Re: [PATCH v2] rust: io: Add a BigEndianMmio wrapper

On Thu Aug 6, 2026 at 7:22 AM BST, Link Mauve wrote:
> This allows the user to read from and write to big-endian MMIO devices,
> such as those found on PowerPC systems.
>
> The implementation is pretty much a copy-paste of the RelaxedMmio type,
> with the business logic changed.
>
> This has been tested with various WIP drivers on the Nintendo Wii, and I
> thought it would be nicer to upstream it before those are completely
> ready, to get some review.

An alternative approach would be to have a generic wrapper `BigEndian<IO>` where
it performs byte-swapping in its `IoCapable` implementation. This is more
composable as it works for all I/O.

Of course, this means that it won't be using the specialized BE I/O impl, but
then the only architectures that have specialized ioread16be are PPC32 and
SPARC, so maybe it's a reasonable trade-off to make?

Best,
Gary

>
> Signed-off-by: Link Mauve <[email protected]>
> ---
>  rust/helpers/io.c | 34 ++++++++++++++++++
>  rust/kernel/io.rs | 88 +++++++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 120 insertions(+), 2 deletions(-)
>
> Changes since v1:
> - Link to v1:
>   https://lore.kernel.org/rust-for-linux/178598049138.1212746.2443151507074972963.b4-review@b4/T/
> - Fix copy/paste mistake in backend docstring (thank you Ethan Plant!)
> - Fix safety comment in macro documentation.
>
> diff --git a/rust/helpers/io.c b/rust/helpers/io.c
> index 7ed9a4f77f1b..3bbb86a00d67 100644
> --- a/rust/helpers/io.c
> +++ b/rust/helpers/io.c
> @@ -120,6 +120,40 @@ __rust_helper void rust_helper_writeq_relaxed(u64 value, void __iomem *addr)
>  }
>  #endif
>  
> +__rust_helper u16 rust_helper_ioread16be(const void __iomem *addr)
> +{
> +	return ioread16be(addr);
> +}
> +
> +__rust_helper u32 rust_helper_ioread32be(const void __iomem *addr)
> +{
> +	return ioread32be(addr);
> +}
> +
> +#ifdef CONFIG_64BIT
> +__rust_helper u64 rust_helper_ioread64be(const void __iomem *addr)
> +{
> +	return ioread64be(addr);
> +}
> +#endif
> +
> +__rust_helper void rust_helper_iowrite16be(u16 value, void __iomem *addr)
> +{
> +	iowrite16be(value, addr);
> +}
> +
> +__rust_helper void rust_helper_iowrite32be(u32 value, void __iomem *addr)
> +{
> +	iowrite32be(value, addr);
> +}
> +
> +#ifdef CONFIG_64BIT
> +__rust_helper void rust_helper_iowrite64be(u64 value, void __iomem *addr)
> +{
> +	iowrite64be(value, addr);
> +}
> +#endif
> +
>  __rust_helper resource_size_t rust_helper_resource_size(struct resource *res)
>  {
>  	return resource_size(res);
> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
> index a38c20ba3d23..ab8b7864298a 100644
> --- a/rust/kernel/io.rs
> +++ b/rust/kernel/io.rs
> @@ -1194,14 +1194,14 @@ impl IoCapable<$ty> for $backend {
>              #[inline]
>              fn io_read(view: <$backend as IoBackend>::View<'_, $ty>) -> $ty {
>                  // SAFETY: `$backend::as_ptr(view)` is a valid pointer for MMIO operations for both
> -                // `MmioBackend` and `RelaxedMmioBackend`.
> +                // `MmioBackend`, `RelaxedMmioBackend` and `BigEndianMmioBackend`.
>                  unsafe { bindings::$read_fn($backend::as_ptr(view).cast_const().cast()) }
>              }
>  
>              #[inline]
>              fn io_write(view: <$backend as IoBackend>::View<'_, $ty>, value: $ty) {
>                  // SAFETY: `$backend::as_ptr(view)` is a valid pointer for MMIO operations for both
> -                // `MmioBackend` and `RelaxedMmioBackend`.
> +                // `MmioBackend`, `RelaxedMmioBackend` and `BigEndianMmioBackend`.
>                  unsafe { bindings::$write_fn(value, $backend::as_ptr(view).cast()) }
>              }
>          }
> @@ -1751,3 +1751,87 @@ macro_rules! io_write {
>  }
>  #[doc(inline)]
>  pub use crate::io_write;
> +
> +/// [`Mmio`] wrapper using big-endian accessors.
> +///
> +/// This type provides an implementation of [`Io`] that uses big-endian I/O MMIO operands instead of
> +/// the regular little-endian ones.
> +///
> +/// See [`Mmio::big_endian`] for a usage example.
> +#[repr(transparent)]
> +pub struct BigEndianMmio<'a, T: ?Sized>(Mmio<'a, T>);
> +
> +impl<T: ?Sized> Copy for BigEndianMmio<'_, T> {}
> +impl<T: ?Sized> Clone for BigEndianMmio<'_, T> {
> +    #[inline]
> +    fn clone(&self) -> Self {
> +        *self
> +    }
> +}
> +
> +/// I/O Backend for memory-mapped I/O using big-endian accessors.
> +pub struct BigEndianMmioBackend;
> +
> +impl IoBackend for BigEndianMmioBackend {
> +    type View<'a, T: ?Sized + KnownSize> = BigEndianMmio<'a, T>;
> +
> +    #[inline]
> +    fn as_ptr<'a, T: ?Sized + KnownSize>(view: Self::View<'a, T>) -> *mut T {
> +        MmioBackend::as_ptr(view.0)
> +    }
> +
> +    #[inline]
> +    unsafe fn project_view<'a, T: ?Sized + KnownSize, U: ?Sized + KnownSize>(
> +        view: Self::View<'a, T>,
> +        ptr: *mut U,
> +    ) -> Self::View<'a, U> {
> +        // SAFETY: Per safety requirement.
> +        BigEndianMmio(unsafe { MmioBackend::project_view(view.0, ptr) })
> +    }
> +}
> +
> +impl<'a, T: ?Sized + KnownSize> IoBase<'a> for BigEndianMmio<'a, T> {
> +    type Backend = BigEndianMmioBackend;
> +    type Target = T;
> +
> +    #[inline]
> +    fn as_view(self) -> BigEndianMmio<'a, T> {
> +        self
> +    }
> +}
> +
> +impl<'a, T: ?Sized> Mmio<'a, T> {
> +    /// Returns a [`BigEndianMmio`] reference that performs big-endian I/O operations.
> +    ///
> +    /// Big-endian makes no change to 8-bit accesses, but will invert the bytes of 16-, 32- and
> +    /// 64-bit accesses, to ensure numbers will be read in their correct order.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```no_run
> +    /// use kernel::io::{
> +    ///     Io,
> +    ///     Mmio,
> +    ///     Region,
> +    ///     BigEndianMmio,
> +    /// };
> +    ///
> +    /// fn do_io(io: Mmio<'_, Region<0x100>>) {
> +    ///     // The access is performed using `ioread32be` instead of `readl`.
> +    ///     let v = io.big_endian().read32(0x10);
> +    /// }
> +    ///
> +    /// ```
> +    #[inline]
> +    pub fn big_endian(self) -> BigEndianMmio<'a, T> {
> +        BigEndianMmio(self)
> +    }
> +}
> +
> +// MMIO regions support 8, 16, and 32-bit accesses.
> +impl_mmio_io_capable!(BigEndianMmioBackend, u8, readb, writeb);
> +impl_mmio_io_capable!(BigEndianMmioBackend, u16, ioread16be, iowrite16be);
> +impl_mmio_io_capable!(BigEndianMmioBackend, u32, ioread32be, iowrite32be);
> +// MMIO regions on 64-bit systems also support 64-bit accesses.
> +#[cfg(CONFIG_64BIT)]
> +impl_mmio_io_capable!(BigEndianMmioBackend, u64, ioread64be, iowrite64be);
>
> base-commit: 1701fda2f58e345c050f4309971bdc07cd6146ba