Using ioctl SPI_IOC_MESSAGE function

Is there documentation / source code explaining how this ioctl "parameter" actually works?
For example - the return positive value indicates number of "characters" processed - transmitted or received?
Does the "function" sends true 4 wire SPI data ( chip select , clock mosi miso ?)

Can it work with interface requiring usage of "register / command" selection?
For example popular "chip " ILI9341 does not truly support standard 4 wire SPI protocol - it requires usage of "C/D".

All ioctls are driver-specific. Check the device documentation or the driver source code.

That is part of my confusion - ioctl is "native kernel module" , driver is something external , build specifically as hardware interface, usually.
I am using ioctl(*spi_cs_fd, SPI_IOC_MESSAGE(length), &spi); to transfer data to a device which requires selection of command or data "mode".

BTW ioctl works fine interfacing with I2C protocol.

The ioctl SPI function I am using does not provide for switching of C/D signal.
I have choices - the crude one - switch the C/D to proper mode BEFORE using the ioctl, or hard one - analyse the SPI_IOC_MESSAGE macro together with the transfer struct it uses to add switching C/D.

And yes, the ioctl(*spi_cs_fd, SPI_IOC_MESSAGE(length), &spi); "talks" standard SPI , the ILI9341 does not.

Would anybody be interested in discussion about SPI_IOC_MESSAGE in detail?
I can "trace" few steps from the initial macro, but still cannot get to actual implementation.
For example - how does ioctl interacts with Raspberry Pi as far as setting SPI pins to input / output?