[PATCH v6 1/8] rust/drm: Add gem::impl_aref_for_gem_obj!

[Lyude Paul]

In the future we're going to be introducing more GEM object types in rust
then just gem::Object<T>. Since all types of GEM objects have refcounting,
let's introduce a macro that we can use in the gem crate in order to copy
this boilerplate implementation for each type: impl_aref_for_gem_obj!().

Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
Reviewed-by: Daniel Almeida <daniel.almeida@xxxxxxxxxxxxx>

---
V5:
* Move .as_raw() call to `let obj` in dec_ref, to ensure that the reference
  to object is not live by the time that we call drm_gem_object_put().
* Add missing #[macro_export] annotation
V6:
* Add missing IntoGEMObject trait bound

 rust/kernel/drm/gem/mod.rs | 53 +++++++++++++++++++++++++++-----------
 1 file changed, 38 insertions(+), 15 deletions(-)

diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
index a7f682e95c018..5c215e83c1b09 100644
--- a/rust/kernel/drm/gem/mod.rs
+++ b/rust/kernel/drm/gem/mod.rs
@@ -15,6 +15,43 @@
 };
 use core::{ops::Deref, ptr::NonNull};
 
+/// A macro for implementing [`AlwaysRefCounted`] for any GEM object type.
+///
+/// Since all GEM objects use the same refcounting scheme.
+#[macro_export]
+macro_rules! impl_aref_for_gem_obj {
+    (
+        impl $( <$( $tparam_id:ident ),+> )? for $type:ty
+        $(
+            where
+                $( $bind_param:path : $bind_trait:path ),+
+        )?
+    ) => {
+        // SAFETY: All gem objects are refcounted
+        unsafe impl $( <$( $tparam_id ),+> )? $crate::types::AlwaysRefCounted for $type
+        where
+            Self: IntoGEMObject,
+            $( $( $bind_param : $bind_trait ),+ )?
+        {
+            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: core::ptr::NonNull<Self>) {
+                // SAFETY: `obj` is a valid pointer to an `Object<T>`.
+                let obj = unsafe { obj.as_ref() }.as_raw();
+
+                // SAFETY: The safety requirements guarantee that the refcount is non-zero.
+                unsafe { bindings::drm_gem_object_put(obj) };
+            }
+        }
+    };
+}
+
+pub(crate) use impl_aref_for_gem_obj;
+
 /// A type alias for retrieving a [`Driver`]s [`DriverFile`] implementation from its
 /// [`DriverObject`] implementation.
 ///
@@ -252,21 +289,7 @@ extern "C" fn free_callback(obj: *mut bindings::drm_gem_object) {
     }
 }
 
-// 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_aref_for_gem_obj!(impl<T> for Object<T> where T: DriverObject);
 
 impl<T: DriverObject> super::private::Sealed for Object<T> {}
 
-- 
2.52.0