Apache NuttX RTOS on Ox64: usleep works OK yay! See https://github.com/lupyuen/nuttx-ox64#fix-the-risc-v-timer-with-opensbi

ox64-nuttx-usleep.log

 0 
switch to partitions #0, OK
mmc0 is current device
Scanning mmc 0:2...
Found /extlinux/extlinux.conf
Retrieving file: /extlinux/extlinux.conf
Select the boot mode
1:.Pine64 0X64 Kernel
2:.Sipeed M1SDock Kernel
Enter choice: 1:.Pine64 0X64 Kernel
Retrieving file: /extlinux/../Image
append: root=PARTLABEL=rootfs rootwait rw rootfstype=ext4 console=ttyS0,2000000 loglevel=8 earlycon=sbi
Retrieving file: /extlinux/../bl808-pine64-ox64.dtb
## Flattened Device Tree blob at 51ff8000
   Booting using the fdt blob at 0x51ff8000
Working FDT set to 51ff8000
   Loading Device Tree to 0000000053f22000, end 0000000053f25fab ... OK
Working FDT set to 53f22000

Starting kernel ...

ABCboard_userled_initialize: 
board_userled_all: ledset=0x0
board_userled_all: led=0, val=0
board_userled_all: led=1, val=0
board_userled_all: led=2, val=0
elf_symname: Symbol has no name
elf_symvalue: SHN_UNDEF: Failed to get symbol name: -3
elf_relocateadd: Section 2 reloc 1: Undefined symbol[0] has no name: -3

NuttShell (NSH) NuttX-12.0.3
nsh> usleep 1
exec_internal: ERROR: Failed to load program 'usleep': -2
nsh> 
nsh> help
exec_internal: ERROR: Failed to load program 'help': -2
help usage:  help [-v] [<cmd>]

    .           cd          exec        ifup        ps          true        
    [           cp          exit        kill        pwd         truncate    
    ?           cmp         false       ls          rm          uname       
    alias       dirname     fdinfo      mkdir       rmdir       umount      
    unalias     dd          free        mkrd        set         unset       
    arp         df          help        mount       sleep       uptime      
    basename    dmesg       hexdump     mv          source      usleep      
    break       echo        ifconfig    pidof       test        xd          
    cat         env         ifdown      printf      time        
nsh> 
nsh> usleep 10
exec_internal: ERROR: Failed to load program 'usleep': -2
nsh> 
nsh> usleep 1000
exec_internal: ERROR: Failed to load program 'usleep': -2
nsh> 
nsh> usleep 10000
exec_internal: ERROR: Failed to load program 'usleep': -2
nsh> 
nsh> usleep 100000
exec_internal: ERROR: Failed to load program 'usleep': -2
nsh> 
nsh> hello
elf_symname: Symbol has no name
elf_symvalue: SHN_UNDEF: Failed to get symbol name: -3
elf_relocateadd: Section 2 reloc 1: Undefined symbol[0] has no name: -3
Hello, World!!
Opening /dev/userleds
Set LED 0 to 1
board_userled_all: ledset=0x1
board_userled_all: led=0, val=1
board_userled_all: led=1, val=0
board_userled_all: led=2, val=0
Waiting...
Set LED 0 to 0
board_userled_all: ledset=0x0
board_userled_all: led=0, val=0
board_userled_all: led=1, val=0
board_userled_all: led=2, val=0
nsh> 
nsh>