TsctlSPIReadDesc

This function initiates a transaction on the SPI buf. The bytes in the Array buf for the length of the Array are filled in with bytes read from the SPI bus during the transaction. The bytes written to the bus during the transaction are undefined. The chip select line to be asserted is specified by adrs. If the value of adrs is negative, the chip select line will not be de-asserted at the end of the read and -adrs is used as address; if adrs is positive then the chip select will be de-asserted at the end of the read. An adrs of 1 corresponds to CS0, an adrs of 2 corresponds to CS1, etc.

A positive value is returned on success, and a negative value on failure.