Re: [PATCH v14 0/7] Rust Abstractions for PWM subsystem with TH1520 PWM driver

[Michal Wilczynski <m.wilczynski@samsung.com>]

On 8/20/25 10:35, 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:
>  - Expose static function pwmchip_release.
>  - 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.
>  - Device Tree Bindings & Nodes: The remaining patches add the necessary
>    DT bindings and nodes for the TH1520 PWM controller, 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 doesn't contain any dependencies that are not currently in
> the mainline kernel anymore.
> 
> ---
> Changes in v14:
> - Re-base on top of 6.17-rc1.
> - Cosmetic change in label function.
> - Link to v13: https://lore.kernel.org/r/20250806-rust-next-pwm-working-fan-for-sending-v13-0-690b669295b6@samsung.com
> 
> Changes in v13:
> - Re-add the T-HEAD TH1520 PWM driver and its device tree bindings, as
>   Iomem series got merged into mainline kernel.
> - Fix Args struct to be consistent with State - no Opaque needed for
>   copies.
> - Replace tuple retur type in the PwmOps trait with dedicated struct
>   for improved clarity.
> - Use build_assert for WfHw size, as it doesn't have to be runtime
>   check.
> - Various cosmetic changes.
> - Link to v12: https://lore.kernel.org/r/20250717-rust-next-pwm-working-fan-for-sending-v12-0-40f73defae0c@samsung.com
> 
> Changes in v12:
>  - Reworked the PWM abstractions to use the subclassing pattern as
>    suggested by reviewers.
>  - pwm::Chip and its driver data are now allocated in a single, contiguous
>    memory block via pwmchip_alloc() sizeof_priv argument.
>  - Chip::new() now uses the pin init API to construct the driver data
>    in place, removing the need for a separate allocation.
>  - The  PwmOps trait is now implemented directly by the driver data struct
>    itself, removing the DrvData associated type and the ForeignOwnable
>    trait.
>  - The custom release handler has been updated to call drop_in_place on the driver
>    data, ensuring destructors are run correctly before the underlying
>    memory is freed.
>  - Moved the pwmchip_release prototype in the C header to a separate
>    section to clarify it is for FFI use only, as requested.
>  - Added a Prerequisite-patch-id trailer to the cover letter to declare
>    the dependency on the PWM_WFHWSIZE patch.
> 
> - Link to v11: https://lore.kernel.org/r/20250710-rust-next-pwm-working-fan-for-sending-v11-0-93824a16f9ec@samsung.com
> 
> Changes in v11:
> - Dropped driver and DT commits, as they don't compile based on publicly
>   known commit.
> - Re-based on top of pwm/for-next.
> - Reverted back to devres::Devres::new_foreign_owned, as pwm/for-next
>   doesn't contain 'register' re-factor, which is present in linux-next,
>   queued for the next merge window. The conflict is trivial, simply
>   change 'new_foreign_owned' -> 'register'.
> - Added list to MAINTAINERS entry as requested.
> - Link to v10: https://lore.kernel.org/r/20250707-rust-next-pwm-working-fan-for-sending-v10-0-d0c5cf342004@samsung.com
> 
> Changes in v10:
>  - Exported the C pwmchip_release function and called it from the custom
>    Rust release_callback to fix a memory leak of the pwm_chip struct.
>  - Removed the PwmOps::free callback, as it is not needed for idiomatic
>    Rust resource management.
>  - Removed the redundant is_null check for drvdata in the release handler,
>    as the Rust API guarantees a valid pointer is always provided.
> 
> - Link to v9: https://lore.kernel.org/r/20250706-rust-next-pwm-working-fan-for-sending-v9-0-42b5ac2101c7@samsung.com
> 
> Changes in v9:
>  - Encapsulated vtable setup in Chip::new(): The Chip::new() function is
>    now generic over the PwmOps implementation. This allows it to create and
>    assign the vtable internally, which simplifies the public API by
>    removing the ops_vtable parameter from Registration::register().
>  - Fixed memory leak with a release handler: A custom release_callback is
>    now assigned to the embedded struct device's release hook. This
>    guarantees that driver specific data is always freed when the chip is
>    destroyed, even if registration fails.
>  - The PwmOpsVTable is now defined as a const associated item to ensure
>    it has a 'static lifetime.
>  - Combined introductory commits: The Device, Chip, and PwmOps abstractions
>    are now introduced in a single commit. This was necessary to resolve the
>    circular dependencies between them and present a clean, compilable unit
>    for review.
> 
> - Link to v8: https://lore.kernel.org/r/20250704-rust-next-pwm-working-fan-for-sending-v8-0-951e5482c9fd@samsung.com
> 
> Changes in v8:
>  - Dropped already accepted commit, re-based on top of linux-next
>  - Reworked the Chip and PwmOps APIs to address the drvdata() type-safety
>    comment. Chip is now generic, and PwmOps uses an associated type
>    to provide compile-time guarantees.
>  - Added a parent device sanity check to Registration::register().
>  - Updated drvdata() to return the idiomatic T::Borrowed<'_>.
>  - added temporary unsafe blocks in the driver, as the current
>    abstraction for Clk is neiter Safe nor Sync. I think eventually
>    proper abstraction for Clk will be added as in a current state it's
>    not very useful.
> 
> - Link to v7: https://lore.kernel.org/r/20250702-rust-next-pwm-working-fan-for-sending-v7-0-67ef39ff1d29@samsung.com
> 
> Changes in v7:
> - Made parent_device function private and moved casts to Device<Bound>
>   there as well.
> - Link to v6: https://lore.kernel.org/r/20250701-rust-next-pwm-working-fan-for-sending-v6-0-2710932f6f6b@samsung.com
> 
> Changes in v6:
>  - Re-based on top of linux-next, dropped two already accepted commits.
>  - After re-basing the IoMem dependent patchset stopped working,
>    reworked it to use similar API like the PCI subsystem (I think it
>    will end up the same). Re-worked the driver for it as well.
>  - Remove the apply and get_state callbacks, and most of the State as
>    well, as the old way of implementing drivers should not be possible
>    in Rust. Left only enabled(), since it's useful for my driver.
>  - Removed the public set_drvdata() method from pwm::Chip
>  - Moved WFHWSIZE to the public include/linux/pwm.h header and renamed it
>    to PWM_WFHWSIZE, allowing bindgen to create safe FFI bindings.
>  - Corrected the ns_to_cycles integer calculation in the TH1520 driver to
>    handle overflow correctly.
>  - Updated the Kconfig entry for the TH1520 driver to select the Rust
>    abstractions for a better user experience.
> 
> - Link to v5: https://lore.kernel.org/r/20250623-rust-next-pwm-working-fan-for-sending-v5-0-0ca23747c23e@samsung.com
> 
> 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 (7):
>       pwm: Export `pwmchip_release` for external use
>       rust: pwm: Add Kconfig and basic data structures
>       rust: pwm: Add complete abstraction layer
>       pwm: Add Rust driver for T-HEAD TH1520 SoC
>       dt-bindings: pwm: thead: Add T-HEAD TH1520 PWM controller
>       riscv: dts: thead: Add PWM controller node
>       riscv: dts: thead: Add PWM fan and thermal control
> 
>  .../devicetree/bindings/pwm/thead,th1520-pwm.yaml  |  48 ++
>  MAINTAINERS                                        |  10 +
>  arch/riscv/boot/dts/thead/th1520-lichee-pi-4a.dts  |  67 ++
>  arch/riscv/boot/dts/thead/th1520.dtsi              |   7 +
>  drivers/pwm/Kconfig                                |  24 +
>  drivers/pwm/Makefile                               |   1 +
>  drivers/pwm/core.c                                 |   3 +-
>  drivers/pwm/pwm_th1520.rs                          | 355 +++++++++
>  include/linux/pwm.h                                |   6 +
>  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                                 | 790 +++++++++++++++++++++
>  14 files changed, 1334 insertions(+), 1 deletion(-)
> ---
> base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
> change-id: 20250524-rust-next-pwm-working-fan-for-sending-552ad2d1b193
> 
> Best regards,


Hi Uwe
Pinging in case this thread got lost in your mailbox.

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

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