The only way to cross clock domains is with a FIFO.

The two-flop synchronizer in isolation (not part of a Asynchronous FIFO) can be safely used when crossing between arbitrary clock domains, including ones of similar but slightly different frequencies.

Clock crossing FIFOs are a source of hold violations.

Grey scale FIFO address coding is used because it leads to smaller adders

If a FIFO is used it must stop accepting data when full or almost full.  This is communicated by wfull.

A 2 flop synchronizer is needed in all crossings,  even if a FIFO is used