linux-kernel - Re: [PATCH v7 4/8] rust: pwm: Add driver operations trait and registration support

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Message-ID: <aGVOXNqnSjCm8fQl@pollux>
Date: Wed, 2 Jul 2025 17:21:00 +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>,
	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>,
	Drew Fustini <fustini@...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
Subject: Re: [PATCH v7 4/8] rust: pwm: Add driver operations trait and
 registration support

On Wed, Jul 02, 2025 at 03:45:32PM +0200, Michal Wilczynski wrote:
> +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 register(
> +        dev: &device::Device<Bound>,
> +        chip: ARef<Chip>,
> +        ops_vtable: &'static PwmOpsVTable,
> +    ) -> Result {

One thing I did miss here: Given that this should give us the guarantee that the
parent device of the Chip is always bound, you have to add a check for this
here, i.e. fail if `dev.as_raw() != chip.parent().as_raw()`.

> +        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::register(dev, registration, GFP_KERNEL)
> +    }
> +}