linux-kernel - Re: [PATCH v4 2/9] rust/drm: Add gem::impl_aref_for_gem

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Message-ID: <aPJDGqsRFzuARlgP@google.com>
Date: Fri, 17 Oct 2025 13:22:34 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Lyude Paul <lyude@...hat.com>
Cc: dri-devel@...ts.freedesktop.org, rust-for-linux@...r.kernel.org, 
	Daniel Almeida <daniel.almeida@...labora.com>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Danilo Krummrich <dakr@...nel.org>, 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>, 
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/9] rust/drm: Add gem::impl_aref_for_gem_obj!

On Thu, Oct 16, 2025 at 05:08:15PM -0400, Lyude Paul 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>
> ---
>  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 20c2769a8c9d6..981fbb931e952 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_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() };
> +
> +                // SAFETY: The safety requirements guarantee that the refcount is non-zero.
> +                unsafe { bindings::drm_gem_object_put(obj.as_raw()) };

I would prefer to move the call to `.as_raw()` to the `let obj` line so
that the reference more clearly expires before the call to
`drm_gem_object_put()`. The reference must not exist during the call to
`drm_gem_object_put()`.

> +            }
> +        }
> +    };
> +}
> +
> +pub(crate) use impl_aref_for_gem_obj;

The usual way to export macros outside the current file is:

1. Annotated with #[macro_export]
2. Export with `pub use impl_aref_for_gem_obj`

Alice