Re: [PATCH v3 04/16] rust: io: perform conversions using `AsRepr`

["Alexandre Courbot" <acourbot@nvidia.com>]

On Wed Aug 19, 2026 at 8:09 PM JST, Gary Guo wrote:
> For types that are layout-compatible with an I/O capable type, we would
> want the ability to use them directly for I/O operations. E.g.
>
>     bitfield! {
>         pub struct Foo(u32) {
>             ...
>         }
>     }
>
>     #[repr(C)]
>     struct Bar {
>         foo: Foo,
>     }
>
>     let mmio: Mmio<'_, Bar> = ...;
>     io_read!(mmio, .foo)
>
> Currently this feature is available from `register!()` macro but not
> otherwise available with `io_read!`, `io_write!`. Support this by
> performing conversions to I/O primitives via the `AsRepr`/`AsReprMut`
> trait.
>
> This makes the `IoLoc::IoType` and `Register::Storage` redundant; thus
> remove them; also convert register methods to use the `read_val` and
> `write_val` instead.

Nice, the redundancy was bothering me a bit so this is clearly a better
design.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>

>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> ---
>  rust/kernel/bitfield.rs    |  10 ++++
>  rust/kernel/io.rs          | 135 +++++++++++++++++++++++++--------------------
>  rust/kernel/io/register.rs |  15 -----
>  rust/macros/io/register.rs |   2 -
>  4 files changed, 86 insertions(+), 76 deletions(-)
>
> diff --git a/rust/kernel/bitfield.rs b/rust/kernel/bitfield.rs
> index a0d089423f21..619c5e2189d1 100644
> --- a/rust/kernel/bitfield.rs
> +++ b/rust/kernel/bitfield.rs
> @@ -308,6 +308,7 @@ macro_rules! bitfield {
>          $(#[$attr])*
>          #[repr(transparent)]
>          #[derive(Clone, Copy, PartialEq, Eq)]
> +        #[derive($crate::prelude::FromBytes, $crate::prelude::IntoBytes)]

Do we need `FromBytes`/`IntoBytes` for every single bitfield type? I
mean that probably doesn't hurt, but if we need them for registers then
we can derive them from the register macro.

<...>
> @@ -498,10 +497,19 @@ fn try_cast<U>(self) -> Result<<Self::Backend as IoBackend>::View<'a, U>>
>      #[inline]
>      fn read_val(self) -> Self::Target
>      where
> -        Self::Backend: IoCapable<Self::Target>,
> -        Self::Target: Sized,
> +        Self::Target: AsReprMut,
> +        Self::Backend: IoCapable<<Self::Target as AsRepr>::Repr>,
>      {
> -        Self::Backend::io_read(self.as_view())
> +        let view = self.as_view();
> +        // SAFETY: `AsRepr` guarantees layout compatibility.
> +        let repr_view = unsafe {
> +            Self::Backend::project_view(
> +                view,
> +                Self::Backend::as_ptr(view).cast::<<Self::Target as AsRepr>::Repr>(),
> +            )
> +        };
> +
> +        Self::Target::from_repr(Self::Backend::io_read(repr_view))
>      }
>  
>      /// Write a value to I/O.
> @@ -520,10 +528,19 @@ fn read_val(self) -> Self::Target
>      #[inline]
>      fn write_val(self, value: Self::Target)
>      where
> -        Self::Backend: IoCapable<Self::Target>,
> -        Self::Target: Sized,
> +        Self::Target: AsRepr,
> +        Self::Backend: IoCapable<<Self::Target as AsRepr>::Repr>,
>      {
> -        Self::Backend::io_write(self.as_view(), value)
> +        let view = self.as_view();
> +        // SAFETY: `AsRepr` guarantees layout compatibility.
> +        let repr_view = unsafe {
> +            Self::Backend::project_view(
> +                view,
> +                Self::Backend::as_ptr(view).cast::<<Self::Target as AsRepr>::Repr>(),
> +            )
> +        };

Can we factorize the two `unsafe` blocks of `read_val` and `write_val`
into a common private helper?