Re: [PATCH 2/3] rust: xarray: abstract `xa_alloc_cyclic`

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Oct 06, 2025 at 07:30:23PM +0300, Onur Özkan wrote:
> Implements `alloc_cyclic` function to `XArray<T>` that
> wraps `xa_alloc_cyclic` safely.
> 
> Resolves a task from the nova/core task list under the "XArray
> bindings [XARR]" section in "Documentation/gpu/nova/core/todo.rst"
> file.
> 
> Signed-off-by: Onur Özkan <work@onurozkan.dev>
> ---
>  rust/kernel/xarray.rs | 43 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 43 insertions(+)
> 
> diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
> index 1b882cd2f58b..4c2fdf53c7af 100644
> --- a/rust/kernel/xarray.rs
> +++ b/rust/kernel/xarray.rs
> @@ -305,6 +305,49 @@ pub fn alloc(
> 
>          Ok(id)
>      }
> +
> +    /// Allocates an empty slot within the given `limit`, storing `value` and cycling from `*next`.
> +    ///
> +    /// May drop the lock if needed to allocate memory, and then reacquire it afterwards.
> +    ///
> +    /// On success, returns the allocated id.
> +    ///
> +    /// On failure, returns the element which was attempted to be stored.
> +    pub fn alloc_cyclic(
> +        &mut self,
> +        limit: bindings::xa_limit,

Could we use a Range<u32> type or similar here? I don't think we want a
bindings type.

> +        next: &mut u32,

So this is a mutable reference because it writes `*id + 1` to next,
taking wrap-around into account? The docs should probably explain that.

> +        value: T,
> +        gfp: alloc::Flags,
> +    ) -> Result<u32, StoreError<T>> {
> +        build_assert!(
> +            T::FOREIGN_ALIGN >= 4,
> +            "pointers stored in XArray must be 4-byte aligned"
> +        );

It should be enough to have this in the constructor. I don't think it's
needed here.

Alice