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

[Alexandre Courbot <acourbot@nvidia.com>]

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`.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/io.rs | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 64 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
index dc894a45bbcc..baa8d3baa20c 100644
--- a/rust/kernel/io.rs
+++ b/rust/kernel/io.rs
@@ -556,9 +556,15 @@ fn io_addr_assert<U>(&self, offset: usize) -> usize {
 
 /// Implements [`IoCapable`] on `$mmio` for `$ty` using `$read_fn` and `$write_fn`.
 macro_rules! impl_mmio_io_capable {
-    ($mmio:ident, $(#[$attr:meta])* $ty:ty, $read_fn:ident, $write_fn:ident) => {
+    (
+        $mmio:ident $(< $($generics:tt),+ >)*,
+        $(#[$attr:meta])* $ty:ty,
+        $read_fn:ident, $write_fn:ident
+    ) => {
         $(#[$attr])*
-        impl<const SIZE: usize> IoCapable<$ty> for $mmio<SIZE> {
+        impl<$($($generics),+,)* const SIZE: usize> IoCapable<$ty>
+            for $mmio<$($($generics),+,)* SIZE>
+        {
             unsafe fn io_read(&self, address: usize) -> $ty {
                 // SAFETY: By the trait invariant `address` is a valid address for MMIO operations.
                 unsafe { bindings::$read_fn(address as *const c_void) }
@@ -695,3 +701,59 @@ pub unsafe fn from_raw(raw: &MmioRaw<SIZE>) -> &Self {
         call_mmio_write(writeq_relaxed) <- u64
     );
 }
+
+/// [`Mmio`] wrapper using relaxed accessors.
+///
+/// This provides an implementation of [`Io`] that uses relaxed I/O MMIO operands instead of the
+/// regular ones.
+///
+/// # Examples
+///
+/// ```no_run
+/// use kernel::io::{Io, Mmio, RelaxedMmio};
+///
+/// fn do_io(io: &Mmio<0x100>) {
+///     let relaxed_io = RelaxedMmio::from(io);
+///
+///     // The access is performed using `readl_relaxed` instead of `readl`.
+///     let v = relaxed_io.read32(0x10);
+/// }
+///
+/// ```
+#[repr(transparent)]
+pub struct RelaxedMmio<'a, const SIZE: usize = 0>(&'a Mmio<SIZE>);
+
+impl<'a, const SIZE: usize> From<&'a Mmio<SIZE>> for RelaxedMmio<'a, SIZE> {
+    fn from(value: &'a Mmio<SIZE>) -> Self {
+        Self(value)
+    }
+}
+
+impl<'a, const SIZE: usize> Io for RelaxedMmio<'a, SIZE> {
+    #[inline]
+    fn addr(&self) -> usize {
+        self.0.addr()
+    }
+
+    #[inline]
+    fn maxsize(&self) -> usize {
+        self.0.maxsize()
+    }
+}
+
+impl<'a, const SIZE: usize> IoKnownSize for RelaxedMmio<'a, SIZE> {
+    const MIN_SIZE: usize = SIZE;
+}
+
+// MMIO regions support 8, 16, and 32-bit accesses.
+impl_mmio_io_capable!(RelaxedMmio<'a>, u8, readb_relaxed, writeb_relaxed);
+impl_mmio_io_capable!(RelaxedMmio<'a>, u16, readw_relaxed, writew_relaxed);
+impl_mmio_io_capable!(RelaxedMmio<'a>, u32, readl_relaxed, writel_relaxed);
+// MMIO regions on 64-bit systems also support 64-bit accesses.
+impl_mmio_io_capable!(
+    RelaxedMmio<'a>,
+    #[cfg(CONFIG_64BIT)]
+    u64,
+    readq_relaxed,
+    writeq_relaxed
+);

-- 
2.52.0