A generated clock should reference the clock that is propagating in its direct
fanin, although the create_generated_clock command lets
you specify any reference clock. This DRC warning is reporting that the generated clock
is associated to a master clock defined farther upstream than the incoming master clock.
In this situation, timing analysis can be more pessimistic and apply additional clock
uncertainty on the paths between the master clock and the generated clock. This can lead
to slightly more difficult timing closure. It is recommended to associate the generated
clock to the master clock source pin that the generated clock is derived.