[PATCH v5 1/8] rust/drm: Add gem::impl_aref_for_gem_obj!

Lyude Paul
2025-10-23T21:26:06Z

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@redhat.com>

---
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

rust/kernel/drm/gem/mod.rs | 54 +++++++++++++++++++++++++++-----------
1 file changed, 39 insertions(+), 15 deletions(-)

diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
index 20c2769a8c9d6..32bff2e8463f4 100644
--- a/rust/kernel/drm/gem/mod.rs
+++ b/rust/kernel/drm/gem/mod.rs
@@ -15,6 +15,44 @@
};
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
+ $( $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.
///
@@ -253,21 +291,7 @@ 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> {}

--
2.51.0