linux-kernel - Re: [PATCH v5 3/9] rust: pwm: Add driver operations trait and registration support

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Message-ID: <8c4284f6-58eb-4d9a-b2e7-e1c66d3fcad6@kernel.org>
Date: Fri, 27 Jun 2025 20:51:51 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Michal Wilczynski <m.wilczynski@...sung.com>
Cc: Uwe Kleine-König <ukleinek@...nel.org>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Drew Fustini <drew@...7.com>,
 Guo Ren <guoren@...nel.org>, Fu Wei <wefu@...hat.com>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Paul Walmsley
 <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>,
 Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
 Marek Szyprowski <m.szyprowski@...sung.com>, Benno Lossin
 <lossin@...nel.org>, Michael Turquette <mturquette@...libre.com>,
 Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org,
 linux-pwm@...r.kernel.org, rust-for-linux@...r.kernel.org,
 linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org,
 linux-clk@...r.kernel.org
Subject: Re: [PATCH v5 3/9] rust: pwm: Add driver operations trait and
 registration support

On 6/23/25 8:08 PM, Michal Wilczynski wrote:
> diff --git a/rust/kernel/pwm.rs b/rust/kernel/pwm.rs
> index 3865b43ec47df6cb0c09bc74a228535512b6b1a8..25bc07a3df1d43467a3a6ec8f2362ae8f770360a 100644
> --- a/rust/kernel/pwm.rs
> +++ b/rust/kernel/pwm.rs
> @@ -8,12 +8,13 @@
>   
>   use crate::{
>       bindings,
> -    device,
> -    error,
> +    device::{self, Bound},
> +    devres::Devres,
> +    error::{self, to_result},
>       prelude::*,
>       types::{ARef, AlwaysRefCounted, ForeignOwnable, Opaque},
>   };
> -use core::{convert::TryFrom, ptr::NonNull};
> +use core::{convert::TryFrom, marker::PhantomData, ptr::NonNull};
>   
>   /// Maximum size for the hardware-specific waveform representation buffer.
>   ///
> @@ -408,3 +409,482 @@ unsafe impl Send for Chip {}
>   // kernel locks, which the C core is responsible for. Any interior mutability is
>   // handled and synchronized by the C kernel code.
>   unsafe impl Sync for Chip {}
> +
> +/// A resource guard that ensures `pwmchip_remove` is called on drop.
> +///
> +/// This struct is intended to be managed by the `devres` framework by transferring its ownership
> +/// via [`Devres::new_foreign_owned`]. This ties the lifetime of the PWM chip registration
> +/// to the lifetime of the underlying device.
> +pub struct Registration {
> +    chip: ARef<Chip>,
> +}
> +
> +impl Registration {
> +    /// Registers a PWM chip with the PWM subsystem.
> +    ///
> +    /// Transfers its ownership to the `devres` framework, which ties its lifetime
> +    /// to the parent device.
> +    /// On unbind of the parent device, the `devres` entry will be dropped, automatically
> +    /// calling `pwmchip_remove`. This function should be called from the driver's `probe`.
> +    pub fn new_foreign_owned(
> +        dev: &device::Device<Bound>,
> +        chip: ARef<Chip>,
> +        ops_vtable: &'static PwmOpsVTable,
> +    ) -> Result {
> +        let c_chip_ptr = chip.as_raw();
> +
> +        // SAFETY: `c_chip_ptr` is valid because the `ARef<Chip>` that owns it exists.
> +        // The vtable pointer is also valid. This sets the `.ops` field on the C struct.
> +        unsafe {
> +            (*c_chip_ptr).ops = ops_vtable.as_raw();
> +        }
> +
> +        // SAFETY: `c_chip_ptr` points to a valid chip with its ops initialized.
> +        // `__pwmchip_add` is the C function to register the chip with the PWM core.
> +        unsafe {
> +            to_result(bindings::__pwmchip_add(c_chip_ptr, core::ptr::null_mut()))?;
> +        }
> +
> +        let registration = Registration { chip };
> +
> +        Devres::new_foreign_owned(dev, registration, GFP_KERNEL)?;
> +
> +        Ok(())

This can just be:

	Devres::new_foreign_owned(dev, registration, GFP_KERNEL)

I.e. no need for the `Ok(())` below.

With that,

	Reviewed-by: Danilo Krummrich <dakr@...nel.org>

for the Registration bits.

@Uwe: Just a head-up if you plan to pick this up for the upcoming merge window,
Devres::new_foreign_owned() will be replaced with devres::register() --
semantics and arguments do not change.

> +    }
> +}
> +
> +impl Drop for Registration {
> +    fn drop(&mut self) {
> +        let chip_raw = self.chip.as_raw();
> +
> +        // SAFETY: `chip_raw` points to a chip that was successfully registered.
> +        // `bindings::pwmchip_remove` is the correct C function to unregister it.
> +        // This `drop` implementation is called automatically by `devres` on driver unbind.
> +        unsafe {
> +            bindings::pwmchip_remove(chip_raw);
> +        }

NIT: You can write this in one line as:

	unsafe { bindings::pwmchip_remove(chip_raw) };