Chapter 2: Leveraging the Regmap API and Simplifying the Code

This chapter introduces the Linux kernel register mapping abstraction layer and shows how to simplify and delegate I/O operations to the regmap subsystem. Dealing with devices, whether they are built-in in the SoC (memory mapped I/O, also known as MMIO) or seated on I2C/SPI buses, consists of accessing (reading/modifying/updating) registers. Regmap became necessary because a lot of device drivers open-coded their register access routines. Regmap stands for Register Map. It was primarily developed for ALSA SoC (ASoC) in order to get rid of redundant open-coded SPI/I2C register access routines in codec drivers. At its origin, regmap provided a set of APIs for reading/writing non-memory-map I/O (for example, I2C and SPI read/write). Since then, MMIO regmap has been upgraded so that we can use regmap to access MMIO.

Nowadays, this framework abstracts I2C, SPI, and MMIO register access, and not only handles locking when necessary...