A System made of CA55,DDR,SYSTEM NOC all interconnected by CCI550.
CA55 read address 0xA from DDR and store in cache.
Then system NoC send read address 0xA to CCI550.
CCI550 snoop filter hit then send snoop command to CA55.
CA55 has 0xA cache clean data. Then CA55 will return snoop data or not?
From a protocol perspective, the Cortex-A55 / DSU are allowed to do either if the line is clean - it can choose to return the line or signal that it has the line but is not returning data.
The ACE specification recommends that for certain transaction types, data is returned even if it is Clean - see Table D5-6 in the in the AXI/ACE Specification Issue H.c.