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

[Danilo Krummrich]

On Sat Aug 23, 2025 at 4:16 PM CEST, Alexandre Courbot wrote:
> On Sat Aug 23, 2025 at 10:57 PM JST, Danilo Krummrich wrote:
>> 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.
>
> That's technically correct, but is also not the intent of the design, at
> least compared to something like Unmapped <-> Mapped. Not a big problem
> if you prefer to keep the current naming though.

I don't mind to name / call it differently, any suggestion?

>>
>>> 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().
>
> Yup. So couldn't similar accessors (marked unsafe of course) be
> convenient?

Absolutely! But I think we want them represented by a common trait that can be
used by SGTable and dma::CoherentAllocation.

>>
>> 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. :)
>
> I've had a few thoughts in that direction as well, but completely agree
> we should debate about this *after* this series is merged. :)

Yeah, let's add this feature subsequently.