On Thu, Jun 26, 2025 at 10:00:42PM +0200, Danilo Krummrich wrote:
> ForeignOwnable::Target defines the payload data of a ForeignOwnable. For
> Arc<T> for instance, ForeignOwnable::Target would just be T.
>
> This is useful for cases where a trait bound is required on the target
> type of the ForeignOwnable. For instance:
>
> fn example<P>(data: P)
> where
> P: ForeignOwnable,
> P::Target: MyTrait,
> {}
>
> Suggested-by: Benno Lossin <lossin@xxxxxxxxxx>
> Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
One nit below:
> ---
> rust/kernel/alloc/kbox.rs | 2 ++
> rust/kernel/sync/arc.rs | 1 +
> rust/kernel/types.rs | 4 ++++
> 3 files changed, 7 insertions(+)
>
> diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
> index c386ff771d50..66fad9777567 100644
> --- a/rust/kernel/alloc/kbox.rs
> +++ b/rust/kernel/alloc/kbox.rs
> @@ -403,6 +403,7 @@ unsafe impl<T: 'static, A> ForeignOwnable for Box<T, A>
> where
> A: Allocator,
> {
> + type Target = T;
> type PointedTo = T;
> type Borrowed<'a> = &'a T;
> type BorrowedMut<'a> = &'a mut T;
> @@ -435,6 +436,7 @@ unsafe impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
> where
> A: Allocator,
> {
> + type Target = T;
> type PointedTo = T;
> type Borrowed<'a> = Pin<&'a T>;
> type BorrowedMut<'a> = Pin<&'a mut T>;
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index c7af0aa48a0a..24fb63597d35 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -374,6 +374,7 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
>
> // SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
> unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
> + type Target = T;
> type PointedTo = ArcInner<T>;
> type Borrowed<'a> = ArcBorrow<'a, T>;
> type BorrowedMut<'a> = Self::Borrowed<'a>;
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 3958a5f44d56..74c787b352a9 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -27,6 +27,9 @@
> /// [`into_foreign`]: Self::into_foreign
> /// [`PointedTo`]: Self::PointedTo
> pub unsafe trait ForeignOwnable: Sized {
> + /// The payload type of the foreign-owned value.
> + type Target;
I think `ForeignOwnable` also implies a `T` maybe get dropped via a
pointer from `into_foreign()`. Not sure it's worth mentioning though.
Regards,
Boqun
> +
> /// Type used when the value is foreign-owned. In practical terms only defines the alignment of
> /// the pointer.
> type PointedTo;
> @@ -128,6 +131,7 @@ unsafe fn try_from_foreign(ptr: *mut Self::PointedTo) -> Option<Self> {
>
> // SAFETY: The `into_foreign` function returns a pointer that is dangling, but well-aligned.
> unsafe impl ForeignOwnable for () {
> + type Target = ();
> type PointedTo = ();
> type Borrowed<'a> = ();
> type BorrowedMut<'a> = ();
> --
> 2.49.0
>
