rust: drm: split Deref for Device context typestates

Split the Deref implementation for drm::Device by context:

  - Device<T> (Normal) dereferences to T::Data.
  - Device<T, Registered> dereferences to Device<T> (Normal).

Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Link: https://patch.msgid.link/20260628145406.2107056-10-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 15 insertions, 1 deletions

diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index eb8146ea1c98..f5342de190f4 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -79,6 +79,9 @@ macro_rules! drm_legacy_fields {
 /// - [`Normal`]: The general-purpose, reference-counted context. A [`Device`] in this context may
 ///   or may not be registered with userspace.
 /// - [`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.
 pub trait DeviceContext: Sealed + Send + Sync + 'static {}
 
 /// The general-purpose, reference-counted [`DeviceContext`].
@@ -320,7 +323,7 @@ impl<T: drm::Driver, C: DeviceContext> Device<T, C> {
     }
 }
 
-impl<T: drm::Driver, C: DeviceContext> Deref for Device<T, C> {
+impl<T: drm::Driver> Deref for Device<T> {
     type Target = T::Data;
 
     fn deref(&self) -> &Self::Target {
@@ -328,6 +331,17 @@ impl<T: drm::Driver, C: DeviceContext> Deref for Device<T, C> {
     }
 }
 
+impl<T: drm::Driver> Deref for Device<T, Registered> {
+    type Target = Device<T>;
+
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: The caller holds a `Device<T, Registered>`, which guarantees all invariants
+        // of the weaker `Normal` context.
+        unsafe { self.assume_ctx() }
+    }
+}
+
 // SAFETY: DRM device objects are always reference counted and the get/put functions
 // satisfy the requirements.
 unsafe impl<T: drm::Driver> AlwaysRefCounted for Device<T> {