Re: [PATCH v7 3/6] rust: io: factor common I/O helpers into Io trait

["Alexandre Courbot" <acourbot@nvidia.com>]

On Fri Nov 21, 2025 at 11:20 PM JST, Alice Ryhl wrote:
> On Wed, Nov 19, 2025 at 01:21:13PM +0200, Zhi Wang wrote:
>> The previous Io<SIZE> type combined both the generic I/O access helpers
>> and MMIO implementation details in a single struct.
>> 
>> To establish a cleaner layering between the I/O interface and its concrete
>> backends, paving the way for supporting additional I/O mechanisms in the
>> future, Io<SIZE> need to be factored.
>> 
>> Factor the common helpers into new {Io, Io64} traits, and move the
>> MMIO-specific logic into a dedicated Mmio<SIZE> type implementing that
>> trait. Rename the IoRaw to MmioRaw and update the bus MMIO implementations
>> to use MmioRaw.
>> 
>> No functional change intended.
>> 
>> Cc: Alexandre Courbot <acourbot@nvidia.com>
>> Cc: Alice Ryhl <aliceryhl@google.com>
>> Cc: Bjorn Helgaas <helgaas@kernel.org>
>> Cc: Danilo Krummrich <dakr@kernel.org>
>> Cc: John Hubbard <jhubbard@nvidia.com>
>> Signed-off-by: Zhi Wang <zhiw@nvidia.com>
>
> I said this on a previous version, but I still don't buy the split
> into IoFallible and IoInfallible.
>
> For one, we're never going to have a method that can accept any Io - we
> will always want to accept either IoInfallible or IoFallible, so the
> base Io trait serves no purpose.
>
> For another, the docs explain that the distinction between them is
> whether the bounds check is done at compile-time or runtime. That is not
> the kind of capability one normally uses different traits to distinguish
> between. It makes sense to have additional traits to distinguish
> between e.g.:
>
> * Whether IO ops can fail for reasons *other* than bounds checks.
> * Whether 64-bit IO ops are possible.
>
> Well ... I guess one could distinguish between whether it's possible to
> check bounds at compile-time at all. But if you can check them at
> compile-time, it should always be possible to check at runtime too, so
> one should be a sub-trait of the other if you want to distinguish
> them. (And then a trait name of KnownSizeIo would be more idiomatic.)
>
> And I'm not really convinced that the current compile-time checked
> traits are a good idea at all. See:
> https://lore.kernel.org/all/DEEEZRYSYSS0.28PPK371D100F@nvidia.com/
>
> If we want to have a compile-time checked trait, then the idiomatic way
> to do that in Rust would be to have a new integer type that's guaranteed
> to only contain integers <= the size. For example, the Bounded integer
> being added elsewhere.

Would that be so different from using an associated const value though?
IIUC the bounded integer type would play the same role, only slightly
differently - by that I mean that if the offset is expressed by an
expression that is not const (such as an indexed access), then the
bounded integer still needs to rely on `build_assert` to be built.