How to get the unique id of the cpu of BPI-M2 Ultra/Berry?

My application is based on a33, which has a unique serial number, and now our company want to move it to A40(BPI). My question is how to get the unique serial number in BPI-M2 Ultra/Berry? I tried the command ‘cat /proc/cpuinfo’, but the it outputs ‘Serial : 0000000000000000’ on the last line.

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For R40 register is 0x5190410c, so command is: devmem2 0x5190410c w|grep Value|sed 's/^.*: 0x/ /'

I got the uid value of FFFFFFFF, what’s wrong? My board is bpi-m2-ultra and the cpu is A40i. Thank you!