Re: [PATCH v6 12/20] rust: io: move `Io` methods to extension trait

[Daniel Almeida <daniel.almeida@collabora.com>]

> On 6 Jul 2026, at 09:44, Gary Guo <gary@garyguo.net> wrote:
> 
> `Io` trait now has a single required method with many more provided
> methods. Provided methods may want to rely on their implementations to not
> be arbitrarily overridden by implementers for correctness or soundness.
> A good example is the `size` method, it may be relied by unsafe code and
> thus must be consistent with the metadata obtained from `as_ptr`.
> 
> Thus, create a new trait to host `size` method, extract existing provided
> methods to the new trait, and provide a blanket implementation. This
> pattern is used extensively in userspace Rust libraries e.g. `tokio` where
> `AsyncRead` has minimum methods and `AsyncReadExt` is what users mostly
> interact with.
> 
> To avoid changing all user imports, the base trait is renamed to `IoBase`
> and the newly added trait takes the existing `Io` name.
> 
> Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
> Suggested-by: Danilo Krummrich <dakr@kernel.org>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> ---
> rust/kernel/devres.rs |  3 ++-
> rust/kernel/io.rs     | 34 ++++++++++++++++++++++++----------
> rust/kernel/io/mem.rs |  6 +++---
> rust/kernel/pci/io.rs |  6 +++---
> 4 files changed, 32 insertions(+), 17 deletions(-)
> 
> diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> index 3545ffc5345d..6e0b845b229b 100644
> --- a/rust/kernel/devres.rs
> +++ b/rust/kernel/devres.rs
> @@ -68,6 +68,7 @@ struct Inner<T> {
> ///     devres::Devres,
> ///     io::{
> ///         Io,
> +///         IoBase,
> ///         Mmio,
> ///         MmioRaw,
> ///         MmioBackend,
> @@ -105,7 +106,7 @@ struct Inner<T> {
> ///     }
> /// }
> ///
> -/// impl<'a, const SIZE: usize> Io<'a> for &'a IoMem<SIZE> {
> +/// impl<'a, const SIZE: usize> IoBase<'a> for &'a IoMem<SIZE> {
> ///    type Backend = MmioBackend;
> ///    type Target = Region<SIZE>;
> ///
> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
> index c7533d605678..ab970e8b2e78 100644
> --- a/rust/kernel/io.rs
> +++ b/rust/kernel/io.rs
> @@ -223,7 +223,7 @@ fn io_view<'a, IO: Io<'a>, U>(
> /// operation.
> pub trait IoBackend {
>     /// View type for this I/O backend.
> -    type View<'a, T: ?Sized + KnownSize>: Io<'a, Backend = Self, Target = T>;
> +    type View<'a, T: ?Sized + KnownSize>: IoBase<'a, Backend = Self, Target = T>;
> 
>     /// Convert a `view` to a raw pointer for projection.
>     ///
> @@ -309,15 +309,12 @@ fn offset(self) -> usize {
> /// Types implementing this trait (e.g. MMIO BARs or PCI config regions)
> /// can perform I/O operations on regions of memory.
> ///
> -/// The [`Io`] trait provides:
> -/// - Method to convert into [`IoBackend::View`].
> -/// - Helper methods for offset validation and address calculation
> -/// - Fallible (runtime checked) accessors for different data widths
> -///
> -/// Which I/O methods are available depends on the associated [`IoBackend`] implementation.
> +/// This trait defines which backend shall be used for I/O operations and provides a method to
> +/// convert into [`IoBackend::View`]. Users should use the [`Io`] trait which provides the actual
> +/// methods to perform I/O operations.
> ///
> /// This should be implemented on cheaply copyable handles, such as references or view types.
> -pub trait Io<'a>: Copy {
> +pub trait IoBase<'a>: Copy {
>     /// Type that defines all I/O operations.
>     type Backend: IoBackend;
> 
> @@ -326,6 +323,21 @@ pub trait Io<'a>: Copy {
> 
>     /// Return a view that covers the full region.
>     fn as_view(self) -> <Self::Backend as IoBackend>::View<'a, Self::Target>;
> +}
> +
> +/// Extension trait to provide I/O operation methods to types that implement [`IoBase`].
> +///
> +/// This trait provides:
> +/// - Helper methods for offset validation and address calculation
> +/// - Fallible (runtime checked) accessors for different data widths
> +///
> +/// Which I/O methods are available depends on the associated [`IoBackend`] implementation.
> +pub trait Io<'a>: IoBase<'a> {
> +    /// Returns the size of this I/O region.
> +    #[inline]
> +    fn size(self) -> usize {
> +        KnownSize::size(Self::Backend::as_ptr(self.as_view()))
> +    }
> 
>     /// Fallible 8-bit read with runtime bounds check.
>     #[inline(always)]
> @@ -779,6 +791,8 @@ fn update<T, L, F>(self, location: L, f: F)
>     }
> }
> 

IIUC this is what makes the methods non-overridable?

If so, consider leaving a comment? i.e.:

// Provided methods may want to rely on their implementations to not be
// arbitrarily overridden by implementers for correctness or soundness, thus,
// enforce it through this blanket implementation

Makes it easier to maintain later IMHO.

> +impl<'a, T: IoBase<'a>> Io<'a> for T {}
> +
> /// A view of memory-mapped I/O region.
> ///
> /// # Invariant
> @@ -819,7 +833,7 @@ unsafe impl<T: ?Sized + Sync> Send for Mmio<'_, T> {}
> // SAFETY: `Mmio<'_, T>` is conceptually `&T` but in I/O memory.
> unsafe impl<T: ?Sized + Sync> Sync for Mmio<'_, T> {}
> 
> -impl<'a, T: ?Sized + KnownSize> Io<'a> for Mmio<'a, T> {
> +impl<'a, T: ?Sized + KnownSize> IoBase<'a> for Mmio<'a, T> {
>     type Backend = MmioBackend;
>     type Target = T;
> 
> @@ -920,7 +934,7 @@ unsafe fn project_view<'a, T: ?Sized + KnownSize, U: ?Sized + KnownSize>(
>     }
> }
> 
> -impl<'a, T: ?Sized + KnownSize> Io<'a> for RelaxedMmio<'a, T> {
> +impl<'a, T: ?Sized + KnownSize> IoBase<'a> for RelaxedMmio<'a, T> {
>     type Backend = RelaxedMmioBackend;
>     type Target = T;
> 
> diff --git a/rust/kernel/io/mem.rs b/rust/kernel/io/mem.rs
> index d9b3189d09b4..e95b769ebe47 100644
> --- a/rust/kernel/io/mem.rs
> +++ b/rust/kernel/io/mem.rs
> @@ -14,7 +14,7 @@
>             Region,
>             Resource, //
>         },
> -        Io,
> +        IoBase,
>         Mmio,
>         MmioBackend,
>         MmioRaw, //
> @@ -210,7 +210,7 @@ pub fn into_devres(self) -> Result<Devres<ExclusiveIoMem<'static, SIZE>>> {
>     }
> }
> 
> -impl<'a, const SIZE: usize> Io<'a> for &'a ExclusiveIoMem<'_, SIZE> {
> +impl<'a, const SIZE: usize> IoBase<'a> for &'a ExclusiveIoMem<'_, SIZE> {
>     type Backend = MmioBackend;
>     type Target = super::Region<SIZE>;
> 
> @@ -292,7 +292,7 @@ fn drop(&mut self) {
>     }
> }
> 
> -impl<'a, const SIZE: usize> Io<'a> for &'a IoMem<'_, SIZE> {
> +impl<'a, const SIZE: usize> IoBase<'a> for &'a IoMem<'_, SIZE> {
>     type Backend = MmioBackend;
>     type Target = super::Region<SIZE>;
> 
> diff --git a/rust/kernel/pci/io.rs b/rust/kernel/pci/io.rs
> index 4be33ecb4192..4d1d0afdc491 100644
> --- a/rust/kernel/pci/io.rs
> +++ b/rust/kernel/pci/io.rs
> @@ -8,8 +8,8 @@
>     device,
>     devres::Devres,
>     io::{
> -        Io,
>         IoBackend,
> +        IoBase,
>         IoCapable,
>         Mmio,
>         MmioBackend,
> @@ -144,7 +144,7 @@ fn io_write(view: ConfigSpace<'_, $ty>, value: $ty) {
> impl_config_space_io_capable!(u16, pci_read_config_word, pci_write_config_word);
> impl_config_space_io_capable!(u32, pci_read_config_dword, pci_write_config_dword);
> 
> -impl<'a, T: ?Sized + KnownSize> Io<'a> for ConfigSpace<'a, T> {
> +impl<'a, T: ?Sized + KnownSize> IoBase<'a> for ConfigSpace<'a, T> {
>     type Backend = ConfigSpaceBackend;
>     type Target = T;
> 
> @@ -267,7 +267,7 @@ fn drop(&mut self) {
>     }
> }
> 
> -impl<'a, const SIZE: usize> Io<'a> for &'a Bar<'_, SIZE> {
> +impl<'a, const SIZE: usize> IoBase<'a> for &'a Bar<'_, SIZE> {
>     type Backend = MmioBackend;
>     type Target = crate::io::Region<SIZE>;
> 
> 
> -- 
> 2.54.0
> 

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>