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

[Danilo Krummrich <dakr@kernel.org>]

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)
> +    }
> +}