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

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Nov 24, 2025 at 12:08:46PM +0200, Zhi Wang wrote:
> On Fri, 21 Nov 2025 14:20:13 +0000
> Alice Ryhl <aliceryhl@google.com> 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.)
> > 
> 
> Thanks a lot for the detailed feedback. Agree with the points. Let's
> keep the IoFallible and IoInfallible traits but not just tie them to the
> bound checks in the docs.

What do you plan to write in the docs instead?

> > 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.
> > 
> 
> Oops, this is a interesting bug. :) I think we can apply the bound
> integer to IoFallible and IoInfallible to avoid possible problems
> mentioned above. E.g. constructing a Bounded interger when constructing
> Mmio and ConfigSpace objects and use them in the boundary checks in the
> trait methods.
> 
> I saw Alex had already had an implementation of Bounded integer [1] in
> rust-next. While my patchset is against driver-core-testing
> branch. Would it be OK that we move on without it and switch to Bounded
> integer when it is landed to driver-core-testing? I am open to
> suggestions. :)

The last -rc of this cycle is already out, so I don't think you need to
worry about branch issues - you won't land it in time for that.

But there is another problem: Bounded only supports the case where the
bound is a power of two, so I don't think it's usable here. You can have
Io regions whose size is not a power of two.

Alice