[v2] Rust Abstractions for PWM subsystem with TH1520 PWM driver

[PATCH v2 0/7] Rust Abstractions for PWM subsystem with TH1520 PWM driver

Posted by Michal Wilczynski 4 months ago

This patch series introduces Rust support for the T-HEAD TH1520 PWM
controller and demonstrates its use for fan control on the Sipeed Lichee
Pi 4A board.

The primary goal of this patch series is to introduce a basic set of
Rust abstractions for the Linux PWM subsystem. As a first user and
practical demonstration of these abstractions, the series also provides
a functional PWM driver for the T-HEAD TH1520 SoC. This allows control
of its PWM channels and ultimately enables temperature controlled fan
support for the Lichee Pi 4A board. This work aims to explore the use of
Rust for PWM drivers and lay a foundation for potential future
Rust based PWM drivers.

The core of this series is a new rust/kernel/pwm.rs module that provides
abstractions for writing PWM chip provider drivers in Rust. This has
been significantly reworked from v1 based on extensive feedback. The key
features of the new abstraction layer include:

- Ownership and Lifetime Management: The pwm::Chip wrapper is managed
by ARef, correctly tying its lifetime to its embedded struct device
reference counter. Chip registration is handled by a pwm::Registration
RAII guard, which guarantees that pwmchip_add is always paired with
pwmchip_remove, preventing resource leaks.

- Modern and Safe API: The PwmOps trait is now based on the modern
waveform API (round_waveform_tohw, write_waveform, etc.) as recommended
by the subsystem maintainer. It is generic over a driver's
hardware specific data structure, moving all unsafe serialization logic
into the abstraction layer and allowing drivers to be written in 100%
safe Rust.

- Ergonomics: The API provides safe, idiomatic wrappers for other PWM
types (State, Args, Device, etc.) and uses standard kernel error
handling patterns.

The series is structured as follows:
- Rust PWM Abstractions: The new safe abstraction layer.
- TH1520 PWM Driver: A new Rust driver for the TH1520 SoC, built on
top of the new abstractions.
- Clock Fix: A necessary fix to the TH1520 clock driver to ensure bus
clocks remain enabled.
- Device Tree Bindings & Nodes: The remaining patches add the necessary
DT bindings and nodes for the TH1520 PWM controller, a thermal
sensor, and the PWM fan configuration for the Lichee Pi 4A board.

Testing:
Tested on the TH1520 SoC. The fan works correctly. The duty/period
calculaties are correct. Fan starts slow when the chip is not hot and
gradually increases the speed when PVT reports higher temperatures.

The patches are based on mainline, with some dependencies which are not
merged yet - platform Io support [1] and math wrapper [2].

Reference repository with all the patches together can be found on
github [3].

[1] - https://lore.kernel.org/rust-for-linux/20250509-topics-tyr-platform_iomem-v8-0-e9f1725a40da@collabora.com/
[2] - https://lore.kernel.org/all/20250609-math-rust-v1-v1-1-285fac00031f@samsung.com/
[3] - https://github.com/mwilczy/linux/commits/rust-next-pwm-working-fan-for-sending-v4

---
Changes in v2:
- Reworked the PWM abstraction layer based on extensive feedback.
- Replaced initial devm allocation with a proper ARef<Chip> lifetime model
using AlwaysRefCounted.
- Implemented a Registration RAII guard to ensure safe chip add/remove.
- Migrated the PwmOps trait from the legacy .apply callback to the modern
waveform API.
- Refactored the TH1520 driver to use the new, safer abstractions.
- Added a patch to mark essential bus clocks as CLK_IGNORE_UNUSED to fix
boot hangs when the PWM and thermal sensors are enabled.
- Link to v1: https://lore.kernel.org/r/20250524-rust-next-pwm-working-fan-for-sending-v1-0-bdd2d5094ff7@samsung.com

---
Michal Wilczynski (7):
rust: Add basic PWM abstractions
pwm: Add Rust driver for T-HEAD TH1520 SoC
clk: thead: Mark essential bus clocks as CLK_IGNORE_UNUSED
dt-bindings: pwm: thead: Add T-HEAD TH1520 PWM controller
riscv: dts: thead: Add PWM controller node
riscv: dts: thead: Add PVT node
riscv: dts: thead: Add PWM fan and thermal control

.../devicetree/bindings/pwm/thead,th1520-pwm.yaml | 48 ++
MAINTAINERS | 8 +
arch/riscv/boot/dts/thead/th1520-lichee-pi-4a.dts | 67 ++
arch/riscv/boot/dts/thead/th1520.dtsi | 18 +
drivers/clk/thead/clk-th1520-ap.c | 5 +-
drivers/pwm/Kconfig | 23 +
drivers/pwm/Makefile | 1 +
drivers/pwm/pwm_th1520.rs | 287 +++++++
rust/bindings/bindings_helper.h | 1 +
rust/helpers/helpers.c | 1 +
rust/helpers/pwm.c | 20 +
rust/kernel/lib.rs | 2 +
rust/kernel/pwm.rs | 864 +++++++++++++++++++++
13 files changed, 1343 insertions(+), 2 deletions(-)
---
base-commit: f153d0d0221f9d3d31872c934800d271ae8a3767
change-id: 20250524-rust-next-pwm-working-fan-for-sending-552ad2d1b193

Best regards,
--
Michal Wilczynski <m.wilczynski@samsung.com>