NuttX is fantastic, just like Linux is… except you cannot run Linux in a fast way in small microcontroller.
From NuttX shell (NSH) I could test if MPU-9250 is working, just reading the WHOAMI (0x75) register:
nsh> i2c get -a 68 -r 75 READ Bus: 1 Addr: 68 Subaddr: 75 Value: 71
NuttX is wonderful!!! (Kudos to Gregory Nutt)