Linux I2C, SPI and GPIO

1. I2C subsystem

   I2C adapters, clients and SMBus.

2. I2C dev interface

   User-space access to I2C buses through /dev/i2c-N ioctls and SMBus transfers.

3. SPI subsystem

   SPI controllers, devices and spidev.

4. SPI userspace API

   Using spidev with SPI_IOC_MESSAGE, clock mode and bits per word from user space.

5. GPIO descriptor consumer

   GPIO descriptors and character device API.

6. GPIO uAPI

   Character device ABI used by libgpiod.

7. PWM

   PWM framework for requesting channels, setting period and duty cycle, and sysfs control.

8. IIO

   Industrial IO sensors and buffers.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.