Re: [PATCH v2 3/5] rust: scatterlist: Add type-state abstraction for sg_table

[Danilo Krummrich]

On Sat Aug 23, 2025 at 3:47 PM CEST, Alexandre Courbot wrote:
> Oops, forgot to mention a couple more things:
>
> On Thu Aug 21, 2025 at 1:52 AM JST, 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 type-state
>> pattern to provide compile-time guarantees about ownership and lifetime.
>
> Is this actually a typestate? From my understanding, the typestate
> pattern implies transitions from one state to the other (such as
> Unmapped -> Mapped), but in this version there are no such transitions
> (the previous ones had, though). We are just using a generic parameter,
> so mentioning typestate sounds a bit misleading to me.

I'd argue that it's still kind of a typestate. You can derive &SGTable (i.e.
&SGTable<Borrowed>) from SGTabe<Owned>. So, technically there is an
uni-directional transition I guess.

> Another random thought, in the owned case, do we want to provide an
> accessor to the provider of the backing pages? Or do we expect the
> caller to take dispositions to keep such a reference if they need to
> access the backing buffer post-mapping?

That's not going to work that easily. Once the backing pages are DMA mapped, the
backing buffer can be accessed safely an more.

See also the safety requirements of dma::CoherentAllocation::as_slice() and
dma::CoherentAllocation::as_slice_mut().

If we want to support that, we have to provide a new type for this and maybe
want to define a common trait for DMA mapped memory accessors, etc.

Not the scope for this series, I believe. :)