[PATCH v15 0/7] Rust Abstractions for PWM subsystem with TH1520 PWM driver

Michal Wilczynski
2025-09-30T12:27:48Z

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 v15:
- Update the TH1520 driver; read the hardware state directly instead of
using state, fix an integer overflow using saturating arithmetic, and
add handling zero period edge cases.
- Add dbg prints, and also update them for using the preferred format
for emiting a waveform.
- Remove the consumer side Args wrapper from the abstraction layer.
- Link to v14: https://lore.kernel.org/r/20250820-rust-next-pwm-working-fan-for-sending-v14-0-df2191621429@samsung.com

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 | 382 +++++++++++
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 | 762 +++++++++++++++++++++
14 files changed, 1333 insertions(+), 1 deletion(-)
---
base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
change-id: 20250524-rust-next-pwm-working-fan-for-sending-552ad2d1b193

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