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

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Nov 26, 2025 at 04:52:05PM +0900, Alexandre Courbot wrote:
> On Tue Nov 25, 2025 at 11:58 PM JST, Alice Ryhl wrote:
> > On Tue, Nov 25, 2025 at 10:44:29PM +0900, Alexandre Courbot wrote:
> >> 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.
> >
> > I mean something like this:
> >
> > trait Io {
> >     const SIZE: usize;
> >     fn write(&mut self, i: Bounded<Self::SIZE>);
> > }
> 
> I have experimented a bit with this idea, and unfortunately expressing
> `Bounded<Self::SIZE>` requires the generic_const_exprs feature and is
> not doable as of today.
> 
> Bounding an integer with an upper/lower bound also proves to be more
> demanding than the current `Bounded` design. For the `MIN` and `MAX`
> constants must be of the same type as the wrapped `T` type, which again
> makes rustc unhappy ("the type of const parameters must not depend on
> other generic parameters"). A workaround would be to use a macro to
> define individual types for each integer type we want to support - or to
> just limit this to `usize`.
> 
> But the requirement for generic_const_exprs makes this a non-starter I'm
> afraid. :/

Can you try this?

trait Io {
    type IdxInt: Int;
    fn write(&mut self, i: Self::IdxInt);
}

then implementers would write:

impl Io for MyIo {
    type IdxInt = Bounded<17>;
}

instead of:
impl Io for MyIo {
    const SIZE = 17;
}

Alice