Failed to access CSR 0xc01 from M-modesbi_trap_error

[Mammon-github]

When I was running linux kernel on sifive unmatched board, I found that get_cycles() always return 0, this function will return the value of CSR_TIME register， It shouldn‘t be zero. but the CSR_CYCLE can be read and working properly.
By reading the SiFive FU740-C000 Manual. I realized that RTC may need a battery, and the problem persisted even after the battery was installed.
Then I add those code in Opensbi：
jj_mtime_addr = 0x0200_BFF8 （mtime register addr in clint）

And error occurs：

I don‘t known why CSR_TIME can't be access.
please help me