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

Daniel Almeida
2025-11-24T15:24:27Z

Hi Lyude,

> On 23 Oct 2025, at 18:22, Lyude Paul <lyude@redhat.com> wrote:
>
> 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 ),+

Don’t we have to match on some type here?

e.g.:

where
Self: IntoGemObject + // or whatever the right trait is
$( $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()) };

…so that we’re sure that things like self.as_raw() exist?

> + }
> +
> + 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
>
>

Perhaps worth it to look at the comment above, but as we only expect two
invocations of this macro, I’d say this is also fine as is.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>