A generated clock must reference the clock propagating through its direct
fan-in, although the create_generated_clock command
allows specifying any reference clock. This DRC warning reports that the generated clock
is associated with a master clock defined farther upstream than the actual incoming
master clock. In this case, timing analysis can become more pessimistic and apply
additional clock uncertainty to paths between the master clock and the generated clock.
This can make timing closure slightly more difficult.