Re: [PATCH v6 RESEND 4/7] rust: io: factor common I/O helpers into Io trait

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Nov 10, 2025 at 10:41:16PM +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 a new Io trait, 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: 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>

This defines three traits:

* Io
* IoInfallible: Io
* IoFallible: Io

This particular split says that there are going to be cases where we
implement IoInfallible only, cases where we implement IoFallible only,
and maybe cases where we implement both.

And the distiction between them is whether the bounds check is runtime
or compile-time.

But this doesn't make much sense to me. Surely any Io resource that can
provide compile-time checked io can also provide runtime-checked io, so
maybe IoFallible should extend IoInfallible?

And why are these separate traits at all? Why not support both
compile-time and runtime-checked IO always?

I noticed also that the trait does not have methods for 64-bit writes,
and that these are left as inherent methods on Mmio.

The traits that would make sense to me are these:

* Io
* Io64: Io

where Io provides everything the three traits you have now provides, and
Io64 provides the 64-bit operations. That way, everything needs to
support operations of various sizes with both compile-time and
runtime-checked bounds, but types may opt-in to providing 64-bit ops.

Thoughts?

Alice