Pinctrl

Pinctrl, short for pin control, is a subsystem used in many System-on-Chip (SoC) environments to manage the multiplexing and configuration of general-purpose input/output pins. Pins on modern SoCs can serve multiple peripheral functions; pinctrl provides a uniform interface for selecting a pin's function, as well as configuring electrical properties and behavior.

In Linux, the pinctrl framework is part of the kernel that exposes banks of pins, pin groups, and functional assignments. It uses a pin controller driver for the specific SoC to apply settings. Pin configurations are typically described in device trees or ACPI tables and include mux settings (which function the pin provides) and pinconf attributes (pull-up/pull-down resistors, drive strength, slew rate, bias, input enable, and hysteresis).

Pin configuration is applied via pinctrl states, such as an initial state when booting and other states for runtime power management or peripheral use. A driver or consumer requests a specific function and configuration through the pinctrl API, which ensures pins are reconfigured atomically or as needed.

Pins are organized into banks and groups, with each group potentially sharing a function, and each function corresponding to a peripheral or GPIO mode. This enables dynamic switching as peripherals are enabled or disabled.

Pinctrl reduces hardware-specific code in drivers, centralizes pin management, and supports power management by allowing pins to be put into low-power configurations.