Message-Id: <DEI7KGLC4AOM.2I3ZMD21X08TO@nvidia.com>
Date: Wed, 26 Nov 2025 09:43:18 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Alice Ryhl" <aliceryhl@...gle.com>, "Alexandre Courbot"
 <acourbot@...dia.com>
Cc: "Zhi Wang" <zhiw@...dia.com>, <rust-for-linux@...r.kernel.org>,
 <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <dakr@...nel.org>, <bhelgaas@...gle.com>, <kwilczynski@...nel.org>,
 <ojeda@...nel.org>, <alex.gaynor@...il.com>, <boqun.feng@...il.com>,
 <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>, <lossin@...nel.org>,
 <a.hindborg@...nel.org>, <tmgross@...ch.edu>, <markus.probst@...teo.de>,
 <helgaas@...nel.org>, <cjia@...dia.com>, <smitra@...dia.com>,
 <ankita@...dia.com>, <aniketa@...dia.com>, <kwankhede@...dia.com>,
 <targupta@...dia.com>, <joelagnelf@...dia.com>, <jhubbard@...dia.com>,
 <zhiwang@...nel.org>
Subject: Re: [PATCH v7 3/6] rust: io: factor common I/O helpers into Io
 trait

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@...dia.com>
>> >> Cc: Alice Ryhl <aliceryhl@...gle.com>
>> >> Cc: Bjorn Helgaas <helgaas@...nel.org>
>> >> Cc: Danilo Krummrich <dakr@...nel.org>
>> >> Cc: John Hubbard <jhubbard@...dia.com>
>> >> Signed-off-by: Zhi Wang <zhiw@...dia.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.

Thanks for elaborating. I really like this idea.

You are right that is would drastically reduce the number of times we
need to rely on `build_assert`, as well as concentrating its use to a
single point (bounded int constructor) instead of having to sprinkle
extra invocations in the Io module.

Now I would also like to also keep the ability to define "integers which
only X LSBs represent the value", so I guess we could have a distinct
type for "integers within a lower and higher bound", since the latter
requires two generic constants vs one for the former.

Maybe we could derive the current `Bounded` and that new type from a
more generic "constrained integer" type, which constraint rule itself is
given as a generic argument? From this we could then easily build all
sort of funny things. That could turn into quite an undertaking though.

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