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

[Alice Ryhl <aliceryhl@google.com>]

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>);
}

You know that Bounded<SIZE> contains a number less than SIZE, so you
know it's in-bounds without any build_assert required.

Yes, if there's a constructor for Bounded that utilizes build_assert,
then you end up with a build_assert to create it. But I think in many
cases it's avoidable depending on where the index comes from.

For example if you iterate all indices 0..SIZE, there could be a way to
directly create Bounded<SIZE> values from an iterator. Or if the index
comes from an ioctl, then you probably runtime check the integer at the
ioctl entrypoint, in which case you want the runtime-checked
constructor.

Alice