Re: [PATCH v5 12/19] rust: drm: Add RegistrationGuard for drm_dev_enter/exit critical sections

On Sun Jun 28, 2026 at 11:53 PM JST, Danilo Krummrich wrote:
<...>
> +impl<T: drm::Driver> Device<T, Ioctl> {
> +    /// Guard against the parent bus device being unbound.
> +    ///
> +    /// Returns a [`RegistrationGuard`] if the device has not been unplugged, [`None`] otherwise.
> +    ///
> +    /// While [`RegistrationGuard`] is held the parent device is guaranteed to be bound.
> +    #[must_use]
> +    pub fn registration_guard(&self) -> Option<RegistrationGuard<'_, T>> {
> +        let mut idx: i32 = 0;

Forgot this nit: since this is used as a parameter of a C function,
don't we prefer to use `ffi::c_int` as the type?

> +        // SAFETY: `self.as_raw()` is a valid pointer to a `struct drm_device`.
> +        if unsafe { bindings::drm_dev_enter(self.as_raw(), &mut idx) } {
> +            // INVARIANT:
> +            // - `idx` is the SRCU index from the successful `drm_dev_enter()` above.
> +            // - The parent bus device is bound: `drm_dev_enter()` succeeded, meaning
> +            //   `drm_dev_unplug()` has not completed; since it is only called from
> +            //   `Registration::drop()` during parent unbind, the parent is still bound.
> +            Some(RegistrationGuard {
> +                // SAFETY: See INVARIANT above; the `Registered` context invariant holds.
> +                dev: unsafe { self.assume_ctx() },
> +                idx,
> +                _not_send: NotThreadSafe,
> +            })
> +        } else {
> +            None
> +        }
> +    }
> +}