rust: drm: return ParentDevice from Device AsRef

Change AsRef for drm::Device to return &T::ParentDevice<device::Normal>
instead of &device::Device, and restrict it to the Normal context.
Device<T, Registered> still gets this through Deref coercion.

This provides access to the typed parent bus device rather than the raw
base device.

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/20260628145406.2107056-15-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/drm/device.rs

@@ -450,11 +450,15 @@ unsafe impl<T: drm::Driver> AlwaysRefCounted for Device<T> {
     }
 }
 
-impl<T: drm::Driver, C: DeviceContext> AsRef<device::Device> for Device<T, C> {
-    fn as_ref(&self) -> &device::Device {
+impl<T: drm::Driver> AsRef<T::ParentDevice<device::Normal>> for Device<T> {
+    fn as_ref(&self) -> &T::ParentDevice<device::Normal> {
         // SAFETY: `bindings::drm_device::dev` is valid as long as the DRM device itself is valid,
         // which is guaranteed by the type invariant.
-        unsafe { device::Device::from_raw((*self.as_raw()).dev) }
+        let dev = unsafe { device::Device::from_raw((*self.as_raw()).dev) };
+
+        // SAFETY: The DRM device was constructed in `UnregisteredDevice::new()` with a parent
+        // device of type `T::ParentDevice`, hence `dev` is contained in a `T::ParentDevice`.
+        unsafe { device::AsBusDevice::from_device(dev) }
     }
 }
 

rust/kernel/drm/driver.rs

@@ -170,7 +170,8 @@ impl<T: Driver> Registration<T> {
     where
         T: 'static,
     {
-        if drm.as_ref().as_raw() != dev.as_raw() {
+        let parent = drm.as_ref();
+        if parent.as_ref().as_raw() != dev.as_raw() {
             return Err(EINVAL);
         }
 

rust/kernel/drm/gem/shmem.rs

@@ -264,7 +264,8 @@ impl<T: DriverObject> Object<T> {
         &'a self,
         dev: &'a device::Device<Bound>,
     ) -> Result<&'a scatterlist::SGTable> {
-        if dev.as_raw() != self.dev().as_ref().as_raw() {
+        let parent = self.dev().as_ref();
+        if dev.as_raw() != parent.as_ref().as_raw() {
             return Err(EINVAL);
         }