Re: [PATCH v5 17/19] rust: drm: Add RegistrationData to drm::Driver

On Sun Jun 28, 2026 at 11:53 PM JST, Danilo Krummrich wrote:
<...>
> diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
> index 5e8b75dab0a6..7a3a0e21e955 100644
> --- a/rust/kernel/drm/device.rs
> +++ b/rust/kernel/drm/device.rs
> @@ -32,6 +32,7 @@
>  };
>  use core::{
>      alloc::Layout,
> +    cell::UnsafeCell,
>      marker::PhantomData,
>      mem,
>      ops::Deref,
> @@ -247,6 +248,9 @@ pub fn new(
>          // SAFETY: `drm_dev` is still private to this function.
>          unsafe { (*drm_dev).driver = const { &Self::VTABLE } };
>  
> +        // SAFETY: `raw_drm` is valid; no concurrent access before registration.
> +        unsafe { (*raw_drm.as_ptr()).registration_data = UnsafeCell::new(NonNull::dangling()) };
> +
>          // SAFETY: The reference count is one, and now we take ownership of that reference as a
>          // `drm::Device`.
>          // INVARIANT: We just created the device above, but have yet to call `drm_dev_register`.
> @@ -270,6 +274,7 @@ pub fn new(
>  pub struct Device<T: drm::Driver, C: DeviceContext = Normal> {
>      dev: Opaque<bindings::drm_device>,
>      data: T::Data,
> +    pub(super) registration_data: UnsafeCell<NonNull<T::RegistrationData<'static>>>,
>      _ctx: PhantomData<C>,
>  }
>  
> @@ -278,6 +283,28 @@ pub(crate) fn as_raw(&self) -> *mut bindings::drm_device {
>          self.dev.get()
>      }
>  
> +    /// Returns a reference to the registration data with lifetime shortened from `'static`.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that:
> +    ///
> +    /// - The parent bus device is bound (e.g. by holding an active `drm_dev_enter()` critical
> +    ///   section via [`RegistrationGuard`]).

In practice, `registration_data_unchecked` is only ever called from a
`Registered` context, so can't we move its definition to an `impl<T>
Device<T, Registered>` block so this invariant is enforced by the type
system?