linux-kernel - Re: [PATCH v5 1/8] rust/drm: Add gem::impl_aref_for_gem

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Message-Id: <1E5BE820-90BA-4135-82A8-05777B738BD5@collabora.com>
Date: Mon, 24 Nov 2025 12:23:51 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Lyude Paul <lyude@...hat.com>
Cc: dri-devel@...ts.freedesktop.org,
 rust-for-linux@...r.kernel.org,
 Alice Ryhl <aliceryhl@...gle.com>,
 Danilo Krummrich <dakr@...nel.org>,
 linux-kernel@...r.kernel.org,
 David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 Trevor Gross <tmgross@...ch.edu>,
 Asahi Lina <lina+kernel@...hilina.net>,
 Shankari Anand <shankari.ak0208@...il.com>
Subject: Re: [PATCH v5 1/8] rust/drm: Add gem::impl_aref_for_gem_obj!

Hi Lyude,

> On 23 Oct 2025, at 18:22, Lyude Paul <lyude@...hat.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@...hat.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@...labora.com>