CPU serial is 0000000000000000 from proc/cpuinfo

I want to use CPU SN as board SN, but serial from proc/cpuinfo is 0. My kernel is 4.4.167 from radxa’s github.
Can anyone tell me how to get CPU serial number?
cat proc/cpuinfo

processor : 5
BogoMIPS : 48.00
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd08
CPU revision : 2

Serial : 0000000000000000


We have solved this issue of cpu serial is zero. Please update kernel to version 4.4.167-18-rockchip.

Hi Stephen,
It works. Thank you!

Gang Deng

When it go to stable branch in Radxa repo?