 (Test environment).
We have referred to the guide and checked the instruction also section 2.3. I believe it had been done properly. We had accessed the following register, but it didn't work.
Please check if the board follow the Table 8 (PCIe recommendations)
and Table 24 (Trace impedance recommendations) of Hardware Development
Guide for the i.MX8MP.