ADR-0002 — Linux driver bus pattern¶

Status: Accepted — 2026-04-22

Context¶

The RPBridge exposes several subsystems (GPIO, I²C, SPI, PWM, IIO ADC) behind a single USB interface. The kernel driver has to spawn one child device per subsystem and bind each to the right subsystem core (gpio_chip, i2c_adapter, spi_controller, pwm_chip, iio_dev).

Two canonical patterns exist in-tree:

MFD + platform_device. Old pattern; used by dln2, cros_ec, stm32-timers, and many others. Works, but is semantically awkward — a USB-attached peripheral is not a platform device — and pulls in a lot of DT/ACPI boilerplate for children that should not need any.
auxiliary_bus (mainline since 5.11). Explicitly designed for "one parent, many-subsystem children" where the children are not independently discoverable. Used by mlx5, ice/irdma, i3c, nvmem composites, and is the pattern the networking maintainers nudge new drivers toward.

Decision¶

Use auxiliary_bus. The RPBridge core driver matches the USB interface, parses the CBOR capability map, and spawns one auxiliary_device per advertised subsystem. Each child has its own auxiliary_driver (compiled into the same rpbridge.ko module in our tree; separable later).

Consequences¶

Positive:

Lighter-weight children (no DT/ACPI code paths).
Matches the mainline-accepted pattern for out-of-tree → upstream submission.
Clean teardown semantics on unplug: the aux_device API frees us from manually tracking child lifetimes.

Negative:

Bumps the minimum supported kernel to 5.11. No practical concern — all distributions we target (Ubuntu 22.04+, Debian 12+, Fedora 40+) ship newer kernels, and we already require 6.13+ for Rust-for-Linux.

Supersedes¶

Supersedes the original architecture draft's "Treiberfamilie nach MFD-Vorbild" section. The MFD posture predates the decision to adopt auxiliary_bus; this ADR is the authoritative record going forward.