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

[Drew Fustini <drew@pdp7.com>]

On Tue, Jun 10, 2025 at 02:52:48PM +0200, 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
> 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].

I'm trying to build your rust-next-pwm-working-fan-for-sending-v4 branch
but I get this error:

$ make W=1 LLVM=1 ARCH=riscv -j16
  CALL    scripts/checksyscalls.sh
.pylintrc: warning: ignored by one of the .gitignore files
  UPD     include/generated/utsversion.h
  CC      init/version-timestamp.o
  KSYMS   .tmp_vmlinux0.kallsyms.S
  AS      .tmp_vmlinux0.kallsyms.o
  LD      .tmp_vmlinux1
ld.lld: error: undefined symbol: rust_build_error
    referenced by pwm_th1520.4789668fc0b4e501-cgu.0
                  drivers/pwm/pwm_th1520.o:(<pwm_th1520::Th1520PwmDriverData as kernel::pwm::PwmOps>::get_state) in archive vmlinux.a
    referenced by pwm_th1520.4789668fc0b4e501-cgu.0
                  drivers/pwm/pwm_th1520.o:(<pwm_th1520::Th1520PwmDriverData as kernel::pwm::PwmOps>::write_waveform) in archive vmlinux.a
    referenced by pwm_th1520.4789668fc0b4e501-cgu.0
                  drivers/pwm/pwm_th1520.o:(<pwm_th1520::Th1520PwmDriverData as kernel::pwm::PwmOps>::write_waveform) in archive vmlinux.a
make[2]: *** [scripts/Makefile.vmlinux:91: vmlinux] Error 1
make[1]: *** [/home/pdp7/linux/Makefile:1241: vmlinux] Error 2
make: *** [Makefile:248: __sub-make] Error 2

I've uploaded the config to:
https://gist.github.com/pdp7/e2c34dd7e4349a54bd67b53254bd3a22

Thanks,
Drew