Re: [PATCH v5 11/19] rust: drm: add Ioctl device context typestate

On Sun Jun 28, 2026 at 11:53 PM JST, Danilo Krummrich wrote:
> Add the Ioctl DeviceContext for DRM devices that have been registered
> with userspace previously.
>
> A Device<T, Ioctl> has been registered at some point, but may be
> concurrently unregistering or already unregistered. drm_dev_enter() can
> guard against this, ensuring the device remains registered for the
> duration of the critical section.
>
> This typestate will be used in ioctl dispatch context where registration
> is guaranteed by the DRM core, and RegistrationGuard can safely be
> acquired.
>
> Reviewed-by: Lyude Paul <[email protected]>
> Signed-off-by: Danilo Krummrich <[email protected]>
> ---
>  rust/kernel/drm/device.rs | 34 +++++++++++++++++++++++++++++++---
>  rust/kernel/drm/mod.rs    |  1 +
>  2 files changed, 32 insertions(+), 3 deletions(-)
>
> diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
> index 86a7fca1d33f..d429b4655449 100644
> --- a/rust/kernel/drm/device.rs
> +++ b/rust/kernel/drm/device.rs
> @@ -74,14 +74,16 @@ macro_rules! drm_legacy_fields {
>  
>  /// A trait implemented by all possible contexts a [`Device`] can be used in.
>  ///
> -/// A [`Device`] can be in one of two contexts:
> +/// A [`Device`] can be in one of the following contexts:
>  ///
>  /// - [`Normal`]: The general-purpose, reference-counted context. A [`Device`] in this context may
>  ///   or may not be registered with userspace.
> +/// - [`Ioctl`]: The device has been registered with userspace at some point; used in ioctl
> +///   dispatch context.
>  /// - [`Registered`]: The device has been registered with userspace at some point.
>  ///
> -/// `Device<T, Registered>` dereferences to `Device<T>` ([`Normal`]), so any method available on a
> -/// [`Normal`] device is also available on a [`Registered`] one.
> +/// Both `Device<T, Ioctl>` and `Device<T, Registered>` dereference to `Device<T>` ([`Normal`]),
> +/// so any method available on a [`Normal`] device is also available in the other contexts.
>  pub trait DeviceContext: Sealed + Send + Sync {}
>  
>  /// The general-purpose, reference-counted [`DeviceContext`].
> @@ -113,6 +115,21 @@ impl DeviceContext for Normal {}
>  impl Sealed for Registered {}
>  impl DeviceContext for Registered {}
>  
> +/// The [`DeviceContext`] of a [`Device`] that has been registered with userspace previously.
> +///
> +/// A [`Device`] in this context has been registered at some point, but may be concurrently
> +/// unregistering or already unregistered. `drm_dev_enter()` can guard against this, ensuring the
> +/// device remains registered for the duration of the critical section.
> +///
> +/// # Invariants
> +///
> +/// A [`Device`] in this context has been registered with userspace via `drm_dev_register()` at
> +/// some point.
> +pub struct Ioctl;
> +
> +impl Sealed for Ioctl {}
> +impl DeviceContext for Ioctl {}
> +
>  /// A [`Device`] which is known at compile-time to be unregistered with userspace.
>  ///
>  /// This type allows performing operations which are only safe to do before userspace registration,
> @@ -342,6 +359,17 @@ fn deref(&self) -> &Self::Target {
>      }
>  }
>  
> +impl<T: drm::Driver> Deref for Device<T, Ioctl> {
> +    type Target = Device<T>;
> +
> +    #[inline]
> +    fn deref(&self) -> &Self::Target {
> +        // SAFETY: The caller holds a `Device<T, Ioctl>`, which guarantees all invariants
> +        // of the weaker `Normal` context.
> +        unsafe { self.assume_ctx() }
> +    }
> +}

Since the `Deref` implementation is the same for `Ioctl` and
`Registered`, do we want to add a private marker trait for these two so
we can factor out this `impl` block?

It wouldn't reduce the LoC count, but would make the logical connection
between the two states explicit and set an anchor for potential future
blocks with a similar relationship.

With or without this:

Reviewed-by: Alexandre Courbot <[email protected]>