1.Yes. The two functions must be initialized in order to read and write via SPI bus. You should also follow the SPI timing described in the BMI160 datasheet.
2.If you were talking about the example "rslt = bmi160_get_regs(reg_addr, &data, len, &sensor);", the len (length) indicates how many bytes of data you have read from the BMI160 reg map.
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