st.chip_cfg.dmp_sample_rate vs st.chip_cfg.sample_rate

After following all of the steps typical of initializing the MPU9150 I call -1 in return when I call mpu_get_sample_rate() and I'm trying to understand why.

After I review the code this is a result of dmp_on state. I'd like to know why this is the behavior. I've also noticed a distinction between st.chip_cfg.dmp_sample_rate and st.chip_cfg.sample_rate but the distinction is not obvious. Can someone help me understand the distinction and the reason for failing reading of rate.

thanks

Reference Embedded_MotionDriver_v5.1 update 5.1.1 inv_mpu.*.