SPI: API

By its features, SPI doesn't properly fit read()/write()
You must always read back as many bytes as you wrote

You may setup a socket interface
For any outcoing packet you receive an incoming one

Still, some devices (e.g. flash) work half duplex
You can write and avoid reading back the empty reply
You can read without filling a write buffer

So my choice is using this function as main engine:

  int spi_xfer(struct spi_dev *dev, enum spi_flags flags,
         const struct spi_ibuf *ibuf, struct spi_obuf *obuf);

This works both for SPI master and SPI slave
And either of ibuf and obuf can be NULL