[PATCH v2 3/6] rust: io: provide Mmio relaxed ops through a wrapper type

Alexandre Courbot
2026-02-06T06:00:45Z

Relaxed I/O accessors for `Mmio` are currently implemented as an extra
set of methods that mirror the ones defined in `Io`, but with the
`_relaxed` suffix.

This makes these methods impossible to use with generic code, which is a
highly plausible proposition now that we have the `Io` trait.

Address this by adding a new `RelaxedMmio` wrapper type for `Mmio` that
provides its own `IoCapable` implementations relying on the relaxed C
accessors. This makes it possible to use relaxed operations on a `Mmio`
simply by wrapping it, and to use `RelaxedMmio` in code generic against
`Io`.

Acked-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
rust/kernel/io.rs | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 62 insertions(+)

diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
index dc894a45bbcc..d5d6e9501453 100644
--- a/rust/kernel/io.rs
+++ b/rust/kernel/io.rs
@@ -695,3 +695,65 @@ pub unsafe fn from_raw(raw: &MmioRaw<SIZE>) -> &Self {
call_mmio_write(writeq_relaxed) <- u64
);
}
+
+/// [`Mmio`] wrapper using relaxed accessors.
+///
+/// This type provides an implementation of [`Io`] that uses relaxed I/O MMIO operands instead of
+/// the regular ones.
+///
+/// See [`Mmio::relaxed`] for a usage example.
+#[repr(transparent)]
+pub struct RelaxedMmio<const SIZE: usize = 0>(Mmio<SIZE>);
+
+impl<const SIZE: usize> Io for RelaxedMmio<SIZE> {
+ #[inline]
+ fn addr(&self) -> usize {
+ self.0.addr()
+ }
+
+ #[inline]
+ fn maxsize(&self) -> usize {
+ self.0.maxsize()
+ }
+}
+
+impl<const SIZE: usize> IoKnownSize for RelaxedMmio<SIZE> {
+ const MIN_SIZE: usize = SIZE;
+}
+
+impl<const SIZE: usize> Mmio<SIZE> {
+ /// Returns a [`RelaxedMmio`] reference that performs relaxed I/O operations.
+ ///
+ /// Relaxed accessors do not provide ordering guarantees with respect to DMA or memory accesses
+ /// and can be used when such ordering is not required.
+ ///
+ /// # Examples
+ ///
+ /// ```no_run
+ /// use kernel::io::{Io, Mmio, RelaxedMmio};
+ ///
+ /// fn do_io(io: &Mmio<0x100>) {
+ /// // The access is performed using `readl_relaxed` instead of `readl`.
+ /// let v = io.relaxed().read32(0x10);
+ /// }
+ ///
+ /// ```
+ pub fn relaxed(&self) -> &RelaxedMmio<SIZE> {
+ // SAFETY: `RelaxedMmio` is `#[repr(transparent)]` over `Mmio`, so `Mmio<SIZE>` and
+ // `RelaxedMmio<SIZE>` have identical layout.
+ unsafe { core::mem::transmute(self) }
+ }
+}
+
+// MMIO regions support 8, 16, and 32-bit accesses.
+impl_mmio_io_capable!(RelaxedMmio, u8, readb_relaxed, writeb_relaxed);
+impl_mmio_io_capable!(RelaxedMmio, u16, readw_relaxed, writew_relaxed);
+impl_mmio_io_capable!(RelaxedMmio, u32, readl_relaxed, writel_relaxed);
+// MMIO regions on 64-bit systems also support 64-bit accesses.
+impl_mmio_io_capable!(
+ RelaxedMmio,
+ #[cfg(CONFIG_64BIT)]
+ u64,
+ readq_relaxed,
+ writeq_relaxed
+);

--
2.53.0