Chapter 3: Delving into the MFD Subsystem and Syscon API

The increasingly dense integration of devices has led to a kind of device that is made up of several other devices or IPs that can achieve a dedicated function. With the advent of this device, a new subsystem appeared in the Linux kernel. These are MFDs, which stands for multi-function devices. These devices are physically seen as standalone devices, but from a software point of view, these are represented in a parent-child relationship, where the children are subdevices.

While some I2C- and SPI-based devices/subdevices might need either some hacks or configurations prior to being added to the system, there are also MMIO-based devices/subdevices where zero conf/hacks are required as they just need to share the main device's register region between subdevices. The simple-mfd helper has then been introduced to handle zero conf/hacks subdevice registering, and syscon has been introduced for sharing a device's memory...