On: Linux Kernel Module Programming Guide

[mytailorisrich]

You need to have the datasheet.

These are usually a Google search away and the way people develop drivers even in professional settings.

For simple devices (e.g. I2C/SPI sensors) the functionality, number of registers, and procedures are usually very limited and it is not difficult.

[zsmi → mytailorisrich]

For simple devices probably user space is just fine. i.e. https://www.kernel.org/doc/Documentation/spi/spidev Usually only if you need an IRQ or something does it require kernel space.