Re: [PATCH v3 3/5] rust: scatterlist: Add abstraction for sg_table

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Aug 25, 2025 at 03:24:42PM +0200, Danilo Krummrich wrote:
> Add a safe Rust abstraction for the kernel's scatter-gather list
> facilities (`struct scatterlist` and `struct sg_table`).
> 
> This commit introduces `SGTable<T>`, a wrapper that uses a generic
> parameter to provide compile-time guarantees about ownership and lifetime.
> 
> The abstraction provides two primary states:
> - `SGTable<Owned<P>>`: Represents a table whose resources are fully
>   managed by Rust. It takes ownership of a page provider `P`, allocates
>   the underlying `struct sg_table`, maps it for DMA, and handles all
>   cleanup automatically upon drop. The DMA mapping's lifetime is tied to
>   the associated device using `Devres`, ensuring it is correctly unmapped
>   before the device is unbound.
> - `SGTable<Borrowed>` (or just `SGTable`): A zero-cost representation of
>   an externally managed `struct sg_table`. It is created from a raw
>   pointer using `SGTable::as_ref()` and provides a lifetime-bound
>   reference (`&'a SGTable`) for operations like iteration.
> 
> The API exposes a safe iterator that yields `&SGEntry` references,
> allowing drivers to easily access the DMA address and length of each
> segment in the list.
> 
> Co-developed-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Overall LGTM. With comments addressed:
Reviewed-by: Alice Ryhl <aliceryhl@google.com>

> +impl RawSGTable {
> +    fn new(
> +        pages: &mut [*mut bindings::page],

This should probably be unsafe due to the raw pointer. Or could we pass
any pointer here?

Alice