[PATCH v5 0/9] Rust Abstractions for PWM subsystem with TH1520 PWM driver

[Michal Wilczynski]

On 6/23/25 20:08, Michal Wilczynski wrote:
> 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
> calculations 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].
> 
> Reference repository with all the patches together can be found on
> github [2].
> 
> [1] - https://lore.kernel.org/rust-for-linux/20250509-topics-tyr-platform_iomem-v8-0-e9f1725a40da@collabora.com/
> [2] - https://github.com/mwilczy/linux/commits/rust-next-pwm-working-fan-for-sending-v9/
> 
> ---
> Changes in v5:
> - Reworked `pwm::Chip` creation to take driver data directly, which
>   allowed making the `chip.drvdata()` accessor infallible
> - added missing `pwm.c` file lost during the commit split (sorry !)
> - Link to v4: https://lore.kernel.org/r/20250618-rust-next-pwm-working-fan-for-sending-v4-0-a6a28f2b6d8a@samsung.com
> 
> Changes in v4:
>  - Reworked the pwm::Registration API to use the devres framework,
>    addressing lifetime issue.
>  - Corrected the PwmOps trait and its callbacks to use immutable references
>    (&Chip, &Device) for improved safety.
>  - Applied various code style and naming cleanups based on feedback
> 
> - Link to v3: https://lore.kernel.org/r/20250617-rust-next-pwm-working-fan-for-sending-v3-0-1cca847c6f9f@samsung.com
> 
> Changes in v3:
>  - Addressed feedback from Uwe by making multiple changes to the TH1520
>    driver and the abstraction layer.
>  - Split the core PWM abstractions into three focused commits to ease
>    review per Benno request.
>  - Confirmed the driver now works correctly with CONFIG_PWM_DEBUG enabled
>    by implementing the full waveform API, which correctly reads the
>    hardware state.
>  - Refactored the Rust code to build cleanly with
>    CONFIG_RUST_BUILD_ASSERT_ALLOW=n, primarily by using the try_* family of
>    functions for IoMem access.
>  - Included several cosmetic changes and cleanups to the abstractions
>    per Miguel review.
> 
> - Link to v2: https://lore.kernel.org/r/20250610-rust-next-pwm-working-fan-for-sending-v2-0-753e2955f110@samsung.com
> 
> 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 (9):
>       rust: pwm: Add Kconfig and basic data structures
>       rust: pwm: Add core 'Device' and 'Chip' object wrappers
>       rust: pwm: Add driver operations trait and registration support
>       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                          | 318 ++++++++
>  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                                 | 890 +++++++++++++++++++++
>  13 files changed, 1400 insertions(+), 2 deletions(-)
> ---
> base-commit: 79b01ff21368605a62b76535ab1ab5f1f726de60
> change-id: 20250524-rust-next-pwm-working-fan-for-sending-552ad2d1b193
> 
> Best regards,

Hi,

Since this v5 patchset hasn't received further comments, I'd like to
inquire about the merge strategy.

The first three patches containing the Rust PWM abstractions have no
outstanding dependencies. Could these be considered for the next merge
window independently?

The fourth patch (the TH1520 driver) depends on the platform IoMem
support [1], which is now at v10. If that series is merged, could the
rest of this series also be considered?

Thanks for your guidance.

[1] - https://lore.kernel.org/rust-for-linux/20250623-topics-tyr-platform_iomem-v10-0-ed860a562940@collabora.com/

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