rust: drm: gem: Implement AlwaysRefCounted for all gem objects automatically

Currently we are requiring AlwaysRefCounted in most trait bounds for gem
objects, and implementing it by hand for our only current type of gem
object. However, all gem objects use the same functions for reference
counting - and all gem objects support reference counting.

We're planning on adding support for shmem gem objects, let's move this
around a bit by instead making IntoGEMObject require AlwaysRefCounted as a
trait bound, and then provide a blanket AlwaysRefCounted implementation for
any object that implements IntoGEMObject so all gem object types can use
the same AlwaysRefCounted implementation. This also makes things less
verbose by making the AlwaysRefCounted trait bound implicit for any
IntoGEMObject bound.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://lore.kernel.org/r/20250513221046.903358-5-lyude@redhat.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 24 insertions, 23 deletions

diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
index e11cf7837bf5..d8765e61c6c2 100644
--- a/rust/kernel/drm/gem/mod.rs
+++ b/rust/kernel/drm/gem/mod.rs
@@ -10,7 +10,7 @@ use crate::{
     drm::driver::{AllocImpl, AllocOps},
     error::{to_result, Result},
     prelude::*,
-    types::{ARef, Opaque},
+    types::{ARef, AlwaysRefCounted, Opaque},
 };
 use core::{mem, ops::Deref, ptr::NonNull};
 
@@ -36,7 +36,7 @@ pub trait BaseDriverObject<T: BaseObject>: Sync + Send + Sized {
 }
 
 /// Trait that represents a GEM object subtype
-pub trait IntoGEMObject: Sized + super::private::Sealed {
+pub trait IntoGEMObject: Sized + super::private::Sealed + AlwaysRefCounted {
     /// Owning driver for this type
     type Driver: drm::Driver;
 
@@ -54,6 +54,26 @@ pub trait IntoGEMObject: Sized + super::private::Sealed {
     unsafe fn as_ref<'a>(self_ptr: *mut bindings::drm_gem_object) -> &'a Self;
 }
 
+// SAFETY: All gem objects are refcounted.
+unsafe impl<T: IntoGEMObject> AlwaysRefCounted for T {
+    fn inc_ref(&self) {
+        // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
+        unsafe { bindings::drm_gem_object_get(self.as_raw()) };
+    }
+
+    unsafe fn dec_ref(obj: NonNull<Self>) {
+        // SAFETY: We either hold the only refcount on `obj`, or one of many - meaning that no one
+        // else could possibly hold a mutable reference to `obj` and thus this immutable reference
+        // is safe.
+        let obj = unsafe { obj.as_ref() }.as_raw();
+
+        // SAFETY:
+        // - The safety requirements guarantee that the refcount is non-zero.
+        // - We hold no references to `obj` now, making it safe for us to potentially deallocate it.
+        unsafe { bindings::drm_gem_object_put(obj) };
+    }
+}
+
 /// Trait which must be implemented by drivers using base GEM objects.
 pub trait DriverObject: BaseDriverObject<Object<Self>> {
     /// Parent `Driver` for this object.
@@ -112,10 +132,7 @@ impl<T: DriverObject> IntoGEMObject for Object<T> {
 }
 
 /// Base operations shared by all GEM object classes
-pub trait BaseObject
-where
-    Self: crate::types::AlwaysRefCounted + IntoGEMObject,
-{
+pub trait BaseObject: IntoGEMObject {
     /// Returns the size of the object in bytes.
     fn size(&self) -> usize {
         // SAFETY: `self.as_raw()` is guaranteed to be a pointer to a valid `struct drm_gem_object`.
@@ -172,7 +189,7 @@ where
     }
 }
 
-impl<T> BaseObject for T where Self: crate::types::AlwaysRefCounted + IntoGEMObject {}
+impl<T: IntoGEMObject> BaseObject for T {}
 
 /// A base GEM object.
 ///
@@ -266,22 +283,6 @@ impl<T: DriverObject> Object<T> {
     }
 }
 
-// SAFETY: Instances of `Object<T>` are always reference-counted.
-unsafe impl<T: DriverObject> crate::types::AlwaysRefCounted for Object<T> {
-    fn inc_ref(&self) {
-        // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
-        unsafe { bindings::drm_gem_object_get(self.as_raw()) };
-    }
-
-    unsafe fn dec_ref(obj: NonNull<Self>) {
-        // SAFETY: `obj` is a valid pointer to an `Object<T>`.
-        let obj = unsafe { obj.as_ref() };
-
-        // SAFETY: The safety requirements guarantee that the refcount is non-zero.
-        unsafe { bindings::drm_gem_object_put(obj.as_raw()) }
-    }
-}
-
 impl<T: DriverObject> super::private::Sealed for Object<T> {}
 
 impl<T: DriverObject> Deref for Object<T> {